Monitoring massive fracture growth at 2-km depths using surface tiltmeter arrays

USGS Publications Warehouse

Wood, M.D.

1979-01-01

Tilt due to massive hydraulic fractures induced in sedimentary rocks at depths of up to 2.2 km have been recorded by surface tiltmeters. Injection of fluid volumes up to 4 ?? 105 liters and masses of propping agent up to 5 ?? 105 kg is designed to produce fractures approximately 1 km long, 50-100 m high and about 1 cm wide. The surface tilt data adequately fit a dislocation model of a tensional fault in a half-space. Theoretical and observational results indicate that maximum tilt occurs at a distance off the strike of the fracture equivalent to 0.4 of the depth to the fracture. Azimuth and extent of the fracture deduced from the geometry of the tilt field agree with other kinds of geophysical measurements. Detailed correlation of the tilt signatures with pumping parameters (pressure, rate, volume, mass) have provided details on asymmetry in geometry and growth rate. Whereas amplitude variations in tilt vary inversely with the square of the depth, changes in flow rate or pressure gradient can produce a cubic change in width. These studies offer a large-scale experimental approach to the study of problems involving fracturing, mass transport, and dilatancy processes. ?? 1979.

Well test mathematical model for fractures network in tight oil reservoirs

NASA Astrophysics Data System (ADS)

Diwu, Pengxiang; Liu, Tongjing; Jiang, Baoyi; Wang, Rui; Yang, Peidie; Yang, Jiping; Wang, Zhaoming

2018-02-01

Well test, especially build-up test, has been applied widely in the development of tight oil reservoirs, since it is the only available low cost way to directly quantify flow ability and formation heterogeneity parameters. However, because of the fractures network near wellbore, generated from artificial fracturing linking up natural factures, traditional infinite and finite conductivity fracture models usually result in significantly deviation in field application. In this work, considering the random distribution of natural fractures, physical model of fractures network is proposed, and it shows a composite model feature in the large scale. Consequently, a nonhomogeneous composite mathematical model is established with threshold pressure gradient. To solve this model semi-analytically, we proposed a solution approach including Laplace transform and virtual argument Bessel function, and this method is verified by comparing with existing analytical solution. The matching data of typical type curves generated from semi-analytical solution indicates that the proposed physical and mathematical model can describe the type curves characteristic in typical tight oil reservoirs, which have up warping in late-term rather than parallel lines with slope 1/2 or 1/4. It means the composite model could be used into pressure interpretation of artificial fracturing wells in tight oil reservoir.

Numerical Simulation of Permeability Change in Wellbore Cement Fractures after Geomechanical Stress and Geochemical Reactions Using X-ray Computed Tomography Imaging

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kabilan, Senthil; Jung, Hun Bok; Kuprat, Andrew P.

X-ray microtomography (XMT) imaging combined with a three-dimensional (3D) computational fluid dynamics (CFD) modeling technique was used to study the effect of geochemical and geomechanical processes on fracture properties in composite Portland cement–basalt caprock core samples. The effect of fluid properties and flow conditions on fracture permeability was numerically studied by using fluids with varying physical properties and simulating different pressure conditions. CFD revealed that the application of geomechanical stress led to increased fluid flow, which resulted in increased fracture permeability. After CO2-reaction, XMT images displayed preferential precipitation of calcium carbonate within the fractures in the cement matrix and lessmore » precipitation in fractures located at the cement–basalt interface. CFD predicted changes in flow characteristics and differences in absolute values of flow properties due to different pressure gradients. CFD was able to highlight the profound effect of fluid properties on flow characteristics and hydraulic properties of fractures. This study demonstrates the applicability of XMT imaging and CFD as powerful tools for characterizing the hydraulic properties of fractures in a number of applications like geologic carbon sequestration and storage, hydraulic fracturing for shale gas production, and enhanced geothermal systems.« less

3D J-Integral Capability in Grizzly

DOE Office of Scientific and Technical Information (OSTI.GOV)

Spencer, Benjamin; Backman, Marie; Chakraborty, Pritam

2014-09-01

This report summarizes work done to develop a capability to evaluate fracture contour J-Integrals in 3D in the Grizzly code. In the current fiscal year, a previously-developed 2D implementation of a J-Integral evaluation capability has been extended to work in 3D, and to include terms due both to mechanically-induced strains and due to gradients in thermal strains. This capability has been verified against a benchmark solution on a model of a curved crack front in 3D. The thermal term in this integral has been verified against a benchmark problem with a thermal gradient. These developments are part of a largermore » effort to develop Grizzly as a tool that can be used to predict the evolution of aging processes in nuclear power plant systems, structures, and components, and assess their capacity after being subjected to those aging processes. The capabilities described here have been developed to enable evaluations of Mode- stress intensity factors on axis-aligned flaws in reactor pressure vessels. These can be compared with the fracture toughness of the material to determine whether a pre-existing flaw would begin to propagate during a pos- tulated pressurized thermal shock accident. This report includes a demonstration calculation to show how Grizzly is used to perform a deterministic assessment of such a flaw propagation in a degraded reactor pressure vessel under pressurized thermal shock conditions. The stress intensity is calculated from J, and the toughness is computed using the fracture master curve and the degraded ductile to brittle transition temperature.« less

Application of the boundary elements method for modeling of the fracture of cylindrical bodies by hydraulic fracturing

NASA Astrophysics Data System (ADS)

Legan, M. A.; Blinov, V. A.; Larichkin, A. Yu; Novoselov, A. N.

2017-10-01

Experimental study of hydraulic fracturing of thick-walled cylinders with a central circular hole was carried out using the machine that creates a high oil pressure. Experiments on the compression fracture of the solid cylinders by diameter and rectangular parallelepipeds perpendicular to the ends were carried out with a multipurpose test machine Zwick / Roell Z100. Samples were made of GF-177 material based on cement. Ultimate stresses in the material under study were determined for three types of stress state: under compression, with a pure shear on the surface of the hole under frecking conditions and under a compound stress state under conditions of diametral compression of a solid cylinder. The value of the critical stress intensity factor of GF-177 material was obtained. The modeling of the fracturing process taking into account the inhomogeneity of the stress state near the hole was carried out using the boundary elements method (in the variant of the fictitious load method) and the gradient fracture criterion. Calculation results of the ultimate pressure were compared with values obtained analytically on the basis of the Lame solution and with experimental data.

Percolation and permeability of fracture networks in Excavated Damaged Zones

NASA Astrophysics Data System (ADS)

Mourzenko, V.; Thovert, J.; Adler, P. M.

2012-12-01

Generally, the excavation process of a gallery generates fractures in its immediate vicinity. The corresponding zone which is called the Excavated Damaged Zone (EDZ), has a larger permeability than the intact surrounding medium. The properties of the EDZ are attracting more and more attention because of their potential importance in repositories of nuclear wastes. The EDZ which is induced by the excavation process may create along the galleries of the repositories a high permeability zone which could directly connect the storage area with the ground surface. Therefore, the studies of its properties are of crucial importance for applications such as the storage of nuclear wastes. Field observations (such as the ones which have been systematically performed at Mont Terri by [1, 2]) suggest that the fracture density is an exponentially decreasing function of the distance to the wall with a characteristic length of about 0.5 m and that the fracture orientation is anisotropic (most fractures are subparallel to the tunnel walls) and well approximated by a Fisher law whose pole is orthogonal to the wall. Numerical samples are generated according to these prescriptions. Their percolation status and hydraulic transmissivity can be calculated by the numerical codes which are detailed in [3]. Percolation is determined by a pseudo diffusion algorithm. Flow determination necessitates the meshing of the fracture networks and the discretisation of the Darcy equation by a finite volume technique; the resulting linear system is solved by a conjugate gradient algorithm. Only the flow properties of the EDZ along the directions which are parallel to the wall are of interest when a pressure gradient parallel to the wall is applied. The transmissivity T which relates the total flow rate per unit width Q along the wall through the whole EDZ to the pressure gradient grad p, is defined by Q = - T grad p/mu where mu is the fluid viscosity. The percolation status and hydraulic transmissivity are systematically determined for a wide range of decay lengths and anisotropy parameters. They can be modeled by comparison with anisotropic fracture networks with a constant density. A heuristic power-law model is proposed which accurately describes the results for the percolation threshold over the whole investigated range of heterogeneity and anisotropy. Then, the data for the EDZ transmissivity are presented. A simple parallel flow model is introduced. The flow properties of the EDZ vary with the distance z from the wall. However, the macroscopic pressure gradient does not depend on z, and the flow lines are in average parallel to the wall. Hence, the overall transmissivity is tentatively estimated by a parallel flow model, where a layer at depth z behaves as a fractured medium with uniform properties corresponding to the state at this position in the EDZ. It yields an explicit analytical expression for the transmissivity as a function of the heterogeneity and anisotropy parameters, and it successfully accounts for all the numerical data. Graphical tools are provided from which first estimates can be quickly and easily obtained. [1] Bossart P. et al, Eng. Geol., vol. 66, 19-38 (2002). [2] Thovert J.-F. et al, Eng. Geol., 117, 39-51 (2011). [3] Adler P.M. et al, Fractured porous media, Oxford U. Press, in press.

Hybrid-dimensional modelling of two-phase flow through fractured porous media with enhanced matrix fracture transmission conditions

NASA Astrophysics Data System (ADS)

Brenner, Konstantin; Hennicker, Julian; Masson, Roland; Samier, Pierre

2018-03-01

In this work, we extend, to two-phase flow, the single-phase Darcy flow model proposed in [26], [12] in which the (d - 1)-dimensional flow in the fractures is coupled with the d-dimensional flow in the matrix. Three types of so called hybrid-dimensional two-phase Darcy flow models are proposed. They all account for fractures acting either as drains or as barriers, since they allow pressure jumps at the matrix-fracture interfaces. The models also permit to treat gravity dominated flow as well as discontinuous capillary pressure at the material interfaces. The three models differ by their transmission conditions at matrix fracture interfaces: while the first model accounts for the nonlinear two-phase Darcy flux conservations, the second and third ones are based on the linear single phase Darcy flux conservations combined with different approximations of the mobilities. We adapt the Vertex Approximate Gradient (VAG) scheme to this problem, in order to account for anisotropy and heterogeneity aspects as well as for applicability on general meshes. Several test cases are presented to compare our hybrid-dimensional models to the generic equi-dimensional model, in which fractures have the same dimension as the matrix, leading to deep insight about the quality of the proposed reduced models.

Numerical study on criteria for design and operation of water curtain system in underground oil storage cavern using site descriptive fracture networks

NASA Astrophysics Data System (ADS)

Moon, Jiwon; Yeo, In Wook

2013-04-01

Underground unlined caverns have been constructed in fractured rocks to stockpile oil and petroleum products, where they are hydraulically contained by natural groundwater pressure. However, for the case that natural groundwater pressure is not maintained at the required level, water curtain boreholes, through which water is injected, are often constructed above the cavern as engineering barrier to secure water pressure enough to overwhelm the operational pressure of the cavern. For secure containment of oil and petroleum products inside the cavern, it is essential to keep water pressure around the cavern higher than operational pressure of the cavern using either natural groundwater pressure or engineering barrier. In the Republic of Korea, a number of underground stockpile bases are being operated by Korea National Oil Corporation (KNOC) and private companies, most of which have water curtain system. The criterion that KNOC adopts for water curtain system design and operation such as the vertical distance from the cavern and operational injection rate is based on the Åberg hypothesis that the vertical hydraulic gradient should be larger than one. The criterion has been used for maintaining oil storage cavern without its thorough review. In this study, systematic numerical works have been done for reviewing the Åberg criterion. As groundwater predominantly takes places through fractures in underground caverns, discrete fracture modeling approach is essential for this study. Fracture data, obtained from boreholes drilled at the stage of site investigation at the Yeosu stockpile base in Korea, were statistically analyzed in terms of orientation and intensity, which were used to generate the site descriptive three dimensional fracture networks. Then, groundwater flow modeling has been carried out for the fracture networks. Constant head boundaries were applied along the circumference of the cavern and water curtain boreholes. Main flow channel and hydraulic connectivity between water curtain boreholes and the caverns have been identified, along which hydraulic heads are monitored to find out whether the required hydraulic pressure is maintained around the cavern. The flow modeling has been repeatedly carried out at different constant head boundary conditions to create the criterion for the optimal operation of water curtain system.

Experimental Study of the Roles of Mechanical and Hydrologic Properties in the Initiation of Natural Hydraulic Fractures

NASA Astrophysics Data System (ADS)

French, M. E.; Goodwin, L. B.; Boutt, D. F.; Lilydahl, H.

2008-12-01

Natural hydraulic fractures (NHFs) are inferred to form where pore fluid pressure exceeds the least compressive stress; i.e., where the hydraulic fracture criterion is met. Although it has been shown that mechanical heterogeneities serve as nuclei for NHFs, the relative roles of mechanical anisotropy and hydrologic properties in initiating NHFs in porous granular media have not been fully explored. We designed an experimental protocol that produces a pore fluid pressure high enough to exceed the hydraulic fracture criterion, allowing us to initiate NHFs in the laboratory. Initially, cylindrical samples 13 cm long and 5 cm in diameter are saturated, σ1 is radial, and σ3 is axial. By dropping the end load (σ3) and pore fluid pressure simultaneously at the end caps, we produce a large pore fluid pressure gradient parallel to the long axis of the sample. This allows us to meet the hydraulic fracture criterion without separating the sample from its end caps. The time over which the pore fluid remains elevated is a function of hydraulic diffusivity. An initial test with a low diffusivity sandstone produced NHFs parallel to bedding laminae that were optimally oriented for failure. To evaluate the relative importance of mechanical heterogeneities such as bedding versus hydraulic properties, we are currently investigating variably cemented St. Peter sandstone. This quartz arenite exhibits a wide range of primary structures, from well developed bedding laminae to locally massive sandstone. Diagenesis has locally accentuated these structures, causing degree of cementation to vary with bedding, and the sandstone locally exhibits concretions that form elliptical rather than tabular heterogeneities. Bulk permeability varies from k=10-12 m2 to k=10-15 m2 and porosity varies from 5% to 28% in this suite of samples. Variations in a single sample are smaller, with permeability varying no more than an order of magnitude within a single core. Air minipermeameter and tracer tests document this variability at the cm scale. Experiments will be performed with σ3 and the pore pressure gradient both perpendicular and parallel to sub-cm scale bedding. The results of these tests will be compared to those of structurally homogeneous samples and samples with elliptical heterogeneities.

Relationships between heat flow, thermal and pressure fields in the Gulf of Mexico

NASA Astrophysics Data System (ADS)

Husson, L.; Henry, P.; Le Pichon, X.

2004-12-01

The thermal field of the Gulf of Mexico (GoM) is restored from a comprehensive temperature-depth database. A striking feature is the systematic sharp gradient increase between 2500 and 4000 m. The analysis of the pressure (fracturation tests and mud weights) indicates a systematic correlation between the pressure and temperature fields, as well as with the thickness of Plio-Pleistocene sedimentary layer, and is interpreted as the fact of cooling from fluid flow in the upper, almost hydrostatically pressured layer. The Nusselt number, that we characterize by the ratio between the near high-P gradient over low-P gradient varies spatially and is correlated to the structural pattern of the GoM; this observation outlines the complex relationships between heat and fluid flows, structure and sedimentation. The deep thermal signal is restored in terms of gradient and heat flow density from a statistical analysis of the thermal data combined to the thermal modelling of about 175 wells. At a regional scale, although the sedimentary cover is warmer in Texas than in Louisiana in terms of temperature, the steady state basal heat flow is higher in Louisiana. In addition, beneath the Corsair Fault, which lay offshore parallel to the Texan coast, the high heat flow suggests a zone of Tertiary lithospheric thinning.

Bioprosthetic Valve Fracture Improves the Hemodynamic Results of Valve-in-Valve Transcatheter Aortic Valve Replacement.

PubMed

Chhatriwalla, Adnan K; Allen, Keith B; Saxon, John T; Cohen, David J; Aggarwal, Sanjeev; Hart, Anthony J; Baron, Suzanne J; Dvir, Danny; Borkon, A Michael

2017-07-01

Valve-in-valve (VIV) transcatheter aortic valve replacement (TAVR) may be less effective in small surgical valves because of patient/prosthesis mismatch. Bioprosthetic valve fracture (BVF) using a high-pressure balloon can be performed to facilitate VIV TAVR. We report data from 20 consecutive clinical cases in which BVF was successfully performed before or after VIV TAVR by inflation of a high-pressure balloon positioned across the valve ring during rapid ventricular pacing. Hemodynamic measurements and calculation of the valve effective orifice area were performed at baseline, immediately after VIV TAVR, and after BVF. BVF was successfully performed in 20 patients undergoing VIV TAVR with balloon-expandable (n=8) or self-expanding (n=12) transcatheter valves in Mitroflow, Carpentier-Edwards Perimount, Magna and Magna Ease, Biocor Epic and Biocor Epic Supra, and Mosaic surgical valves. Successful fracture was noted fluoroscopically when the waist of the balloon released and by a sudden drop in inflation pressure, often accompanied by an audible snap. BVF resulted in a reduction in the mean transvalvular gradient (from 20.5±7.4 to 6.7±3.7 mm Hg, P <0.001) and an increase in valve effective orifice area (from 1.0±0.4 to 1.8±0.6 cm 2 , P <0.001). No procedural complications were reported. BVF can be performed safely in small surgical valves to facilitate VIV TAVR with either balloon-expandable or self-expanding transcatheter valves and results in reduced residual transvalvular gradients and increased valve effective orifice area. © 2017 American Heart Association, Inc.

Overpressure Prediction From Seismic Data: Implications on Drilling Safety

NASA Astrophysics Data System (ADS)

Osinowo, O. O.; Oladunjoye, M. A.; Olayinka, A. I.

2007-12-01

High rate of sediment influx into the Niger Delta via river Niger coupled with high rate of basin subsidence, very thick clayey members of Agbada and Akata Formations as well as prevailing presence of growth faults had been identified as the main factors responsible for overpressure generation and preservation in the Niger Delta basin. Analysis of porosity dependent parameters such as interval transit times and interval velocities derived from the seismic records of a field in the Western Niger Delta revealed the presence of overpressured formation at depth of 8670 feet, which is the top of the overpressured zone. The plot of interval transit times against depth gave a positive deflection from normal at the region of overpressure while interval velocity plot gave negative deflection; the ratio of this deviation in both cases is as high as 1.52. Pressure gradient in the upper, normally pressured part of the field was determined to be 0.465 psi/ft., which is within the established normal pressure gradient range in Niger Delta, while the abnormal formation pressure gradient in the overpressured region was determined to be 0.96 psi/ft., and this is also within the published abnormal pressure gradient range of 0.71 to 1.1 psi/ft. in Niger Delta. Formation fracture pressure gradients were determined from the formation pressure information to be 0.66psi/ft. in the upper part of the field and 1.2psi/ft. in the overpressured horizon. Mud weight window (MWW); mud density range necessary to prevent formation kick without initiating hydraulic fracturing was determined to be 10.2 to 12.5lbm/gal in the upper part of the field and 22.1 to 22.63lbm/gal in the overpressured horizon. MWW is indispensable for the selection of the mud pump type, capacity, pumping rate and mud densities at different formation pressure regimes. Overpressure prediction is also requisite for drilling program design, casing design as well as rig capacity choice before spudding. It is necessary to reduce well construction risk, save drilling hour as well as cut down drilling cost. If adequate predictions are not taken however, drilling hazards known as blowout may occur. Blowout, an uncontrollable flow of formation fluid into the well has made oil exploration and exploitation activities in Niger Delta, Southern Nigeria, a curse for the people rather than a blessing because considerable numbers of wells blew out during well construction activities, hence the characteristic oil spill which had degraded the environment, making fishing operation, a source of livelihood of the people difficult. Therefore the need for overpressure prediction as a guide for safe drilling, especially in unfamiliar exploration environments.

Nonlinear flow model of multiple fractured horizontal wells with stimulated reservoir volume including the quadratic gradient term

NASA Astrophysics Data System (ADS)

Ren, Junjie; Guo, Ping

2017-11-01

The real fluid flow in porous media is consistent with the mass conservation which can be described by the nonlinear governing equation including the quadratic gradient term (QGT). However, most of the flow models have been established by ignoring the QGT and little work has been conducted to incorporate the QGT into the flow model of the multiple fractured horizontal (MFH) well with stimulated reservoir volume (SRV). This paper first establishes a semi-analytical model of an MFH well with SRV including the QGT. Introducing the transformed pressure and flow-rate function, the nonlinear model of a point source in a composite system including the QGT is linearized. Then the Laplace transform, principle of superposition, numerical discrete method, Gaussian elimination method and Stehfest numerical inversion are employed to establish and solve the seepage model of the MFH well with SRV. Type curves are plotted and the effects of relevant parameters are analyzed. It is found that the nonlinear effect caused by the QGT can increase the flow capacity of fluid flow and influence the transient pressure positively. The relevant parameters not only have an effect on the type curve but also affect the error in the pressure calculated by the conventional linear model. The proposed model, which is consistent with the mass conservation, reflects the nonlinear process of the real fluid flow, and thus it can be used to obtain more accurate transient pressure of an MFH well with SRV.

Regional maps of subsurface geopressure gradients of the onshore and offshore Gulf of Mexico basin

USGS Publications Warehouse

Burke, Lauri A.; Kinney, Scott A.; Dubiel, Russell F.; Pitman, Janet K.

2013-01-01

The U.S. Geological Survey created a comprehensive geopressure-gradient model of the regional pressure system spanning the onshore and offshore Gulf of Mexico basin, USA. This model was used to generate ten maps that included (1) five contour maps characterizing the depth to the surface defined by the first occurrence of isopressure gradients ranging from 0.60 psi/ft to 1.00 psi/ft, in 0.10-psi/ft increments; and (2) five supporting maps illustrating the spatial density of the data used to construct the contour maps. These contour maps of isopressure-gradients at various increments enable the identification and quantification of the occurrence, magnitude, location, and depth of the subsurface pressure system, which allows for the broad characterization of regions exhibiting overpressured, underpressured, and normally pressured strata. Identification of overpressured regions is critical for exploration and evaluation of potential undiscovered hydrocarbon accumulations based on petroleum-generation pressure signatures and pressure-retention properties of reservoir seals. Characterization of normally pressured regions is essential for field development decisions such as determining the dominant production drive mechanisms, evaluating well placement and drainage patterns, and deciding on well stimulation methods such as hydraulic fracturing. Identification of underpressured regions is essential for evaluating the feasibility of geological sequestration and long-term containment of fluids such as supercritical carbon dioxide for alternative disposal methods of greenhouse gases. This study is the first, quantitative investigation of the regional pressure systems of one of the most important petroleum provinces in the United States. Although this methodology was developed for pressure studies in the Gulf of Mexico basin, it is applicable to any basin worldwide.

Experimental and Model Studies on Loading Path-Dependent and Nonlinear Gas Flow Behavior in Shale Fractures

NASA Astrophysics Data System (ADS)

Li, Honglian; Lu, Yiyu; Zhou, Lei; Tang, Jiren; Han, Shuaibin; Ao, Xiang

2018-01-01

Interest in shale gas as an energy source is growing worldwide. Because the rock's natural fracture system can contribute to gas production, it is important to understand the flow behavior of natural fractures in shale. Previous studies on the flow characteristics in shale fractures were limited and did not consider the effect of nonlinearity. To understand the basic mechanics of the gas flow behavior in shale fractures, laboratory investigations with consideration of the fluid pressure gradient, the confining stress, the loading history and the fracture geometry were conducted in this paper. Izbash's equation was used to analyze the nonlinearity of the flow. The results show that the behavior of the friction factors is similar to that shown in flow tests in smooth and rough pipes. The increase of the confining stress and the irreversible damage to the shale decreased the hydraulic aperture and increased the relative roughness. Thus, turbulent flow could appear at a low Reynolds number, resulting in a significant pressure loss. The limits of the cubic law and the existing correction factor for transmissivity are discussed. It is found that the previous friction models overestimate the friction factor in the laminar regime and underestimate the friction factor in the turbulent regime. For this reason, a new friction model based on a linear combination of the Reynolds number and the relative roughness was developed.

High-pressure injection of dissolved oxygen for hydrocarbon remediation in a fractured dolostone aquifer.

PubMed

Greer, K D; Molson, J W; Barker, J F; Thomson, N R; Donaldson, C R

2010-10-21

A field experiment was completed at a fractured dolomite aquifer in southwestern Ontario, Canada, to assess the delivery of supersaturated dissolved oxygen (supersaturated with respect to ambient conditions) for enhanced bioremediation of petroleum hydrocarbons in groundwater. The injection lasted for 1.5h using iTi's gPro® oxygen injection technology at pressures of up to 450 kPa and at concentrations of up to 34 mg O₂/L. A three-dimensional numerical model for advective-dispersive transport of dissolved oxygen within a discretely-fractured porous medium was calibrated to the observed field conditions under a conservative (no-consumption) scenario. The simulation demonstrated that oxygen rapidly filled the local intersecting fractures as well as the porous matrix surrounding the injection well. Following injection, the local fractures were rapidly flushed by the natural groundwater flow system but slow back-diffusion ensured a relatively longer residence time in the matrix. A sensitivity analysis showed significant changes in behaviour with varying fracture apertures and hydraulic gradients. Applying the calibrated model to a 7-day continuous injection scenario showed oxygen residence times (at the 3mg/L limit), within a radius of 2-4m from the injection well, of up to 100 days. This study has demonstrated that supersaturated dissolved oxygen can be effectively delivered to this type of a fractured and porous bedrock system at concentrations and residence times potentially sufficient for enhanced aerobic biodegradation. Copyright © 2010 Elsevier B.V. All rights reserved.

Unconventional Liquid Flow in Low-Permeability Media: Theory and Revisiting Darcy's Law

NASA Astrophysics Data System (ADS)

Liu, H. H.; Chen, J.

2017-12-01

About 80% of fracturing fluid remains in shale formations after hydraulic fracturing and the flow back process. It is critical to understand and accurately model the flow process of fracturing fluids in a shale formation, because the flow has many practical applications for shale gas recovery. Owing to the strong solid-liquid interaction in low-permeability media, Darcy's law is not always adequate for describing liquid flow process in a shale formation. This non-Darcy flow behavior (characterized by nonlinearity of the relationship between liquid flux and hydraulic gradient), however, has not been given enough attention in the shale gas community. The current study develops a systematic methodology to address this important issue. We developed a phenomenological model for liquid flow in shale (in which liquid flux is a power function of pressure gradient), an extension of the conventional Darcy's law, and also a methodology to estimate parameters for the phenomenological model from spontaneous imbibition tests. The validity of our new developments is verified by satisfactory comparisons of theoretical results and observations from our and other research groups. The relative importance of this non-Darcy liquid flow for hydrocarbon production in unconventional reservoirs remains an issue that needs to be further investigated.

Borehole Stability in High-Temperature Formations

NASA Astrophysics Data System (ADS)

Yan, Chuanliang; Deng, Jingen; Yu, Baohua; Li, Wenliang; Chen, Zijian; Hu, Lianbo; Li, Yang

2014-11-01

In oil and gas drilling or geothermal well drilling, the temperature difference between the drilling fluid and formation will lead to an apparent temperature change around the borehole, which will influence the stress state around the borehole and tend to cause borehole instability in high geothermal gradient formations. The thermal effect is usually not considered as a factor in most of the conventional borehole stability models. In this research, in order to solve the borehole instability in high-temperature formations, a calculation model of the temperature field around the borehole during drilling is established. The effects of drilling fluid circulation, drilling fluid density, and mud displacement on the temperature field are analyzed. Besides these effects, the effect of temperature change on the stress around the borehole is analyzed based on thermoelasticity theory. In addition, the relationships between temperature and strength of four types of rocks are respectively established based on experimental results, and thermal expansion coefficients are also tested. On this basis, a borehole stability model is established considering thermal effects and the effect of temperature change on borehole stability is also analyzed. The results show that the fracture pressure and collapse pressure will both increase as the temperature of borehole rises, and vice versa. The fracture pressure is more sensitive to temperature. Temperature has different effects on collapse pressures due to different lithological characters; however, the variation of fracture pressure is unrelated to lithology. The research results can provide a reference for the design of drilling fluid density in high-temperature wells.

Assessing Hydrogen Assisted Cracking Modes in High Strength Steel Welds

DTIC Science & Technology

1988-12-01

posed theoretical hydrogen assisted cracking mechanisms. It was found that the microplasticity theory of Beachem can best describe how the stress...precludes an internal pressure gradient as the driv- ing force for crack growth. The adsorption theory of Petch and Stables3 and further modifications4...the adsorption theory. In addition, fracture surfaces indicate rapid void formation and coales- cence at low temperatures where the rate of surface

Toughness-Dominated Regime of Hydraulic Fracturing in Cohesionless Materials

NASA Astrophysics Data System (ADS)

Germanovich, L. N.; Hurt, R. S.; Ayoub, J.; Norman, W. D.

2011-12-01

This work examines the mechanisms of hydraulic fracturing in cohesionless particulate materials with geotechnical, geological, and petroleum applications. For this purpose, experimental techniques have been developed, and used to quantify the initiation and propagation of hydraulic fractures in saturated particulate materials. The fracturing liquid is injected into particulate materials, which are practically cohesionless. The liquid flow is localized in thin self-propagating crack-like conduits. By analogy we call them 'cracks' or 'hydraulic fractures.' When a fracture propagates in a solid, new surfaces are created by breaking material bonds. Consequently, the material is in tension at the fracture tip. Because the particulate material is already 'fractured,' no new surface is created and no fracturing process per se is involved. Therefore, the conventional fracture mechanics principles cannot be directly applied. Based on the laboratory observations, performed on three particulate materials (Georgia Red Clay, silica flour, and fine sand, and their mixtures), this work offers physical concepts to explain the observed phenomena. The goal is to determine the controlling parameters of fracture behavior and to quantify their effects. An important conclusion of our work is that all parts of the cohesionless particulate material (including the tip zone of hydraulic fracture) are likely to be in compression. The compressive stress state is an important characteristic of hydraulic fracturing in particulate materials with low, or no, cohesion (such as were used in our experiments). At present, two kinematic mechanisms of fracture propagation, consistent with the compressive stress regime, can be offered. The first mechanism is based on shear bands propagating ahead of the tip of an open fracture. The second is based on the tensile strain ahead of the fracture tip and reduction of the effective stresses to zero within the leak-off zone. Scaling indicates that in our experiments, there is a high pressure gradient in the leak-off zone in the direction normal to the fracture. Fluid pressure does not decrease considerably along the fracture, however, due to the relatively wide fracture aperture. This suggests that hydraulically induced fractures in unconsolidated materials may be considered to be within the toughness-dominated regime of hydraulic fracturing. Our results indicate that the primary influence on peak or initiation pressure comes from the remote stresses. However, fracture morphology changes significantly with other chosen parameters (stress, flow rate, rheology and permeability). Additionally, an important characteristic feature of fractures in our experiments is the frequent bluntness of the fracture tip, which suggests that plastic deformation at the fracture tip is important. Modeling shows that large openings at the fracture tip correspond to relatively large 'effective' fracture (surface) energy, which can be orders of magnitude greater than for typical (solid) rocks.

Fault reactivation and seismicity risk from CO2 sequestration in the Chinshui gas field, NW Taiwan

NASA Astrophysics Data System (ADS)

Sung, Chia-Yu; Hung, Jih-Hao

2015-04-01

The Chinshui gas field located in the fold-thrust belt of western Taiwan was a depleted reservoir. Recently, CO2 sequestration has been planned at shallower depths of this structure. CO2 injection into reservoir will generate high fluid pressure and trigger slip on reservoir-bounding faults. We present detailed in-situ stresses from deep wells in the Chinshui gas field and evaluated the risk of fault reactivation for underground CO2 injection. The magnitudes of vertical stress (Sv), formation pore pressure (Pf) and minimum horizontal stress (Shmin) were obtained from formation density logs, repeat formation tests, sonic logs, mud weight, and hydraulic fracturing including leak-off tests and hydraulic fracturing. The magnitude of maximum horizontal stress (SHmax) was constrained by frictional limit of critically stressed faults. Results show that vertical stress gradient is about 23.02 MPa/km (1.02 psi/ft), and minimum horizontal stress gradient is 18.05 MPa/km (0.80 psi/ft). Formation pore pressures were hydrostatic at depths 2 km, and increase with a gradient of 16.62 MPa/km (0.73 psi/ft). The ratio of fluid pressure and overburden pressure (λp) is 0.65. The upper bound of maximum horizontal stress constrained by strike-slip fault stress regime (SHmax>Sv>Shmin) and coefficient of friction (μ=0.6) is about 18.55 MPa/km (0.82 psi/ft). The orientation of maximum horizontal stresses was calculated from four-arm caliper tools through the methodology suggested by World Stress Map (WMS). The mean azimuth of preferred orientation of borehole breakouts are in ~65。N. Consequently, the maximum horizontal stress axis trends in 155。N and sub-parallel to the far-field plate-convergence direction. Geomechanical analyses of the reactivation of pre-existing faults was assessed using 3DStress and Traptester software. Under current in-situ stress, the middle block fault has higher slip tendency, but still less than frictional coefficient of 0.6 a common threshold value for motion on incohesive faults. The results also indicate that CO2 injection in the Chinshui gas field will not compromise the stability of faults.

Influence of Turbulent Flow and Fractal Scaling on Effective Permeability of Fracture Network

NASA Astrophysics Data System (ADS)

Zhu, J.

2017-12-01

A new approach is developed to calculate hydraulic gradient dependent effective permeability of a fractal fracture network where both laminar and turbulent flows may occur in individual fractures. A critical fracture length is used to distinguish flow characteristics in individual fractures. The developed new solutions can be used for the case of a general scaling relationship, an extension to the linear scaling. We examine the impact on the effective permeability of the network of fractal fracture network characteristics, which include the fractal scaling coefficient and exponent, fractal dimension, ratio of minimum over maximum fracture lengths. Results demonstrate that the developed solution can explain more variations of the effective permeability in relation to the fractal dimensions estimated from the field observations. At high hydraulic gradient the effective permeability decreases with the fractal scaling exponent, but increases with the fractal scaling exponent at low gradient. The effective permeability increases with the scaling coefficient, fractal dimension, fracture length ratio and maximum fracture length.

High serum total cholesterol is a long-term cause of osteoporotic fracture.

PubMed

Trimpou, P; Odén, A; Simonsson, T; Wilhelmsen, L; Landin-Wilhelmsen, K

2011-05-01

Risk factors for osteoporotic fractures were evaluated in 1,396 men and women for a period of 20 years. Serum total cholesterol was found to be an independent osteoporotic fracture risk factor whose predictive power improves with time. The purpose of this study was to evaluate long-term risk factors for osteoporotic fracture. A population random sample of men and women aged 25-64 years (the Gothenburg WHO MONICA project, N = 1,396, 53% women) was studied prospectively. The 1985 baseline examination recorded physical activity at work and during leisure time, psychological stress, smoking habits, coffee consumption, BMI, waist/hip ratio, blood pressure, total, HDL and LDL cholesterol, triglycerides, and fibrinogen. Osteoporotic fractures over a period of 20 years were retrieved from the Gothenburg hospital registers. Poisson regression was used to analyze the predictive power for osteoporotic fracture of each risk factor. A total number of 258 osteoporotic fractures occurred in 143 participants (10.2%). As expected, we found that previous fracture, smoking, coffee consumption, and lower BMI each increase the risk for osteoporotic fracture independently of age and sex. More unexpectedly, we found that the gradient of risk of serum total cholesterol to predict osteoporotic fracture significantly increases over time (p = 0.0377). Serum total cholesterol is an independent osteoporotic fracture risk factor whose predictive power improves with time. High serum total cholesterol is a long-term cause of osteoporotic fracture.

Thermal and pressure histories of the Malay Basin, offshore Malaysia

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yusoff, W.I.; Swarbrick, R.E.

1994-07-01

The Malay Basin is a Neogene intracratonic basin characterized by high heat flow and rapid sedimentation; moderate to high overpressure is common in deeper reservoirs. Thermal conductivity and temperature data from 55 wells have been used to reassess the areal and vertical heat-flow distribution within the basin. Anomalously high temperatures have been observed in some sandstone intervals above the overpressured reservoir section. A narrow to rather abrupt pressure transition zone could be recognized. All hydrocarbon-filled reservoirs seemed to be associated with high heat flow (i.e., about 90 mW/m[sup 2]). Overpressure in some wells is approaching critical fracture pressure (i.e., 0.85more » psi/ft. pressure gradient) in the region. In the central part of the basin, the overpressured sections are found within the shallower (<2000 m) hydrocarbon-bearing units. Selective studies of the temporal development of the pore pressure indicated that overpressure development is associated with episodes of rapid sedimentation. A preliminary fluid flow model supported by pressure modeling is proposed whereby hot fluids are currently being expelled from deeper overpressured sandstone and mudrocks through a fractured seal induced by overpressure. The latter is caused by relatively rapid burial since late Tertiary times. Hydrocarbon migration may have been aided by this fluid movement.« less

On the mechanical interaction between a fluid-filled fracture and the earth's surface

USGS Publications Warehouse

Pollard, D.D.; Holzhausen, G.

1979-01-01

The mechanical interaction between a fluid-filled fracture (e.g., hydraulic fracture joint, or igneous dike) and the earth's surface is analyzed using a two-dimensional elastic solution for a slit of arbitrary inclination buried beneath a horizontal free surface and subjected to an arbitrary pressure distribution. The solution is obtained by iteratively superimposing two fundamental sets of analytical solutions. For uniform internal pressure the slit behaves essentially as if it were in an infinite region if the depth-to-center is three times greater than the half-length. For shallower slits interaction with the free surface is pronounced: stresses and displacements near the slit differ by more than 10% from values for the deeply buried slit. The following changes are noted as the depth-to-center decreases: 1. (1) the mode I stress intensity factor increases for both ends of the slit, but more rapidly at the upper end; 2. (2) the mode II stress-intensity factor is significantly different from zero (except for vertical slits) suggesting propagation out of the original plane of the slit; 3. (3) displacements of the slit wall are asymmetric such that the slit gaps open more widely near the upper end. Similar changes are noted if fluid density creates a linear pressure gradient that is smaller than the lithostatic gradient. Under such conditions natural fractures should propagate preferentially upward toward the earth's surface requiring less pressure as they grow in length. If deformation near the surface is of interest, the model should account explicitly for the free surface. Stresses and displacements at the free surface are not approximated very well by values calculated along a line in an infinite region, even when the slit is far from the line. As depth-to-center of a shallow pressurized slit decreases, the following changes are noted: 1. (1) displacements of the free surface increase to the same order of magnitude as the displacements of the slit walls, 2. (2) tensile stresses of magnitude greater than the pressure in the slit are concentrated along the free surface. The relative surface displacements over a shallow vertical slit are downward over the slit and upward to both sides of this area. The tensile stress acting parallel to the free surface over a shallow vertical slit is concentrated in two maxima adjacent to a point of very low stress immediately over the slit. The solution is used to estimate the length-to-depth ratio at which igneous sills have gained sufficient leverage on overlying strata to bend these strata upward and form a laccolith. The pronounced mode II stress intensity associated with shallow horizontal slits explains the tendency for some sills to climb to higher stratigraphie horizons as they grow in length. The bimodal tensile stress concentration over shallow vertical slits correlates qualitatively with the distribution of cracks and normal faults which flank fissure eruptions on volcanoes. The solution may be used to analyze surface displacements and tilts over massive hydraulic fractures in oil fields and to understand the behavior of hydraulic fractures in granite quarries. ?? 1979.

Extreme hydrothermal conditions at an active plate-bounding fault.

PubMed

Sutherland, Rupert; Townend, John; Toy, Virginia; Upton, Phaedra; Coussens, Jamie; Allen, Michael; Baratin, Laura-May; Barth, Nicolas; Becroft, Leeza; Boese, Carolin; Boles, Austin; Boulton, Carolyn; Broderick, Neil G R; Janku-Capova, Lucie; Carpenter, Brett M; Célérier, Bernard; Chamberlain, Calum; Cooper, Alan; Coutts, Ashley; Cox, Simon; Craw, Lisa; Doan, Mai-Linh; Eccles, Jennifer; Faulkner, Dan; Grieve, Jason; Grochowski, Julia; Gulley, Anton; Hartog, Arthur; Howarth, Jamie; Jacobs, Katrina; Jeppson, Tamara; Kato, Naoki; Keys, Steven; Kirilova, Martina; Kometani, Yusuke; Langridge, Rob; Lin, Weiren; Little, Timothy; Lukacs, Adrienn; Mallyon, Deirdre; Mariani, Elisabetta; Massiot, Cécile; Mathewson, Loren; Melosh, Ben; Menzies, Catriona; Moore, Jo; Morales, Luiz; Morgan, Chance; Mori, Hiroshi; Niemeijer, Andre; Nishikawa, Osamu; Prior, David; Sauer, Katrina; Savage, Martha; Schleicher, Anja; Schmitt, Douglas R; Shigematsu, Norio; Taylor-Offord, Sam; Teagle, Damon; Tobin, Harold; Valdez, Robert; Weaver, Konrad; Wiersberg, Thomas; Williams, Jack; Woodman, Nick; Zimmer, Martin

2017-06-01

Temperature and fluid pressure conditions control rock deformation and mineralization on geological faults, and hence the distribution of earthquakes. Typical intraplate continental crust has hydrostatic fluid pressure and a near-surface thermal gradient of 31 ± 15 degrees Celsius per kilometre. At temperatures above 300-450 degrees Celsius, usually found at depths greater than 10-15 kilometres, the intra-crystalline plasticity of quartz and feldspar relieves stress by aseismic creep and earthquakes are infrequent. Hydrothermal conditions control the stability of mineral phases and hence frictional-mechanical processes associated with earthquake rupture cycles, but there are few temperature and fluid pressure data from active plate-bounding faults. Here we report results from a borehole drilled into the upper part of the Alpine Fault, which is late in its cycle of stress accumulation and expected to rupture in a magnitude 8 earthquake in the coming decades. The borehole (depth 893 metres) revealed a pore fluid pressure gradient exceeding 9 ± 1 per cent above hydrostatic levels and an average geothermal gradient of 125 ± 55 degrees Celsius per kilometre within the hanging wall of the fault. These extreme hydrothermal conditions result from rapid fault movement, which transports rock and heat from depth, and topographically driven fluid movement that concentrates heat into valleys. Shear heating may occur within the fault but is not required to explain our observations. Our data and models show that highly anomalous fluid pressure and temperature gradients in the upper part of the seismogenic zone can be created by positive feedbacks between processes of fault slip, rock fracturing and alteration, and landscape development at plate-bounding faults.

Extreme hydrothermal conditions at an active plate-bounding fault

NASA Astrophysics Data System (ADS)

Sutherland, Rupert; Townend, John; Toy, Virginia; Upton, Phaedra; Coussens, Jamie; Allen, Michael; Baratin, Laura-May; Barth, Nicolas; Becroft, Leeza; Boese, Carolin; Boles, Austin; Boulton, Carolyn; Broderick, Neil G. R.; Janku-Capova, Lucie; Carpenter, Brett M.; Célérier, Bernard; Chamberlain, Calum; Cooper, Alan; Coutts, Ashley; Cox, Simon; Craw, Lisa; Doan, Mai-Linh; Eccles, Jennifer; Faulkner, Dan; Grieve, Jason; Grochowski, Julia; Gulley, Anton; Hartog, Arthur; Howarth, Jamie; Jacobs, Katrina; Jeppson, Tamara; Kato, Naoki; Keys, Steven; Kirilova, Martina; Kometani, Yusuke; Langridge, Rob; Lin, Weiren; Little, Timothy; Lukacs, Adrienn; Mallyon, Deirdre; Mariani, Elisabetta; Massiot, Cécile; Mathewson, Loren; Melosh, Ben; Menzies, Catriona; Moore, Jo; Morales, Luiz; Morgan, Chance; Mori, Hiroshi; Niemeijer, Andre; Nishikawa, Osamu; Prior, David; Sauer, Katrina; Savage, Martha; Schleicher, Anja; Schmitt, Douglas R.; Shigematsu, Norio; Taylor-Offord, Sam; Teagle, Damon; Tobin, Harold; Valdez, Robert; Weaver, Konrad; Wiersberg, Thomas; Williams, Jack; Woodman, Nick; Zimmer, Martin

2017-06-01

Temperature and fluid pressure conditions control rock deformation and mineralization on geological faults, and hence the distribution of earthquakes. Typical intraplate continental crust has hydrostatic fluid pressure and a near-surface thermal gradient of 31 ± 15 degrees Celsius per kilometre. At temperatures above 300-450 degrees Celsius, usually found at depths greater than 10-15 kilometres, the intra-crystalline plasticity of quartz and feldspar relieves stress by aseismic creep and earthquakes are infrequent. Hydrothermal conditions control the stability of mineral phases and hence frictional-mechanical processes associated with earthquake rupture cycles, but there are few temperature and fluid pressure data from active plate-bounding faults. Here we report results from a borehole drilled into the upper part of the Alpine Fault, which is late in its cycle of stress accumulation and expected to rupture in a magnitude 8 earthquake in the coming decades. The borehole (depth 893 metres) revealed a pore fluid pressure gradient exceeding 9 ± 1 per cent above hydrostatic levels and an average geothermal gradient of 125 ± 55 degrees Celsius per kilometre within the hanging wall of the fault. These extreme hydrothermal conditions result from rapid fault movement, which transports rock and heat from depth, and topographically driven fluid movement that concentrates heat into valleys. Shear heating may occur within the fault but is not required to explain our observations. Our data and models show that highly anomalous fluid pressure and temperature gradients in the upper part of the seismogenic zone can be created by positive feedbacks between processes of fault slip, rock fracturing and alteration, and landscape development at plate-bounding faults.

A simplified model to evaluate the effect of fluid rheology on non-Newtonian flow in variable aperture fractures

NASA Astrophysics Data System (ADS)

Felisa, Giada; Ciriello, Valentina; Longo, Sandro; Di Federico, Vittorio

2017-04-01

Modeling of non-Newtonian flow in fractured media is essential in hydraulic fracturing operations, largely used for optimal exploitation of oil, gas and thermal reservoirs. Complex fluids interact with pre-existing rock fractures also during drilling operations, enhanced oil recovery, environmental remediation, and other natural phenomena such as magma and sand intrusions, and mud volcanoes. A first step in the modeling effort is a detailed understanding of flow in a single fracture, as the fracture aperture is typically spatially variable. A large bibliography exists on Newtonian flow in single, variable aperture fractures. Ultimately, stochastic modeling of aperture variability at the single fracture scale leads to determination of the flowrate under a given pressure gradient as a function of the parameters describing the variability of the aperture field and the fluid rheological behaviour. From the flowrate, a flow, or 'hydraulic', aperture can then be derived. The equivalent flow aperture for non-Newtonian fluids of power-law nature in single, variable aperture fractures has been obtained in the past both for deterministic and stochastic variations. Detailed numerical modeling of power-law fluid flow in a variable aperture fracture demonstrated that pronounced channelization effects are associated to a nonlinear fluid rheology. The availability of an equivalent flow aperture as a function of the parameters describing the fluid rheology and the aperture variability is enticing, as it allows taking their interaction into account when modeling flow in fracture networks at a larger scale. A relevant issue in non-Newtonian fracture flow is the rheological nature of the fluid. The constitutive model routinely used for hydro-fracturing modeling is the simple, two-parameter power-law. Yet this model does not characterize real fluids at low and high shear rates, as it implies, for shear-thinning fluids, an apparent viscosity which becomes unbounded for zero shear rate and tends to zero for infinite shear rate. On the contrary, the four-parameter Carreau constitutive equation includes asymptotic values of the apparent viscosity at those limits; in turn, the Carreau rheological equation is well approximated by the more tractable truncated power-law model. Results for flow of such fluids between parallel walls are already available. This study extends the adoption of the truncated power-law model to variable aperture fractures, with the aim of understanding the joint influence of rheology and aperture spatial variability. The aperture variation, modeled within a stochastic or deterministic framework, is taken to be one-dimensional and perpendicular to the flow direction; for stochastic modeling, the influence of different distribution functions is examined. Results are then compared with those obtained for pure power-law fluids for different combinations of model parameters. It is seen that the adoption of the pure power law model leads to significant overestimation of the flowrate with respect to the truncated model, more so for large external pressure gradient and/or aperture variability.

Stress Fracture Etiology as Dependent on Mechanically Induced Fluid Flow

DTIC Science & Technology

2004-08-01

polymethylmethacry- 1994: Weinbaum et al., 1994: Hillsley and Frangos , late. Two Steinmann pins, 4nmn in diameter and 92 mm 1994: Frangos et al., 1996...fluorescent labeling analyses, in which new bone ( Frangos et al., 1996, Rubin et al., 1997, Jacobs et al., formation and intracortical remodeling were...components, i.e., been supported by mounting in vitro experimental work pressure gradients, a close source driving fluid velocity ( Frangos et al

30 CFR 250.1617 - Application for permit to drill.

Code of Federal Regulations, 2012 CFR

2012-07-01

...) Formation fracture gradients; (iii) Potential lost circulation zones; (iv) Mud weights; (v) Casing setting... various casing strings, fracture gradients of the exposed formations, casing setting depths, and cementing...

30 CFR 250.1617 - Application for permit to drill.

Code of Federal Regulations, 2013 CFR

2013-07-01

...) Formation fracture gradients; (iii) Potential lost circulation zones; (iv) Mud weights; (v) Casing setting... various casing strings, fracture gradients of the exposed formations, casing setting depths, and cementing...

30 CFR 250.1617 - Application for permit to drill.

Code of Federal Regulations, 2014 CFR

2014-07-01

...) Formation fracture gradients; (iii) Potential lost circulation zones; (iv) Mud weights; (v) Casing setting... various casing strings, fracture gradients of the exposed formations, casing setting depths, and cementing...

Barodontalgia as a differential diagnosis: symptoms and findings.

PubMed

Robichaud, Roland; McNally, Mary E

2005-01-01

This paper provides a review of the literature concerning the etiology and manifestations of barodontalgia, as well as important clinical considerations for its management. Barodontalgia is characterized by exposure to a pressure gradient, such as that experienced by underwater divers, aviation personnel and air travellers. This form of dental pain is generally marked by a predisposing dental pathology such as acute or chronic periapical infection, caries, deep or failing restorations, residual dental cysts, sinusitis or a history of recent surgery. Studies indicate that severity of barodontalgia and the resulting deterioration of dental health correlates with duration of barometric stress. Restorative materials are also affected by pressure gradients. Resin is indicated as a luting agent of choice for cementing fixed prostheses in populations at risk for barodontalgia. Under the influence of pressure gradients, resin cements maintain original bond strength and demonstrate the least amount of microleakage compared with other cements. The key to avoiding barodontalgia is good oral health. Clinicians must pay close attention to areas of dentin exposure, caries, fractured cusps, the integrity of restorations and periapical pathology in those at risk. The Fédération dentaire internationale describes 4 classes of barodontalgia based on signs and symptoms and provides specific and valuable recommendations for therapeutic intervention.

Fracture analysis of CAD-CAM high-density polymers used for interim implant-supported fixed, cantilevered prostheses.

PubMed

Yilmaz, Burak; Alp, Gülce; Seidt, Jeremy; Johnston, William M; Vitter, Roger; McGlumphy, Edwin A

2018-01-06

The load-to-fracture performance of computer-assisted design and computer-assisted manufacturing (CAD-CAM) high-density polymer (HDP) materials in cantilevers is unknown. The purposes of this in vitro study were to evaluate the load-to-fracture performance of CAD-CAM-fabricated HDPs and to compare that with performance of autopolymerized and injection-molded acrylic resins. Specimens from 8 different brands of CAD-CAM HDPs, including Brylic Solid (BS); Brylic Gradient (BG); AnaxCAD Temp EZ (AE); AnaxCAD Temp Plus (AP); Zirkonzahn Temp Basic (Z); GDS Tempo-CAD (GD); Polident (Po); Merz M-PM-Disc (MAT); an autopolymerized acrylic resin, Imident (Conv) and an injection-molded acrylic resin, SR-IvoBase High Impact (Inj) were evaluated for load-to-fracture analysis (n=5). CAD-CAM specimens were milled from poly(methyl methacrylate) (PMMA) blocks measuring 7 mm in buccolingual width, 8 mm in occlusocervical thickness, and 30 mm in length. A wax pattern was prepared in the same dimensions used for CAD-CAM specimens, flasked, and boiled out. Autopolymerizing acrylic resin was packed and polymerized in a pressure container for 30 minutes. An identical wax pattern was flasked and boiled out, and premeasured capsules were injected (SR-IvoBase) and polymerized under hydraulic pressure for 35 minutes for the injection-molded PMMA. Specimens were thermocycled 5000 times (5°C to 55°C) and fixed to a universal testing machine to receive static loads on the 10-mm cantilever, vertically at a 1 mm/min crosshead speed until fracture occurred. Maximum load-to-fracture values were recorded. ANOVA was used to analyze the maximum force values. Significant differences among materials were analyzed by using the Ryan-Einot-Gabriel-Welsch multiple range test (α=.05). Statistically significant differences were found among load-to-fracture values of different HDPs (P<.001). GD and Po materials had significantly higher load-to-fracture values than other materials (P<.001), and no statistically significant differences were found between GD and Po. The lowest load-to-fracture values were observed for autopolymerized and BG materials, which were significantly lower than those of GD, Po, AE, AP, Z, MAT, Inj, and BS. The load-to-fracture value of autopolymerized acrylic resin was not significantly different from that of BG CAD-CAM polymer. GD and Po CAD-CAM materials had the highest load-to-fracture values. AE, AP, Z, MAT, and BS CAD-CAM polymers and injection-molded acrylic resin had similar load-to-fracture values, which were higher than those of BG and autopolymerized acrylic resin. Autopolymerized acrylic resin load-to-fracture value was similar to that of BG CAD-CAM polymer, which is colored in a gradient pattern. Copyright © 2017 Editorial Council for the Journal of Prosthetic Dentistry. Published by Elsevier Inc. All rights reserved.

Permeability Evolution of Fractured Anhydrite Caused by Chemical and Mechanical Alteration

NASA Astrophysics Data System (ADS)

Detwiler, R. L.; Elkhoury, J. E.; Ameli, P.

2011-12-01

Geologic carbon sequestration requires competent structural seals (caprock) to prevent leakage over decadal time scales. Injection of large volumes of CO2 perturbs the target formation from chemical and mechanical equilibrium leading to the possible creation or enhancement of leakage pathways. We investigate the potential for leakage pathways (fractures) to grow over time under reservoir conditions in a series of anhydrite (Ca2SO4) cores. To simulate a potential leakage event in the laboratory, we fractured and jacketed the cores, and placed them in a flow-through reactor vessel. A high-pressure syringe pump applied confining stresses ranging from 7 to 17 MPa and another syringe pump pushed water through the sample at a constant flow rate with pressure control at the outlet. Effluent was sampled periodically and analyzed for Ca2+ and SO42- using an ion chromatograph. Before and after each experiment, we characterized the surfaces of the fractures using a high-resolution optical profilometer and a scanning electron microscope. Careful alignment of the surfaces during optical profiling allowed reproduction of the fracture aperture before and after each experiment. We present results from several experiments each carried out under different conditions in similar fractured anhydrite cores. One involved a well-mated pre-existing fracture and results showed that the permeability of the fractured core was similar to the intact rock matrix (O(10-18 m2); chemical alteration of the core was largely limited to the inflow face of the core and the fracture surfaces remained largely unaltered. To enhance permeability during subsequent experiments, we imposed a small (380 μm) shear displacement between the fracture surfaces resulting in a four-order-of-magnitude increase in initial permeability. The first of these was run at a constant flow rate of 0.6 ml/min for a period of 7 days. The measured pressure gradient within the core increased slowly for a period of 4 days followed by a rapid increase in differential pressure corresponding to a two-order-of-magnitude decrease in permeability. During the experiment, the diameter of the core decreased by ~300 μm at the inlet and a skin of gypsum (Ca2SO42H2O) was created along the length of the fracture. Dissolution of anhydrite and transition to gypsum of additional anhydrite weakened the fracture surfaces leading to closure of the fracture with a corresponding reduction in aperture and permeability. Additional experiments explore the influence of flow at a lower flow rate, which, in the absence of a large confining stress, has been shown to lead to the development of dissolution channels or wormholes.

Mechanisms of hydrocarbon migration in Mahakam delta, Kalimantan, Indonesia

DOE Office of Scientific and Technical Information (OSTI.GOV)

Durand, B.; Bessereau, G.; Ungerer, P.H.

1986-05-01

In the Mahakam delta, hydrocarbons formed from landplant debris, either dispersed in clays or concentrated in coal levels. The hydrocarbon zone is located partly or entirely in overpressured zones. Hydrocarbon migration is primarily a polyphasic mechanism, i.e., water and hydrocarbons move in separate phases. When hydrocarbon generation occurs in normally pressured zones, hydrocarbons are easily expelled to close carrier beds. Then they migrate toward the top of structures through a network of abundant interconnected sand bodies. However, most hydrocarbons are generated in overpressured zones, in which they move preferentially toward the structural highs. Simultaneously, excess pressure is transmitted to themore » top of the structures because of the sedimentary load in the synclines, which results in a high pressure gradient at the top. This pressure gradient facilitates hydrocarbon filtration from overpressured zones to normally pressured zones, or it may cause hydraulic fracturing, which provides avenues for migration. Gas-rich hydrocarbons formed in deep overpressured zones, probably in a single phase owing to high temperature and pressures. The passage from overpressured zones to normally pressured zones resulted in decreased temperature and pressure, which produced several hydrocarbon phases by retrograde condensation. Finally, lighter hydrocarbons pooled above the heaviest ones. These mechanisms have been simulated by a numerical model of basin evolution, including a two-phase migration modulus, and by a numerical model of retrograde condensation.« less

A homogenized localizing gradient damage model with micro inertia effect

NASA Astrophysics Data System (ADS)

Wang, Zhao; Poh, Leong Hien

2018-07-01

The conventional gradient enhancement regularizes structural responses during material failure. However, it induces a spurious damage growth phenomenon, which is shown here to persist in dynamics. Similar issues were reported with the integral averaging approach. Consequently, the conventional nonlocal enhancement cannot adequately describe the dynamic fracture of quasi-brittle materials, particularly in the high strain rate regime, where a diffused damage profile precludes the development of closely spaced macrocracks. To this end, a homogenization theory is proposed to translate the micro processes onto the macro scale. Starting with simple elementary models at the micro scale to describe the fracture mechanisms, an additional kinematic field is introduced to capture the variations in deformation and velocity within a unit cell. An energetic equivalence between micro and macro is next imposed to ensure consistency at the two scales. The ensuing homogenized microforce balance resembles closely the conventional gradient expression, albeit with an interaction domain that decreases with damage, complemented by a micro inertia effect. Considering a direct single pressure bar example, the homogenized model is shown to resolve the non-physical responses obtained with conventional nonlocal enhancement. The predictive capability of the homogenized model is furthermore demonstrated by considering the spall tests of concrete, with good predictions on failure characteristics such as fragmentation profiles and dynamic tensile strengths, at three different loading rates.

Proppant backflow: Mechanical and flow considerations

DOE Office of Scientific and Technical Information (OSTI.GOV)

McLennan, John; Walton, Ian; Moore, Joseph

2015-09-01

One of the concerns of using proppant in geothermal wells, and particularly in enhanced geothermal systems, is proppant flowback. Particulate proppant maintain post-closure conductivity in hydraulically opened fractures. If that proppant is displaced from the near-wellbore region, either due to overflushing during stimulation or flowback to the wellbore at any time, the reduced fracture width chokes the injection or production. Two intermediate-scale laboratory analogs of a propped hydraulic fracture were prepared, and fluid was flowed through a normally stressed, propped fracture into a central wellbore. The tests were conducted in a polyaxial load frame. Acoustic/microseismic activity was measured during themore » injection programs. In one scenario—radial flow through a transverse fracture to a wellbore—the results suggest the creation of flow channels and nominally intact propped zones around the channels, maintaining fracture aperture. In the other—linear flow through a longitudinal fracture into a wellbore—there was substantially more proppant removal. The measurements have shown a greater tendency for proppant flowback in a linear flow situation (proppant movement is kinematically more restricted for radial convergent flow). The pressure gradients causing flow are exceedingly small and restraining flowback will be difficult. Convergent flow relationships could be an issue for injector wells, which will experience fluid flowback during hard shutdowns.« less

Triaxial Permeability Device

DTIC Science & Technology

1988-01-01

potential for particle migration which can lead to piping, clogging, or hydraulic fracturing . Since the chemical conductivity under the high gradient...U~ .. .. - > 63 consolidation or expansion, it may be concluded that this gradient did not cause piping, clogging, or hydraulic fracturing for

Communicating Risks Associated with Onshore Oil and Gas Development: A Reflective Case Study from a Public Health Perspective

NASA Astrophysics Data System (ADS)

Blake, U.

2017-12-01

The growth of onshore oil and natural gas development (OGD) over the past decade has led to an increase in environmental health concerns caused by potential risks associated with industry activities. While the industry has shown commitment to protecting the environment and the health and safety of all who share it (through the development of standards and best practices, and the support of relevant research projects), the negative rhetoric persists. The persistence can be related to differences in the perception of risks, complicated by the uncertainties associated with published and unpublished scientific findings, which are experienced and communicated differently by scientists, regulators, industry and the public. For example, to industry operators, hydraulic fracturing is a "well completion" operation, during which fracturing fluid is pumped down a well to the target formation, under high pressure (pressure high enough to exceed the formation fracture gradient). However, to the general public, the term hydraulic fracturing represents the entire onshore exploration and production operation (from pad development to the production of the resource). This difference in language is the tip of the iceberg that represents the challenges industry faces when communicating its operations to the public. By reflecting on industry experiences, this presentation offers a reflective case study that relates the challenges of navigating and communicating within a narrative of purported health effects, to risk perception and communication theories.

Aperture-Tolerant, Chemical-Based Methods to Reduce Channeling

DOE Office of Scientific and Technical Information (OSTI.GOV)

Randall S. Seright

2007-09-30

This final technical progress report describes work performed from October 1, 2004, through May 16, 2007, for the project, 'Aperture-Tolerant, Chemical-Based Methods to Reduce Channeling'. We explored the potential of pore-filling gels for reducing excess water production from both fractured and unfractured production wells. Several gel formulations were identified that met the requirements--i.e., providing water residual resistance factors greater than 2,000 and ultimate oil residual resistance factors (F{sub rro}) of 2 or less. Significant oil throughput was required to achieve low F{sub rro} values, suggesting that gelant penetration into porous rock must be small (a few feet or less) formore » existing pore-filling gels to provide effective disproportionate permeability reduction. Compared with adsorbed polymers and weak gels, strong pore-filling gels can provide greater reliability and behavior that is insensitive to the initial rock permeability. Guidance is provided on where relative-permeability-modification/disproportionate-permeability-reduction treatments can be successfully applied for use in either oil or gas production wells. When properly designed and executed, these treatments can be successfully applied to a limited range of oilfield excessive-water-production problems. We examined whether gel rheology can explain behavior during extrusion through fractures. The rheology behavior of the gels tested showed a strong parallel to the results obtained from previous gel extrusion experiments. However, for a given aperture (fracture width or plate-plate separation), the pressure gradients measured during the gel extrusion experiments were much higher than anticipated from rheology measurements. Extensive experiments established that wall slip and first normal stress difference were not responsible for the pressure gradient discrepancy. To explain the discrepancy, we noted that the aperture for gel flow (for mobile gel wormholing through concentrated immobile gel within the fracture) was much narrower than the width of the fracture. The potential of various approaches were investigated for improving sweep in parts of the Daqing Oil Field that have been EOR targets. Possibilities included (1) gel treatments that are directed at channeling through fractures, (2) colloidal dispersion gels, (3) reduced polymer degradation, (4) more viscous polymer solutions, and (5) foams and other methods. Fractures were present in a number of Daqing wells (both injectors and producers). Because the fractures were narrow far from the wellbore, severe channeling did not occur. On the contrary, fractures near the wellbore aided reservoir sweep. In the February 2006 issue of the Journal of Petroleum Technology, a 'Distinguished-Author-Series' paper claimed that a process using aqueous colloidal dispersion gels (CDG gels) performed superior to polymer flooding. Unfortunately, this claim is misleading and generally incorrect. Colloidal dispersion gels, in their present state of technological development, should not be advocated as an improvement to, or substitute for, polymer flooding.« less

DOE Office of Scientific and Technical Information (OSTI.GOV)

Balhoff, Matthew; Tavassoli, Shayan; Fei Ho, Jostine

The potential leakage of hydrocarbon fluids or CO 2 out of subsurface formations through wells with fractured cement or debonded microannuli is a primary concern in oil and gas production and CO 2 storage. The presence of fractures in a cement annulus with apertures on the order of 10–300 microns can pose a significant leakage danger with effective permeability in the range of 0.1–1 mD (millidarcy). Leakage pathways with small apertures are often difficult to repair using conventional oilfield cement, thus a low-viscosity sealant that can be easily placed into these fractures while providing an effective seal is desired. Themore » development of a novel application using pH-triggered polymeric sealants could potentially be the solution to plugging these fractures and that was the research aim of this study. The application is based on the transport and reaction of a low-pH poly(acrylic acid) polymer through fractures in strongly alkaline cement. The pH-sensitive microgels viscosify upon neutralization with cement to become highly swollen gels with substantial yield stress that can block fluid flow. Experiments in a cement fracture determined the effects of the viscosification and gel deposition via real-time visual observation and measurements of pressure gradient and effluent pH. While the pH-triggered gelling mechanism and rheology measurements of the neutralized polymer gel show promising results, the polymer solution in contact with cement undergoes an undesirable reaction known as polymer syneresis. Syneresis is caused by the release of calcium cation from cement that collapses the polymer network. Syneresis produces an unstable calcium-precipitation byproduct that is detrimental to the strength and stability of the gel in place. As a result, gel-sealed leakage pathways that subjected to various degrees of syneresis often failed to hold back pressures. Several chemicals were studied to inhibit polymer syneresis and tested for pretreatment of cement cores to remove calcium and prevent syneresis during polymer placement. A chelating agent, sodium triphosphate (Na 5P 3O 10), was found to successfully eliminate syneresis without compromising the injectivity of polymer solution during placement. Polymer gel strength is determined by recording the maximum holdback pressure gradients during liquid breakthrough tests after various periods of pretreatment and polymer shut-in time. Cores pretreated with Na 5P 3O 10 successfully held up to an average of 80 psi/ft, which is significantly greater than the expected threshold value of about 0.1-5 psi/ft required to prevent flow in a typical CO 2 leakage scenario. The use of such inexpensive, pH-triggered poly-acrylic acid polymer allows long-term robust seal of leaky wellbores under high pH conditions.« less

Horizontal exploitation of the Upper Cretaceous Austin Chalk of south Texas

DOE Office of Scientific and Technical Information (OSTI.GOV)

Borkowski, R.; Hand, L.; Dickerson, D.

1990-05-01

Horizontal drilling in the fractured Austin Chalk of south Texas has proven to be a viable technology for exploiting reserve opportunities in mature trends as well as in frontier areas. To date, the results of an interdisciplinary approach to the regional analysis of structure and stress regimes combined with studies of the depositional characteristics of the Austin Chalk and Eagleford Shale have been a success. Productive characteristics of the Austin Chalk indicate the influence of regional fractures on the preferential flow direction and partitioning in the Pearsall field area of the trend. Well bore orientation and inclination are designed suchmore » that multiple fracture swarms at several stratigraphic horizons are intersected with a single horizontal well bore. As a result of the greater frequency of fracture contacts with the well bore, there is a significant increase in the ultimate recovery of hydrocarbons in place. Conventional vertical drilling techniques are frequently ineffective at encountering these laterally partitioned fracture sets, resulting in lower volumes of recoverable hydrocarbons. Additionally, horizontal well bores may increase ultimate recovery of hydrocarbons by lowering the pressure gradient to the well bore and maximizing the reservoir energy.« less

Role of Geomechanics in Assessing the Feasibility of CO2 Sequestration in Depleted Hydrocarbon Sandstone Reservoirs

NASA Astrophysics Data System (ADS)

Fang, Zhi; Khaksar, Abbas

2013-05-01

Carbon dioxide (CO2) sequestration in depleted sandstone hydrocarbon reservoirs could be complicated by a number of geomechanical problems associated with well drilling, completions, and CO2 injection. The initial production of hydrocarbons (gas or oil) and the resulting pressure depletion as well as associated reduction in horizontal stresses (e.g., fracture gradient) narrow the operational drilling mud weight window, which could exacerbate wellbore instabilities while infill drilling. Well completions (casing, liners, etc.) may experience solids flowback to the injector wells when injection is interrupted due to CO2 supply or during required system maintenance. CO2 injection alters the pressure and temperature in the near wellbore region, which could cause fault reactivation or thermal fracturing. In addition, the injection pressure may exceed the maximum sustainable storage pressure, and cause fracturing and fault reactivation within the reservoirs or bounding formations. A systematic approach has been developed for geomechanical assessments for CO2 storage in depleted reservoirs. The approach requires a robust field geomechanical model with its components derived from drilling and production data as well as from wireline logs of historical wells. This approach is described in detail in this paper together with a recent study on a depleted gas field in the North Sea considered for CO2 sequestration. The particular case study shows that there is a limitation on maximum allowable well inclinations, 45° if aligning with the maximum horizontal stress direction and 65° if aligning with the minimum horizontal stress direction, beyond which wellbore failure would become critical while drilling. Evaluation of sanding risks indicates no sand control installations would be needed for injector wells. Fracturing and faulting assessments confirm that the fracturing pressure of caprock is significantly higher than the planned CO2 injection and storage pressures for an ideal case, in which the total field horizontal stresses increase with the reservoir re-pressurization in a manner opposite to their reduction with the reservoir depletion. However, as the most pessimistic case of assuming the total horizontal stresses staying the same over the CO2 injection, faulting could be reactivated on a fault with the least favorable geometry once the reservoir pressure reaches approximately 7.7 MPa. In addition, the initial CO2 injection could lead to a high risk that a fault with a cohesion of less than 5.1 MPa could be activated due to the significant effect of reduced temperature on the field stresses around the injection site.

Laboratory hydraulic fracturing experiments in intact and pre-fractured rock

USGS Publications Warehouse

Zoback, M.D.; Rummel, F.; Jung, R.; Raleigh, C.B.

1977-01-01

Laboratory hydraulic fracturing experiments were conducted to investigate two factors which could influence the use of the hydrofrac technique for in-situ stress determinations; the possible dependence of the breakdown pressure upon the rate of borehole pressurization, and the influence of pre-existing cracks on the orientation of generated fractures. The experiments have shown that while the rate of borehole pressurization has a marked effect on breakdown pressures, the pressure at which hydraulic fractures initiate (and thus tensile strength) is independent of the rate of borehole pressurization when the effect of fluid penetration is negligible. Thus, the experiments indicate that use of breakdown pressures rather than fracture initiation pressures may lead to an erroneous estimate of tectonic stresses. A conceptual model is proposed to explain anomalously high breakdown pressures observed when fracturing with high viscosity fluids. In this model, initial fracture propagation is presumed to be stable due to large differences between the borehole pressure and that within the fracture. In samples which contained pre-existing fractures which were 'leaky' to water, we found it possible to generate hydraulic fractures oriented parallel to the direction of maximum compression if high viscosity drilling mud was used as the fracturing fluid. ?? 1977.

Phase-field modelling of ductile fracture: a variational gradient-extended plasticity-damage theory and its micromorphic regularization

PubMed Central

Teichtmeister, S.; Aldakheel, F.

2016-01-01

This work outlines a novel variational-based theory for the phase-field modelling of ductile fracture in elastic–plastic solids undergoing large strains. The phase-field approach regularizes sharp crack surfaces within a pure continuum setting by a specific gradient damage modelling. It is linked to a formulation of gradient plasticity at finite strains. The framework includes two independent length scales which regularize both the plastic response as well as the crack discontinuities. This ensures that the damage zones of ductile fracture are inside of plastic zones, and guarantees on the computational side a mesh objectivity in post-critical ranges. PMID:27002069

Field-gradient partitioning for fracture and frictional contact in the material point method: Field-gradient partitioning for fracture and frictional contact in the material point method [Fracture and frictional contact in material point method using damage-field gradients for velocity-field partitioning

DOE Office of Scientific and Technical Information (OSTI.GOV)

Homel, Michael A.; Herbold, Eric B.

Contact and fracture in the material point method require grid-scale enrichment or partitioning of material into distinct velocity fields to allow for displacement or velocity discontinuities at a material interface. We present a new method which a kernel-based damage field is constructed from the particle data. The gradient of this field is used to dynamically repartition the material into contact pairs at each node. Our approach avoids the need to construct and evolve explicit cracks or contact surfaces and is therefore well suited to problems involving complex 3-D fracture with crack branching and coalescence. A straightforward extension of this approachmore » permits frictional ‘self-contact’ between surfaces that are initially part of a single velocity field, enabling more accurate simulation of granular flow, porous compaction, fragmentation, and comminution of brittle materials. Finally, numerical simulations of self contact and dynamic crack propagation are presented to demonstrate the accuracy of the approach.« less

Field-gradient partitioning for fracture and frictional contact in the material point method: Field-gradient partitioning for fracture and frictional contact in the material point method [Fracture and frictional contact in material point method using damage-field gradients for velocity-field partitioning

DOE PAGES

Homel, Michael A.; Herbold, Eric B.

2016-08-15

Contact and fracture in the material point method require grid-scale enrichment or partitioning of material into distinct velocity fields to allow for displacement or velocity discontinuities at a material interface. We present a new method which a kernel-based damage field is constructed from the particle data. The gradient of this field is used to dynamically repartition the material into contact pairs at each node. Our approach avoids the need to construct and evolve explicit cracks or contact surfaces and is therefore well suited to problems involving complex 3-D fracture with crack branching and coalescence. A straightforward extension of this approachmore » permits frictional ‘self-contact’ between surfaces that are initially part of a single velocity field, enabling more accurate simulation of granular flow, porous compaction, fragmentation, and comminution of brittle materials. Finally, numerical simulations of self contact and dynamic crack propagation are presented to demonstrate the accuracy of the approach.« less

Elevated temperature crack growth

NASA Technical Reports Server (NTRS)

Malik, S. N.; Vanstone, R. H.; Kim, K. S.; Laflen, J. H.

1987-01-01

The objective of the Elevated Temperature Crack Growth Program is to evaluate proposed nonlinear fracture mechanics methods for application to hot section components of aircraft gas turbine engines. Progress during the past year included linear-elastic fracture mechanics data reduction on nonlinear crack growth rate data on Alloy 718. The bulk of the analytical work centered on thermal gradient problems and proposed fracture mechanics parameters. Good correlation of thermal gradient experimental displacement data and finite element prediction was obtained.

A study of TiB2/TiB gradient coating by laser cladding on titanium alloy

NASA Astrophysics Data System (ADS)

Lin, Yinghua; Lei, Yongping; Li, Xueqiao; Zhi, Xiaohui; Fu, Hanguang

2016-07-01

TiB2/TiB gradient coating has been fabricated by a laser cladding technique on the surface of a Ti-6Al-4V substrate using TiB2 powder as the cladding material. The microstructure and mechanical properties of the gradient coating were analyzed by SEM, EPMA, XRD, TEM and an instrument to measure hardness. With the increasing distance from the coating surface, the content of TiB2 particles gradually decreased, but the content of TiB short fibers gradually increased. Meanwhile, the micro-hardness and the elastic modulus of the TiB2/TiB coating showed a gradient decreasing trend, but the fracture toughness showed a gradient increasing trend. The fracture toughness of the TiB2/TiB coating between the center and the bottom was improved, primarily due to the debonding of TiB2 particles and the high fracture of TiB short fibers, and the fracture position of TiB short fiber can be moved to an adjacent position. However, the debonding of TiB2 particles was difficult to achieve at the surface of the TiB2/TiB coating.

Quantitative measurement of channel-block hydraulic interactions by experimental saturation of a large, natural, fissured rock mass.

PubMed

Guglielmi, Y; Mudry, J

2001-01-01

The hydrodynamic behavior of fissured media relies on the relationships between a few very conductive fractures (channels) and the remaining low-conductivity fractures and matrix (blocks). We made a quantitative measurement of those relationships and their effect on water drainage and storage in a 19,000 m3 natural reservoir consisting of karstified limestones. This reservoir was artificially saturated with water by closing a water gate on the main outlet during a varying time (delta t) fixed by the operator. The water gate was completely or partly closed until the water pressure reached a particular specified value. If the water gate was left completely closed long enough, the water pressure was fixed by the elevation of temporary outlets at the site boundaries. The water elevation within the reservoir was monitored by means of pressure cells located on single fractures representative of the bedding plane and the two families of fractures of the massif network. The comparison of pressure variations with the network geometry allows us to identify a typical double permeability characterized by a few very conductive channels along 10 vertical faults. These channels limit blocks consisting of low-conductivity bedding planes and a rather impervious matrix. Depending on the closure interval, delta t, of the water gate, the total volume of water stored in the reservoir can vary from 0.8 m3 (delta t = 5 minutes) to 18.6 m3 (delta t = 2 days). Such a variance of storage versus closure time is explained by the reservoir's double permeability that is characterized by blocks that saturate much more slowly than channels. If plotted versus time, this injected volume fits a power relationship, according to the saturation state of the blocks. In less than 34 minutes, storage is close to zero in the blocks and to 1.6 to 2 m3 in the channels. For closing times shorter than 1 hour, only 1% of the volume that flows in the channels is stored into the blocks. Depending on the water pressure and for a given delta t = 3000 minutes, the volume of water stored is controlled by the geometry of the joint network and of the aquifer boundaries. Such an experiment shows that the flow is concentrated in about 10% of the fractured network (channels). The water storage that takes place in the 90% remaining fractures (blocks) depends mainly on time during which pressure remains high into the 10% channels. The accurate modeling of such typical double-permeability media needs a careful study of the geometry of the channels whose narrowings modify the flow and induce dam effects that maintain a high pressure gradient with surrounding blocks.

Trace Elements in Basalts From the Siqueiros Fracture Zone: Implications for Melt Migration Models

NASA Astrophysics Data System (ADS)

Pickle, R. C.; Forsyth, D. W.; Saal, A. E.; Nagle, A. N.; Perfit, M. R.

2008-12-01

Incompatible trace element (ITE) ratios in MORB from a variety of locations may provide insights into the melt migration process by constraining aggregated melt compositions predicted by mantle melting and flow models. By using actual plate geometries to create a 3-D thermodynamic mantle model, melt volumes and compositions at all depths and locations may be calculated and binned into cubes using the pHMELTS algorithm [Asimow et al., 2004]. These melts can be traced from each cube to the surface assuming several migration models, including a simplified pressure gradient model and one in which melt is guided upwards by a low permeability compacted layer. The ITE ratios of all melts arriving at the surface are summed, averaged, and compared to those of the actual sample compositions from the various MOR locales. The Siqueiros fracture zone at 8° 20' N on the East Pacific Rise (EPR) comprises 4 intra-transform spreading centers (ITSCs) across 140 km of offset between two longer spreading ridges, and is an excellent study region for several reasons. First, an abundance of MORB data is readily available, and the samples retrieved from ITSCs are unlikely to be aggregated in a long-lived magma chamber or affected by along-axis transport, so they represent melts extracted locally from the mantle. Additionally, samples at Siqueiros span a compositional range from depleted to normal MORB within the fracture zone yet have similar isotopic compositions to samples collected from the 9-10° EPR. This minimizes the effect of assuming a uniform source composition in our melting model despite a heterogeneous mantle, allowing us to consistently compare the actual lava composition with that predicted by our model. Finally, it has been demonstrated with preliminary migration models that incipient melts generated directly below an ITSC may not necessarily erupt at that ITSC but migrate laterally towards a nearby ridge due to enhanced pressure gradients. The close proximity of the ITSCs at Siqueiros to the large ridges bounding the fracture zone provide a good opportunity to model this phenomenon and may help explain the variable ITE ratios found between samples collected within the transform and those near the ridges.

Consequences of Fluid Lag in Three-Dimensional Hydraulic Fractures

NASA Astrophysics Data System (ADS)

Advani (Deceased), S. H.; Lee, T. S.; Dean, R. H.; Pak, C. K.; Avasthi, J. M.

1997-04-01

Research investigations on three-dimensional (3-D) rectangular hydraulic fracture configurations with varying degrees of fluid lag are reported. This paper demonstrates that a 3-D fracture model coupled with fluid lag (a small region of reduced pressure) at the fracture tip can predict very large excess pressure measurements for hydraulic fracture processes. Predictions of fracture propagation based on critical stress intensity factors are extremely sensitive to the pressure profile at the tip of a propagating fracture. This strong sensitivity to the pressure profile at the tip of a hydraulic fracture is more strongly pronounced in 3-D models versus 2-D models because 3-D fractures are clamped at the top and bottom, and pressures in the 3-D fractures that are far removed from the fracture tip have little effect on the stress intensity factor at the fracture tip. This rationale for the excess pressure mechanism is in marked contrast to the crack tip process damage zone assumptions and attendant high rock fracture toughness value hypotheses advanced in the literature. A comparison with field data is presented to illustrate the proposed fracture fluid pressure sensitivity phenomenon. This paper does not attempt to calculate the length of the fluid lag region in a propagating fracture but instead attempts to show that the pressure profile at the tip of the propagating fracture plays a major role in fracture propagation, and this role is magnified in 3-D models. Int. J. Numer. Anal. Meth. Geomech., vol. 21, 229-240 (1997).

Phase-field modelling of ductile fracture: a variational gradient-extended plasticity-damage theory and its micromorphic regularization.

PubMed

Miehe, C; Teichtmeister, S; Aldakheel, F

2016-04-28

This work outlines a novel variational-based theory for the phase-field modelling of ductile fracture in elastic-plastic solids undergoing large strains. The phase-field approach regularizes sharp crack surfaces within a pure continuum setting by a specific gradient damage modelling. It is linked to a formulation of gradient plasticity at finite strains. The framework includes two independent length scales which regularize both the plastic response as well as the crack discontinuities. This ensures that the damage zones of ductile fracture are inside of plastic zones, and guarantees on the computational side a mesh objectivity in post-critical ranges. © 2016 The Author(s).

Stability of a penny-shaped geothermal reservoir in the earth's crust

DOE Office of Scientific and Technical Information (OSTI.GOV)

Abe, H.; Sekine, H.

1982-06-01

The theoretical analysis of a penny-shaped geothermal reservoir in the earth's crust subject to linear tectonic stress gradients has been made on the basis of the three dimensional theory of elasticity. The condition for stability of a reservoir requires K/sub 1/ < K /SUB c/ , where K/sub 1/ and K /SUB c/ are, respectively, the stress intensity factor for the opening mode and the fracture toughness of the surrounding rock. From this condition the upper critical pressure being necessary for the reservoir stability is obtained and is shown graphically.

Compartment syndrome as a complication of a stab wound to the thigh: a case report and review of the literature

PubMed Central

Gillooly, John J; Hacker, Andrew; Patel, Vipul

2007-01-01

Acute compartment syndrome of the thigh is a rare but potentially devastating condition, in which the pressure within the osseofascial compartment rises above the capillary perfusion gradient, leading to cellular anoxia, muscle ischaemia and death. Early diagnosis and treatment is essential to prevent long term disability. It is most often associated with crush injuries and femoral fracture. We present a previously unreported case of thigh compartment syndrome following a stab injury, treated by emergent fasciotomy. PMID:17954836

Mode-I Fracture Toughness Testing and Coupled Cohesive Zone Modeling at In Situ P, T, and Chemical (H2O-CO2-NaCl) Conditions

NASA Astrophysics Data System (ADS)

Dewers, T. A.; Choens, R. C., II; Regueiro, R. A.; Eichhubl, P.; Bryan, C. R.; Rinehart, A. J.; Su, J. C.; Heath, J. E.

2017-12-01

Propagation of mode I cracks is fundamental to subsurface engineering endeavors, but the majority of fracture toughness measurements are performed at ambient conditions. A novel testing apparatus was used to quantify the relationship between supercritical carbon dioxide (scCO2), water vapor, and fracture toughness in analogs for reservoir rock and caprock lithologies at temperature and pressure conditions relevant to geologic carbon storage. Samples of Boise Sandstone and Marcellus Shale were subject to fracture propagation via a novel short rod fracture toughness tester composed of titanium and Hastelloy® and designed to fit inside a pressure vessel. The tester is controlled by a hydraulically-driven ram and instrumented with a LVDT to monitor displacement. We measure fracture toughness under conditions of dry supercritical CO2 (scCO2), scCO2-saturated brine, and scCO2 with varying water content ( 25%, 90%, and 100% humidity) at 13.8 MPa and 70oC. Water film development as a function of humidity is determined in situ during the experiments with a quartz crystal microbalance. Two orientations of the Marcellus are included in the testing matrix. Dry CO2 has a negligible to slightly strengthening effect compared to a control, however hydrous scCO2 can decrease the fracture toughness, and the effect increases with increasing humidity, which likely is due to capillary condensation of reactive water films at nascent crack tips and associated subcritical weakening. A 2D poromechanical finite element model with cohesive surface elements (CSEs) and a chemo-plasticity phenomenology is being used to describe the chemical weakening/softening effects observed in the testing. The reductions in fracture toughness seen in this study could be important in considerations of borehole stability, in situ stress measurements, changes in fracture gradient, and reservoir caprock integrity during CO2 injection and storage. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia LLC, a wholly owned subsidiary of Honeywell International Inc. for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-NA0003525.

Fracturing mechanics before valve-in-valve therapy of small aortic bioprosthetic heart valves.

PubMed

Johansen, Peter; Engholt, Henrik; Tang, Mariann; Nybo, Rasmus F; Rasmussen, Per D; Nielsen-Kudsk, Jens Erik

2017-10-13

Patients with degraded bioprosthetic heart valves (BHV) who are not candidates for valve replacement may benefit from transcatheter valve-in-valve (VIV) therapy. However, in smaller-sized surgical BHV the resultant orifice may become too narrow. To overcome this, the valve frame can be fractured by a high-pressure balloon prior to VIV. However, knowledge on fracture pressures and mechanics are prerequisites. The aim of this study was to identify the fracture pressures needed in BHV, and to describe the fracture mechanics. Commonly used BHV of small sizes were mounted on a high-pressure balloon situated in a biplane fluoroscopic system with a high-speed camera. The instant of fracture was captured along with the balloon pressure. The valves were inspected for material protrusion and later dissected for fracture zone investigation and description. The valves with a polymer frame fractured at a lower pressure (8-10 atm) than those with a metal stent (19-26 atm). None of the fractured valves had elements protruding. VIV procedures in small-sized BHV may be performed after prior fracture of the valve frame by high-pressure balloon dilatation. This study provides tentative guidelines for expected balloon sizes and pressures for valve fracturing.

Mode I Fracture Toughness of Rock - Intrinsic Property or Pressure-Dependent?

NASA Astrophysics Data System (ADS)

Stoeckhert, F.; Brenne, S.; Molenda, M.; Alber, M.

2016-12-01

The mode I fracture toughness of rock is usually regarded as an intrinsic material parameter independent of pressure. However, most fracture toughness laboratory tests are conducted only at ambient pressure. To investigate fracture toughness of rock under elevated pressures, sleeve fracturing laboratory experiments were conducted with various rock types and a new numerical method was developed for the evaluation of these experiments. The sleeve fracturing experiments involve rock cores with central axial boreholes that are placed in a Hoek triaxial pressure cell to apply an isostatic confining pressure. A polymere tube is pressurized inside these hollow rock cylinders until they fail by tensile fracturing. Numerical simulations incorporating fracture mechanical models are used to obtain a relation between tensile fracture propagation and injection pressure. These simulations indicate that the magnitude of the injection pressure at specimen failure is only depending on the fracture toughness of the tested material, the specimen dimensions and the magnitude of external loading. The latter two are known parameters in the experiments. Thus, the fracture toughness can be calculated from the injection pressure recorded at specimen breakdown. All specimens had a borehole diameter to outer diameter ratio of about 1:10 with outer diameters of 40 and 62 mm. The length of the specimens was about two times the diameter. Maximum external loading was 7.5 MPa corresponding to maximum injection pressures at specimen breakdown of about 100 MPa. The sample set tested in this work includes Permian and Carboniferous sandstones, Jurassic limestones, Triassic marble, Permian volcanic rocks and Devonian slate from Central Europe. The fracture toughness values determined from the sleeve fracturing experiments without confinement using the new numerical method were found to be in good agreement with those from Chevron bend testing according to the ISRM suggested methods. At elevated confining pressures, the results indicate a significant positive correlation between fracture toughness and confining pressure for most tested rock types.

A review of the multiwell experiment in tight gas sandstones of the Mesaverde Group, Piceance Basin, Colorado

USGS Publications Warehouse

Nelson, P.H.

2002-01-01

The Cretaceous Iles and Williams Fork Formations of the Mesaverde Group contain important reservoir and source rocks for basin-centered gas accumulations in the Piceance Basin of northwestern Colorado. The sandstones in these formations have very low permeability, so low that successful production of gas requires the presence of fractures. To increase gas production, the natural fracture system of these "tight gas sandstones" must be augmented by inducing artificial fractures, while minimizing the amount of formation damage due to introduced fluids. The Multiwell Experiment was undertaken to provide geological characterization, obtain physical property data, and perform stimulation experiments in the Iles and Williams Fork Formations. Three vertical wells and one follow-up slant well were drilled, logged, partially cored, tested for gas production, stimulated in various manners, and tested again. Drawing from published reports and papers, this review paper presents well log, core, and test data from the Multiwell Experiment while emphasizing the geological controls on gas production at the site. Gas production is controlled primarily by a set of regional fractures trending west-northwest. The fractures are vertical, terminating at lithologic boundaries within and at the upper and lower boundaries of sandstone beds. Fractures formed preferentially in sandstones where in situ stress and fracture gradients are lower than in shales and mudstones. The fractures cannot be identified adequately in vertical wellbores; horizontal wells are required. Because present-day maximum horizontal stress is aligned with the regional fractures, artificial fractures induced by pressuring the wellbore form parallel to the regional fractures rather than linking them, with consequent limitations upon enhancement of gas production.

Longitudinal Fracture Analysis of a Two-Dimensional Functionally Graded Beam

NASA Astrophysics Data System (ADS)

Rizov, V.

2017-11-01

Longitudinal fracture in a two-dimensional functionally graded beam is analyzed. The modulus of elasticity varies continuously in the beam cross-section. The beam is clamped in its right-hand end. The external loading consists of one longitudinal force applied at the free end of the lower crack arm. The longitudinal crack is located in the beam mid-plane. The fracture is studied in terms of the strain energy release rate. The solution derived is used to elucidate the effects of material gradients along the height as well as along the width of the beam cross-section on the fracture behaviour. The results obtained indicate that the fracture in two-dimensional functionally graded beams can be regulated efficiently by employing appropriate material gradients.

Integration of fracturing dynamics and pressure transient analysis for hydraulic fracture evaluation

DOE Office of Scientific and Technical Information (OSTI.GOV)

Arihara, N.; Abbaszadeh, M.; Wright, C.A.

This paper presents pre- and post-fracture pressure transient analysis, combined with net fracture pressure interpretation, for a well in a naturally fractured geothermal reservoir. Integrated analysis was performed to achieve a consistent interpretation of the created fracture geometry, propagation, conductivity, shrinkage, reservoir flow behavior, and formation permeability characteristics. The interpreted data includes two-rate pre-frac injection tests, step-rate injection tests, a series of pressure falloff tests, and the net fracturing pressure from a massive fracture treatment. Pressure transient analyses were performed utilizing advanced well test interpretation techniques and a thermal reservoir simulator with fracture propagation option. Hydraulic fracture propagation analysis wasmore » also performed Milt a generalized 3-D dynamic fracture growth model simulator. Three major conclusions resulted from the combined analysis: (1) that an increasing number of hydraulic fractures were being simultaneously propagated during the fracture treatment. (2) that the reservoir behaved as a composite reservoir Keith the outer region permeability being greater than the permeability of the region immediately surrounding the wellbore, and (3) that the created fractures extended into the outer region during the fracture treatment but retreated to the inner region several days after stimulation had ceased. These conclusions were apparent from independent pressure transient analysis and from independent hydraulic fracture propagation analysis. Integrated interpretation, however, increased the confidence in these conclusions and greatly aided the quantification of the created hydraulic fracture geometry and characterization of the reservoir permeability.« less

In-situ stress measurements at DOE's Multi-Well Experiment site, Mesaverde Group, Rifle, CO

DOE Office of Scientific and Technical Information (OSTI.GOV)

Warpinski, N.R.

Measurements of the vertical distribution of the minimum principal in situ stress in the lower Mesaverde group (7300 to 8100 ft depth) at DOE's Multi-Well Experiment site have been made by conducting small-volume, hydraulic-fracture stress tests through perforations. Accurate, reproducible results were obtained by conducting a number of repeat injections in each zone of interest using a specially designed pump system, modified high-resolution electronic equipment, and a down-hole shut-off tool with a bottom-hole pressure transducer. Stress tests were conducted in marine sandstones and shales as well as a coal, mudstone, and sandstone in a paludal depositional environment; these tests providemore » a detailed stress distribution in this region. The stress magnitudes were found to be dependent on lithology. Marine shales above and below the blanket sands have large horizontal stress - near lithostatic with a frac gradient greater than 1.0 psi/ft (23 kPa/m). This indicates that these rocks do not behave elastically and processes such as creep and possibly fracturing are the dominant mechanisms controlling the stress state. On the other hand, sandstones and siltstones have much lower stresses with a frac gradient of 0.85 to 0.9 psi/ft (19 to 20 kPa/m). Containment of hydraulic fractures would be expected under these conditions. Only three data points were obtained from the paludal interval; no significant stress differences were observed in the different lithologies. 19 references, 8 figures, 1 table.« less

Experiment 2033. Injection Test of Upper EE-3 Fracture Zone

DOE Office of Scientific and Technical Information (OSTI.GOV)

Grigsby, Charles O.

1983-09-12

This experiment is designed to investigate the apparent lithologic boundary between the low-opening-pressure fracture system (upper EE-3 fracture and Phase I system) and the high-opening-pressure fracture system (lower fracture in EE-3 and in EE-2). The experiment will test for resistence to breakthrough into the lower EE-2 fracture system at relatively low pressure and will define the veting behavior of the low pressure system.

Sensitivity analysis of coupled processes and parameters on the performance of enhanced geothermal systems.

PubMed

Pandey, S N; Vishal, Vikram

2017-12-06

3-D modeling of coupled thermo-hydro-mechanical (THM) processes in enhanced geothermal systems using the control volume finite element code was done. In a first, a comparative analysis on the effects of coupled processes, operational parameters and reservoir parameters on heat extraction was conducted. We found that significant temperature drop and fluid overpressure occurred inside the reservoirs/fracture that affected the transport behavior of the fracture. The spatio-temporal variations of fracture aperture greatly impacted the thermal drawdown and consequently the net energy output. The results showed that maximum aperture evolution occurred near the injection zone instead of the production zone. Opening of the fracture reduced the injection pressure required to circulate a fixed mass of water. The thermal breakthrough and heat extraction strongly depend on the injection mass flow rate, well distances, reservoir permeability and geothermal gradients. High permeability caused higher water loss, leading to reduced heat extraction. From the results of TH vs THM process simulations, we conclude that appropriate coupling is vital and can impact the estimates of net heat extraction. This study can help in identifying the critical operational parameters, and process optimization for enhanced energy extraction from a geothermal system.

Numerical Borehole Breakdown Investigations using XFEM

NASA Astrophysics Data System (ADS)

Beckhuis, Sven; Leonhart, Dirk; Meschke, Günther

2016-04-01

During pressurization of a wellbore a typical downhole pressure record shows the following regimes: first the applied wellbore pressure balances the reservoir pressure, then after the compressive circumferential hole stresses are overcome, tensile stresses are induced on the inside surface of the hole. When the magnitude of these stresses reach the tensile failure stress of the surrounding rock medium, a fracture is initiated and propagates into the reservoir. [1] In standard theories this pressure, the so called breakdown pressure, is the peak pressure in the down-hole pressure record. However experimental investigations [2] show that the breakdown did not occur even if a fracture was initiated at the borehole wall. Drilling muds had the tendency to seal and stabilize fractures and prevent fracture propagation. Also fracture mechanics analysis of breakdown process in mini-frac or leak off tests [3] show that the breakdown pressure could be either equal or larger than the fracture initiation pressure. In order to gain a deeper understanding of the breakdown process in reservoir rock, numerical investigations using the extended finite element method (XFEM) for hydraulic fracturing of porous materials [4] are discussed. The reservoir rock is assumed to be pre-fractured. During pressurization of the borehole, the injection pressure, the pressure distribution and the position of the highest flux along the fracture for different fracturing fluid viscosities are recorded and the influence of the aforementioned values on the stability of fracture propagation is discussed. [1] YEW, C. H. (1997), "Mechanics of Hydraulic Fracturing", Gulf Publishing Company [2] MORITA, N.; BLACK, A. D.; FUH, G.-F. (1996), "Borehole Breakdown Pressure with Drilling Fluids". International Journal of Rock Mechanics and Mining Sciences 33, pp. 39-51 [3] DETOURNAY, E.; CARBONELL, R. (1996), "Fracture Mechanics Analysis of the Breakdown Process in Minifrac or Leakoff Test", Society of Petroleum Engineers, Inc. [4] MESCHKE, G.; Leonhart, D. (2015), "A generalized finite element method for hydro-mechanically coupled analysis of hydraulic fracturing problems using space-time variant enrichment functions." Computer Methods in Applied Mechanics and Engineering, 290:438 - 465

Numerical Model of Hydraulic Fracturing Fluid Transport in the Subsurface with Pressure Transient, Density Effects, and Imbibition

NASA Astrophysics Data System (ADS)

Birdsell, D.; Rajaram, H.; Dempsey, D.; Viswanathan, H.

2014-12-01

Understanding the transport of hydraulic fracturing (HF) fluid that is injected into the deep subsurface for shale gas extraction is important to ensure that shallow drinking water aquifers are not contaminated from an environmental and public health perspective and to understand formation damage from an oil and gas production perspective. Upward pressure gradients, permeable pathways such as faults or improperly abandoned wellbores, and the density contrast of the HF fluid to the surrounding brine encourages upward HF fluid migration. In contrast, the very low shale permeability and the imbibition of water into partially-saturated shale may sequester much of the HF fluid. Using the Finite Element Heat and Mass Transfer Code (FEHM), single-phase flow and transport simulations are performed to quantify how much HF fluid is removed via the wellbore as flowback and produced water and how much reaches overlying aquifers; imbibition is calculated with a semi-analytical one-dimensional solution and treated as a sink term. The travel time for HF fluid to reach the shallow aquifers is highly dependent on the amount of water imbibed and the suction applied to the well. If imbibition rates and suction are small, the pressure transient due to injection and the density contrast allows rapid upward plume migration at early times. The density contrast diminishes considerably within tens to hundreds of years as mixing occurs. We present estimates of HF fluid migration to shallow aquifers during the first 1,000 years after hydraulic fracturing begins for ranges of subsurface properties.

Non-Newtonian fluid flow in 2D fracture networks

NASA Astrophysics Data System (ADS)

Zou, L.; Håkansson, U.; Cvetkovic, V.

2017-12-01

Modeling of non-Newtonian fluid (e.g., drilling fluids and cement grouts) flow in fractured rocks is of interest in many geophysical and industrial practices, such as drilling operations, enhanced oil recovery and rock grouting. In fractured rock masses, the flow paths are dominated by fractures, which are often represented as discrete fracture networks (DFN). In the literature, many studies have been devoted to Newtonian fluid (e.g., groundwater) flow in fractured rock using the DFN concept, but few works are dedicated to non-Newtonian fluids.In this study, a generalized flow equation for common non-Newtonian fluids (such as Bingham, power-law and Herschel-Bulkley) in a single fracture is obtained from the analytical solutions for non-Newtonian fluid discharge between smooth parallel plates. Using Monte Carlo sampling based on site characterization data for the distribution of geometrical features (e.g., density, length, aperture and orientations) in crystalline fractured rock, a two dimensional (2D) DFN model is constructed for generic flow simulations. Due to complex properties of non-Newtonian fluids, the relationship between fluid discharge and the pressure gradient is nonlinear. A Galerkin finite element method solver is developed to iteratively solve the obtained nonlinear governing equations for the 2D DFN model. Using DFN realizations, simulation results for different geometrical distributions of the fracture network and different non-Newtonian fluid properties are presented to illustrate the spatial discharge distributions. The impact of geometrical structures and the fluid properties on the non-Newtonian fluid flow in 2D DFN is examined statistically. The results generally show that modeling non-Newtonian fluid flow in fractured rock as a DFN is feasible, and that the discharge distribution may be significantly affected by the geometrical structures as well as by the fluid constitutive properties.

Effect of property gradients on enamel fracture in human molar teeth.

PubMed

Barani, Amir; Bush, Mark B; Lawn, Brian R

2012-11-01

A model for the fracture of tooth enamel with graded elastic modulus and toughness is constructed using an extended finite element modeling (XFEM) package. The property gradients are taken from literature data on human molars, with maximum in modulus at the outer enamel surface and in toughness at the inner surface. The tooth is modeled as a brittle shell (enamel) and a compliant interior (dentin), with occlusal loading from a hard, flat contact at the cusp. Longitudinal radial (R) and margin (M) cracks are allowed to extend piecewise along the enamel walls under the action of an incrementally increasing applied load. A simple stratagem is deployed in which fictitious temperature profiles generate the requisite property gradients. The resulting XFEM simulations demonstrate that the crack fronts become more segmented as the property gradients become more pronounced, with enhanced propagation at the outer surface and inhibited propagation at the inner. Whereas the growth history of the cracks is profoundly influenced by the gradients, the ultimate critical loads required to attain full fractures are relatively unaffected. Some implications concerning dentistry are considered. Copyright © 2012 Elsevier Ltd. All rights reserved.

A Numerical Study on Small-Scale Permeability Creation Associated with Fluid Pressure Induced Inelastic Shearing

NASA Astrophysics Data System (ADS)

Vogler, D.; Amann, F.; Bayer, P.

2014-12-01

Anthropogenic perturbations in a rock mass at great depth cause a complex thermal-hydro-mechanical (THM) response. This is of particular relevance when dealing with enhanced geothermal systems (EGS) and unconventional oil and gas recovery utilizing hydraulic fracturing. Studying the key THM coupled processes associated with specific reservoir characteristics in an EGS are of foremost relevance to establish a heat exchanger able to achieve the target production rate.Many reservoirs are naturally low permeable, and the target permeability can only be achieved through the creation of new fractures or inelastic and dilatant shearing of pre-existing discontinuities. The latter process, which is considered to irreversibly increase the apertures of pre-existing discontinuities, has been shown to be especially important for EGS. Common constitutive equations linking the change in hydraulic aperture and the change in mechanical aperture are based on the basic formulation of the cubic law, which linearly relates the flow rate in a fracture to the pressure gradient. However, HM-coupled laboratory investigations demonstrate, that the relation between the mechanical and the hydraulic aperture as assumed in the cubic law, is not valid when dealing with very small initial apertures, which are likely to occur at great depth. In a current study, we investigate the relevance of this discrepancy for the early stage of permeability creation in an EGS, where massive fluid injections trigger largely irreversible in-elastic shearing of critically stressed discontinuities. Understanding small-scale effects in fractures in EGS during fluid injection is crucial to predict reservoir fluid production rates and seismic events.Our study aims to implement an empirical constitutive law in an existing discrete fracture code, and calibrate this against experimental data showing the irreversible shearing induced permeability changes. This empirical relation will later be used to quantify the relevance of uncertainties in reservoir characterisation such as discrete fracture networks (DFN) and in-situ state of stress.

Timing of porosity destruction related to pressure-solution in limestones

NASA Astrophysics Data System (ADS)

Beaudoin, Nicolas; Koehn, Daniel; Aharonov, Einat; Boyce, Adrian; Billi, Andrea; Hamilton, Andrea

2017-04-01

Among effects that affect sedimentary rocks during diagenesis, pressure-solution has a very strong impact on the physical properties of rocks such as porosity and permeability. Intergranular pressure-solution results in rough or wavy surfaces called stylolites, which are very common in sedimentary basins, especially in limestone. According to the opening of the system, dissolved material can precipitate locally, leading to the destruction of the porosity around the stylolite. That can namely occur during the development of sedimentary stylolites, when no fracture of fault can allow dissolved material to flow away before precipitating again. This contribution aims at unravelling the depth at which the material dissolved during compaction precipitated in the open porosity, adding new data to discuss when pressure-solution starts to be an efficient mechanism of deformation in limestone during strata burial in sedimentary basins. We report the results of the study of cements that fill the fractures developed at the tips of stylolites in a sample of dolostone from the Jurassic Calcare Massiccio formation, coming from the Umbria-Marche area (Italy). The fractures developed from stylolite-induced stress, and the filling cements' oxygen and carbon isotopic values range between 10.6‰ to -6.1‰ PDB and -8.2‰ to -0.6‰ PDB, respectively. Considering a closed system, we use fractionation equations to convert δ18O values into temperature, which shows that the material put in solution during pressure-solution precipitated at a temperature ranging from 18°C to 39°C. Temperature range and geothermal gradient estimates suggest that the mechanism of pressure-solution actually was primarily active at depth as low as 1 km. In the studied sample, up to 18% of the original volume has been dissolved on stylolites, and that volume loss would have occurred in the first 2 km of the burial history. This natural example feeds the growing body of evidence that stylolites can start developing at a very low depth level. Our results suggest that the porosity in sedimentary rocks can be destroyed very early during burial, both by dissolution and by precipitation, which make the pressure-solution mechanism's impact on fluid flow in basin likely to be underestimated.

Paving the road for hydraulic fracturing in Paleozoic tight gas reservoirs in Abu Dhabi

NASA Astrophysics Data System (ADS)

Alzarouni, Asim

This study contributes to the ongoing efforts of Abu Dhabi National Oil Company (ADNOC) to improve gas production and supply in view of increasing demand and diminishing conventional gas reservoirs in the region. The conditions of most gas reservoirs with potentially economical volumes of gas in Abu Dhabi are tight abrasive deep sand reservoirs at high temperature and pressures. Thus it inevitably tests the limit of both conventional thinking and technology. Accurate prediction of well performance is a major challenge that arises during planning phase. The primary aim is to determine technical feasibility for the implementation of the hydraulic fracture technology in a new area. The ultimate goal is to make economical production curves possible and pave the road to tap new resource of clean hydrocarbon energy source. The formation targeted in this study is characterized by quartzitic sandstone layers and variably colored shale and siltstones with thin layers of anhydrites. It dates back from late Permian to Carboniferous age. It forms rocks at the lower reservoir permeability ranging from 0.2 to less than 1 millidarcy (mD). When fractured, the expected well flow in Abu Dhabi offshore deep gas wells will be close to similar tight gas reservoir in the region. In other words, gas production can be described as transient initially with high rates and rapidly declining towards a pseudo-steady sustainable flow. The study results estimated fracturing gradient range from 0.85 psi/ft to 0.91 psi/ft. In other words, the technology can be implemented successfully to the expected rating without highly weighted brine. Hence, it would be a remarkable step to conduct the first hydraulic fracturing successfully in Abu Dhabi which can pave the road to tapping on a clean energy resource. The models predicted a remarkable conductivity enhancement and an increase of production between 3 to 4 times after fracturing. Moreover, a sustainable rate above 25 MMSCFD between 6 to 10 years is predicted based on a single well model. The forecasts also show that most of the contribution will come from one zone and therefore optimized operational cost can be achieved in future. Once pressures during a diagnostic injection test are known prior to the main hydraulic fracturing treatment, precise calibration will enable accurate design of fracture geometry and containment for full field development. The feasibility of hydraulic fracture is based on available offset well data. The biggest two challenges in Abu-Dhabi at this stage are high depths and high temperatures as well as offshore conditions. For this reason, a higher well pressure envelop and fracturing string installation is envisaged as a necessity in a future well where unknown tectonic stress could result in higher fracturing load. Finally the study recommends drilling a candidate well designed for the implementation of hydraulic fracturing. This well should consider required pressure rating for the fracturing string. Thermal design considerations will also play a role during production due to high temperature. A dipole or multi pole sonic log from the same well is essential to confirm in situ stresses. The planned well will be in the crest at close proximity to studied offset wells to minimize uncertainty where tested wells produced dry gas and to avoid drilling to watered zones down the flank of the reservoir.

Diffusion-coupled cohesive interface simulations of stress corrosion intergranular cracking in polycrystalline materials

DOE Office of Scientific and Technical Information (OSTI.GOV)

Pu, Chao; Gao, Yanfei; Wang, Yanli

To study the stress corrosion intergranular cracking mechanism, a diffusion-coupled cohesive zone model (CZM) is proposed for the simulation of the stress-assisted diffusional process along grain boundaries and the mechanical response of grain boundary sliding and separation. This simulation methodology considers the synergistic effects of impurity diffusion driven by pressure gradient and degradation of grain boundary strength by impurity concentration. The diffusion-coupled CZM is combined with crystal plasticity finite element model (CPFEM) to simulate intergranular fracture of polycrystalline material under corrosive environment. Significant heterogeneity of the stress field and extensive impurity accumulation is observed at grain boundaries and junction points.more » Deformation mechanism maps are constructed with respect to the grain boundary degradation factor and applied strain rate, which dictate the transition from internal to near-surface intergranular fracture modes under various strain amplitudes and grain sizes.« less

Acoustic emission analysis of crack resistance and fracture behavior of 20GL steel having the gradient microstructure and strength

NASA Astrophysics Data System (ADS)

Nikulin, S.; Nikitin, A.; Belov, V.; Rozhnov, A.; Turilina, V.; Anikeenko, V.; Khatkevich, V.

2017-07-01

The crack resistances as well as fracture behavior of 20GL steel quenched with a fast-moving water stream and having gradient microstructure and strength are analyzed. Crack resistance tests with quenched and normalized flat rectangular specimens having different cut lengths loaded by three-point bending with acoustic emission measurements have been performed. The critical J-integral has been used as the crack resistance parameter of the material. Quenching with a fast moving water stream leads to gradient (along a specimen wall thickness) strengthening of steel due to highly refined gradient microstructure formation of the troostomartensite type. Quenching with a fast-moving water stream increases crack resistance Jc , of 20GL steel by a factor of ∼ 1.5. The fracture accrues gradually with the load in the normalized specimens while the initiated crack is hindered in the variable ductility layer and further arrested in the more ductile core in the quenched specimens.

The Influence of Hydraulic Fracturing on Carbon Storage Performance

NASA Astrophysics Data System (ADS)

Fu, Pengcheng; Settgast, Randolph R.; Hao, Yue; Morris, Joseph P.; Ryerson, Frederick J.

2017-12-01

Conventional principles of the design and operation of geologic carbon storage (GCS) require injecting CO2 below the caprock fracturing pressure to ensure the integrity of the storage complex. In nonideal storage reservoirs with relatively low permeability, pressure buildup can lead to hydraulic fracturing of the reservoir and caprock. While the GCS community has generally viewed hydraulic fractures as a key risk to storage integrity, a carefully designed stimulation treatment under appropriate geologic conditions could provide improved injectivity while maintaining overall seal integrity. A vertically contained hydraulic fracture, either in the reservoir rock or extending a limited height into the caprock, provides an effective means to access reservoir volume far from the injection well. Employing a fully coupled numerical model of hydraulic fracturing, solid deformation, and matrix fluid flow, we study the enabling conditions, processes, and mechanisms of hydraulic fracturing during CO2 injection. A hydraulic fracture's pressure-limiting behavior dictates that the near-well fluid pressure is only slightly higher than the fracturing pressure of the rock and is insensitive to injection rate and mechanical properties of the formation. Although a fracture contained solely within the reservoir rock with no caprock penetration, would be an ideal scenario, poroelastic principles dictate that sustaining such a fracture could lead to continuously increasing pressure until the caprock fractures. We also investigate the propagation pattern and injection pressure responses of a hydraulic fracture propagating in a caprock subjected to heterogeneous in situ stress. The results have important implications for the use of hydraulic fracturing as a tool for managing storage performance.

Effect of fluid penetration on tensile failure during fracturing of an open-hole wellbore

NASA Astrophysics Data System (ADS)

Zeng, Fanhui; Cheng, Xiaozhao; Guo, Jianchun; Chen, Zhangxin; Tao, Liang; Liu, Xiaohua; Jiang, Qifeng; Xiang, Jianhua

2018-06-01

It is widely accepted that a fracture can be induced at a wellbore surface when the fluid pressure overcomes the rock tensile strength. However, few models of this phenomenon account for the fluid penetration effect. A rock is a typical permeable, porous medium, and the transmission of pressure from a wellbore to the surrounding rock temporally and spatially perturbs the effective stresses. In addition, these induced stresses influence the fracture initiation pressure. To gain a better understanding of the penetration effect on the initiation pressure of a permeable formation, a comprehensive formula is presented to study the effects of the in situ stresses, rock mechanical properties, injection rate, rock permeability, fluid viscosity, fluid compressibility and wellbore size on the magnitude of the initiation pressure during fracturing of an open-hole wellbore. In this context, the penetration effect is treated as a consequence of the interaction among these parameters by using Darcy’s law of radial flow. A fully coupled analytical procedure is developed to show how the fracturing fluid infiltrates the rock around the wellbore and considerably reduces the magnitude of the initiation pressure. Moreover, the calculation results are validated by hydraulic fracturing experiments in hydrostone. An exhaustive sensitivity study is performed, indicating that the local fluid pressure induced from a seepage effect strongly influences the fracture evolution. For permeable reservoirs, a low injection rate and a low viscosity of the injected fluid have a significant impact on the fracture initiation pressure. In this case, the Hubbert and Haimson equations to predict the fracture initiation pressure are not valid. The open-hole fracture initiation pressure increases with the fracturing fluid viscosity and fluid compressibility, while it decreases as the rock permeability, injection rate and wellbore size increase.

Method of fracturing a geological formation

DOEpatents

Johnson, James O.

1990-01-01

An improved method of fracturing a geological formation surrounding a well bore is disclosed. A relatively small explosive charge is emplaced in a well bore and the bore is subsequently hydraulically pressurized to a pressure less than the formation breakdown pressure and preferably greater than the fracture propagation pressure of the formation. The charge is denoted while the bore is so pressurized, resulting in the formation of multiple fractures in the surrounding formation with little or no accompanying formation damage. Subsequent hydraulic pressurization can be used to propagate and extend the fractures in a conventional manner. The method is useful for stimulating production of oil, gas and possibly water from suitable geologic formations.

Pinedale unit MHF experiments. Final report

DOE Office of Scientific and Technical Information (OSTI.GOV)

Not Available

1976-10-01

Three MHF experiments have been performed in a tight reservoir in the Northern Green River Basin at depths between 8,000 and 12,000 feet. A total of 894,190 gallons of fluid and 2,715,000 pounds of sand were pumped in three stages in two wells with the limited entry technique. Fluid viscosities were designed to give propped lengths of 1,000 to 1,500 feet and proppant sand beds having heights greater than 50 percent of the thickness of each sandstone fractured. The experiments included laboratory research to design limited entry with perforations through one and two strings of casing. Field data analysis tomore » determine fracture gradients and extent of perforation erosion has been complicated by a dependence of friction pressure in tubular goods upon sand concentration and by an apparent large variation in minimum principal in-situ stress between sandstones simultaneously fractured with the limited entry technique. A high proppant concentration was used to assure that production would be limited to reservoir characteristics, rather than fracture conductivity. Comparison was made with results of prior hydraulic fractures propped with a partial monolayer. Resulting production capacity to date has been only about one-fifth that projected in the National Gas Survey report. Evaluation of the resulting production capability and the cost of the hydraulic fracture treatmnet indicates that the stimulation technique employed is not commercially feasible at this time for the reservoir conditions tested. 10 fig, 6 tables.« less

Multiple fracturing experiments: propellant and borehole considerations

DOE Office of Scientific and Technical Information (OSTI.GOV)

Cuderman, J F

1982-01-01

The technology for multiple fracturing of a wellbore, using progressively burning propellants, is being developed to enhance natural gas recovery. Multiple fracturing appears especially attractive for stimulating naturally fractured reservoirs such as Devonian shales where it is expected to effectively intersect existing fractures and connect them to a wellbore. Previous experiments and modeling efforts defined pressure risetimes required for multiple fracturing as a function of borehole diameter, but identified only a weak dependence on peak pressure attained. Typically, from four to eight equally spaced major fractures occur as a function of pressure risetime and in situ stress orientation. The presentmore » experiments address propellant and rock response considerations required to achieve the desired pressure risetimes for reliable multiple fracturing.« less

Hydromechanics in dentine: role of dentinal tubules and hydrostatic pressure on mechanical stress-strain distribution.

PubMed

Kishen, A; Vedantam, S

2007-10-01

This investigation is to understand the role of free water in the dentinal tubules on the mechanical integrity of bulk dentine. Three different experiments were conducted in this study. In experiment 1, three-dimensional models of dentine with gradient elastic modulus, homogenous elastic modulus, and with and without hydrostatic pressure were simulated using the finite element method. Static compressive loads of 15, 50 and 100 N were applied and the distribution of the principal stresses, von Mises stresses, and strains in loading direction were determined. In experiment 2, experimental compression testing of fully hydrated and partially dehydrated dentine (21 degrees C for 72 h) was conducted using a Universal testing machine. In experiment 3, Fourier transform infrared spectroscopic analysis of hydrated and partially dehydrated dentine was carried out. The finite element analysis revealed that the dentine model with simulated hydrostatic pressure displayed residual tensile stresses and strains in the inner region adjacent to the root canal. When external compressive loads were applied to the model, the residual stresses and strains counteracted the applied loads. Similarly the hydrated specimens subjected to experimental compression loads showed greater toughness when compared to the partially dehydrated specimens. The stress at fracture was significantly higher in partially dehydrated specimens (p=0.014), while the strain at fracture was significantly higher in hydrated dentine specimens (p=0.037). These experiments highlighted the distinct role of free water in the dentinal tubules and hydrostatic pressure on the stress-strain distribution within the bulk dentine.

Tomographic evidence for enhanced fracturing and permeability within the relatively aseismic Nemaha Fault Zone, Oklahoma

NASA Astrophysics Data System (ADS)

Stevens, N. T.; Keranen, K. M.; Lambert, C.

2017-12-01

Recent earthquakes in north central Oklahoma are dominantly hosted on unmapped basement faults away from and outside of the largest regional structure, the Nemaha Fault Zone (NFZ) [Lambert, 2016]. The NFZ itself remains largely aseismic, despite the presence of disposal wells and numerous faults. Here we present results from double-difference tomography using TomoDD [Zhang and Thurber, 2003] for the NFZ and the surrounding region, utilizing a seismic catalog of over 10,000 local events acquired by 144 seismic stations deployed between 2013 and 2017. Inversion results for shallow crustal depth, beneath the 2-3 km sedimentary cover, show compressional wavespeeds (Vp) of >6 km/sec and shear wavespeeds (Vs) >4 km/sec outside the NFZ, consistent with crystalline rock. Along the western margin of the NFZ, both Vp and Vs are reduced, and Vp/Vs gradients parallel the trend of major faults, suggesting enhanced fault density and potentially enhanced fluid pressure within the study region. Enhanced fracture density within the NFZ, and associated permeability enhancement, could reduce the effect of regional fluid pressurization from injection wells, contributing to the relative aseismicity of the NFZ.

Vertebral end-plate fractures as a result of high rate pressure loading in the nucleus of the young adult porcine spine.

PubMed

Brown, Stephen H M; Gregory, Diane E; McGill, Stuart M

2008-01-01

In a healthy spine, end-plate fractures occur from excessive pressurization of the intervening nucleus. Younger spines are most susceptible to such type of injury due to the highly hydraulic nature of their intervertebral discs. The purpose of this paper was to confirm this fracture mechanism of the healthy spine through the pressurization of the nucleus in the absence of external compressive loading. Sixteen functional porcine spine units were dissected and both injection and pressure transducer needles were inserted into the nucleus of the intervertebral disc. Hydraulic fluid was rapidly injected into the nucleus until failure occurred. Peak pressure and rate of pressure development were monitored. Spine units were dissected to determine the type and location of fracture. Fifteen of the 16 spine units fractured (the remaining unit had a degenerated disc). Of the 15 fractures, 13 occurred at the posterior margin of the end-plate along the lines of the growth plates. A slightly exponential relationship was found between peak pressure and its rate of development (R(2) = 0.544). Also, in each of the growth-plate fractured specimens, nuclear material was forcefully emitted, during fracture, from the intervertebral disc into the vertebral foramen. The posterior end-plate fractures produced here are similar to those often seen in young adult humans. This provides insight into a mechanism of fracture development through pressurization of the nucleus that might be seen in older adolescents and younger adults during athletic events or mild trauma.

Porous media fracturing dynamics: stepwise crack advancement and fluid pressure oscillations

NASA Astrophysics Data System (ADS)

Cao, Toan D.; Hussain, Fazle; Schrefler, Bernhard A.

2018-02-01

We present new results explaining why fracturing in saturated porous media is not smooth and continuous but is a distinct stepwise process concomitant with fluid pressure oscillations. All exact solutions and almost all numerical models yield smooth fracture advancement and fluid pressure evolution, while recent experimental results, mainly from the oil industry, observation from geophysics and a very few numerical results for the quasi-static case indeed reveal the stepwise phenomenon. We summarize first these new experiments and these few numerical solutions for the quasi-static case. Both mechanical loading and pressure driven fractures are considered because their behaviours differ in the direction of the pressure jumps. Then we explore stepwise crack tip advancement and pressure fluctuations in dynamic fracturing with a hydro-mechanical model of porous media based on the Hybrid Mixture Theory. Full dynamic analyses of examples dealing with both hydraulic fracturing and mechanical loading are presented. The stepwise fracture advancement is confirmed in the dynamic setting as well as in the pressure fluctuations, but there are substantial differences in the frequency contents of the pressure waves in the two loading cases. Comparison between the quasi-static and fully dynamic solutions reveals that the dynamic response gives much more information such as the type of pressure oscillations and related frequencies and should be applied whenever there is a doubt about inertia forces playing a role - the case in most fracturing events. In the absence of direct relevant dynamic tests on saturated media some experimental results on dynamic fracture in dry materials, a fast hydraulic fracturing test and observations from geophysics confirm qualitatively the obtained results such as the type of pressure oscillations and the substantial difference in the behaviour under the two loading cases.

Nonequilibrium capillarity effects in multiphase flow through small volume fractured porous media

NASA Astrophysics Data System (ADS)

Tang, M.; Zhan, H.; Lu, S.

2017-12-01

Analyzing and understanding the capillary pressure curves in fractured porous media is a crucial subject in a number of industrial applications, such as crude oil recovery in the fractured reservoir, CO2 sequestration in fractured brine aquifers and shale gas development. Many studies have observed the significant nonequilibrium capillarity effects in multiphase flow through porous media and proposed that conventional equilibrium capillary pressure may not accurately describe transient two-phase flow behavior under dynamical conditions. To date, only several laboratory experiments and numerical models have been conducted into investigating the characteristic of nonequilibrium capillary pressure in unfractured porous media, a clear picture of the effects of fractures on the dynamic capillary pressure in fractured porous media remains elusive. In this study, four digital porous models were built based on CT image data from ZEISS Xradia 520 Versa CT scanning, a series of direct simulations of multiphase flow in fractured porous media were carried out based on lattice Boltzmann method and three-dimensional porous models. The results show that both the aperture and orientation of the fractures have significant effects on the nonequilibrium capillary pressure coefficients and multiphase flow behaviors. The nonequilibrium capillary pressure coefficients in fractured porous media are one to two orders of magnitude lower than unfractured porous media. This study presents a new direct simulation based methodology for the detailed analysis of nonequilibrium capillary pressure in fractured porous media.

Foot pressures during gait: a comparison of techniques for reducing pressure points.

PubMed

Lawless, M W; Reveal, G T; Laughlin, R T

2001-07-01

Various methods have been used to redistribute plantar surface foot pressure in patients with foot ulcers. This study was conducted to determine the effectiveness of four modalities (fracture walker, fracture walker with insert, and open and closed toe total contact casts) in reducing plantar foot pressure. Ten healthy, normal volunteer subjects had an F-scan sensor (ultra thin shoe insert pressure monitor) placed under the right foot. They then ambulated on a flat surface, maintaining their normal gait. Dynamic plantar pressures were averaged over 10 steps at four different sites (plantar surface of great toe, first metatarsal head, base of fifth metatarsal, and plantar heel). All subjects repeated this sequence under five different testing conditions (barefoot, with a fracture walker, fracture walker with arch support insert, open and closed toe total contact cast). Each subject's barefoot pressures were then compared with the pressures during the different modalities. All four treatment modalities significantly reduced (p < 0.05) plantar pressure at the first metatarsal head (no method was superior). The fracture walker, fracture walker with insert, and open toe total contact cast significantly reduced pressure at the heel. Pressures at the base of the fifth metatarsal and great toe were not significantly reduced with any treatment form. The fracture walker, with and without arch support, and total contact cast can effectively reduce plantar pressure at the heel and first metatarsal head.

Ground-water hydrology and subsurface migration of radioisotopes at a low-level solid radioactive-waste disposal site, West Valley, New York

USGS Publications Warehouse

Prudic, David E.; Randall, Allan D.

1977-01-01

Burial trenches for disposal of solid radioactive waste at West Valley, N.Y., are excavated in till that has very low hydraulic conductivity (about 5 x 10 to the minus 8th power centimeters per second). Fractures and root tubes with chemically oxidized and (or) reduced soil in their walls extend to 3 to 4.5 meters below natural land surface. Preliminary simulations of pressure heads with a digital model suggest that hydraulic conductivity is an order of magnitude greater in the fractured till near land surface than at greater depth. Hydraulic gradients are predominantly downward, even beneath small valleys. The upper part of a body of underlying lacustrine silt is unsaturated; in the lower, saturated part, slow lateral flow may occur. In the older trenches, water began to build up in 1971, overflowed briefly in 1975, and was pumped out in 1975-76. Water levels rose abruptly during major rainstorms in mid-1975, indicating rapid infiltration through cracks in the cover material. The new trenches have maintained low, stable water levels, perhaps because of thicker, more compact cover and less waste settlement; pressure heads near these trenches are low, locally approaching zero, perhaps because of slight infiltration and limited near-surface storage. Peak tritium concentrations in test-hole cores (generally 0.00001 to 0.001 microcuries per milliliter) were found within 3 meters of land surface and are attributed to surface contamination. Concentrations declined rapidly with depth within the fractured till; secondary peaks found at about 9 meters in three holes are attributed to lateral migration from trenches. Other radioisotopes were detected only near land surface. Samples from the walls of shallow fractures revealed no accumulation of radioisotopes. (Woodard-USGS)

Cross-hole fracture connectivity assessed using hydraulic responses during liner installations in crystalline bedrock boreholes

NASA Astrophysics Data System (ADS)

Persaud, Elisha; Levison, Jana; Pehme, Peeter; Novakowski, Kentner; Parker, Beth

2018-01-01

In order to continually improve the current understanding of flow and transport in crystalline bedrock environments, developing and improving fracture system characterization techniques is an important area of study. The presented research examines the installation of flexible, impermeable FLUTe™ liners as a means for assessing cross-hole fracture connectivity. FLUTe™ liners are used to generate a new style of hydraulic pulse, with pressure response monitored in a nearby network of open boreholes drilled in gneissic rock of the Canadian Shield in eastern Ontario, Canada. Borehole liners were installed in six existing 10-15 cm diameter boreholes located 10-35 m apart and drilled to depths ranging between 25-45 m. Liner installation tests were completed consecutively with the number of observation wells available for each test ranging between one and six. The collected pressure response data have been analyzed to identify significant groundwater flow paths between source and observation boreholes as well as to estimate inter-well transmissivity and storativity using a conventional type-curve analysis. While the applied solution relies on a number of general assumptions, it has been found that reasonable comparison can be made to previously completed pulse interference and pumping tests. Results of this research indicate areas where method refinement is necessary, but, nonetheless, highlight the potential for use in crystalline bedrock environments. This method may provide value to future site characterization efforts given that it is complementary to, and can be used in conjunction with, other currently employed borehole liner applications, such as the removal of cross-connection at contaminated sites and the assessment of discrete fracture distributions when boreholes are sealed, recreating natural hydraulic gradient conditions.

40 CFR 146.88 - Injection well operating requirements.

Code of Federal Regulations, 2011 CFR

2011-07-01

..., the owner or operator must ensure that injection pressure does not exceed 90 percent of the fracture pressure of the injection zone(s) so as to ensure that the injection does not initiate new fractures or propagate existing fractures in the injection zone(s). In no case may injection pressure initiate fractures...

40 CFR 146.88 - Injection well operating requirements.

Code of Federal Regulations, 2014 CFR

2014-07-01

..., the owner or operator must ensure that injection pressure does not exceed 90 percent of the fracture pressure of the injection zone(s) so as to ensure that the injection does not initiate new fractures or propagate existing fractures in the injection zone(s). In no case may injection pressure initiate fractures...

40 CFR 146.88 - Injection well operating requirements.

Code of Federal Regulations, 2013 CFR

2013-07-01

..., the owner or operator must ensure that injection pressure does not exceed 90 percent of the fracture pressure of the injection zone(s) so as to ensure that the injection does not initiate new fractures or propagate existing fractures in the injection zone(s). In no case may injection pressure initiate fractures...

40 CFR 146.88 - Injection well operating requirements.

Code of Federal Regulations, 2012 CFR

2012-07-01

..., the owner or operator must ensure that injection pressure does not exceed 90 percent of the fracture pressure of the injection zone(s) so as to ensure that the injection does not initiate new fractures or propagate existing fractures in the injection zone(s). In no case may injection pressure initiate fractures...

Modeling flow and transport in fracture networks using graphs

NASA Astrophysics Data System (ADS)

Karra, S.; O'Malley, D.; Hyman, J. D.; Viswanathan, H. S.; Srinivasan, G.

2018-03-01

Fractures form the main pathways for flow in the subsurface within low-permeability rock. For this reason, accurately predicting flow and transport in fractured systems is vital for improving the performance of subsurface applications. Fracture sizes in these systems can range from millimeters to kilometers. Although modeling flow and transport using the discrete fracture network (DFN) approach is known to be more accurate due to incorporation of the detailed fracture network structure over continuum-based methods, capturing the flow and transport in such a wide range of scales is still computationally intractable. Furthermore, if one has to quantify uncertainty, hundreds of realizations of these DFN models have to be run. To reduce the computational burden, we solve flow and transport on a graph representation of a DFN. We study the accuracy of the graph approach by comparing breakthrough times and tracer particle statistical data between the graph-based and the high-fidelity DFN approaches, for fracture networks with varying number of fractures and degree of heterogeneity. Due to our recent developments in capabilities to perform DFN high-fidelity simulations on fracture networks with large number of fractures, we are in a unique position to perform such a comparison. We show that the graph approach shows a consistent bias with up to an order of magnitude slower breakthrough when compared to the DFN approach. We show that this is due to graph algorithm's underprediction of the pressure gradients across intersections on a given fracture, leading to slower tracer particle speeds between intersections and longer travel times. We present a bias correction methodology to the graph algorithm that reduces the discrepancy between the DFN and graph predictions. We show that with this bias correction, the graph algorithm predictions significantly improve and the results are very accurate. The good accuracy and the low computational cost, with O (104) times lower times than the DFN, makes the graph algorithm an ideal technique to incorporate in uncertainty quantification methods.

Modeling flow and transport in fracture networks using graphs.

PubMed

Karra, S; O'Malley, D; Hyman, J D; Viswanathan, H S; Srinivasan, G

2018-03-01

Fractures form the main pathways for flow in the subsurface within low-permeability rock. For this reason, accurately predicting flow and transport in fractured systems is vital for improving the performance of subsurface applications. Fracture sizes in these systems can range from millimeters to kilometers. Although modeling flow and transport using the discrete fracture network (DFN) approach is known to be more accurate due to incorporation of the detailed fracture network structure over continuum-based methods, capturing the flow and transport in such a wide range of scales is still computationally intractable. Furthermore, if one has to quantify uncertainty, hundreds of realizations of these DFN models have to be run. To reduce the computational burden, we solve flow and transport on a graph representation of a DFN. We study the accuracy of the graph approach by comparing breakthrough times and tracer particle statistical data between the graph-based and the high-fidelity DFN approaches, for fracture networks with varying number of fractures and degree of heterogeneity. Due to our recent developments in capabilities to perform DFN high-fidelity simulations on fracture networks with large number of fractures, we are in a unique position to perform such a comparison. We show that the graph approach shows a consistent bias with up to an order of magnitude slower breakthrough when compared to the DFN approach. We show that this is due to graph algorithm's underprediction of the pressure gradients across intersections on a given fracture, leading to slower tracer particle speeds between intersections and longer travel times. We present a bias correction methodology to the graph algorithm that reduces the discrepancy between the DFN and graph predictions. We show that with this bias correction, the graph algorithm predictions significantly improve and the results are very accurate. The good accuracy and the low computational cost, with O(10^{4}) times lower times than the DFN, makes the graph algorithm an ideal technique to incorporate in uncertainty quantification methods.

Modeling flow and transport in fracture networks using graphs

DOE PAGES

Karra, S.; O'Malley, D.; Hyman, J. D.; ...

2018-03-09

Fractures form the main pathways for flow in the subsurface within low-permeability rock. For this reason, accurately predicting flow and transport in fractured systems is vital for improving the performance of subsurface applications. Fracture sizes in these systems can range from millimeters to kilometers. Although modeling flow and transport using the discrete fracture network (DFN) approach is known to be more accurate due to incorporation of the detailed fracture network structure over continuum-based methods, capturing the flow and transport in such a wide range of scales is still computationally intractable. Furthermore, if one has to quantify uncertainty, hundreds of realizationsmore » of these DFN models have to be run. To reduce the computational burden, we solve flow and transport on a graph representation of a DFN. We study the accuracy of the graph approach by comparing breakthrough times and tracer particle statistical data between the graph-based and the high-fidelity DFN approaches, for fracture networks with varying number of fractures and degree of heterogeneity. Due to our recent developments in capabilities to perform DFN high-fidelity simulations on fracture networks with large number of fractures, we are in a unique position to perform such a comparison. We show that the graph approach shows a consistent bias with up to an order of magnitude slower breakthrough when compared to the DFN approach. We show that this is due to graph algorithm's underprediction of the pressure gradients across intersections on a given fracture, leading to slower tracer particle speeds between intersections and longer travel times. We present a bias correction methodology to the graph algorithm that reduces the discrepancy between the DFN and graph predictions. We show that with this bias correction, the graph algorithm predictions significantly improve and the results are very accurate. In conclusion, the good accuracy and the low computational cost, with O(10 4) times lower times than the DFN, makes the graph algorithm an ideal technique to incorporate in uncertainty quantification methods.« less

Modeling flow and transport in fracture networks using graphs

DOE Office of Scientific and Technical Information (OSTI.GOV)

Karra, S.; O'Malley, D.; Hyman, J. D.

Fractures form the main pathways for flow in the subsurface within low-permeability rock. For this reason, accurately predicting flow and transport in fractured systems is vital for improving the performance of subsurface applications. Fracture sizes in these systems can range from millimeters to kilometers. Although modeling flow and transport using the discrete fracture network (DFN) approach is known to be more accurate due to incorporation of the detailed fracture network structure over continuum-based methods, capturing the flow and transport in such a wide range of scales is still computationally intractable. Furthermore, if one has to quantify uncertainty, hundreds of realizationsmore » of these DFN models have to be run. To reduce the computational burden, we solve flow and transport on a graph representation of a DFN. We study the accuracy of the graph approach by comparing breakthrough times and tracer particle statistical data between the graph-based and the high-fidelity DFN approaches, for fracture networks with varying number of fractures and degree of heterogeneity. Due to our recent developments in capabilities to perform DFN high-fidelity simulations on fracture networks with large number of fractures, we are in a unique position to perform such a comparison. We show that the graph approach shows a consistent bias with up to an order of magnitude slower breakthrough when compared to the DFN approach. We show that this is due to graph algorithm's underprediction of the pressure gradients across intersections on a given fracture, leading to slower tracer particle speeds between intersections and longer travel times. We present a bias correction methodology to the graph algorithm that reduces the discrepancy between the DFN and graph predictions. We show that with this bias correction, the graph algorithm predictions significantly improve and the results are very accurate. In conclusion, the good accuracy and the low computational cost, with O(10 4) times lower times than the DFN, makes the graph algorithm an ideal technique to incorporate in uncertainty quantification methods.« less

Pore pressure migration during hydraulic stimulation due to permeability enhancement by low-pressure subcritical fracture slip

NASA Astrophysics Data System (ADS)

Mukuhira, Yusuke; Moriya, Hirokazu; Ito, Takatoshi; Asanuma, Hiroshi; Häring, Markus

2017-04-01

Understanding the details of pressure migration during hydraulic stimulation is important for the design of an energy extraction system and reservoir management, as well as for the mitigation of hazardous-induced seismicity. Based on microseismic and regional stress information, we estimated the pore pressure increase required to generate shear slip on an existing fracture during stimulation. Spatiotemporal analysis of pore pressure migration revealed that lower pore pressure migrates farther and faster and that higher pore pressure migrates more slowly. These phenomena can be explained by the relationship between fracture permeability and stress state criticality. Subcritical fractures experience shear slip following smaller increases of pore pressure and promote migration of pore pressure because of their enhanced permeability. The difference in migration rates between lower and higher pore pressures suggests that the optimum wellhead pressure is the one that can stimulate relatively permeable fractures, selectively. Its selection optimizes economic benefits and minimizes seismic risk.

Fluid identification based on P-wave anisotropy dispersion gradient inversion for fractured reservoirs

NASA Astrophysics Data System (ADS)

Zhang, J. W.; Huang, H. D.; Zhu, B. H.; Liao, W.

2017-10-01

Fluid identification in fractured reservoirs is a challenging issue and has drawn increasing attentions. As aligned fractures in subsurface formations can induce anisotropy, we must choose parameters independent with azimuths to characterize fractures and fluid effects such as anisotropy parameters for fractured reservoirs. Anisotropy is often frequency dependent due to wave-induced fluid flow between pores and fractures. This property is conducive for identifying fluid type using azimuthal seismic data in fractured reservoirs. Through the numerical simulation based on Chapman model, we choose the P-wave anisotropy parameter dispersion gradient (PADG) as the new fluid factor. PADG is dependent both on average fracture radius and fluid type but independent on azimuths. When the aligned fractures in the reservoir are meter-scaled, gas-bearing layer could be accurately identified using PADG attribute. The reflection coefficient formula for horizontal transverse isotropy media by Rüger is reformulated and simplified according to frequency and the target function for inverting PADG based on frequency-dependent amplitude versus azimuth is derived. A spectral decomposition method combining Orthogonal Matching Pursuit and Wigner-Ville distribution is used to prepare the frequency-division data. Through application to synthetic data and real seismic data, the results suggest that the method is useful for gas identification in reservoirs with meter-scaled fractures using high-qualified seismic data.

Influences of hydraulic gradient, surface roughness, intersecting angle, and scale effect on nonlinear flow behavior at single fracture intersections

NASA Astrophysics Data System (ADS)

Li, Bo; Liu, Richeng; Jiang, Yujing

2016-07-01

Fluid flow tests were conducted on two crossed fracture models for which the geometries of fracture segments and intersections were measured by utilizing a visualization technique using a CCD (charged coupled device) camera. Numerical simulations by solving the Navier-Stokes equations were performed to characterize the fluid flow at fracture intersections. The roles of hydraulic gradient, surface roughness, intersecting angle, and scale effect in the nonlinear fluid flow behavior through single fracture intersections were investigated. The simulation results of flow rate agreed well with the experimental results for both models. The experimental and simulation results showed that with the increment of the hydraulic gradient, the ratio of the flow rate to the hydraulic gradient, Q/J, decreases and the relative difference of Q/J between the calculation results employing the Navier-Stokes equations and the cubic law, δ, increases. When taking into account the fracture surface roughness quantified by Z2 ranging 0-0.42 for J = 1, the value of δ would increase by 0-10.3%. The influences of the intersecting angle on the normalized flow rate that represents the ratio of the flow rate in a segment to the total flow rate, Ra, and the ratio of the hydraulic aperture to the mechanical aperture, e/E, are negligible when J < 10-3, whereas their values change significantly when J > 10-2. Based on the regression analysis on simulation results, a mathematical expression was proposed to quantify e/E, involving variables of J and Rr, where Rr is the radius of truncating circles centered at an intersection. For E/Rr > 10-2, e/E varies significantly and the scale of model has large impacts on the nonlinear flow behavior through intersections, while for E/Rr < 10-3, the scale effect is negligibly small. Finally, a necessary condition to apply the cubic law to fluid flow through fracture intersections is suggested as J < 10-3, E/Rr < 10-3, and Z2 = 0.

Transient pressure analysis of a volume fracturing well in fractured tight oil reservoirs

NASA Astrophysics Data System (ADS)

Lu, Cheng; Wang, Jiahang; Zhang, Cong; Cheng, Minhua; Wang, Xiaodong; Dong, Wenxiu; Zhou, Yingfang

2017-12-01

This paper presents a semi-analytical model to simulate transient pressure curves for a vertical well with a reconstructed fracture network in fractured tight oil reservoirs. In the proposed model, the reservoir is a composite system and contains two regions. The inner region is described as a formation with a finite conductivity hydraulic fracture network and the flow in the fracture is assumed to be linear, while the outer region is modeled using the classical Warren-Root model where radial flow is applied. The transient pressure curves of a vertical well in the proposed reservoir model are calculated semi-analytically using the Laplace transform and Stehfest numerical inversion. As shown in the type curves, the flow is divided into several regimes: (a) linear flow in artificial main fractures; (b) coupled boundary flow; (c) early linear flow in a fractured formation; (d) mid radial flow in the semi-fractures of the formation; (e) mid radial flow or pseudo steady flow; (f) mid cross-flow; (g) closed boundary flow. Based on our newly proposed model, the effects of some sensitive parameters, such as elastic storativity ratio, cross-flow coefficient, fracture conductivity and skin factor, on the type curves were also analyzed extensively. The simulated type curves show that for a vertical fractured well in a tight reservoir, the elastic storativity ratios and crossflow coefficients affect the time and the degree of crossflow respectively. The pressure loss increases with an increase in the fracture conductivity. To a certain extent, the effect of the fracture conductivity is more obvious than that of the half length of the fracture on improving the production effect. With an increase in the wellbore storage coefficient, the fluid compressibility is so large that it might cover the early stage fracturing characteristics. Linear or bilinear flow may not be recognized, and the pressure and pressure derivative gradually shift to the right. With an increase in the skin effect, the pressure loss increases gradually.

Bioprosthetic Valve Fracture to Facilitate Transcatheter Valve-in-Valve Implantation.

PubMed

Allen, Keith B; Chhatriwalla, Adnan K; Cohen, David J; Saxon, John T; Aggarwal, Sanjeev; Hart, Anthony; Baron, Suzanne; Davis, J Russell; Pak, Alex F; Dvir, Danny; Borkon, A Michael

2017-11-01

Valve-in-valve transcatheter aortic valve replacement is less effective in small surgical bioprostheses. We evaluated the feasibility of bioprosthetic valve fracture with a high-pressure balloon to facilitate valve-in-valve transcatheter aortic valve replacement. In vitro bench testing on aortic tissue valves was performed on 19-mm and 21-mm Mitroflow (Sorin, Milan, Italy), Magna and Magna Ease (Edwards Lifesciences, Irvine, CA), Trifecta and Biocor Epic (St. Jude Medical, Minneapolis, MN), and Hancock II and Mosaic (Medtronic, Minneapolis, MN). High-pressure balloons Tru Dilation, Atlas Gold, and Dorado (C.R. Bard, Murray Hill, NJ) were used to determine which valves could be fractured and at what pressure fracture occurred. Mitroflow, Magna, Magna Ease, Mosaic, and Biocor Epic surgical valves were successfully fractured using high-pressures balloon 1 mm larger than the labeled valve size whereas Trifecta and Hancock II surgical valves could not be fractured. Only the internal valve frame was fractured, and the sewing cuff was never disrupted. Manufacturer's rated burst pressures for balloons were exceeded, with fracture pressures ranging from 8 to 24 atmospheres depending on the surgical valve. Testing further demonstrated that fracture facilitated the expansion of previously constrained, underexpanded transcatheter valves (both balloon and self-expanding) to the manufacturer's recommended size. Bench testing demonstrates that the frame of most, but not all, bioprosthetic surgical aortic valves can be fractured using high-pressure balloons. The safety of bioprosthetic valve fracture to optimize valve-in-valve transcatheter aortic valve replacement in small surgical valves requires further clinical investigation. Copyright © 2017 The Society of Thoracic Surgeons. Published by Elsevier Inc. All rights reserved.

Fifty shades of gradients: does the pressure gradient in venous sinus stenting for idiopathic intracranial hypertension matter? A systematic review.

PubMed

McDougall, Cameron M; Ban, Vin Shen; Beecher, Jeffrey; Pride, Lee; Welch, Babu G

2018-03-02

OBJECTIVE The role of venous sinus stenting (VSS) for idiopathic intracranial hypertension (IIH) is not well understood. The aim of this systematic review is to attempt to identify subsets of patients with IIH who will benefit from VSS based on the pressure gradients of their venous sinus stenosis. METHODS MEDLINE/PubMed was searched for studies reporting venous pressure gradients across the stenotic segment of the venous sinus, pre- and post-stent pressure gradients, and clinical outcomes after VSS. Findings are reported according to the PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) guidelines. RESULTS From 32 eligible studies, a total of 186 patients were included in the analysis. Patients who had favorable outcomes had higher mean pressure gradients (22.8 ± 11.5 mm Hg vs 17.4 ± 8.0 mm Hg, p = 0.033) and higher changes in pressure gradients after stent placement (19.4 ± 10.0 mm Hg vs 12.0 ± 6.0 mm Hg, p = 0.006) compared with those with unfavorable outcomes. The post-stent pressure gradients between the 2 groups were not significantly different (2.8 ± 4.0 mm Hg vs 2.7 ± 2.0 mm Hg, p = 0.934). In a multivariate stepwise logistic regression controlling for age, sex, body mass index, CSF opening pressure, pre-stent pressure gradient, and post-stent pressure gradient, the change in pressure gradient with stent placement was found to be an independent predictor of favorable outcome (p = 0.028). Using a pressure gradient of 21 as a cutoff, 81/86 (94.2%) of patients with a gradient > 21 achieved favorable outcomes, compared with 82/100 (82.0%) of patients with a gradient ≤ 21 (p = 0.022). CONCLUSIONS There appears to be a relationship between the pressure gradient of venous sinus stenosis and the success of VSS in IIH. A randomized controlled trial would help elucidate this relationship and potentially guide patient selection.

Numerical Modelling of Extended Leak-Off Test with a Pre-Existing Fracture

NASA Astrophysics Data System (ADS)

Lavrov, A.; Larsen, I.; Bauer, A.

2016-04-01

Extended leak-off test (XLOT) is one of the few techniques available for stress measurements in oil and gas wells. Interpretation of the test is often difficult since the results depend on a multitude of factors, including the presence of natural or drilling-induced fractures in the near-well area. Coupled numerical modelling of XLOT has been performed to investigate the pressure behaviour during the flowback phase as well as the effect of a pre-existing fracture on the test results in a low-permeability formation. Essential features of XLOT known from field measurements are captured by the model, including the saw-tooth shape of the pressure vs injected volume curve, and the change of slope in the pressure vs time curve during flowback used by operators as an indicator of the bottomhole pressure reaching the minimum in situ stress. Simulations with a pre-existing fracture running from the borehole wall in the radial direction have revealed that the results of XLOT are quite sensitive to the orientation of the pre-existing fracture. In particular, the fracture initiation pressure and the formation breakdown pressure increase steadily with decreasing angle between the fracture and the minimum in situ stress. Our findings seem to invalidate the use of the fracture initiation pressure and the formation breakdown pressure for stress measurements or rock strength evaluation purposes.

Coupled Thermo-Hydro-Chemical (THC) Modeling of Hypogene Karst Evolution in a Prototype Mountain Hydrologic System

NASA Astrophysics Data System (ADS)

Chaudhuri, A.; Rajaram, H.; Viswanathan, H. S.; Zyvoloski, G.

2011-12-01

Hypogene karst systems are believed to develop when water flowing upward against the geothermal gradient dissolves limestone as it cools. We present a comprehensive THC model incorporating time-evolving fluid flow, heat transfer, buoyancy effects, multi-component reactive transport and aperture/permeability change to investigate the origin of hypogene karst systems. Our model incorporates the temperature and pressure dependence of the solubility and dissolution kinetics of calcite. It also allows for rigorous representation of temperature-dependent fluid density and its influence on buoyancy forces at various stages of karstification. The model is applied to investigate karstification over geological time scales in a prototype mountain hydrologic system. In this system, a high water table maintained by mountain recharge, drives flow downward through the country rock and upward via a high-permeability fault/fracture. The pressure boundary conditions are maintained constant in time. The fluid flux through the fracture remains nearly constant even though the fracture aperture and permeability increase by dissolution, largely because the permeability of the country rock is not altered significantly due to slower dissolution rates. However, karstification by fracture dissolution is not impeded even though the fluid flux stays nearly constant. Forced and buoyant convection effects arise due to the increased permeability of the evolving fracture system. Since in reality the aperture varies significantly within the fracture plane, the initial fracture aperture is modeled as a heterogeneous random field. In such a heterogeneous aperture field, the water initially flows at a significant rate mainly through preferential flow paths connecting the relatively large aperture zones. Dissolution is more prominent at early time along these flow paths, and the aperture grows faster within these paths. With time, the aperture within small sub-regions of these preferential flow paths grows to a point where the permeability is large enough for the onset of buoyant convection. As a result, a multitude of buoyant convection cells form that take on a two-dimensional (2D) maze-like appearance, which could represent a 2D analog of the three-dimensional (3D) mazework pattern widely thought to be characteristic of hypogene cave systems. Although computational limitations limited us to 2D, we suggest that similar process interactions in a 3D network of fractures and faults could produce a 3D mazework.

Hydraulic fracturing and permeability enhancement in granite from subcritical/brittle to supercritical/ductile conditions

NASA Astrophysics Data System (ADS)

Watanabe, Noriaki; Egawa, Motoki; Sakaguchi, Kiyotoshi; Ishibashi, Takuya; Tsuchiya, Noriyoshi

2017-06-01

Hydraulic fracturing experiments were conducted at 200-450°C by injecting water into cylindrical granite samples containing a borehole at an initial effective confining pressure of 40 MPa. Intensive fracturing was observed at all temperatures, but the fracturing characteristics varied with temperature, perhaps due to differences in the water viscosity. At the lowest considered temperature (200°C), fewer fractures propagated linearly from the borehole, and the breakdown pressure was twice the confining pressure. However, these characteristics disappeared with increasing temperature; the fracture pattern shifted toward the formation of a greater number of shorter fractures over the entire body of the sample, and the breakdown pressure decreased greatly. Hydraulic fracturing significantly increased the permeability at all temperatures, and this permeability enhancement was likely to form a productive geothermal reservoir even at the highest considered temperature, which exceeded both the brittle-ductile transition temperature of granite and the critical temperature of water.

Biofilm-induced calcium carbonate precipitation: application in the subsurface

NASA Astrophysics Data System (ADS)

Phillips, A. J.; Eldring, J.; Lauchnor, E.; Hiebert, R.; Gerlach, R.; Mitchell, A. C.; Esposito, R.; Cunningham, A. B.; Spangler, L.

2012-12-01

We have investigated mitigation strategies for sealing high permeability regions, like fractures, in the subsurface. This technology has the potential to, for example, improve the long-term security of geologically-stored carbon dioxide (CO2) by sealing fractures in cap rocks or to mitigate leakage pathways to prevent contamination of overlying aquifers from hydraulic fracturing fluids. Sealing technologies using low-viscosity fluids are advantageous since they potentially reduce the necessary injection pressures and increase the radius of influence around injection wells. In this technology, aqueous solutions and suspensions are used to promote microbially-induced mineral precipitation which can be applied in subsurface environments. To this end, a strategy was developed to twice seal a hydraulically fractured, 74 cm (2.4') diameter Boyles Sandstone core, collected in North-Central Alabama, with biofilm-induced calcium carbonate (CaCO3) precipitates under ambient pressures. Sporosarcina pasteurii biofilms were established and calcium and urea containing reagents were injected to promote saturation conditions favorable for CaCO3 precipitation followed by growth reagents to resuscitate the biofilm's ureolytic activity. Then, in order to evaluate this process at relevant deep subsurface pressures, a novel high pressure test vessel was developed to house the 74 cm diameter core under pressures as high as 96 bar (1,400 psi). After determining that no impact to the fracture permeability occurred due to increasing overburden pressure, the fractured core was sealed under subsurface relevant pressures relating to 457 meters (1,500 feet) below ground surface (44 bar (650 psi) overburden pressure). After fracture sealing under both ambient and subsurface relevant pressure conditions, the sandstone core withstood three times higher well bore pressure than during the initial fracturing event, which occurred prior to biofilm-induced CaCO3 mineralization. These studies suggest biofilm-induced CaCO3 precipitation technologies may potentially seal and strengthen high permeability regions or fractures (either natural or induced) in the subsurface. Novel high pressure test vessel to investigate biogeochemical processes under relevant subsurface scales and pressures.

Investigation of post hydraulic fracturing well cleanup physics in the Cana Woodford Shale

NASA Astrophysics Data System (ADS)

Lu, Rong

Hydraulic fracturing was first carried out in the 1940s and has gained popularity in current development of unconventional resources. Flowing back the fracturing fluids is critical to a frac job, and determining well cleanup characteristics using the flowback data can help improve frac design. It has become increasingly important as a result of the unique flowback profiles observed in some shale gas plays due to the unconventional formation characteristics. Computer simulation is an efficient and effective way to tackle the problem. History matching can help reveal some mechanisms existent in the cleanup process. The Fracturing, Acidizing, Stimulation Technology (FAST) Consortium at Colorado School of Mines previously developed a numerical model for investigating the hydraulic fracturing process, cleanup, and relevant physics. It is a three-dimensional, gas-water, coupled fracture propagation-fluid flow simulator, which has the capability to handle commonly present damage mechanisms. The overall goal of this research effort is to validate the model on real data and to investigate the dominant physics in well cleanup for the Cana Field, which produces from the Woodford Shale in Oklahoma. To achieve this goal, first the early time delayed gas production was explained and modeled, and a simulation framework was established that included all three relevant damage mechanisms for a slickwater fractured well. Next, a series of sensitivity analysis of well cleanup to major reservoir, fracture, and operational variables was conducted; five of the Cana wells' initial flowback data were history matched, specifically the first thirty days' gas and water producing rates. Reservoir matrix permeability, net pressure, Young's modulus, and formation pressure gradient were found to have an impact on the gas producing curve's shape, in different ways. Some moderately good matches were achieved, with the outcome of some unknown reservoir information being proposed using the corresponding inputs from the history matching study. It was also concluded that extended shut-in durations after fracturing all the stages do not delay production in the overall situation. The success of history matching will further knowledge of well cleanup characteristics in the Cana Field, enable the future usage of this tool in other hydraulically fractured gas wells, and help operators optimize the flowback operations. Future improvements can be achieved by further developing the current simulator so that it has the capability of optimizing its grids setting every time the user changes the inputs, which will result in better stability when the relative permeability setting is modified.

Hybrid-finite-element analysis of some nonlinear and 3-dimensional problems of engineering fracture mechanics

NASA Technical Reports Server (NTRS)

Atluri, S. N.; Nakagaki, M.; Kathiresan, K.

1980-01-01

In this paper, efficient numerical methods for the analysis of crack-closure effects on fatigue-crack-growth-rates, in plane stress situations, and for the solution of stress-intensity factors for arbitrary shaped surface flaws in pressure vessels, are presented. For the former problem, an elastic-plastic finite element procedure valid for the case of finite deformation gradients is developed and crack growth is simulated by the translation of near-crack-tip elements with embedded plastic singularities. For the latter problem, an embedded-elastic-singularity hybrid finite element method, which leads to a direct evaluation of K-factors, is employed.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Hoak, T.E.; Decker, A.D.

Mesaverde Group reservoirs in the Piceance Basin, Western Colorado contain a large reservoir base. Attempts to exploit this resource base are stymied by low permeability reservoir conditions. The presence of abundant natural fracture systems throughout this basin, however, does permit economic production. Substantial production is associated with fractured reservoirs in Divide Creek, Piceance Creek, Wolf Creek, White River Dome, Plateau, Shire Gulch, Grand Valley, Parachute and Rulison fields. Successful Piceance Basin gas production requires detailed information about fracture networks and subsurface gas and water distribution in an overall gas-centered basin geometry. Assessment of these three parameters requires an integrated basinmore » analysis incorporating conventional subsurface geology, seismic data, remote sensing imagery analysis, and an analysis of regional tectonics. To delineate the gas-centered basin geometry in the Piceance Basin, a regional cross-section spanning the basin was constructed using hydrocarbon and gamma radiation logs. The resultant hybrid logs were used for stratigraphic correlations in addition to outlining the trans-basin gas-saturated conditions. The magnitude of both pressure gradients (paludal and marine intervals) is greater than can be generated by a hydrodynamic model. To investigate the relationships between structure and production, detailed mapping of the basin (top of the Iles Formation) was used to define subtle subsurface structures that control fractured reservoir development. The most productive fields in the basin possess fractured reservoirs. Detailed studies in the Grand Valley-Parachute-Rulison and Shire Gulch-Plateau fields indicate that zones of maximum structural flexure on kilometer-scale structural features are directly related to areas of enhanced production.« less

Pore Pressure and Field stress variation from Salt Water Injection; A case Study from Beaver Lodge Field in Williston Basin

NASA Astrophysics Data System (ADS)

Mohammed, R. A.; Khatibi, S.

2017-12-01

One of the major concerns in producing from oil and gas reservoirs in North American Basins is the disposal of high salinity salt water. It is a misconception that Hydro frack triggers Earthquakes, but due to the high salinity and density of water being pumped to the formation that has pore space of the rock already filled, which is not the case in Hydro-frack or Enhanced Oil Recovery in which fracturing fluid is pumped into empty pore space of rocks in depleted reservoirs. A review on the Bakken history showed that the concerns related to induce seismicity has increased over time due to variations in Pore pressure and In-situ stress that have shown steep changes in the region over the time. In this study, we focused on Pore pressure and field Stress variations in lower Cretaceous Inyan Kara and Mississippian Devonian Bakken, Inyan Kara is the major source for class-II salt-water disposal in the basin. Salt-water disposal is the major cause for induced seismicity. A full field study was done on Beaver Lodge Field, which has many salt-water disposal wells Adjacent to Oil and Gas Wells. We analyzed formation properties, stresses, pore-pressure, and fracture gradient profile in the field and. The constructed Mechanical Earth Model (MEM) revealed changes in pore pressure and stresses over time due to saltwater injection. Well drilled in the past were compared to recently drilled wells, which showed much stress variations. Safe mud weight Window of wells near proximity of injection wells was examined which showed many cases of wellbore instabilities. Results of this study will have tremendous impact in studying environmental issues and the future drilling and Fracking operations.

The effects of confining pressure and stress difference on static fatigue of granite

NASA Technical Reports Server (NTRS)

Kranz, R. L.

1979-01-01

Samples of Barre granite were creep tested at room temperature at confining pressures up to 2 kilobars. The time to fracture increased with decreasing stress difference at every pressure, but the rate of change of fracture time with respect to the stress difference increased with pressure. At 87% of the short-term fracture strength, the time to fracture increased from about 4 minutes at atmospheric pressure to longer than one day at 2 Kb of pressure. The inelastic volumetric strain at the onset of tertiary creep, delta, was constant within 25% at any particular pressure but increased with pressure in a manner analogous to the increase of strength with pressure. At the onset of tertiary creep, the number of cracks and their average length increased with pressure. The crack angle and crack length spectra were quite similar, however, at each pressure at the onset of tertiary creep.

Design of experimental system for supercritical CO2 fracturing under confining pressure conditions

NASA Astrophysics Data System (ADS)

Wang, H.; Lu, Q.; Li, X.; Yang, B.; Zheng, Y.; Shi, L.; Shi, X.

2018-03-01

Supercritical CO2 has the characteristics of low viscosity, high diffusion and zero surface tension, and it is considered as a new fluid for non-polluting and non-aqueous fracturing which can be used for shale gas development. Fracturing refers to a method of utilizing the high-pressure fluid to generate fractures in the rock formation so as to improve the oil and gas flow conditions and increase the oil and gas production. In this article, a new type of experimental system for supercritical CO2 fracturing under confining pressure conditions is designed, which is based on characteristics of supercritical CO2, shale reservoir and down-hole environment. The experimental system consists of three sub-systems, including supercritical CO2 generation system, supercritical CO2 fracturing system and data analysis system. It can be used to simulate supercritical CO2 fracturing under geo-stress conditions, thus to study the rock initiation pressure, the formation of the rock fractures, fractured surface morphology and so on. The experimental system has successfully carried out a series of supercritical CO2 fracturing experiments. The experimental results confirm the feasibility of the experimental system and the high efficiency of supercritical CO2 in fracturing tight rocks.

Measurement of width and pressure in a propagating hydraulic fracture

DOE Office of Scientific and Technical Information (OSTI.GOV)

Warpinski, N.R.

Measurements of width and pressure in a propagating hydraulic fracture have been made in tests conducted at DOE's Nevada Test Site. This was accomplished by creating an instrumented fracture at a tunnel complex (at a depth of 1400 ft) where realistic in situ conditions prevail, particularly with respect to stress and geologic features such as natural fractures and material anisotropy. Analyses of these data show that the pressure drop along the fracture length is much larger than predicted by viscous theory and currently in use in models today. This is apparently due to the tortuosity of the fracture path, multiplemore » fracture strands, roughness and sharp turns (corners) in the flow path due to natural fractures and rock property variations. It suggests that fracture design models need to be updated to include a more realistic friction factor so that fracture lengths are not overestimated.« less

Insights to Engineered Geothermal System Performance Using Gringarten-Witherspoon-Ohnishi Analytical Solutions and Fracture Network Models

NASA Astrophysics Data System (ADS)

Doe, T.; McLaren, R.; Finilla, A.

2017-12-01

An enduring legacy of Paul Witherspoon and his students and colleagues has been both the development of geothermal energy and the bases of modern fractured-rock hydrogeology. One of the seminal contributions to the geothermal field was Gringarten, Witherspoon, and Ohnishi's analytical models for enhanced geothermal systems. Although discrete fracture network (DFN) modeling developed somewhat independently in the late 1970s, Paul Witherspoon's foresight in promoting underground in situ testing at the Stripa Mine in Sweden was a major driver in Lawrence Berkeley Laboratory's contributions to its development.This presentation looks extensions of Gringarten's analytical model into discrete fracture network modeling as a basis for providing further insights into the challenges and opportunities of engineered geothermal systems. The analytical solution itself has many insightful applications beyond those presented in the original paper. The definition of dimensionless time by itself shows that thermal breakthrough has a second power dependence on surface area and on flow rate. The fracture intensity also plays a strong role, as it both increases the surface area and decrease his flow rate per fracture. The improvement of EGS performance with fracture intensity reaches a limit where thermal depletion of the rock lags only slightly behind the thermal breakthrough of cold water in the fracture network.Simple network models, which couple a DFN generator (FracMan) with a hydrothermally coupled flow solver (HydroGeoSphere) expand on Gringarten's concepts to show that realistic heterogeneity of spacing and transmissivity significantly degrades EGS performance. EGS production in networks of stimulated fractures initially follows Gringarten's type curves, with a later deviation is the smaller rock blocks thermally deplete and the entire stimulated volume acts as a single sink. Three-dimensional models of EGS performance show the critical importance of the relative magnitudes of fluid pressure and stress gradients, preferential growth and aperture enhancement may change with depth creating preferential pathways through rock this cooler than the injection depth.

On the relation between phase-field crack approximation and gradient damage modelling

NASA Astrophysics Data System (ADS)

Steinke, Christian; Zreid, Imadeddin; Kaliske, Michael

2017-05-01

The finite element implementation of a gradient enhanced microplane damage model is compared to a phase-field model for brittle fracture. Phase-field models and implicit gradient damage models share many similarities despite being conceived from very different standpoints. In both approaches, an additional differential equation and a length scale are introduced. However, while the phase-field method is formulated starting from the description of a crack in fracture mechanics, the gradient method starts from a continuum mechanics point of view. At first, the scope of application for both models is discussed to point out intersections. Then, the analysis of the employed mathematical methods and their rigorous comparison are presented. Finally, numerical examples are introduced to illustrate the findings of the comparison which are summarized in a conclusion at the end of the paper.

Volcanic conduit failure as a trigger to magma fragmentation

NASA Astrophysics Data System (ADS)

Lavallée, Y.; Benson, P. M.; Heap, M. J.; Flaws, A.; Hess, K.-U.; Dingwell, D. B.

2012-01-01

In the assessment of volcanic risk, it is often assumed that magma ascending at a slow rate will erupt effusively, whereas magma ascending at fast rate will lead to an explosive eruption. Mechanistically viewed, this assessment is supported by the notion that the viscoelastic nature of magma (i.e., the ability of magma to relax at an applied strain rate), linked via the gradient of flow pressure (related to discharge rate), controls the eruption style. In such an analysis, the physical interactions between the magma and the conduit wall are commonly, to a first order, neglected. Yet, during ascent, magma must force its way through the volcanic edifice/structure, whose presence and form may greatly affect the stress field through which the magma is trying to ascend. Here, we demonstrate that fracturing of the conduit wall via flow pressure releases an elastic shock resulting in fracturing of the viscous magma itself. We find that magma fragmentation occurred at strain rates seven orders of magnitude slower than theoretically anticipated from the applied axial strain rate. Our conclusion, that the discharge rate cannot provide a reliable indication of ascending magma rheology without knowledge of conduit wall stability, has important ramifications for volcanic hazard assessment. New numerical simulations are now needed in order to integrate magma/conduit interaction into eruption models.

Wellbore pressure transducer

DOEpatents

Shuck, Lowell Z.

1979-01-01

Subterranean earth formations containing energy values are subjected to hydraulic fracturing procedures to enhance the recovery of the energy values. These fractures are induced in the earth formation by pumping liquid into the wellbore penetrating the earth formation until the pressure of the liquid is sufficient to fracture the earth formation adjacent to the wellbore. The present invention is directed to a transducer which is positionable within the wellbore to generate a signal indicative of the fracture initiation useful for providing a timing signal to equipment for seismic mapping of the fracture as it occurs and for providing a measurement of the pressure at which the fracture is initiated.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ahmad Ghassemi

Geothermal energy is recovered by circulating water through heat exchange areas within a hot rock mass. Geothermal reservoir rock masses generally consist of igneous and metamorphic rocks that have low matrix permeability. Therefore, cracks and fractures play a significant role in extraction of geothermal energy by providing the major pathways for fluid flow and heat exchange. Therefore, knowledge of the conditions leading to formation of fractures and fracture networks is of paramount importance. Furthermore, in the absence of natural fractures or adequate connectivity, artificial fractures are created in the reservoir using hydraulic fracturing. Multiple fractures are preferred because of themore » large size necessary when using only a single fracture. Although the basic idea is rather simple, hydraulic fracturing is a complex process involving interactions of high pressure fluid injections with a stressed hot rock mass, mechanical interaction of induced fractures with existing natural fractures, and the spatial and temporal variations of in-situ stress. As a result, it is necessary to develop tools that can be used to study these interactions as an integral part of a comprehensive approach to geothermal reservoir development, particularly enhanced geothermal systems. In response to this need we have developed advanced poro-thermo-chemo-mechanical fracture models for rock fracture research in support of EGS design. The fracture propagation models are based on a regular displacement discontinuity formulation. The fracture propagation studies include modeling interaction of induced fractures. In addition to the fracture propagation studies, two-dimensional solution algorithms have been developed and used to estimate the impact of pro-thermo-chemical processes on fracture permeability and reservoir pressure. Fracture permeability variation is studied using a coupled thermo-chemical model with quartz reaction kinetics. The model is applied to study quartz precipitation/dissolution, as well as the variation in fracture aperture and pressure. Also, a three-dimensional model of injection/extraction has been developed to consider the impact poro- and thermoelastic stresses on fracture slip and injection pressure. These investigations shed light on the processes involved in the observed phenomenon of injection pressure variation (e.g., in Coso), and allow the assessment of the potential of thermal and chemical stimulation strategies.« less

Non-invasive assessment of pulsatile intracranial pressure with phase-contrast magnetic resonance imaging

PubMed Central

Lindstrøm, Erika Kristina; Vatnehol, Svein Are Sirirud; Mardal, Kent-André; Emblem, Kyrre Eeg; Eide, Per Kristian

2017-01-01

Invasive monitoring of pulsatile intracranial pressure can accurately predict shunt response in patients with idiopathic normal pressure hydrocephalus, but may potentially cause complications such as bleeding and infection. We tested how a proposed surrogate parameter for pulsatile intracranial pressure, the phase-contrast magnetic resonance imaging derived pulse pressure gradient, compared with its invasive counterpart. In 22 patients with suspected idiopathic normal pressure hydrocephalus, preceding invasive intracranial pressure monitoring, and any surgical shunt procedure, we calculated the pulse pressure gradient from phase-contrast magnetic resonance imaging derived cerebrospinal fluid flow velocities obtained at the upper cervical spinal canal using a simplified Navier-Stokes equation. Repeated measurements of the pulse pressure gradient were also undertaken in four healthy controls. Of 17 shunted patients, 16 responded, indicating high proportion of “true” normal pressure hydrocephalus in the patient cohort. However, there was no correlation between the magnetic resonance imaging derived pulse pressure gradient and pulsatile intracranial pressure (R = -.18, P = .43). Pulse pressure gradients were also similar in patients and healthy controls (P = .26), and did not differ between individuals with pulsatile intracranial pressure above or below established thresholds for shunt treatment (P = .97). Assessment of pulse pressure gradient at level C2 was therefore not found feasible to replace invasive monitoring of pulsatile intracranial pressure in selection of patients with idiopathic normal pressure hydrocephalus for surgical shunting. Unlike invasive, overnight monitoring, the pulse pressure gradient from magnetic resonance imaging comprises short-term pressure fluctuations only. Moreover, complexity of cervical cerebrospinal fluid flow and -pulsatility at the upper cervical spinal canal may render the pulse pressure gradient a poor surrogate marker for intracranial pressure pulsations. PMID:29190788

Non-invasive assessment of pulsatile intracranial pressure with phase-contrast magnetic resonance imaging.

PubMed

Ringstad, Geir; Lindstrøm, Erika Kristina; Vatnehol, Svein Are Sirirud; Mardal, Kent-André; Emblem, Kyrre Eeg; Eide, Per Kristian

2017-01-01

Invasive monitoring of pulsatile intracranial pressure can accurately predict shunt response in patients with idiopathic normal pressure hydrocephalus, but may potentially cause complications such as bleeding and infection. We tested how a proposed surrogate parameter for pulsatile intracranial pressure, the phase-contrast magnetic resonance imaging derived pulse pressure gradient, compared with its invasive counterpart. In 22 patients with suspected idiopathic normal pressure hydrocephalus, preceding invasive intracranial pressure monitoring, and any surgical shunt procedure, we calculated the pulse pressure gradient from phase-contrast magnetic resonance imaging derived cerebrospinal fluid flow velocities obtained at the upper cervical spinal canal using a simplified Navier-Stokes equation. Repeated measurements of the pulse pressure gradient were also undertaken in four healthy controls. Of 17 shunted patients, 16 responded, indicating high proportion of "true" normal pressure hydrocephalus in the patient cohort. However, there was no correlation between the magnetic resonance imaging derived pulse pressure gradient and pulsatile intracranial pressure (R = -.18, P = .43). Pulse pressure gradients were also similar in patients and healthy controls (P = .26), and did not differ between individuals with pulsatile intracranial pressure above or below established thresholds for shunt treatment (P = .97). Assessment of pulse pressure gradient at level C2 was therefore not found feasible to replace invasive monitoring of pulsatile intracranial pressure in selection of patients with idiopathic normal pressure hydrocephalus for surgical shunting. Unlike invasive, overnight monitoring, the pulse pressure gradient from magnetic resonance imaging comprises short-term pressure fluctuations only. Moreover, complexity of cervical cerebrospinal fluid flow and -pulsatility at the upper cervical spinal canal may render the pulse pressure gradient a poor surrogate marker for intracranial pressure pulsations.

40 CFR 146.13 - Operating, monitoring and reporting requirements.

Code of Federal Regulations, 2013 CFR

2013-07-01

... pressure in the injection zone during injection does not initiate new fractures or propagate existing fractures in the injection zone. In no case shall injection pressure initiate fractures in the confining...

40 CFR 146.13 - Operating, monitoring and reporting requirements.

Code of Federal Regulations, 2010 CFR

2010-07-01

... pressure in the injection zone during injection does not initiate new fractures or propagate existing fractures in the injection zone. In no case shall injection pressure initiate fractures in the confining...

40 CFR 146.13 - Operating, monitoring and reporting requirements.

Code of Federal Regulations, 2011 CFR

2011-07-01

... pressure in the injection zone during injection does not initiate new fractures or propagate existing fractures in the injection zone. In no case shall injection pressure initiate fractures in the confining...

40 CFR 146.13 - Operating, monitoring and reporting requirements.

Code of Federal Regulations, 2014 CFR

2014-07-01

... pressure in the injection zone during injection does not initiate new fractures or propagate existing fractures in the injection zone. In no case shall injection pressure initiate fractures in the confining...

40 CFR 146.13 - Operating, monitoring and reporting requirements.

Code of Federal Regulations, 2012 CFR

2012-07-01

... pressure in the injection zone during injection does not initiate new fractures or propagate existing fractures in the injection zone. In no case shall injection pressure initiate fractures in the confining...

Measurement of width and pressure in a propagating hydraulic fracture

DOE Office of Scientific and Technical Information (OSTI.GOV)

Warpinski, N.R.

Measurements of width and pressure in a propagating hydraulic fracture have been made in tests conducted at the U.S. DOE's Nevada test site. This was accomplished by creating an ''instrumented fracture'' at a tunnel complex (at a depth of 1,400 ft (425 m)) where realistic insitu conditions prevail, particularly with respect to stress and geologic features such as natural fractures and material anisotropy. Analyses of these data show that the pressure drop along the fracture length is much larger than predicted by viscous theory, which currently is used in models. This apparently is caused by the tortuosity of the fracturemore » path, multiple fracture strands, roughness, and sharp turns (corners) in the flow path resulting from natural fractures and rock property variations. It suggests that fracture design models need to be updated to include a more realistic friction factor so that fracture lengths are not overestimated.« less

Measurement of width and pressure in a propagating hydraulic fracture

DOE Office of Scientific and Technical Information (OSTI.GOV)

Warpinski, N.R.

Measurements of width and pressure in a propagating hydraulic fracture have been made in tests conducted at DOE's Nevada Test Site. This was accomplished by creating an ''instrumented fracture'' at a tunnel complex (at a depth of 1400 ft) where realistic in-situ conditions prevail, particularly with respect to stress and geologic features such as natural fractures and material anisotropy. Analyses of these data show that the pressure drop along the fracture length is much larger than predicted by viscous theory and currently in use in models today. This is apparently due to the tortuosity of the fracture path, multiple fracturemore » strands, roughness and sharp turns (corners) in the flow path due to natural fractures and rock property variations. It suggests that fracture design models need to be updated to include a more realistic friction factor so that fracture lengths are not overestimated.« less

Correlations to predict frictional pressure loss of hydraulic-fracturing slurry in coiled tubing

DOE Office of Scientific and Technical Information (OSTI.GOV)

Shah, S.; Zhoi, Y.X.; Bailey, M.

2009-08-15

Compared with conventional-tubing fracturing, coiled-tubing (CT) fracturing has several advantages. CT fracturing has become an effective stimulation technique for multizone oil and gas wells. It is also an attractive production-enhancement method for multiseam coalbed-methane wells, and wells with bypassed zones. The excessive frictional pressure loss through CT has been a concern in fracturing. The small diameter of the string limits the cross-sectional area open to flow. Furthermore, the tubing curvature causes secondary flow and results in extra flow resistance. This increased frictional pressure loss results in high surface pumping pressure. The maximum possible pump rate and sand concentration, therefore, havemore » to be reduced. To design a CT fracturing job properly, it is essential to predict the frictional pressure loss through the tubing accurately. This paper presents correlations for the prediction of frictional pressure loss of fracturing slurries in straight tubing and CT. They are developed on the basis of full-scale slurry-flow tests with 11/2-in. CT and slurries prepared with 35 lbm/1,000 gal of guar gel. The extensive experiments were conducted at the full-scale CT-flow test facility. The proposed correlations have been verified with the experimental data and actual field CT-fracturing data. Case studies of wells recently fractured are provided to demonstrate the application of the correlations. The correlations will be useful to the CT engineers in their hydraulics design calculations.« less

Experimental Investigation on the Basic Law of the Fracture Spatial Morphology for Water Pressure Blasting in a Drillhole Under True Triaxial Stress

NASA Astrophysics Data System (ADS)

Huang, Bingxiang; Li, Pengfeng

2015-07-01

The present literature on the morphology of water pressure blasting fractures in drillholes is not sufficient and does not take triaxial confining stress into account. Because the spatial morphology of water pressure blasting fractures in drillholes is not clear, the operations lack an exact basis. Using a large true triaxial water pressure blasting experimental system and an acoustic emission 3-D positioning system, water pressure blasting experiments on cement mortar test blocks (300 mm × 300 mm × 300 mm) were conducted to study the associated basic law of the fracture spatial morphology. The experimental results show that water pressure blasting does not always generate bubble pulsation. After water pressure blasting under true triaxial stress, a crushed compressive zone and a blasting fracture zone are formed from the inside, with the blasting section of the naked drillhole as the center, to the outside. The shape of the outer edges of the two zones is ellipsoidal. The range of the blasting fracture is large in the radial direction of the drillhole, where the surrounding pressure is large, i.e., the range of the blasting fracture in the drillhole radial cross-section is approximately ellipsoidal. The rock near the drillhole wall is affected by a tensile stress wave caused by the test block boundary reflection, resulting in more flake fractures appearing in the fracturing crack surface in the drillhole axial direction and parallel to the boundary surface. The flake fracture is thin, presenting a small-range flake fracture. The spatial morphology of the water pressure blasting fracture in the drillhole along the axial direction is similar to a wide-mouth Chinese bottle: the crack extent is large near the drillhole orifice, gradually narrows inward along the drillhole axial direction, and then increases into an approximate ellipsoid in the internal naked blasting section. Based on the causes of the crack generation, the blasting cracks are divided into three zones: the blasting shock zone, the axial extension zone, and the orifice influence zone. The explosion shock zone is the range that is directly impacted by the explosive shock waves. The axial extension zone is the axial crack area with uniform width, which is formed when the blasting fracture in the edge of the explosion shock zone extends along the drillhole wall. The extension of the orifice influence zone is very large because the explosion stress waves reflect at the free face and generate tensile stress waves. In the water pressure blasting of the drillhole, the sealing section should be lengthened to allow the drillhole blasting cracks to extend sufficiently under the long-time effect of the blasting stress field of quasi-hydrostatic pressure.

In-Situ Ultra Low Frequency Poroelastic Response of a Natural Macro-Fracture

NASA Astrophysics Data System (ADS)

Guglielmi, Y.; Cappa, F.; Rutqvist, J.; Tsang, C.; Gaffet, S.

2008-12-01

The seismic visibility of macro-fractures filled with fluids is a central problem in the exploration of thermo- hydro-mechanical and chemical processes that occur in Earth' s subsurface. Most studies have been concerned (1) with cracks of a small size relative to the seismic wavelength (2) with "core-sized" samples of single macro-fractures. In comparison, in-situ studies of macro-fractures are very rare and no real estimate is made of the relevance of this convenient "core-sized" data to in-situ reservoirs in general. In this study, we present a new experimental approach to in-situ characterize mechanical and hydraulic properties of fractures using the innovative HPPP protocol. This protocol allows simultaneous high-frequency (120.2 Hz) sampling of normal displacement and fluid pressure in a borehole intersecting the fracture. We show preliminary results conducted in a single fracture vertically embedded in a carbonate reservoir that contains 3 sets of macro-fractures with an average 2m spacing. Two HPPP probes were set, spaced one meter vertically in the fracture. Two types of ULF seismic sources are applied: a fluid pressure pulse injected in the fracture and a hammer hit at a point located 5m far from the fracture plane. There is a highly non-linear variation of fracture normal displacement-versus- fluid pressure as a function of frequency, the higher the frequency, the lower the displacement spectral amplitude is. The pressure pulse and the hammer hit allow exploring the fracture poroelastic response in the [0 - 3Hz] frequency range. The fracture plays the role of a "low-pass" filter for fluid pressure waves; only a quasi-static pressure signal being registered at the receiver. The displacement wave propagation is more complex resulting in uncoupled quasi-static-pressure-2Hz-deformation signals at the receiver. For low magnitude seismic sources (low amplitude pulse and seismic wave), the fracture natural resonance is amplified resulting in separate signals power spectral peaks. When fluid pressure is enough increased, hydraulic diffusion takes place at frequencies lower than 1.2 Hz. Poroelastic effects related to static hydraulic diffusion and to wave propagation were described separately using a linear elastic model where the fracture was treated as a displacement discontinuity across which stresses are continuous but displacement are discontinuous. It appears that the dynamic fracture normal stiffness at 2 to 3 Hz is a factor of 2.8 higher than the static stiffness although the fracture displays a high hydraulic aperture of 10-4 m. This surprising result is related to a high heterogeneity of the fracture channel network with a large porosity/permeability contrast that does not allow fluid displacement under dynamic loading. The HPPP approach appears as a possibility to in-situ characterize such fractures static to seismic poroelastic heterogeneous properties.

Relationship between exercise pressure gradient and haemodynamic progression of aortic stenosis.

PubMed

Ringle, Anne; Levy, Franck; Ennezat, Pierre-Vladimir; Le Goffic, Caroline; Castel, Anne-Laure; Delelis, François; Menet, Aymeric; Malaquin, Dorothée; Graux, Pierre; Vincentelli, André; Tribouilloy, Christophe; Maréchaux, Sylvestre

We hypothesized that large exercise-induced increases in aortic mean pressure gradient can predict haemodynamic progression during follow-up in asymptomatic patients with aortic stenosis. We retrospectively identified patients with asymptomatic moderate or severe aortic stenosis (aortic valve area<1.5cm 2 or<1cm 2 ) and normal ejection fraction, who underwent an exercise stress echocardiography at baseline with a normal exercise test and a resting echocardiography during follow-up. The relationship between exercise-induced increase in aortic mean pressure gradient and annualised changes in resting mean pressure gradient during follow-up was investigated. Fifty-five patients (mean age 66±15 years; 45% severe aortic stenosis) were included. Aortic mean pressure gradient significantly increased from rest to peak exercise (P<0.001). During a median follow-up of 1.6 [1.1-3.2] years, resting mean pressure gradient increased from 35±13mmHg to 48±16mmHg, P<0.0001. Median annualised change in resting mean pressure gradient during follow-up was 5 [2-11] mmHg. Exercise-induced increase in aortic mean pressure gradient did correlate with annualised changes in mean pressure gradient during follow-up (r=0.35, P=0.01). Hemodynamic progression of aortic stenosis was faster in patients with large exercise-induced increase in aortic mean pressure gradient (≥20mmHg) as compared to those with exercise-induced increase in aortic mean pressure gradient<20mmHg (median annualised increase in mean pressure gradient 19 [6-28] vs. 4 [2-10] mmHg/y respectively, P=0.002). Similar results were found in the subgroup of 30 patients with moderate aortic stenosis. Large exercise-induced increases in aortic mean pressure gradient correlate with haemodynamic progression of stenosis during follow-up in patients with asymptomatic aortic stenosis. Further studies are needed to fully establish the role of ESE in the decision-making process in comparison to other prognostic markers in asymptomatic patients with aortic stenosis. Copyright © 2017 Elsevier Masson SAS. All rights reserved.

Use of Pressure-Redistributing Support Surfaces among Elderly Hip Fracture Patients across the Continuum of Care: Adherence to Pressure Ulcer Prevention Guidelines

ERIC Educational Resources Information Center

Baumgarten, Mona; Margolis, David; Orwig, Denise; Hawkes, William; Rich, Shayna; Langenberg, Patricia; Shardell, Michelle; Palmer, Mary H.; McArdle, Patrick; Sterling, Robert; Jones, Patricia S.; Magaziner, Jay

2010-01-01

Purpose: To estimate the frequency of use of pressure-redistributing support surfaces (PRSS) among hip fracture patients and to determine whether higher pressure ulcer risk is associated with greater PRSS use. Design and Methods: Patients (n = 658) aged [greater than or equal] 65 years who had surgery for hip fracture were examined by research…

Using Fully Coupled Hydro-Geomechanical Numerical Test Bed to Study Reservoir Stimulation with Low Hydraulic Pressure

DOE Data Explorer

Fu, Pengcheng; Johnson, Scott M.; Carrigan, Charles R.

2012-01-31

This paper documents our effort to use a fully coupled hydro-geomechanical numerical test bed to study using low hydraulic pressure to stimulate geothermal reservoirs with existing fracture network. In this low pressure stimulation strategy, fluid pressure is lower than the minimum in situ compressive stress, so the fractures are not completely open but permeability improvement can be achieved through shear dilation. We found that in this low pressure regime, the coupling between the fluid phase and the rock solid phase becomes very simple, and the numerical model can achieve a low computational cost. Using this modified model, we study the behavior of a single fracture and a random fracture network.

The fracture strength and frictional strength of Weber Sandstone

USGS Publications Warehouse

Byerlee, J.D.

1975-01-01

The fracture strength and frictional strength of Weber Sandstone have been measured as a function of confining pressure and pore pressure. Both the fracture strength and the frictional strength obey the law of effective stress, that is, the strength is determined not by the confining pressure alone but by the difference between the confining pressure and the pore pressure. The fracture strength of the rock varies by as much as 20 per cent depending on the cement between the grains, but the frictional strength is independent of lithology. Over the range 0 2 kb, ??=0??5 + 0??6??n. This relationship also holds for other rocks such as gabbro, dunite, serpentinite, granite and limestone. ?? 1975.

Assessing the P-wave attenuation and phase velocity characteristics of fractured media based on creep and relaxation tests

NASA Astrophysics Data System (ADS)

Milani, Marco; Germán Rubino, J.; Müller, Tobias M.; Quintal, Beatriz; Holliger, Klaus

2014-05-01

Fractures are present in most geological formations and they tend to dominate not only their mechanical but also, and in particular, their hydraulic properties. For these reasons, the detection and characterization of fractures are of great interest in several fields of Earth sciences. Seismic attenuation has been recognized as a key attribute for this purpose, as both laboratory and field experiments indicate that the presence of fractures typically produces significant energy dissipation and that this attribute tends to increase with increasing fracture density. This energy loss is generally considered to be primarily due to wave-induced pressure diffusion between the fractures and the embedding porous matrix. That is, due to the strong compressibility contrast between these two domains, the propagation of seismic waves can generate a strong fluid pressure gradient and associated pressure diffusion, which leads to fluid flow and in turn results in frictional energy dissipation. Numerical simulations based on Biot's poroelastic wave equations are computationally very expensive. Alternative approaches consist in performing numerical relaxation or creep tests on representative elementary volumes (REV) of the considered medium. These tests are typically based on Biot's consolidation equations. Assuming that the heterogeneous poroelastic medium can be replaced by an effective, homogeneous viscoelastic solid, these numerical creep and relaxation tests allow for computing the equivalent seismic P-wave attenuation and phase velocity. From a practical point of view, an REV is typically characterized by the smallest volume for which rock physical properties are statistically stationary and representative of the probed medium in its entirety. A more general definition in the context of wavefield attributes is to consider an REV as the smallest volume over which the P-wave attenuation and phase velocity dispersion are independent of the applied boundary conditions. That is, the corresponding results obtained from creep and relaxation tests must be equivalent. For most analyses of media characterized by patchy saturation or double-porosity-type structures these two definitions are equivalent. It is, however, not clear whether this equivalence remains true in the presence of strong material contrasts as those prevailing in fractured rocks. In this work, we explore this question for periodically fractured media. To this end, we build a medium composed of infinite replicas of a unit volume containing one fracture. This unit volume coincides with the smallest possible volume that is statistically representative of the whole. Then, we perform several creep and relaxation tests on samples composed of an increasing number of these unit volumes. We find that the wave field signatures determined from relaxation tests are independent from the number of unit volumes. Conversely, the P-wave attenuation and phase velocity characteristics inferred from creep tests are different and vary with the number of unit volumes considered. Quite interestingly, the creep test results converge with those of the relaxation tests as the number of unit volumes increases. These findings are expected to have direct implications for corresponding laboratory measurements as well as for our understanding of seismic wave propagation in fractured media.

Altered disc pressure profile after an osteoporotic vertebral fracture is a risk factor for adjacent vertebral body fracture

PubMed Central

Tzermiadianos, Michael N.; Renner, Susan M.; Phillips, Frank M.; Hadjipavlou, Alexander G.; Zindrick, Michael R.; Havey, Robert M.; Voronov, Michael

2008-01-01

This study investigated the effect of endplate deformity after an osteoporotic vertebral fracture in increasing the risk for adjacent vertebral fractures. Eight human lower thoracic or thoracolumbar specimens, each consisting of five vertebrae were used. To selectively fracture one of the endplates of the middle VB of each specimen a void was created under the target endplate and the specimen was flexed and compressed until failure. The fractured vertebra was subjected to spinal extension under 150 N preload that restored the anterior wall height and vertebral kyphosis, while the fractured endplate remained significantly depressed. The VB was filled with cement to stabilize the fracture, after complete evacuation of its trabecular content to ensure similar cement distribution under both the endplates. Specimens were tested in flexion-extension under 400 N preload while pressure in the discs and strain at the anterior wall of the adjacent vertebrae were recorded. Disc pressure in the intact specimens increased during flexion by 26 ± 14%. After cementation, disc pressure increased during flexion by 15 ± 11% in the discs with un-fractured endplates, while decreased by 19 ± 26.7% in the discs with the fractured endplates. During flexion, the compressive strain at the anterior wall of the vertebra next to the fractured endplate increased by 94 ± 23% compared to intact status (p < 0.05), while it did not significantly change at the vertebra next to the un-fractured endplate (18.2 ± 7.1%, p > 0.05). Subsequent flexion with compression to failure resulted in adjacent fracture close to the fractured endplate in six specimens and in a non-adjacent fracture in one specimen, while one specimen had no adjacent fractures. Depression of the fractured endplate alters the pressure profile of the damaged disc resulting in increased compressive loading of the anterior wall of adjacent vertebra that predisposes it to wedge fracture. This data suggests that correction of endplate deformity may play a role in reducing the risk of adjacent fractures. PMID:18795344

Roughness-Dominated Hydraulic Fracture Propagation

NASA Astrophysics Data System (ADS)

Garagash, D.

2015-12-01

Current understanding suggests that the energy to propagate a hydraulic fracture is defined by the viscous fluid pressure drop along the fracture channel, while the energy dissipation in the immediate vicinity of the fracture front (i.e. fracture toughness) is negligible. This status quo relies on the assumption of Poiseuille flow in the fracture, which transmissivity varies as cube of the aperture. We re-evaluate this assumption in the vicinity of the fracture tip, where the aperture roughness and/or branching of the fracture path may lead to very significant deviations from the cubic law. Existing relationships suggest rough fracture transmissivity power laws ~ wr with 4.5 ≤ r ≤ 6, when aperture w is smaller than the roughness. Solving for the tip region of a steadily propagating hydraulic fracture with the "rough fracture" transmissivity, we are able to show (a) larger energy dissipation than predicted by the Poiseuille flow model; (b) localization of the fluid pressure drop into the low-transmissivity, rough tip region; and (c) emergence of potentially preeminent "toughness-dominated" fracture propagation regime where most of the energy is dissipated at the tip and can be described in the context of classical fracture mechanics by invoking the effective fracture toughness dependent upon the details of the pressure drop in the rough tip. We establish that the ratio of the roughness scale wc to the viscous aperture scale wμ = μVE / σ02, controls the pressure drop localization. (Here V - propagation speed, μ - fluid viscosity, E - rock modulus, and σ0 - in-situ stress). For a range of industrial fracturing fluids (from slick-water to linear gels) and treatment conditions, wc/wμ is large, suggesting a fully-localized pressure drop and energy dissipation. The latter is adequately described by the effective toughness - a function of the propagation velocity, confining stress and material parameters, which estimated values are much larger than the "dry" rock fracture toughness measured in the lab. Using the effective, velocity-dependent fracture toughness to predict the evolution of a penny-shape fracture, we are able to show how/when the classical viscosity-dominated and toughness-dominated solutions based upon the Poiseuille law and the "dry", laboratory fracture toughness values, respectively, may become inadequate.

Image-based modeling of the flow transition from a Berea rock matrix to a propped fracture

NASA Astrophysics Data System (ADS)

Sanematsu, P.; Willson, C. S.; Thompson, K. E.

2013-12-01

In the past decade, new technologies and advances in horizontal hydraulic fracturing to extract oil and gas from tight rocks have raised questions regarding the physics of the flow and transport processes that occur during production. Many of the multi-dimensional details of flow from the rock matrix into the fracture and within the proppant-filled fracture are still unknown, which leads to unreliable well production estimations. In this work, we use x-ray computed micro tomography (XCT) to image 30/60 CarboEconoprop light weight ceramic proppant packed between berea sandstone cores (6 mm in diameter and ~2 mm in height) under 4000 psi (~28 MPa) loading stress. Image processing and segmentation of the 6 micron voxel resolution tomography dataset into solid and void space involved filtering with anisotropic diffusion (AD), segmentation using an indicator kriging (IK) algorithm, and removal of noise using a remove islands and holes program. Physically-representative pore network structures were generated from the XCT images, and a representative elementary volume (REV) was analyzed using both permeability and effective porosity convergence. Boundary conditions were introduced to mimic the flow patterns that occur when fluid moves from the matrix into the proppant-filled fracture and then downstream within the proppant-filled fracture. A smaller domain, containing Berea and proppants close to the interface, was meshed using an in-house unstructured meshing algorithm that allows different levels of refinement. Although most of this domain contains proppants, the Berea section accounted for the majority of the elements due to mesh refinement in this region of smaller pores. A finite element method (FEM) Stokes flow model was used to provide more detailed insights on the flow transition from rock matrix to fracture. Results using different pressure gradients are used to describe the flow transition from the Berea rock matrix to proppant-filled fracture.

Snap, Crackle, Pop: Dilational fault breccias record seismic slip below the brittle-plastic transition

NASA Astrophysics Data System (ADS)

Melosh, Ben L.; Rowe, Christie D.; Smit, Louis; Groenewald, Conrad; Lambert, Christopher W.; Macey, Paul

2014-10-01

Off-fault dynamic tensile cracks form behind an earthquake rupture front with distinct orientation and spacing. These cracks explode the wall rock and create breccias, which we hypothesize will preserve a unique fingerprint of dynamic rupture. Identification of these characteristic breccias may enable a new tool for identifying paleoseismic slip surfaces in the rock record. Using previous experimental and theoretical predictions, we develop a field-based model of dynamic dilational breccia formation. Experimental studies find that secondary tensile fracture networks comprise closely spaced fractures at angles of 70-90° from a slip surface, as well as fractures that branch at angles of ∼ 30 ° from a primary mode I fracture. The Pofadder Shear Zone, in Namibia and South Africa, preserves breccias formed in the brittle-ductile transition zone displaying fracture patterns consistent with those described above. Fracture spacing is approximately two orders of magnitude less than predicted by quasi-static models. Breccias are clast-supported, monomict and can display an abrupt transition from fracture network crackle breccia to mosaic breccia textures. Brecciation occurs by the intersection of off-fault dynamic fractures and wall rock fabric; this is in contrast to previous models of fluid pressure gradient-driven failure ;implosion breccias;. This mechanism tends to form many similar sized clasts with particle size distributions that may not display self-similarity; where self-similarity is observed the distributions have relatively low D-values of 1.47 ± 0.37, similar to other studies of dynamic processes. We measure slip distances at dilational breccia stepovers, estimating earthquake magnitudes between Mw 2.8-5.8 and associated rupture lengths of 0.023-3.3 km. The small calculated rupture dimensions, in combination with our geologic observations, suggest that some earthquakes nucleated within the quartz-plastic transitional zone and potentially record deep seismic slip.

Numerical simulation on ferrofluid flow in fractured porous media based on discrete-fracture model

NASA Astrophysics Data System (ADS)

Huang, Tao; Yao, Jun; Huang, Zhaoqin; Yin, Xiaolong; Xie, Haojun; Zhang, Jianguang

2017-06-01

Water flooding is an efficient approach to maintain reservoir pressure and has been widely used to enhance oil recovery. However, preferential water pathways such as fractures can significantly decrease the sweep efficiency. Therefore, the utilization ratio of injected water is seriously affected. How to develop new flooding technology to further improve the oil recovery in this situation is a pressing problem. For the past few years, controllable ferrofluid has caused the extensive concern in oil industry as a new functional material. In the presence of a gradient in the magnetic field strength, a magnetic body force is produced on the ferrofluid so that the attractive magnetic forces allow the ferrofluid to be manipulated to flow in any desired direction through the control of the external magnetic field. In view of these properties, the potential application of using the ferrofluid as a new kind of displacing fluid for flooding in fractured porous media is been studied in this paper for the first time. Considering the physical process of the mobilization of ferrofluid through porous media by arrangement of strong external magnetic fields, the magnetic body force was introduced into the Darcy equation and deals with fractures based on the discrete-fracture model. The fully implicit finite volume method is used to solve mathematical model and the validity and accuracy of numerical simulation, which is demonstrated through an experiment with ferrofluid flowing in a single fractured oil-saturated sand in a 2-D horizontal cell. At last, the water flooding and ferrofluid flooding in a complex fractured porous media have been studied. The results showed that the ferrofluid can be manipulated to flow in desired direction through control of the external magnetic field, so that using ferrofluid for flooding can raise the scope of the whole displacement. As a consequence, the oil recovery has been greatly improved in comparison to water flooding. Thus, the ferrofluid flooding is a large potential method for enhanced oil recovery in the future.

A Review of Large-Scale Fracture Experiments Relevant to Pressure Vessel Integrity Under Pressurized Thermal Shock Conditions

DOE Office of Scientific and Technical Information (OSTI.GOV)

Pugh, C.E.

2001-01-29

Numerous large-scale fracture experiments have been performed over the past thirty years to advance fracture mechanics methodologies applicable to thick-wall pressure vessels. This report first identifies major factors important to nuclear reactor pressure vessel (RPV) integrity under pressurized thermal shock (PTS) conditions. It then covers 20 key experiments that have contributed to identifying fracture behavior of RPVs and to validating applicable assessment methodologies. The experiments are categorized according to four types of specimens: (1) cylindrical specimens, (2) pressurized vessels, (3) large plate specimens, and (4) thick beam specimens. These experiments were performed in laboratories in six different countries. This reportmore » serves as a summary of those experiments, and provides a guide to references for detailed information.« less

Fluid Lavage of Open Wounds (FLOW): A Multicenter, Blinded, Factorial Trial Comparing Alternative Irrigating Solutions and Pressures in Patients with Open Fractures

DTIC Science & Technology

2013-10-01

Multicenter, Blinded, Factorial Trial Comparing Alternative Irrigating Solutions and Pressures in Patients with Open Fractures PRINCIPAL...Solutions and Pressures in Patients with Open Fractures 5b. GRANT NUMBER W81XWH-12-1-0530 5c. PROGRAM ELEMENT NUMBER 6. AUTHOR(S) Kyle J. Jeray...important initial step in preventing infection in open fractures . However, there is little clinical evidence as to the best irrigation methods and additives

Surface self-potential patterns related to transmissive fracture trends during a water injection test

NASA Astrophysics Data System (ADS)

DesRoches, A. J.; Butler, K. E.; MacQuarrie, K. TB

2018-03-01

Variations in self-potential (SP) signals were recorded over an electrode array during a constant head injection test in a fractured bedrock aquifer. Water was injected into a 2.2 m interval isolated between two inflatable packers at 44 m depth in a vertical well. Negative SP responses were recorded on surface corresponding to the start of the injection period with strongest magnitudes recorded in electrodes nearest the well. SP response decreased in magnitude at electrodes further from the well. Deflation of the packer system resulted in a strong reversal in the SP signal. Anomalous SP patterns observed at surface at steady state were found to be aligned with dominant fracture strike orientations found within the test interval. Numerical modelling of fluid and current flow within a simplified fracture network showed that azimuthal patterns in SP are mainly controlled by transmissive fracture orientations. The strongest SP gradients occur parallel to hydraulic gradients associated with water flowing out of the transmissive fractures into the tighter matrix and other less permeable cross-cutting fractures. Sensitivity studies indicate that increasing fracture frequency near the well increases the SP magnitude and enhances the SP anomaly parallel to the transmissive set. Decreasing the length of the transmissive fractures leads to more fluid flow into the matrix and into cross-cutting fractures proximal to the well, resulting in a more circular and higher magnitude SP anomaly. Results from the field experiment and modelling provide evidence that surface-based SP monitoring during constant head injection tests has the ability to identify groundwater flow pathways within a fractured bedrock aquifer.

The permeability of fractured rocks in pressurised volcanic and geothermal systems.

PubMed

Lamur, A; Kendrick, J E; Eggertsson, G H; Wall, R J; Ashworth, J D; Lavallée, Y

2017-07-21

The connectivity of rocks' porous structure and the presence of fractures influence the transfer of fluids in the Earth's crust. Here, we employed laboratory experiments to measure the influence of macro-fractures and effective pressure on the permeability of volcanic rocks with a wide range of initial porosities (1-41 vol. %) comprised of both vesicles and micro-cracks. We used a hand-held permeameter and hydrostatic cell to measure the permeability of intact rock cores at effective pressures up to 30 MPa; we then induced a macro-fracture to each sample using Brazilian tensile tests and measured the permeability of these macro-fractured rocks again. We show that intact rock permeability increases non-linearly with increasing porosity and decreases with increasing effective pressure due to compactional closure of micro-fractures. Imparting a macro-fracture both increases the permeability of rocks and their sensitivity to effective pressure. The magnitude of permeability increase induced by the macro-fracture is more significant for dense rocks. We finally provide a general equation to estimate the permeability of intact and fractured rocks, forming a basis to constrain fluid flow in volcanic and geothermal systems.

Water Pressure Effects on Strength and Deformability of Fractured Rocks Under Low Confining Pressures

NASA Astrophysics Data System (ADS)

Noorian Bidgoli, Majid; Jing, Lanru

2015-05-01

The effect of groundwater on strength and deformation behavior of fractured crystalline rocks is one of the important issues for design, performance and safety assessments of surface and subsurface rock engineering problems. However, practical difficulties make the direct in situ and laboratory measurements of these properties of fractured rocks impossible at present, since effects of complex fracture system hidden inside the rock masses cannot be accurately estimated. Therefore, numerical modeling needs to be applied. The overall objective of this paper is to deepen our understanding on the validity of the effective stress concept, and to evaluate the effects of water pressure on strength and deformation parameters. The approach adopted uses discrete element methods to simulate the coupled stress-deformation-flow processes in a fractured rock mass with model dimensions at a representative elementary volume (REV) size and realistic representation of fracture system geometry. The obtained numerical results demonstrate that water pressure has significant influence on the strength, but with minor effects on elastic deformation parameters, compared with significant influence by the lateral confining pressure. Also, the classical effective stress concept to fractured rock can be quite different with that applied in soil mechanics. Therefore, one should be cautious when applying the classical effective stress concept to fractured rock media.

Experiments and Simulations of Fully Hydro-Mechanically Coupled Response of Rough Fractures Exposed to High-Pressure Fluid Injection

NASA Astrophysics Data System (ADS)

Vogler, D.; Settgast, R. R.; Annavarapu, C.; Madonna, C.; Bayer, P.; Amann, F.

2018-02-01

In this work, we present the application of a fully coupled hydro-mechanical method to investigate the effect of fracture heterogeneity on fluid flow through fractures at the laboratory scale. Experimental and numerical studies of fracture closure behavior in the presence of heterogeneous mechanical and hydraulic properties are presented. We compare the results of two sets of laboratory experiments on granodiorite specimens against numerical simulations in order to investigate the mechanical fracture closure and the hydro-mechanical effects, respectively. The model captures fracture closure behavior and predicts a nonlinear increase in fluid injection pressure with loading. Results from this study indicate that the heterogeneous aperture distributions measured for experiment specimens can be used as model input for a local cubic law model in a heterogeneous fracture to capture fracture closure behavior and corresponding fluid pressure response.

In Situ Triaxial Testing To Determine Fracture Permeability and Aperture Distribution for CO2 Sequestration in Svalbard, Norway.

PubMed

Van Stappen, Jeroen F; Meftah, Redouane; Boone, Marijn A; Bultreys, Tom; De Kock, Tim; Blykers, Benjamin K; Senger, Kim; Olaussen, Snorre; Cnudde, Veerle

2018-04-17

On Svalbard, Arctic Norway, an unconventional siliciclastic reservoir, relying on (micro)fractures for enhanced fluid flow in a low-permeable system, is investigated as a potential CO 2 sequestration site. The fractures' properties at depth are, however, poorly understood. High resolution X-ray computed tomography (micro-CT) imaging allows one to visualize such geomaterials at reservoir conditions. We investigated reservoir samples from the De Geerdalen Formation on Svalbard to understand the influence of fracture closure on the reservoir fluid flow behavior. Small rock plugs were brought to reservoir conditions, while permeability was measured through them during micro-CT imaging. Local fracture apertures were quantified down to a few micrometers wide. The permeability measurements were complemented with fracture permeability simulations based on the obtained micro-CT images. The relationship between fracture permeability and the imposed confining pressure was determined and linked to the fracture apertures. The investigated fractures closed due to the increased confining pressure, with apertures reducing to approximately 40% of their original size as the confining pressure increased from 1 to 10 MPa. This coincides with a permeability drop of more than 90%. Despite their closure, fluid flow is still controlled by the fractures at pressure conditions similar to those at the proposed storage depth of 800-1000 m.

Intra-articular calcaneal fractures: effect of open reduction and internal fixation on the contact characteristics of the subtalar joint.

PubMed

Mulcahy, D M; McCormack, D M; Stephens, M M

1998-12-01

Intra-articular calcaneal fractures are associated with significant long-term morbidity, and considerable controversy exists regarding the optimum method of treating them. The contact characteristics in the intact subtalar joint were determined at known loads and for different positions of the ankle and subtalar joint, using pressure-sensitive film (Super Low; Fuji, Itochu Canada Ltd, Montreal, Quebec). We measured the contact area to joint area ratio (pressure > 5 kg force/cm2 [kgf/cm2]) which normalizes for differences in joint size and the ratio of high pressure zone (>20 kgf/cm2) as a reflection of overall increase in joint pressure. Three simulated fracture patterns were then created and stabilized with either 1 or 2 mm of articular incongruity. Eight specimens were prepared with a primary fracture line through the posterior facet, eight with a joint depression-type fracture, and six with a central joint depression fracture. A measure of 1 to 2 mm of incongruity in the posterior facet for all three fracture patterns produced significant unloading of the depressed fragment, with a redistribution of the overall pattern of pressure distribution to parts of the facet that were previously unloaded.

Quantifying the Effects of Spatial Uncertainty in Fracture Permeability on CO2 Leakage through Columbia River Basalt Flow Interiors

NASA Astrophysics Data System (ADS)

Gierzynski, A.; Pollyea, R.

2016-12-01

Recent studies suggest that continental flood basalts may be suitable for geologic carbon sequestration, due to fluid-rock reactions that mineralize injected CO2 on relatively short time-scales. Flood basalts also possess a morphological structure conducive to injection, with alternating high-permeability (flow margin) and low-permeability (flow interior) layers. However, little information exists on the behavior of CO2 migration within field-scale fracture networks, particularly within flow interiors and at conditions near the critical point for CO2. In this study, numerical simulation is used to investigate the influence of fracture permeability uncertainty during gravity-driven CO2 migration within a jointed basalt flow interior as CO2 undergoes phase change from supercritical fluid to a subcritical phase. The model domain comprises a 2D fracture network mapped with terrestrial LiDAR scans of Columbia River Basalt acquired near Starbuck, WA. The model domain is 5 m × 5 m with bimodal heterogeneity (fracture and matrix), and initial conditions corresponding to a hydrostatic pressure gradient between 750 and 755 m depth. Under these conditions, the critical point for CO2 occurs 1.5 m above the bottom of the domain. For this model scenario, CO2 enters the base of the fracture network at 0.5 MPa overpressure, and matrix permeability is assumed constant. Fracture permeability follows a lognormal distribution on the basis of fracture aperture values from literature. In order to account for spatial uncertainty, the lognormal fracture permeability distribution is randomly located in the model domain and CO2 migration is simulated within the same fracture network for 50 equally probable realizations. Model results suggest that fracture connectivity, which is independent of permeability distribution, governs the path taken by buoyant CO2 as it rises through the flow interior; however, the permeability distribution strongly governs the CO2 flux magnitude. In particular, this research shows that even where fracture networks are sufficiently connected, CO2 flux is often inhibited by a cell of lower permeability, analogous to an obstruction or asperity in a natural fracture. This impresses the importance of considering spatial uncertainty in fracture apertures when modeling CO2 leakage through a caprock.

CT scanning and flow measurements of shale fractures after multiple shearing events

DOE PAGES

Crandall, Dustin; Moore, Johnathan; Gill, Magdalena; ...

2017-11-05

A shearing apparatus was used in conjunction with a Hassler-style core holder to incrementally shear fractured shale cores while maintaining various confining pressures. Computed tomography scans were performed after each shearing event, and were used to obtain information on evolving fracture geometry. Fracture transmissivity was measured after each shearing event to understand the hydrodynamic response to the evolving fracture structure. The digital fracture volumes were used to perform laminar single phase flow simulations (local cubic law with a tapered plate correction model) to qualitatively examine small scale flow path variations within the altered fractures. Fractures were found to generally increasemore » in aperture after several shear slip events, with corresponding transmissivity increases. Lower confining pressure resulted in a fracture more prone to episodic mechanical failure and sudden changes in transmissivity. Conversely, higher confining pressures resulted in a system where, after an initial setting of the fracture surfaces, changes to the fracture geometry and transmissivity occurred gradually. Flow paths within the fractures are largely controlled by the location and evolution of zero aperture locations. Lastly, a reduction in the number of primary flow pathways through the fracture, and an increase in their width, was observed during all shearing tests.« less

CT scanning and flow measurements of shale fractures after multiple shearing events

DOE Office of Scientific and Technical Information (OSTI.GOV)

Crandall, Dustin; Moore, Johnathan; Gill, Magdalena

A shearing apparatus was used in conjunction with a Hassler-style core holder to incrementally shear fractured shale cores while maintaining various confining pressures. Computed tomography scans were performed after each shearing event, and were used to obtain information on evolving fracture geometry. Fracture transmissivity was measured after each shearing event to understand the hydrodynamic response to the evolving fracture structure. The digital fracture volumes were used to perform laminar single phase flow simulations (local cubic law with a tapered plate correction model) to qualitatively examine small scale flow path variations within the altered fractures. Fractures were found to generally increasemore » in aperture after several shear slip events, with corresponding transmissivity increases. Lower confining pressure resulted in a fracture more prone to episodic mechanical failure and sudden changes in transmissivity. Conversely, higher confining pressures resulted in a system where, after an initial setting of the fracture surfaces, changes to the fracture geometry and transmissivity occurred gradually. Flow paths within the fractures are largely controlled by the location and evolution of zero aperture locations. Lastly, a reduction in the number of primary flow pathways through the fracture, and an increase in their width, was observed during all shearing tests.« less

Initiation and propagation of a PKN hydraulic fracture in permeable rock: Toughness dominated regime

NASA Astrophysics Data System (ADS)

Sarvaramini, E.; Garagash, D.

2011-12-01

The present work investigates the injection of a low-viscosity fluid into a pre-existing fracture with constrained height (PKN), as in waterflooding or supercritical CO2 injection. Contrary to conventional hydraulic fracturing, where 'cake build up' limits diffusion to a small zone, the low viscosity fluid allows for diffusion over a wider range of scales. Over large injection times the pattern becomes 2 or 3-D, necessitating a full-space diffusion modeling. In addition, the dissipation of energy associated with fracturing of rock dominates the energy needed for the low-viscosity fluid flow into the propagating crack. As a result, the fracture toughness is important in evaluating both the initiation and the ensuing propagation of these fractures. Classical PKN hydraulic fracturing model, amended to account for full-space leak-off and the toughness [Garagash, unpublished 2009], is used to evaluate the pressure history and fluid leak-off volume during the injection of low viscosity fluid into a pre-existing and initially stationary. In order to find the pressure history, the stationary crack is first subject to a step pressure increase. The response of the porous medium to the step pressure increase in terms of fluid leak-off volume provides the fundamental solution, which then can be used to find the transient pressurization using Duhamel theorem [Detournay & Cheng, IJSS 1991]. For the step pressure increase an integral equation technique is used to find the leak-off rate history. For small time the solution must converge to short time asymptote, which corresponds to 1-D diffusion pattern. However, as the diffusion length in the zone around the fracture increases the assumption of a 1-D pattern is no longer valid and the diffusion follows a 2-D pattern. The solution to the corresponding integral equation gives the leak-off rate history, which is used to find the cumulative leak-off volume. The transient pressurization solution is obtained using global conservation of fluid injected into the fracture. With increasing pressure in the fracture due to the fluid injection, the energy release rate eventually becomes equal to the toughness and fracture propagates. The evolution of the fracture length is established using the method similar to the one employed for the stationary crack.

Pressure vessel fracture, fatigue, and life management: PVP-Volume 233

DOE Office of Scientific and Technical Information (OSTI.GOV)

Bhandari, S.; Milella, P.P.; Pennell, W.E.

1992-01-01

This volume contains papers relating to the structural integrity assessment of pressure vessels and piping, with special emphasis on the effects of aging. The papers are organized in the following five areas: (1) pressure vessel life management; (2) fracture characterization using local and dual-parameter approaches; (3) stratification and thermal fatigue; (4) creep, fatigue, and fracture; and (5) integrated approach to integrity assessment of pressure components. Separate abstracts were prepared for 39 papers in this conference.

The Influence of Fracturing Fluids on Fracturing Processes: A Comparison Between Water, Oil and SC-CO2

NASA Astrophysics Data System (ADS)

Wang, Jiehao; Elsworth, Derek; Wu, Yu; Liu, Jishan; Zhu, Wancheng; Liu, Yu

2018-01-01

Conventional water-based fracturing treatments may not work well for many shale gas reservoirs. This is due to the fact that shale gas formations are much more sensitive to water because of the significant capillary effects and the potentially high contents of swelling clay, each of which may result in the impairment of productivity. As an alternative to water-based fluids, gaseous stimulants not only avoid this potential impairment in productivity, but also conserve water as a resource and may sequester greenhouse gases underground. However, experimental observations have shown that different fracturing fluids yield variations in the induced fracture. During the hydraulic fracturing process, fracturing fluids will penetrate into the borehole wall, and the evolution of the fracture(s) then results from the coupled phenomena of fluid flow, solid deformation and damage. To represent this, coupled models of rock damage mechanics and fluid flow for both slightly compressible fluids and CO2 are presented. We investigate the fracturing processes driven by pressurization of three kinds of fluids: water, viscous oil and supercritical CO2. Simulation results indicate that SC-CO2-based fracturing indeed has a lower breakdown pressure, as observed in experiments, and may develop fractures with greater complexity than those developed with water-based and oil-based fracturing. We explore the relation between the breakdown pressure to both the dynamic viscosity and the interfacial tension of the fracturing fluids. Modeling demonstrates an increase in the breakdown pressure with an increase both in the dynamic viscosity and in the interfacial tension, consistent with experimental observations.

Group invariant solution for a pre-existing fluid-driven fracture in impermeable rock

NASA Astrophysics Data System (ADS)

Fitt, A. D.; Mason, D. P.; Moss, E. A.

2007-11-01

The propagation of a two-dimensional fluid-driven fracture in impermeable rock is considered. The fluid flow in the fracture is laminar. By applying lubrication theory a partial differential equation relating the half-width of the fracture to the fluid pressure is derived. To close the model the PKN formulation is adopted in which the fluid pressure is proportional to the half-width of the fracture. By considering a linear combination of the Lie point symmetries of the resulting non-linear diffusion equation the boundary value problem is expressed in a form appropriate for a similarity solution. The boundary value problem is reformulated as two initial value problems which are readily solved numerically. The similarity solution describes a preexisting fracture since both the total volume and length of the fracture are initially finite and non-zero. Applications in which the rate of fluid injection into the fracture and the pressure at the fracture entry are independent of time are considered.

Failure of a gas well to respond to a foam hydraulic fracturing treatment

DOE Office of Scientific and Technical Information (OSTI.GOV)

Rauscher, B.D.

1996-12-31

Well No. 1 (not the real name of the well) is not producing gas at maximum capacity following a foam hydraulic fracturing treatment performed upon completion of the well in 1987. The failure of the stimulation treatment, which has affected other wells throughout the field, was due to a combination of three factors: (1) downward fracture growth and proppant settling during injection (2) embedment due to a high pressure drawdown in the wellbore during flowback procedures, and (3) poor cleanup of the fracture fluid due to high capillary pressures. The following are recommendations to help improve future fracturing treatments throughoutmore » the field: (1) Fracture at lower treating pressures; (2) Improve perforating techniques; (3) Change flowback procedures; and (4) Evaluate using N{sub 2} as a fracture fluid.« less

Hydrogeological investigation at Site 5, Willow Grove Naval Air Station/Joint Reserve Base, Horsham Township, Montgomery County, Pennsylvania

USGS Publications Warehouse

Sloto, Ronald A.

2002-01-01

The U.S. Geological Survey conducted borehole geophysical logging, collected and analyzed water-level data, and sampled sections of a rock core to determine the concentration of volatile organic compounds in the aquifer matrix of the Stockton Formation. Borehole geophysical logs were run in three monitor wells. At well 05MW04I, the vertical gradient was upward at depths above 42 feet below land surface (ft bls), downward between 42 and 82 ft bls, and upward below 82 ft bls. At well 05MW05I, a downward vertical gradient was present. At well 05MW12I, the vertical gradient was downward above 112 ft bls and upward below 112 ft bls.Three water-bearing fractures in a 17-foot long rock core from 23.5 to 40.5 ft bls were identified and sampled. Three samples were analyzed from each water-bearing fracture—at the fracture face, 2 centimeters (cm) below the fracture, and 4 cm below the fracture. Fifteen compounds were detected; however, concentrations of seven compounds were less than 1 microgram per kilogram (mg/kg) when detected. Concentrations of benzene (from 0.39 to 3.3 mg/kg), 1,1-dichloroethene (1,1-DCE) (from 0.15 to 13 mg/kg), 1,1,1-trichloroethane (TCA) (from 0.17 to 22 mg/kg), and trichloroethylene (TCE) (from 0.092 to 9.6 mg/kg) were detected in all samples. The highest concentrations detected were for toluene, which was detected at a concentration of 32 and 86 mg/kg in the samples from unweathered sandstone at 2 and 4 cm below the fracture, respectively. Concentrations generally decreased with distance below the fracture in the mudstone samples. Concentrations of benzene and toluene increased with distance below the fractures in the unweathered sandstone samples. Concentrations of 1,1-DCE, TCA, and TCE were higher in the mudstone samples than in the samples from sandstone. Toluene concentrations were higher in unweathered sandstone than in weathered sandstone or mudstone.The effect of the pumping of Horsham Water and Sewer Authority public supply well 26 (HWSA-26), 0.2 mile southwest of the base boundary, on groundwater levels on the base was determined by shutting the well down for 6 days to allow water levels to recover. Water levels in 22 nearby wells were measured. The only well (02MW01I) that showed an unambiguous response to the shutdown of well HWSA-26 is 1,350 feet directly along strike from well HWSA-26. The recovery of well 05MW11I in response to the shutdown of well HWSA-26 is masked by recharge from snowmelt but probably does not exceed about 0.2 feet on the basis of the water level in well 05MW11I, which showed a response to the pumping of well HWSA-26 that ranged from 0.5 to 0.15 foot.Horizontal gradients differ with depth, and the rate and direction of ground-water flow and contaminant movement is depth dependent. The potentiometric-surface map for water levels measured in wells screened between 5 and 44 ft bls in the aquifer shows a ground-water mound that is the high point on a regional ground-water divide. From this divide, ground water flows both northwest toward Park Creek and southeast toward Pennypack Creek. The hydraulic gradient around this mound is relatively flat to the southeast and particularly flat to the northwest. The potentiometric-surface map for water levels measured in wells screened between 40 and 100 ft bls in the aquifer shows a very flat hydraulic gradient. Differences in the elevation of the potentiometric surface are less than 2 feet. The potentiometric-surface map for water levels measured in wells screened between 105 and 179 ft bls in the aquifer shows a steep hydraulic gradient between Sites 5 and 2 and a relatively flat hydraulic gradient between Sites 5 and 3. Water levels measured on October 7, 1999, showed downward vertical head gradients for all well clusters at Site 5. Vertical gradients ranged from 0.01 at well cluster 05MW10 to 0.2 at cluster 05MW11. Most gradients were between 0.01 and 0.026. Vertical head gradients vary with time. The variability is caused by a difference in the magnitude of water-level fluctuations between shallow and the deep fractures. The difference in the magnitude of water-level fluctuations is because of differences in lithology and aquifer storativity.

Flow-related Right Ventricular - Pulmonary Arterial Pressure Gradients during Exercise.

PubMed

Wright, Stephen P; Opotowsky, Alexander R; Buchan, Tayler A; Esfandiari, Sam; Granton, John T; Goodman, Jack M; Mak, Susanna

2018-06-06

The assumption of equivalence between right ventricular and pulmonary arterial systolic pressure is fundamental to several assessments of right ventricular or pulmonary vascular hemodynamic function. Our aims were to 1) determine whether systolic pressure gradients develop across the right ventricular outflow tract in healthy adults during exercise, 2) examine the potential correlates of such gradients, and 3) consider the effect of such gradients on calculated indices of right ventricular function. Healthy untrained and endurance-trained adult volunteers were studied using right-heart catheterization at rest and during submaximal cycle ergometry. Right ventricular and pulmonary artery pressures were simultaneously transduced, and cardiac output was determined by thermodilution. Systolic pressures, peak and mean gradients, and indices of chamber, vascular, and valve function were analyzed offline. Summary data are reported as mean ± standard deviation or median [interquartile range]. No significant right ventricular outflow tract gradients were observed at rest (mean gradient = 4 [3-5] mmHg), and calculated effective orifice area was 3.6±1.0 cm2. Right ventricular systolic pressure increases during exercise were greater than that of pulmonary artery systolic pressure. Accordingly, mean gradients developed during light exercise (8 [7-9] mmHg) and increased during moderate exercise (12 [9-14] mmHg, p < 0.001). The magnitude of the mean gradient was linearly related to cardiac output (r2 = 0.70, p < 0.001). In healthy adults without pulmonic stenosis, systolic pressure gradients develop during exercise, and the magnitude is related to blood flow rate.

Variations of permeability and pore size distribution of porous media with pressure.

PubMed

Chen, Quan; Kinzelbach, Wolfgang; Ye, Chaohui; Yue, Yong

2002-01-01

Porosity and permeability of porous and fractured geological media decrease with the exploitation of formation fluids such as petroleum, natural gas, or ground water. This may result in ground subsidence and a decrease of recovery of petroleum, natural gas, or ground water. Therefore, an evaluation of the behavior of permeability and porosity under formation fluid pressure changes is important to petroleum and ground water industries. This study for the first time establishes a method, which allows for the measurement of permeability, porosity, and pore size distribution of cores simultaneously. From the observation of the pore size distribution by low-field nuclear magnetic resonance (NMR) relaxation time spectrometry the mechanisms of pressure-dependent porosity and permeability change can be derived. This information cannot be obtained by traditional methods. As the large-size pores or fractures contribute significantly to the permeability, their change consequently leads to a large permeability change. The contribution of fractures to permeability is even larger than that of pores. Thus, the permeability of the cores with fractures decreased more than that of cores without fractures during formation pressure decrease. Furthermore, it did not recover during formation pressure increase. It can be concluded that in fractures, mainly plastic deformation takes place, while matrix pores mainly show elastic deformation. Therefore, it is very important to keep an appropriate formation fluid pressure during the exploitation of ground water and petroleum in a fractured formation.

Mechanical models for dikes: A third school of thought

NASA Astrophysics Data System (ADS)

Townsend, Meredith R.; Pollard, David D.; Smith, Richard P.

2017-04-01

Geological and geophysical data from continental volcanic centers and giant radial swarms, and from oceanic shield volcanoes and rift zones, indicate that dikes propagate laterally for distances that can be 10 to over 100 times their height. What traps dikes within the shallow lithosphere and promotes these highly eccentric shapes? Gravity-induced stress gradients in the surrounding rock and pressure gradients in the magma are the primary loading mechanisms; pressure gradients due to magma flow are secondary to insignificant, because the flow direction is dominantly horizontal. This configuration of vertical, blade-shaped dikes with horizontal dike propagation and magma flow is fundamentally different from the two dike model configurations described in a recent review paper as two schools of thought for mechanical models of dikes. In School I, a dike is disconnected from its source and ascends under the influence of buoyancy. In School II, a dike is connected to a magma reservoir and is driven upward by magma flux from the source. We review the geological and geophysical data supporting the vertical dike - horizontal flow/propagation configuration and suggest the abundance and veracity of these data in many different geological settings, and the modeling results that address this physical process, warrant adding this as a third school of thought. A new analytical solution for the boundary-value problem of a homogeneous, isotropic, and linear elastic solid with a vertical, fluid-filled crack is used to investigate the effects of gravitationally induced stress and pressure gradients on the aperture distribution, dike-tip stress intensity, and stable height. Model results indicate that in a homogeneous crust, dikes can achieve stable heights greater than a kilometer only if the host rock fracture toughness KIC 100 MPa · m1/2. However, density stratification of the crust is an effective mechanism for trapping kilometer-scale dikes even if the host rock is very weak (KIC = 0). This analysis may explain why vertical dikes propagate laterally for great distances, but reside within a narrow range of depths in the crust.

Effects of varying pulsatile lavage pressure on cancellous bone structure and fracture healing.

PubMed

Polzin, Britton; Ellis, Thomas; Dirschl, Douglas R

2006-04-01

To study the effects of variations in pulsatile lavage irrigation pressure on the rate of new bone formation and the degree to which cellular elements are removed from cancellous bone after fracture. A previously described intraarticular fracture model was used for 29 New Zealand white rabbits that underwent osteotomy of the medial femoral condyle. Fractures were irrigated with high-pressure pulsatile lavage at a fixed distance and volume, but at nozzle pressures varying from 20 to 70 psi. Fractures were reduced and stabilized, and animals euthanized 14 days after fracture. Fluorescent bone staining was used to determine the rate of new bone formation in the osteotomy site. At the time of euthanasia, the nonoperated knees of 12 rabbits underwent osteotomy and irrigation using the same protocol. These specimens were sent for immediate scanning electron microscopy to determine the amount of cellular material removed from the bony trabeculae. In the first week after irrigation, there was no significant difference in the amount of new bone formation between the 20- and 30-psi groups, but there were significant differences between these groups and the 50- and 70-psi groups. There were no significant differences between any of the groups in the amount of new bone formed during the second week after irrigation. No structural damage to the bony trabeculae was observed in any specimen irrigated at any of the pressures used. There was a direct correlation between percentage of the trabecula completely cleared of cellular material and irrigation pressure; there were statistically significant differences between each of the groups. There are presently no recommended guidelines as to the optimal irrigation pressure, and this study is the first to address the effects of variations in pressure on bone healing. The results of this study indicate that early new bone formation in an intraarticular fracture rabbit model is inhibited by irrigation pressure of 50 psi or greater. Additionally, this study demonstrates a direct relationship between irrigation pressure and the amount of cellular material removed from the trabecula at the irrigation site. Surgeons should be aware of the potentially detrimental effects of using irrigation pressures at or above 50 psi in the treatment of fractures.

Linear Elastic and Cohesive Fracture Analysis to Model Hydraulic Fracture in Brittle and Ductile Rocks

NASA Astrophysics Data System (ADS)

Yao, Yao

2012-05-01

Hydraulic fracturing technology is being widely used within the oil and gas industry for both waste injection and unconventional gas production wells. It is essential to predict the behavior of hydraulic fractures accurately based on understanding the fundamental mechanism(s). The prevailing approach for hydraulic fracture modeling continues to rely on computational methods based on Linear Elastic Fracture Mechanics (LEFM). Generally, these methods give reasonable predictions for hard rock hydraulic fracture processes, but still have inherent limitations, especially when fluid injection is performed in soft rock/sand or other non-conventional formations. These methods typically give very conservative predictions on fracture geometry and inaccurate estimation of required fracture pressure. One of the reasons the LEFM-based methods fail to give accurate predictions for these materials is that the fracture process zone ahead of the crack tip and softening effect should not be neglected in ductile rock fracture analysis. A 3D pore pressure cohesive zone model has been developed and applied to predict hydraulic fracturing under fluid injection. The cohesive zone method is a numerical tool developed to model crack initiation and growth in quasi-brittle materials considering the material softening effect. The pore pressure cohesive zone model has been applied to investigate the hydraulic fracture with different rock properties. The hydraulic fracture predictions of a three-layer water injection case have been compared using the pore pressure cohesive zone model with revised parameters, LEFM-based pseudo 3D model, a Perkins-Kern-Nordgren (PKN) model, and an analytical solution. Based on the size of the fracture process zone and its effect on crack extension in ductile rock, the fundamental mechanical difference of LEFM and cohesive fracture mechanics-based methods is discussed. An effective fracture toughness method has been proposed to consider the fracture process zone effect on the ductile rock fracture.

Swarms of repeating long-period earthquakes at Shishaldin Volcano, Alaska, 2001-2004

USGS Publications Warehouse

Petersen, Tanja

2007-01-01

During 2001–2004, a series of four periods of elevated long-period seismic activity, each lasting about 1–2 months, occurred at Shishaldin Volcano, Aleutian Islands, Alaska. The time periods are termed swarms of repeating events, reflecting an abundance of earthquakes with highly similar waveforms that indicate stable, non-destructive sources. These swarms are characterized by increased earthquake amplitudes, although the seismicity rate of one event every 0.5–5 min has remained more or less constant since Shishaldin last erupted in 1999. A method based on waveform cross-correlation is used to identify highly repetitive events, suggestive of spatially distinct source locations. The waveform analysis shows that several different families of similar events co-exist during a given swarm day, but generally only one large family dominates. A network of hydrothermal fractures may explain the events that do not belong to a dominant repeating event group, i.e. multiple sources at different locations exist next to a dominant source. The dominant waveforms exhibit systematic changes throughout each swarm, but some of these waveforms do reappear over the course of 4 years indicating repeatedly activated source locations. The choked flow model provides a plausible trigger mechanism for the repeating events observed at Shishaldin, explaining the gradual changes in waveforms over time by changes in pressure gradient across a constriction within the uppermost part of the conduit. The sustained generation of Shishaldin's long-period events may be attributed to complex dynamics of a multi-fractured hydrothermal system: the pressure gradient within the main conduit may be regulated by temporarily sealing and reopening of parallel flow pathways, by the amount of debris within the main conduit and/or by changing gas influx into the hydrothermal system. The observations suggest that Shishaldin's swarms of repeating events represent time periods during which a dominant source is activated.

Research on Annular Frictional Pressure Loss of Hydraulic-Fracturing in Buckling Coiled Tubing

NASA Astrophysics Data System (ADS)

Liu, Bin; Cai, Meng; Li, Junliang; Xu, Yongquan; Wang, Peng

2018-01-01

Compared with conventional hydraulic fracturing, coiled tubing (CT) annular delivery sand fracturing technology is a new method to enhance the recovery ratio of low permeability reservoir. Friction pressure loss through CT has been a concern in fracturing. The small diameter of CT limits the cross-sectional area open to flow, therefore, to meet large discharge capacity, annular delivery sand technology has been gradually developed in oilfield. Friction pressure is useful for determining the required pump horsepower and fracturing construction design programs. Coiled tubing can buckle when the axial compressive load acting on the tubing is greater than critical buckling load, then the geometry shape of annular will change. Annular friction pressure loss elevates dramatically with increasing of discharge capacity, especially eccentricity and CT buckling. Despite the frequency occurrence of CT buckling in oilfield operations, traditionally annular flow frictional pressure loss considered concentric and eccentric annuli, not discussing the effects of for discharge capacity and sand ratio varying degree of CT buckling. The measured data shows that the factors mentioned above cannot be ignored in the prediction of annular pressure loss. It is necessary to carry out analysis of annulus flow pressure drop loss in coiled tubing annular with the methods of theoretical analysis and numerical simulation. Coiled tubing buckling has great influence on pressure loss of fracturing fluid. Therefore, the correlations have been developed for turbulent flow of Newtonian fluids and Two-phase flow (sand-liquid), and that improve the friction pressure loss estimation in coiled tubing operations involving a considerable level of buckling. Quartz sand evidently increases pressure loss in buckling annular, rising as high as 40%-60% more than fresh water. Meanwhile, annulus flow wetted perimeter increases with decreasing helical buckling pitch of coiled tubing, therefore, the annulus flow frictional pressure loss rapidly increases with decreasing helical buckling pitch. The research achievement provides theoretical guidance for coiled tubing annular delivery sand fracturing operation and design.

Study of pore pressure reaction on hydraulic fracturing

NASA Astrophysics Data System (ADS)

Trimonova, Mariia; Baryshnikov, Nikolay; Turuntaev, Sergey; Zenchenko, Evgeniy; Zenchenko, Petr

2017-04-01

We represent the results of the experimental study of the hydraulic fracture propagation influence on the fluid pore pressure. Initial pore pressure was induced by injection and production wells. The experiments were carried out according to scaling analysis based on the radial model of the fracture. All required geomechanical and hydrodynamical properties of a sample were derived from the scaling laws. So, gypsum was chosen as a sample material and vacuum oil as a fracturing fluid. The laboratory setup allows us to investigate the samples of cylindrical shape. It can be considered as an advantage in comparison with standard cubic samples, because we shouldn't consider the stress field inhomogeneity induced by the corners. Moreover, we can set 3D-loading by this setting. Also the sample diameter is big enough (43cm) for placing several wells: the fracturing well in the center and injection and production wells on two opposite sides of the central well. The experiment consisted of several stages: a) applying the horizontal pressure; b) applying the vertical pressure; c) water solution injection in the injection well with a constant pressure; d) the steady state obtaining; e) the oil injection in the central well with a constant rate. The pore pressure was recorded in the 15 points along bottom side of the sample during the whole experiment. We observe the pore pressure change during all the time of the experiment. First, the pore pressure changed due to water injection. Then we began to inject oil in the central well. We compared the obtained experimental data on the pore pressure changes with the solution of the 2D single-phase equation of pore-elasticity, and we found significant difference. The variation of the equation parameters couldn't help to resolve the discrepancy. After the experiment, we found that oil penetrated into the sample before and after the fracture initiation. This fact encouraged us to consider another physical process - the oil-water displacement. Have taken into account the phenomenon, we could find the parameter values for the best matching the experimental data with the analytical one. After such a comparison, we could estimate the permeability variation in the different directions due to changes in the pore pressure during fracturing. Thus it was found that for the correct solution of hydrodynamic problems in relation with hydraulic fracturing (for example, to estimate the production rate of the fractured well) one should take into account the change of the permeability in the vicinity of the fracture and solve nonlinear pore-elasticity problem.

The Effect of Loading Rate on Hydraulic Fracturing in Synthetic Granite - a Discrete Element Study

NASA Astrophysics Data System (ADS)

Tomac, I.; Gutierrez, M.

2015-12-01

Hydraulic fracture initiation and propagation from a borehole in hard synthetic rock is modeled using the two dimensional Discrete Element Method (DEM). DEM uses previously established procedure for modeling the strength and deformation parameters of quasi-brittle rocks with the Bonded Particle Model (Itasca, 2004). A series of simulations of laboratory tests on granite in DEM serve as a reference for synthetic rock behavior. Fracturing is enabled by breaking parallel bonds between DEM particles as a result of the local stress state. Subsequent bond breakage induces fracture propagation during a time-stepping procedure. Hydraulic fracturing occurs when pressurized fluid induces hoop stresses around the wellbore which cause rock fracturing and serves for geo-reservoir permeability enhancement in oil, gas and geothermal industries. In DEM, a network of fluid pipes and reservoirs is used for mathematical calculation of fluid flow through narrow channels between DEM particles, where the hydro-mechanical coupling is fully enabled. The fluid flow calculation is superimposed with DEM stress-strain calculation at each time step. As a result, the fluid pressures during borehole pressurization in hydraulic fracturing, as well as, during the fracture propagation from the borehole, can be simulated. The objective of this study is to investigate numerically a hypothesis that fluid pressurization rate, or the fluid flow rate, influences upon character, shape and velocity of fracture propagation in rock. The second objective is to better understand and define constraints which are important for successful fracture propagation in quasi-brittle rock from the perspective of flow rate, fluid density, viscosity and compressibility relative to the rock physical properties. Results from this study indicate that not only too high fluid flow rates cause fracture arrest and multiple fracture branching from the borehole, but also that the relative compressibility of fracturing fluid and rock plays a significant role in fracture propagation velocity. Fluid viscosity effects are similar to the loading rate effects, because in both cases the rapid buildup of the pressure in the wellbore in absence of the inflow of the fluid into initiated fracture causes induction of multiple simultaneous fracture branches at the wellbore wall.

Controls on the Karaha-Telaga Bodas geothermal reservoir, Indonesia

USGS Publications Warehouse

Nemcok, M.; Moore, J.N.; Christensen, Carl; Allis, R.; Powell, T.; Murray, B.; Nash, G.

2007-01-01

Karaha-Telaga Bodas is a partially vapor-dominated, fracture-controlled geothermal system located adjacent to Galunggung Volcano in western Java, Indonesia. The geothermal system consists of: (1) a caprock, ranging from several hundred to 1600 m in thickness, and characterized by a steep, conductive temperature gradient and low permeability; (2) an underlying vapor-dominated zone that extends below sea level; and (3) a deep liquid-dominated zone with measured temperatures up to 353 ??C. Heat is provided by a tabular granodiorite stock encountered at about 3 km depth. A structural analysis of the geothermal system shows that the effective base of the reservoir is controlled either by the boundary between brittle and ductile deformational regimes or by the closure and collapse of fractures within volcanic rocks located above the brittle/ductile transition. The base of the caprock is determined by the distribution of initially low-permeability lithologies above the reservoir; the extent of pervasive clay alteration that has significantly reduced primary rock permeabilities; the distribution of secondary minerals deposited by descending waters; and, locally, by a downward change from a strike-slip to an extensional stress regime. Fluid-producing zones are controlled by both matrix and fracture permeabilities. High matrix permeabilities are associated with lacustrine, pyroclastic, and epiclastic deposits. Productive fractures are those showing the greatest tendency to slip and dilate under the present-day stress conditions. Although the reservoir appears to be in pressure communication across its length, fluid, and gas chemistries vary laterally, suggesting the presence of isolated convection cells. ?? 2006 CNR.

Hydro-mechanical coupled simulation of hydraulic fracturing using the eXtended Finite Element Method (XFEM)

NASA Astrophysics Data System (ADS)

Youn, Dong Joon

This thesis presents the development and validation of an advanced hydro-mechanical coupled finite element program analyzing hydraulic fracture propagation within unconventional hydrocarbon formations under various conditions. The realistic modeling of hydraulic fracturing is necessarily required to improve the understanding and efficiency of the stimulation technique. Such modeling remains highly challenging, however, due to factors including the complexity of fracture propagation mechanisms, the coupled behavior of fracture displacement and fluid pressure, the interactions between pre-existing natural and initiated hydraulic fractures and the formation heterogeneity of the target reservoir. In this research, an eXtended Finite Element Method (XFEM) scheme is developed allowing for representation of single or multiple fracture propagations without any need for re-meshing. Also, the coupled flows through the fracture are considered in the program to account for their influence on stresses and deformations along the hydraulic fracture. In this research, a sequential coupling scheme is applied to estimate fracture aperture and fluid pressure with the XFEM. Later, the coupled XFEM program is used to estimate wellbore bottomhole pressure during fracture propagation, and the pressure variations are analyzed to determine the geometry and performance of the hydraulic fracturing as pressure leak-off test. Finally, material heterogeneity is included into the XFEM program to check the effect of random formation property distributions to the hydraulic fracture geometry. Random field theory is used to create the random realization of the material heterogeneity with the consideration of mean, standard deviation, and property correlation length. These analyses lead to probabilistic information on the response of unconventional reservoirs and offer a more scientific approach regarding risk management for the unconventional reservoir stimulation. The new stochastic approach combining XFEM and random field is named as eXtended Random Finite Element Method (XRFEM). All the numerical analysis codes in this thesis are written in Fortran 2003, and these codes are applicable as a series of sub-modules within a suite of finite element codes developed by Smith and Griffiths (2004).

Simulation of naturally fractured reservoirs

DOE Office of Scientific and Technical Information (OSTI.GOV)

Saidi, A.M.

1983-11-01

A three-dimensional, three-phase reservoir simulator was developed to study the behavior of fully or partially fractured reservoirs. It is also demonstrated, that when a fractured reservoir is subject to a relatively large rate of pressure drop and/or it composed of relatively large blocks, the pseudo steady-state pressure concept gives large errors as compared with transient fromulation. In addition, when gravity drainage and imbibitum processes, which is the most important mechanism in the fractured reservoirs, are represented by a ''lumped parameter'' even larger errors can be produced in exchange flow between matrix and fractures. For these reasons, the matrix blocks aremore » gridded and the transfer between matrix and fractures are calculated using pressure and diffusion transient concept. In this way the gravity drainage is also calculated accurately. As the matrix-fracture exchange flow depends on the location of each matrix grid relative to the GOC and/or WOC in fracture, the exchange flow equation are derived and given for each possible case. The differential equation describing the flow of water, oil, and gas within the matrix and fracture system, each of which may contain six unknowns, are presented. The two sets of equations are solved implicitly for pressure water, and gas stauration in both matrix and fractures. The first twenty two years of the history of Haft Kel field was successfully matched with this model and the results are included.« less

Hyper-localized carbon mineralization in diffusion-limited basalt fractures

NASA Astrophysics Data System (ADS)

Menefee, A. H.; Giammar, D.; Ellis, B. R.

2017-12-01

Basalt formations could enable secure carbon sequestration through mineral trapping. CO2 injection acidifies formation brines and drives dissolution of the host rock, which releases divalent metal cations that combine with dissolved carbonate ions to form stable carbonate minerals. Here, a series of high-pressure flow-through experiments was conducted to evaluate how transport limitations and geochemical gradients drive microscale carbonation reactions in fractured basalts. To isolate advection- and diffusion-controlled zones, surfaces of saw-cut basalt cores were milled to create one primary flow channel adjoined by four dead-end fracture pathways. In the first experiment, a representative basalt brine (6.3 mM NaHCO3) equilibrated with CO2 (100ºC, 10 MPa) was injected at 1 mL/h under 20 MPa confining stress. The second experiment was conducted under the same physical conditions but [NaHCO3] was elevated to 640 mM, and in the third, temperature was also raised to 150ºC. Effluent chemistry was monitored via ICP-MS to infer dissolution trends and calibrate reactive transport models. Reacted cores were characterized using x-ray computed tomography (xCT), optical microscopy, scanning electron microscopy, and Raman spectroscopy. Carbonation occurred in all experiments but increased in experiments with higher alkalinity and higher temperature. At low [NaHCO3], secondary precipitate coatings formed distinct reaction fronts that varied with distance into dead-end fractures. Reactive transport modeling demonstrated that these reactions fronts were due to sharp gradients in pH and dissolved inorganic carbon. Carbonation was restricted to transport-limited vugs and pores between the confined core surfaces and was highly localized on reactive primary mineral grains (e.g. pyroxene) that contributed major divalent cations. Increasing [NaHCO3] by two orders of magnitude significantly enhanced carbonation and promoted Mg and Fe uptake into carbonates. While xCT scans revealed clays filling the advective path, no permeability changes were measured. Our coupled experiment-modeling approach further elucidates the geochemical conditions controlling carbonation reactions and extends unique microstructural observations to implications for long-term CO2 mineralization in basalt reservoirs.

Design and Implementation of Energized Fracture Treatment in Tight Gas Sands

DOE Office of Scientific and Technical Information (OSTI.GOV)

Mukul Sharma; Kyle Friehauf

2009-12-31

Hydraulic fracturing is essential for producing gas and oil at an economic rate from low permeability sands. Most fracturing treatments use water and polymers with a gelling agent as a fracturing fluid. The water is held in the small pore spaces by capillary pressure and is not recovered when drawdown pressures are low. The un-recovered water leaves a water saturated zone around the fracture face that stops the flow of gas into the fracture. This is a particularly acute problem in low permeability formations where capillary pressures are high. Depletion (lower reservoir pressures) causes a limitation on the drawdown pressuremore » that can be applied. A hydraulic fracturing process can be energized by the addition of a compressible, sometimes soluble, gas phase into the treatment fluid. When the well is produced, the energized fluid expands and gas comes out of solution. Energizing the fluid creates high gas saturation in the invaded zone, thereby facilitating gas flowback. A new compositional hydraulic fracturing model has been created (EFRAC). This is the first model to include changes in composition, temperature, and phase behavior of the fluid inside the fracture. An equation of state is used to evaluate the phase behavior of the fluid. These compositional effects are coupled with the fluid rheology, proppant transport, and mechanics of fracture growth to create a general model for fracture creation when energized fluids are used. In addition to the fracture propagation model, we have also introduced another new model for hydraulically fractured well productivity. This is the first and only model that takes into account both finite fracture conductivity and damage in the invaded zone in a simple analytical way. EFRAC was successfully used to simulate several fracture treatments in a gas field in South Texas. Based on production estimates, energized fluids may be required when drawdown pressures are smaller than the capillary forces in the formation. For this field, the minimum CO{sub 2} gas quality (volume % of gas) recommended is 30% for moderate differences between fracture and reservoir pressures (2900 psi reservoir, 5300 psi fracture). The minimum quality is reduced to 20% when the difference between pressures is larger, resulting in additional gas expansion in the invaded zone. Inlet fluid temperature, flow rate, and base viscosity did not have a large impact on fracture production. Finally, every stage of the fracturing treatment should be energized with a gas component to ensure high gas saturation in the invaded zone. A second, more general, sensitivity study was conducted. Simulations show that CO{sub 2} outperforms N{sub 2} as a fluid component because it has higher solubility in water at fracturing temperatures and pressures. In fact, all gas components with higher solubility in water will increase the fluid's ability to reduce damage in the invaded zone. Adding methanol to the fracturing solution can increase the solubility of CO{sub 2}. N{sub 2} should only be used if the gas leaks-off either during the creation of the fracture or during closure, resulting in gas going into the invaded zone. Experimental data is needed to determine if the gas phase leaks-off during the creation of the fracture. Simulations show that the bubbles in a fluid traveling across the face of a porous medium are not likely to attach to the surface of the rock, the filter cake, or penetrate far into the porous medium. In summary, this research has created the first compositional fracturing simulator, a useful tool to aid in energized fracture design. We have made several important and original conclusions about the best practices when using energized fluids in tight gas sands. The models and tools presented here may be used in the future to predict behavior of any multi-phase or multi-component fracturing fluid system.« less

Structural, petrophysical and geomechanical characterization of the Becancour CO2 storage pilot site (Quebec, Canada)

NASA Astrophysics Data System (ADS)

Konstantinovskaya, E.; Malo, M.; Claprood, M.; Tran-Ngoc, T. D.; Gloaguen, E.; Lefebvre, R.

2012-04-01

The Paleozoic sedimentary succession of the St. Lawrence Platform was characterized to estimate the CO2 storage capacity, the caprock integrity and the fracture/fault stability at the Becancour pilot site. Results are based on the structural interpretation of 25 seismic lines and analysis of 11 well logs and petrophysical data. The three potential storage units of Potsdam, Beekmantown and Trenton saline aquifers are overlain by a multiple caprock system of Utica shales and Lorraine siltstones. The NE-SW regional normal faults dipping to the SE affect the subhorizontal sedimentary succession. The Covey Hill (Lower Potsdam) was found to be the only unit with significant CO2 sequestration potential, since these coarse-grained poorly-sorted fluvial-deltaic quartz-feldspar sandstones are characterized by the highest porosity, matrix permeability and net pay thickness and have the lowest static Young modulus, Poisson's ratio and compressive strength relative to other units. The Covey Hill is located at depths of 1145-1259 m, thus injected CO2 would be in supercritical state according to observed salinity, temperature and fluid pressure. The calcareous Utica shale of the regional seal is more brittle and has higher Young modulus and lower Poisson's ratio than the overlying Lorraine shale. The 3D geological model is kriged using the tops of the geological formations recorded at wells and picked travel times as external drift. The computed CO2 storage capacity in the Covey Hill sandstones is estimated by the volumetric and compressibility methods as 0.22 tons/km2 with storage efficiency factor E 2.4% and 0.09 tons/km2 with E 1%, respectively. A first set of numerical radial simulations of CO2 injection into the Covey Hill were carried out with TOUGH2/ECO2N. A geomechanical analysis of the St. Lawrence Platform sedimentary basin provides the maximum sustainable fluid pressures for CO2 injection that will not induce tensile fracturing and shear reactivation along pre-existing fractures and faults in the caprock. The regional stresses/pressure gradients estimated for the Paleozoic sedimentary basin (depths < 4 km) indicate a strike-slip stress regime. The average maximum horizontal stress orientation (SHmax) is estimated N62.8°E±4.0° in the Becancour-Notre Dame area. The high-angle NE-SW Yamaska normal fault is oriented at 16.7° to the SHmax orientation in the Becancour site. The slip tendency along the fault in this area is estimated to be 0.47 based on the stress magnitude and rock strength evaluations for the borehole breakout intervals in local wells. The regional pore pressure-stress coupling ratio under assumed parameters is about 0.5-0.65 and may contribute to reduce the risk of shear reactivation of faults and fractures. The maximum sustainable fluid pressure that would not cause opening of vertical tensile fractures during CO2 operations is about 18.5-20 MPa at a depth of 1 km.

Visualization of hydraulic connections using Borehole Array around LPG Underground Storage Cavern

NASA Astrophysics Data System (ADS)

Shimo, M.; Mashimo, H.; Maejima, T.; Aoki, K.

2006-12-01

This paper presents a systematic approach to visualize the hydraulic connections within the fractured rock mass around the underground LPG storage caverns using array of water injection boreholes. By taking advantage that water injection boreholes are located so as to cover the storage caverns, a complete sketch of hydraulic conditions around the caverns, such as locations of water conducting fractures, hydraulic conductivity and groundwater pressure can be obtained. Applicability of the proposed techniques have been tested in an on-going construction project operated by JOGMEC, Japan Oil, Gas and Metals National Corporation, at Namikata, Western part of Japan. Three 26m x 30m x 485m caverns, located at 150 - 200 m below the ground surface in a granitic rock, are under construction. By systematically monitoring the pressure responses between the neighboring boreholes during drilling of total 387 boreholes around the two propane caverns, a spatial profile of the hydraulic connections and hydraulic conductivity around the caverns has been successfully obtained. Locations of localized depressurized zones created during an arch excavation have been detected by monitoring pressure in each borehole after stopping water supply to that borehole temporarily. Measurement has been conducted using each one of the 302 boreholes, one at a time. Observation shows that there is a clear correlation between total pressure drop and pressure gradient versus time curve on semi-logarithmic plot, dH/log10t, as expected by the numerical prediction. Regions where dH/log10t is larger than a certain criteria, determined by a numerical simulation for flow around a cavern in a rock with uniform hydraulic conductivity, have been evaluated as a depressurized zone caused by insufficient water supply, possibly due to existence of the high permeable zones. Separate pore pressure measurement around the caverns also supports this interpretation that a low pressure is prevailing near the borehole where a large value of dH/log10t is obtained. As a countermeasure to avoid further depressurization, installation of additional water injection hole was conducted. The same observation was then repeated. It is recognized that dH/log10t has recovered above the criteria, showing that the local water balance has been improved. Finally it is concluded that the proposed rather simple but space encompassing observation is applicable to groundwater management during construction and also provides useful information for creating a hydrogeological model, considering a fracture network system, that will be used for the evaluation of the cavern performance as a storage tank.

Image Analysis of Proppant Performance in Pressurized Fractures

NASA Astrophysics Data System (ADS)

Crandall, D.; Smith, M. M.; Carroll, S.; Walsh, S. D.; Gill, M.; Moore, J.; Tennant, B.; Aines, R. D.

2014-12-01

Proppants are small particles used to prop or hold open subsurface fractures to permit fluid flow through these pathways. In many oil and gas well applications, the most common proppant materials are sand, ceramic particles, resin-coated sands, glass beads or even walnut shells. More dense proppants require additives to create viscous fluids which can transport them further along wells and into fractures, but are generally preferred over neutrally buoyant options due to their increased strength. Currently, proppant strength and generation of broken fragments ("fines") is analyzed via a standardized crush test between parallel plates. To augment this type of information, we present here the results of various experiments involving resin-coated proppants held at increasing pressures in fractured samples of Marcellus shale. The shale/proppant samples were imaged continuously with an industrial tomography scanner during pressurization up to 10,000psi. This technique allows for in situ characterization of fracture/proppant interactions and fracture void volume and average aperture with varying confining pressures.

In situ measurements of hydraulic fracture behavior, PTE-3. Final report

DOE Office of Scientific and Technical Information (OSTI.GOV)

Warpinski, N.R.

Measurements of width and pressure in a propagating hydraulic fracture have been made in tests conducted at DOE`s Nevada Test Site. This was accomplished by creating an "instrumented fracture" at a tunnel complex (at a depth of 1400 ft) where realistic in situ conditions prevail, particularly with respect to stress and geologic features such as natural fractures and material anisotropy. Analyses of these data show that the pressure drop along the fracture length is much larger than predicted by viscous theory currently in use in models today. This is apparently due to the tortuosity of the fracture path, multiple fracturemore » strands, roughness, and sharp turns (corners) in the flow path due to natural fractures and rock property variations. It suggests that fracture design models need to be updated to include a more realistic friction factor so that fracture lengths are not overestimated. The width and pressure profiles near the crack tip have been investigated in some detail, including the length of the unwetted region and the tapering of the crack tip. The overall fracture behavior has been compared with published fracture models. Mineback of the fracture provided evidence of the geometry of the fracture and details of surface features. 35 refs., 89 figs., 30 tabs.« less

A quantitative investigation of the fracture pump-in/flowback test

DOE Office of Scientific and Technical Information (OSTI.GOV)

Plahn, S.V.; Nolte, K.G.; Thompson, L.G.

1997-02-01

Fracture-closure pressure is an important parameter for fracture treatment design and evaluation. The pump-in/flowback (PIFB) test is frequently used to estimate its magnitude. The test is attractive because bottomhole pressures (BHP`s) during flowback develop a distinct and repeatable signature. This is in contrast to the pump-in/shut-in test, where strong indications of fracture closure are rarely seen. Various techniques are used to extract closure pressure from the flowback-pressure response. Unfortunately, these techniques give different estimates for closure pressure, and their theoretical bases are not well established. The authors present results that place the PIFB test on a firmer foundation. A numericalmore » model is used to simulate the PIFB test and glean physical mechanisms contributing to the response. On the basis of their simulation results, they propose interpretation techniques that give better estimates of closure pressure than existing techniques.« less

A quantitative investigation of the fracture pump-in/flowback test

DOE Office of Scientific and Technical Information (OSTI.GOV)

Plahn, S.V.; Nolte, K.G.; Miska, S.

1995-12-31

Fracture closure pressure is an important parameter for fracture treatment design and evaluation. The pump-in/flowback (PIFB) test is frequently used to estimate its magnitude. The test is attractive because bottomhole pressures during flowback develop a distinct and repeatable signature. This is in contrast to the pump-in/shut-in test where strong indications of fracture closure are rarely seen. Various techniques exist for extracting closure pressure from the flowback pressure response. Unfortunately, these procedures give different estimates for closure pressure and their theoretical bases are not well established. We present results that place the PIFB test on a more solid foundation. A numericalmore » model is used to simulate the PIFB test and glean physical mechanisms contributing to the response. Based on our simulation results, we propose an interpretation procedure which gives better estimates for closure pressure than existing techniques.« less

The effect of a microscale fracture on dynamic capillary pressure of two-phase flow in porous media

NASA Astrophysics Data System (ADS)

Tang, Mingming; Lu, Shuangfang; Zhan, Hongbin; Wenqjie, Guo; Ma, Huifang

2018-03-01

Dynamic capillary pressure (DCP) effects, which is vital for predicting multiphase flow behavior in porous media, refers to the injection rate dependence capillary pressure observed during non-equilibrium displacement experiments. However, a clear picture of the effects of microscale fractures on DCP remains elusive. This study quantified the effects of microscale fractures on DCP and simulated pore-scale force and saturation change in fractured porous media using the multiphase lattice Boltzmann method (LBM). Eighteen simulation cases were carried out to calculate DCP as a function of wetting phase saturation. The effects of viscosity ratio and fracture orientation, aperture and length on DCP and DCP coefficient τ were investigated, where τ refers to the ratio of the difference of DCP and static capillary pressure (SCP) over the rate of wetting-phase saturation change versus time. Significant differences in τ values were observed between unfractured and fractured porous media. The τ values of fractured porous media were 1.1  × 104 Pa ms to 5.68 × 105 Pa ms, which were one or two orders of magnitude lower than those of unfractured porous media with a value of 4 × 106 Pa. ms. A horizontal fracture had greater effects on DCP and τ than a vertical fracture, given the same fracture aperture and length. This study suggested that a microscale fracture might result in large magnitude changes in DCP for two-phase flow.

Statistics of pressure and pressure gradient in homogeneous isotropic turbulence

NASA Technical Reports Server (NTRS)

Gotoh, T.; Rogallo, R. S.

1994-01-01

The statistics of pressure and pressure gradient in stationary isotropic turbulence are measured within direct numerical simulations at low to moderate Reynolds numbers. It is found that the one-point pdf of the pressure is highly skewed and that the pdf of the pressure gradient is of stretched exponential form. The power spectrum of the pressure P(k) is found to be larger than the corresponding spectrum P(sub G)(k) computed from a Gaussian velocity field having the same energy spectrum as that of the DNS field. The ratio P(k)/P(sub G)(k), a measure of the pressure-field intermittence, grows with wavenumber and Reynolds number as -R(sub lambda)(exp 1/2)log(k/k(sub d)) for k less than k(sub d)/2 where k(sub d) is the Kolmogorov wavenumber. The Lagrangian correlations of pressure gradient and velocity are compared and the Lagrangian time scale of the pressure gradient is observed to be much shorter than that of the velocity.

Optimal disturbances in boundary layers subject to streamwise pressure gradient

NASA Technical Reports Server (NTRS)

Ashpis, David E.; Tumin, Anatoli

2003-01-01

An analysis of the optimal non-modal growth of perturbations in a boundary layer in the presence of a streamwise pressure gradient is presented. The analysis is based on PSE equations for an incompressible fluid. Examples with Falkner-Scan profiles indicate that a favorable pressure gradient decreases the non-modal growth, while an unfavorable pressure gradient leads to an increase of the amplification. It is suggested that the transient growth mechanism be utilized to choose optimal parameters of tripping elements on a low-pressure turbine (LPT) airfoil. As an example, a boundary layer flow with a streamwise pressure gradient corresponding to the pressure distribution over a LPT airfoil is considered. It is shown that there is an optimal spacing of the tripping elements and that the transient growth effect depends on the starting point.

Fluid driven fracture mechanics in highly anisotropic shale: a laboratory study with application to hydraulic fracturing

NASA Astrophysics Data System (ADS)

Gehne, Stephan; Benson, Philip; Koor, Nick; Enfield, Mark

2017-04-01

The finding of considerable volumes of hydrocarbon resources within tight sedimentary rock formations in the UK led to focused attention on the fundamental fracture properties of low permeability rock types and hydraulic fracturing. Despite much research in these fields, there remains a scarcity of available experimental data concerning the fracture mechanics of fluid driven fracturing and the fracture properties of anisotropic, low permeability rock types. In this study, hydraulic fracturing is simulated in a controlled laboratory environment to track fracture nucleation (location) and propagation (velocity) in space and time and assess how environmental factors and rock properties influence the fracture process and the developing fracture network. Here we report data on employing fluid overpressure to generate a permeable network of micro tensile fractures in a highly anisotropic shale ( 50% P-wave velocity anisotropy). Experiments are carried out in a triaxial deformation apparatus using cylindrical samples. The bedding planes are orientated either parallel or normal to the major principal stress direction (σ1). A newly developed technique, using a steel guide arrangement to direct pressurised fluid into a sealed section of an axially drilled conduit, allows the pore fluid to contact the rock directly and to initiate tensile fractures from the pre-defined zone inside the sample. Acoustic Emission location is used to record and map the nucleation and development of the micro-fracture network. Indirect tensile strength measurements at atmospheric pressure show a high tensile strength anisotropy ( 60%) of the shale. Depending on the relative bedding orientation within the stress field, we find that fluid induced fractures in the sample propagate in two of the three principal fracture orientations: Divider and Short-Transverse. The fracture progresses parallel to the bedding plane (Short-Transverse orientation) if the bedding plane is aligned (parallel) with the direction of σ1. Conversely, the crack plane develops perpendicular to the bedding plane, if the bedding plane is orientated normal to σ1. Fracture initiation pressures are higher in the Divider orientation ( 24MPa) than in the Short-Transverse orientation ( 14MPa) showing a tensile strength anisotropy ( 42%) comparable to ambient tensile strength results. We then use X-Ray Computed Tomography (CT) 3D-images to evaluate the evolved fracture network in terms of fracture pattern, aperture and post-test water permeability. For both fracture orientations, very fine, axial fractures evolve over the entire length of the sample. For the fracturing in the Divider orientation, it has been observed, that in some cases, secondary fractures are branching of the main fracture. Test data from fluid driven fracturing experiments suggest that fracture pattern, fracture propagation trajectories and fracturing fluid pressure (initiation and propagation pressure) are predominantly controlled by the interaction between the anisotropic mechanical properties of the shale and the anisotropic stress environment. The orientation of inherent rock anisotropy relative to the principal stress directions seems to be the main control on fracture orientation and required fracturing pressure.

Arterial Pressure Gradients during Upright Posture and 30 deg Head Down Tilt

NASA Technical Reports Server (NTRS)

Sanchez, E. R; William, J. M.; Ueno, T.; Ballard, R. E.; Hargens, A. R.; Holton, Emily M. (Technical Monitor)

1997-01-01

Gravity alters local blood pressure within the body so that arterial pressures in the head and foot are lower and higher, respectively, than that at heart level. Furthermore, vascular responses to local alterations of arterial pressure are probably important to maintain orthostatic tolerance upon return to the Earth after space flight. However, it has been difficult to evaluate the body's arterial pressure gradient due to the lack of noninvasive technology. This study was therefore designed to investigate whether finger arterial pressure (FAP), measured noninvasively, follows a normal hydrostatic pressure gradient above and below heart level during upright posture and 30 deg head down tilt (HDT). Seven healthy subjects gave informed consent and were 19 to 52 years old with a height range of 158 to 181 cm. A Finapres device measured arterial pressure at different levels of the body by moving the hand from 36 cm below heart level (BH) to 72 cm above heart level (AH) in upright posture and from 36 cm BH to 48 cm AH during HDT in increments of 12 cm. Mean FAP creased by 85 mmHg transitioning from BH to AH in upright posture, and the pressure gradient calculated from hydrostatic pressure difference (rho(gh)) was 84 mmHg. In HDT, mean FAP decreased by 65 mmHg from BH to AH, and the calculated pressure gradient was also 65 mmHg. There was no significant difference between the measured FAP gradient and the calculated pressure gradient, although a significant (p = 0.023) offset was seen for absolute arterial pressure in upright posture. These results indicate that arterial pressure at various levels can be obtained from the blood pressure at heart level by calculating rho(gh) + an offset. The offset equals the difference between heart level and the site of measurement. In summary, we conclude that local blood pressure gradients can be measured by noninvasive studies of FAP.

Uncertainty based pressure reconstruction from velocity measurement with generalized least squares

NASA Astrophysics Data System (ADS)

Zhang, Jiacheng; Scalo, Carlo; Vlachos, Pavlos

2017-11-01

A method using generalized least squares reconstruction of instantaneous pressure field from velocity measurement and velocity uncertainty is introduced and applied to both planar and volumetric flow data. Pressure gradients are computed on a staggered grid from flow acceleration. The variance-covariance matrix of the pressure gradients is evaluated from the velocity uncertainty by approximating the pressure gradient error to a linear combination of velocity errors. An overdetermined system of linear equations which relates the pressure and the computed pressure gradients is formulated and then solved using generalized least squares with the variance-covariance matrix of the pressure gradients. By comparing the reconstructed pressure field against other methods such as solving the pressure Poisson equation, the omni-directional integration, and the ordinary least squares reconstruction, generalized least squares method is found to be more robust to the noise in velocity measurement. The improvement on pressure result becomes more remarkable when the velocity measurement becomes less accurate and more heteroscedastic. The uncertainty of the reconstructed pressure field is also quantified and compared across the different methods.

Evaluation of production tests in oil wells stimulated by massive acid fracturing offshore Qatar

DOE Office of Scientific and Technical Information (OSTI.GOV)

McDonald, S.W.

This paper presents the evaluation of pressure-buildup data from production tests in wells that have been stimulated by massive acid fracturing. Fracture type curves are used in combination with conventional semilog analysis techniques. Fracture characteristics are calculated from a match of the early-time pressure data with the type curves, and reservoir characteristics are calculated from a conventional semilog plot of late-time data. Unexpectedly high formation permeabilities are evaluated, and fracture half-lengths are much shorter than design values.

On the impact of adverse pressure gradient on the supersonic turbulent boundary layer

NASA Astrophysics Data System (ADS)

Wang, Qian-Cheng; Wang, Zhen-Guo; Zhao, Yu-Xin

2016-11-01

By employing the particle image velocimetry, the mean and turbulent characteristics of a Mach 2.95 turbulent boundary layer are experimentally investigated without the impact of curvature. The physical mechanism with which the streamwise adverse pressure gradient affects the supersonic boundary layer is revealed. The data are compared to that of the concave boundary layer with similar streamwise distributions of wall static pressure to clarify the separate impacts of the adverse pressure gradient and the concave curvature. The logarithmic law is observed to be well preserved for both of the cases. The dip below the logarithmic law is not observed in present investigation. Theoretical analysis indicates that it could be the result of compromise between the opposite impacts of the compression wave and the increased turbulent intensity. Compared to the zero pressure gradient boundary layer, the principal strain rate and the turbulent intensities are increased by the adverse pressure gradient. The shear layer formed due the hairpin packets could be sharpened by the compression wave, which leads to higher principal strain rate and the associated turbulent level. Due to the additional impact of the centrifugal instability brought by the concave wall, even higher turbulent intensities than that of the adverse pressure gradient case are introduced. The existence of velocity modes within the zero pressure gradient boundary layer suggests that the large scale motions are statistically well organized. The generation of new velocity modes due to the adverse pressure gradient indicates that the turbulent structure is changed by the adverse pressure gradient, through which more turbulence production that cannot be effectively predicted by the Reynolds-stress transport equations could be brought.

A numerical approach for assessing effects of shear on equivalent permeability and nonlinear flow characteristics of 2-D fracture networks

NASA Astrophysics Data System (ADS)

Liu, Richeng; Li, Bo; Jiang, Yujing; Yu, Liyuan

2018-01-01

Hydro-mechanical properties of rock fractures are core issues for many geoscience and geo-engineering practices. Previous experimental and numerical studies have revealed that shear processes could greatly enhance the permeability of single rock fractures, yet the shear effects on hydraulic properties of fractured rock masses have received little attention. In most previous fracture network models, single fractures are typically presumed to be formed by parallel plates and flow is presumed to obey the cubic law. However, related studies have suggested that the parallel plate model cannot realistically represent the surface characters of natural rock fractures, and the relationship between flow rate and pressure drop will no longer be linear at sufficiently large Reynolds numbers. In the present study, a numerical approach was established to assess the effects of shear on the hydraulic properties of 2-D discrete fracture networks (DFNs) in both linear and nonlinear regimes. DFNs considering fracture surface roughness and variation of aperture in space were generated using an originally developed code DFNGEN. Numerical simulations by solving Navier-Stokes equations were performed to simulate the fluid flow through these DFNs. A fracture that cuts through each model was sheared and by varying the shear and normal displacements, effects of shear on equivalent permeability and nonlinear flow characteristics of DFNs were estimated. The results show that the critical condition of quantifying the transition from a linear flow regime to a nonlinear flow regime is: 10-4 〈 J < 10-3, where J is the hydraulic gradient. When the fluid flow is in a linear regime (i.e., J < 10-4), the relative deviation of equivalent permeability induced by shear, δ2, is linearly correlated with J with small variations, while for fluid flow in the nonlinear regime (J 〉 10-3), δ2 is nonlinearly correlated with J. A shear process would reduce the equivalent permeability significantly in the orientation perpendicular to the sheared fracture as much as 53.86% when J = 1, shear displacement Ds = 7 mm, and normal displacement Dn = 1 mm. By fitting the calculated results, the mathematical expression for δ2 is established to help choose proper governing equations when solving fluid flow problems in fracture networks.

The subsurface impact of hydraulic fracturing in shales- Perspectives from the well and reservoir

NASA Astrophysics Data System (ADS)

ter Heege, Jan; Coles, Rhys

2017-04-01

It has been identified that the main risks of subsurface shale gas operations in the U.S.A. and Canada are associated with (1) drilling and well integrity, (2) hydraulic fracturing, and (3) induced seismicity. Although it is unlikely that hydraulic fracturing operations result in direct pathways of enhanced migration between stimulated fracture disturbed rock volume and shallow aquifers, operations may jeopardize well integrity or induce seismicity. From the well perspective, it is often assumed that fluid injection leads to the initiation of tensile (mode I) fractures at different perforation intervals along the horizontal sections of shale gas wells if pore pressure exceeds the minimum principal stress. From the reservoir perspective, rise in pore pressure resulting from fluid injection may lead to initiation of tensile fractures, reactivation of shear (mode II) fractures if the criterion for failure in shear is exceeded, or combinations of different fracturing modes. In this study, we compare tensile fracturing simulations using conventional well-based models with shear fracturing simulations using a fractured shale model with characteristic fault populations. In the fractured shale model, stimulated permeability is described by an analytical model that incorporates populations of reactivated faults and that combines 3D permeability tensors for layered shale matrix, damage zone and fault core. Well-based models applied to wells crosscutting the Posidonia Shale Formation are compared to generic fractured shale models, and fractured shale models are compared to micro-seismic data from the Marcellus Shale. Focus is on comparing the spatial distribution of permeability, stimulated reservoir volume and seismicity, and on differences in fracture initiation pressure and fracture orientation for tensile and shear fracturing end-members. It is shown that incorporation of fault populations (for example resulting from analysis of 3D seismics or outcrops) in hydraulic fracturing models provides better constraints on well pressures, stimulated fracture disturbed volume and induced seismicity. Thereby, it helps assessing the subsurface impact of hydraulic fracturing in shales and mitigating risks associated with loss of loss of well integrity, loss of fracture containment, and induced seismicity.

Joint Use of ERT, Tracer, and Numerical Techniques to Image Preferential Flow Paths in a Fractured Granite Aquifer

NASA Astrophysics Data System (ADS)

Sanaga, S.; Vijay, S.; Kbvn, P.; Peddinti, S. R.; P S L, S.

2017-12-01

Fractured geologic media poses formidable challenges to hydrogeologists due of the strenuous mapping of fracture-matrix system and quantification of flow and transport processes. In this research, we demonstrated the efficacy of tracer-ERT studies coupled with numerical simulations to delineate preferential flow paths in a fractured granite aquifer of Deccan traps in India. A series of natural gradient saline tracer experiments were conducted from a depth window of 18 to 22 m in an injection well located inside the IIT Hyderabad campus. Tracer migration was monitored in a time-lapse mode using two cross-sectional surface ERT profiles placed in the direction of flow gradient. Dynamic changes in sub-surface electrical properties inferred via resistivity anomalies were used to highlight preferential flow paths of the study area. ERT-derived tracer breakthrough curves were in agreement with geochemical sample measurements (R2=0.74). Fracture geometry and hydraulic properties derived from ERT and pumping tests were then used to evaluate two mathematical conceptualizations that are relevant to fractured aquifers. Results of numerical analysis conclude that a dual continuum model that combines matrix and fracture systems through a flow exchange term has outperformed equivalent continuum model in reproducing tracer concentrations at the monitoring wells (evident by decrease in RMSE from 199 mg/l to 65 mg/l). A sensitivity analysis of the model parameters reveals that spatial variability in hydraulic conductivity, local-scale dispersion, and flow exchange at fracture-matrix interface have a profound effect on model simulations. Keywords: saline tracer, ERT, fractured granite, groundwater, preferential flow, numerical simulation

Gradient Tempering Of Bearing Races

NASA Technical Reports Server (NTRS)

Parr, Richardson A.

1991-01-01

Gradient-tempering process increases fracture toughness and resistance to stress-corrosion cracking of ball-bearing races made of hard, strong steels and subject to high installation stresses and operation in corrosive media. Also used in other applications in which local toughening of high-strength/low-toughness materials required.

Experimental and numerical simulation of dissolution and precipitation: implications for fracture sealing at Yucca Mountain, Nevada

NASA Astrophysics Data System (ADS)

Dobson, Patrick F.; Kneafsey, Timothy J.; Sonnenthal, Eric L.; Spycher, Nicolas; Apps, John A.

2003-05-01

Plugging of flow paths caused by mineral precipitation in fractures above the potential repository at Yucca Mountain, Nevada could reduce the probability of water seeping into the repository. As part of an ongoing effort to evaluate thermal-hydrological-chemical (THC) effects on flow in fractured media, we performed a laboratory experiment and numerical simulations to investigate mineral dissolution and precipitation under anticipated temperature and pressure conditions in the repository. To replicate mineral dissolution by vapor condensate in fractured tuff, water was flowed through crushed Yucca Mountain tuff at 94 °C. The resulting steady-state fluid composition had a total dissolved solids content of about 140 mg/l; silica was the dominant dissolved constituent. A portion of the steady-state mineralized water was flowed into a vertically oriented planar fracture in a block of welded Topopah Spring Tuff that was maintained at 80 °C at the top and 130 °C at the bottom. The fracture began to seal with amorphous silica within 5 days. A 1-D plug-flow numerical model was used to simulate mineral dissolution, and a similar model was developed to simulate the flow of mineralized water through a planar fracture, where boiling conditions led to mineral precipitation. Predicted concentrations of the major dissolved constituents for the tuff dissolution were within a factor of 2 of the measured average steady-state compositions. The mineral precipitation simulations predicted the precipitation of amorphous silica at the base of the boiling front, leading to a greater than 50-fold decrease in fracture permeability in 5 days, consistent with the laboratory experiment. These results help validate the use of a numerical model to simulate THC processes at Yucca Mountain. The experiment and simulations indicated that boiling and concomitant precipitation of amorphous silica could cause significant reductions in fracture porosity and permeability on a local scale. However, differences in fluid flow rates and thermal gradients between the experimental setup and anticipated conditions at Yucca Mountain need to be factored into scaling the results of the dissolution/precipitation experiments and associated simulations to THC models for the potential Yucca Mountain repository.

CONFORMANCE IMPROVEMENT USING GELS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Randall S. Seright

2003-09-01

This report describes work performed during the second year of the project, ''Conformance Improvement Using Gels.'' The project has two objectives. The first objective is to identify gel compositions and conditions that substantially reduce flow through fractures that allow direct channeling between wells, while leaving secondary fractures open so that high fluid injection and production rates can be maintained. The second objective is to optimize treatments in fractured production wells, where the gel must reduce permeability to water much more than that to oil. Pore-level images from X-ray computed microtomography were re-examined for Berea sandstone and porous polyethylene. This analysismore » suggests that oil penetration through gel-filled pores occurs by a gel-dehydration mechanism, rather than a gel-ripping mechanism. This finding helps to explain why aqueous gels can reduce permeability to water more than to oil. We analyzed a Cr(III)-acetate-HPAM gel treatment in a production well in the Arbuckle formation. The availability of accurate pressure data before, during, and after the treatment was critical for the analysis. After the gel treatment, water productivity was fairly constant at about 20% of the pre-treatment value. However, oil productivity was stimulated by a factor of 18 immediately after the treatment. During the six months after the treatment, oil productivity gradually decreased to approach the pre-treatment value. To explain this behavior, we proposed that the fracture area open to oil flow was increased substantially by the gel treatment, followed by a gradual closing of the fractures during subsequent production. For a conventional Cr(III)-acetate-HPAM gel, the delay between gelant preparation and injection into a fracture impacts the placement, leakoff, and permeability reduction behavior. Formulations placed as partially formed gels showed relatively low pressure gradients during placement, and yet substantially reduced the flow capacity of fractures (with widths from 1 to 4 mm) during brine and oil flow after placement. Regardless of gel age before placement, very little gel washed out from the fractures during brine or oil flow. However, increased brine or oil flow rate and cyclic injection of oil and water significantly decreased the level of permeability reduction. A particular need exists for gels that can plug large apertures (e.g., wide fractures and vugs). Improved mechanical strength and stability were demonstrated (in 1- to 4-mm-wide fractures) for a gel that contained a combination of high- and low-molecular weight polymers. This gel reduced the flow capacity of 2- and 4-mm-wide fractures by 260,000. In a 1-mm-wide fracture, it withstood 26 psi/ft without allowing any brine flow through the fracture. Cr(III)-acetate-HPAM gels exhibited disproportionate permeability reduction in fractures. The effect was most pronounced when the gel was placed as gelant or partially formed gels. The effect occurred to a modest extent with concentrated gels and with gels that were ''fully formed'' when placed. The effect was not evident in tubes. We explored swelling polymers for plugging fractures. Polymer suspensions were quickly prepared and injected. In concept, the partially dissolved polymer would lodge and swell to plug the fracture. For three types of swelling polymers, behavior was promising. However, additional development is needed before their performance will be superior to that of conventional gels.« less

Influence of pressure change during hydraulic tests on fracture aperture.

PubMed

Ji, Sung-Hoon; Koh, Yong-Kwon; Kuhlman, Kristopher L; Lee, Moo Yul; Choi, Jong Won

2013-03-01

In a series of field experiments, we evaluate the influence of a small water pressure change on fracture aperture during a hydraulic test. An experimental borehole is instrumented at the Korea Atomic Energy Research Institute (KAERI) Underground Research Tunnel (KURT). The target fracture for testing was found from the analyses of borehole logging and hydraulic tests. A double packer system was developed and installed in the test borehole to directly observe the aperture change due to water pressure change. Using this packer system, both aperture and flow rate are directly observed under various water pressures. Results indicate a slight change in fracture hydraulic head leads to an observable change in aperture. This suggests that aperture change should be considered when analyzing hydraulic test data from a sparsely fractured rock aquifer. © 2012, The Author(s). Groundwater © 2012, National Ground Water Association.

Stress-induced, time-dependent fracture closure at hydrothermal conditions

USGS Publications Warehouse

Beeler, N.M.; Hickman, S.H.

2004-01-01

Time-dependent closure of fractures in quartz was measured in situ at 22-530??C temperature and 0.1-150 MPa water pressure. Unlike previous crack healing and rock permeability studies, in this study, fracture aperture is monitored directly and continuously using a windowed pressure vessel, a long-working-distance microscope, and reflected-light interferometry. Thus the fracture volume and geometry can be measured as a function of time, temperature, and water pressure. Relatively uniform closure occurs rapidly at temperatures and pressures where quartz becomes significantly soluble in water. During closure the aperture is reduced by as much as 80% in a few hours. We infer that this closure results from the dissolution of small particles or asperities that prop the fracture open. The driving force for closure via dissolution of the prop is the sum of three chemical potential terms: (1) the dissolution potential, proportional to the logarithm of the degree of undersaturation of the solution; (2) the coarsening potential, proportional to the radius of curvature of the prop; and (3) the pressure solution potential, proportional to the effective normal stress at the contact between propping particles and the fracture wall. Our observations suggest that closure is controlled by a pressure solution-like process. The aperture of dilatant fractures and microcracks in the Earth that are similar to those in our experiments, such as ones generated from thermal stressing or brittle failure during earthquake rupture and slip, will decrease rapidly with time, especially if the macroscopic stress is nonhydrostatic.

Stress-induced, time-dependent fracture closure at hydrothermal conditions

NASA Astrophysics Data System (ADS)

Beeler, N. M.; Hickman, S. H.

2004-02-01

Time-dependent closure of fractures in quartz was measured in situ at 22-530°C temperature and 0.1-150 MPa water pressure. Unlike previous crack healing and rock permeability studies, in this study, fracture aperture is monitored directly and continuously using a windowed pressure vessel, a long-working-distance microscope, and reflected-light interferometry. Thus the fracture volume and geometry can be measured as a function of time, temperature, and water pressure. Relatively uniform closure occurs rapidly at temperatures and pressures where quartz becomes significantly soluble in water. During closure the aperture is reduced by as much as 80% in a few hours. We infer that this closure results from the dissolution of small particles or asperities that prop the fracture open. The driving force for closure via dissolution of the prop is the sum of three chemical potential terms: (1) the dissolution potential, proportional to the logarithm of the degree of undersaturation of the solution; (2) the coarsening potential, proportional to the radius of curvature of the prop; and (3) the pressure solution potential, proportional to the effective normal stress at the contact between propping particles and the fracture wall. Our observations suggest that closure is controlled by a pressure solution-like process. The aperture of dilatant fractures and microcracks in the Earth that are similar to those in our experiments, such as ones generated from thermal stressing or brittle failure during earthquake rupture and slip, will decrease rapidly with time, especially if the macroscopic stress is nonhydrostatic.

Cost and Performance Report; In-Situ Remediation of MTBE Contaminated Aquifers Using Propane Biosparging

DTIC Science & Technology

2003-12-01

potential problem is decreasing the permeability by pneumatic or hydraulic fracturing . Pneumatic fracturing (PF) uses the injection of pressurized air...to increase the biodegradation of the contaminants. Hydraulic fracturing is similar to PF with the exception that water is used and at pressures as

In Situ Remediation of MTBE-Contaminated Aquifers Using Propane Biosparging

DTIC Science & Technology

2003-09-01

problem is decreasing the permeability by pneumatic or hydraulic fracturing . Pneumatic fracturing (PF) uses the injection of pressurized air 47 (up to...increase the biodegradation of the contaminants. Hydraulic fracturing is similar to PF with the exception that water is used and at pressures as high as

Inhaled Beta Agonist Bronchodilator Does Not Affect Trans-diaphragmatic Pressure Gradient but Decreases Lower Esophageal Sphincter Retention Pressure in Patients with Chronic Obstructive Pulmonary Disease (COPD) and Gastroesophageal Reflux Disease (GERD).

PubMed

Del Grande, Leonardo M; Herbella, Fernando A M; Bigatao, Amilcar M; Jardim, Jose R; Patti, Marco G

2016-10-01

Chronic obstructive pulmonary disease (COPD) patients have a high incidence of gastroesophageal reflux disease (GERD) whose pathophysiology seems to be linked to an increased trans-diaphragmatic pressure gradient and not to a defective esophagogastric barrier. Inhaled beta agonist bronchodilators are a common therapy used by patients with COPD. This drug knowingly not only leads to a decrease in the lower esophageal sphincter (LES) resting pressure, favoring GERD, but also may improve ventilatory parameters, therefore preventing GERD. This study aims to evaluate the effect of inhaled beta agonist bronchodilators on the trans-diaphragmatic pressure gradient and the esophagogastric barrier. We studied 21 patients (mean age 67 years, 57 % males) with COPD and GERD. All patients underwent high-resolution manometry and esophageal pH monitoring. Abdominal and thoracic pressure, trans-diaphragmatic pressure gradient (abdominal-thoracic pressure), and the LES retention pressure (LES basal pressure-transdiaphragmatic gradient) were measured before and 5 min after inhaling beta agonist bronchodilators. The administration of inhaled beta agonist bronchodilators leads to the following: (a) a simultaneous increase in abdominal and thoracic pressure not affecting the trans-diaphragmatic pressure gradient and (b) a decrease in the LES resting pressure with a reduction of the LES retention pressure. In conclusion, inhaled beta agonist bronchodilators not only increase the thoracic pressure but also lead to an increased abdominal pressure favoring GERD by affecting the esophagogastric barrier.

75 FR 72653 - Alternate Fracture Toughness Requirements for Protection Against Pressurized Thermal Shock Events...

Federal Register 2010, 2011, 2012, 2013, 2014

2010-11-26

..., Criminal penalties, Fire protection, Intergovernmental relations, Nuclear power plants and reactors... Requirements for Protection Against Pressurized Thermal Shock Events; Correction AGENCY: Nuclear Regulatory... fracture toughness requirements for protection against pressurized thermal shock (PTS) events for...

Pore-scale mechanisms of gas flow in tight sand reservoirs

DOE Office of Scientific and Technical Information (OSTI.GOV)

Silin, D.; Kneafsey, T.J.; Ajo-Franklin, J.B.

2010-11-30

Tight gas sands are unconventional hydrocarbon energy resource storing large volume of natural gas. Microscopy and 3D imaging of reservoir samples at different scales and resolutions provide insights into the coaredo not significantly smaller in size than conventional sandstones, the extremely dense grain packing makes the pore space tortuous, and the porosity is small. In some cases the inter-granular void space is presented by micron-scale slits, whose geometry requires imaging at submicron resolutions. Maximal Inscribed Spheres computations simulate different scenarios of capillary-equilibrium two-phase fluid displacement. For tight sands, the simulations predict an unusually low wetting fluid saturation threshold, at whichmore » the non-wetting phase becomes disconnected. Flow simulations in combination with Maximal Inscribed Spheres computations evaluate relative permeability curves. The computations show that at the threshold saturation, when the nonwetting fluid becomes disconnected, the flow of both fluids is practically blocked. The nonwetting phase is immobile due to the disconnectedness, while the permeability to the wetting phase remains essentially equal to zero due to the pore space geometry. This observation explains the Permeability Jail, which was defined earlier by others. The gas is trapped by capillarity, and the brine is immobile due to the dynamic effects. At the same time, in drainage, simulations predict that the mobility of at least one of the fluids is greater than zero at all saturations. A pore-scale model of gas condensate dropout predicts the rate to be proportional to the scalar product of the fluid velocity and pressure gradient. The narrowest constriction in the flow path is subject to the highest rate of condensation. The pore-scale model naturally upscales to the Panfilov's Darcy-scale model, which implies that the condensate dropout rate is proportional to the pressure gradient squared. Pressure gradient is the greatest near the matrix-fracture interface. The distinctive two-phase flow properties of tight sand imply that a small amount of gas condensate can seriously affect the recovery rate by blocking gas flow. Dry gas injection, pressure maintenance, or heating can help to preserve the mobility of gas phase. A small amount of water can increase the mobility of gas condensate.« less

Constraining geometrical, hydrodynamical and mechanical properties of a fault zone at hourly time scales from ground surface tilt data

NASA Astrophysics Data System (ADS)

Schuite, Jonathan; Longuevergne, Laurent; Bour, Olivier; Burbey, Thomas J.; Boudin, Frédéric

2017-04-01

Flow through reservoirs such as fractured media is powered by pressure gradients which also generate measurable poroelastic deformation of the rock body. The combined analysis of ground surface deformation and sub-surface fluid pressure provides valuable insights of a reservoir's structure and hydromechanical properties, which are of interest for deep-seated CO2 or nuclear waste storage for instance. Amongst all surveying tools, surface tiltmeters offer the possibility to grasp hydraulically-induced deformation over a broad range of time scales with a remarkable precision (1 nanoradian). Here, we investigate the information content of transient surface tilt generated by flow in a kilometer scale sub-vertical fault zone and its surrounding fractured rock matrix. Our approach involves the combined analysis of field data and results of a fully coupled poroelastic model, where fault and matrix are represented as equivalent homogeneous domains. The signature of pressure changes in the fault zone due to pumping cycles is clearly recognizable in field tilt data and we aim to explain the peculiar features that appear in: 1) tilt time series alone from a set of 4 instruments; 2) the ratio of tilt over pressure. With the model, we evidence that the shape of tilt measurements on both sides of a fault zone is sensitive to its diffusivity and its elastic modulus. In particular, we show a few well placed tiltmeters (on each side of a fault) give more information on the medium's properties than well spatialized surface displacement maps. Furthermore, the ratio of tilt over pressure predominantly encompasses information about the system's dynamic behavior and extent of the fault zone, and allows separating contributions of flow in the different compartments. Hence, tiltmeters are well suited to characterize hydromechanical processes associated to fault zone hydrogeology at short time scales, where space-borne surveying methods fail to seize any deformation signal.

Shake, Rupture And Flow: Hydraulic Constraints On The Formation Of Europa’s Chaos

NASA Astrophysics Data System (ADS)

Schmidt, Britney E.; Gooch, B. T.; Blankenship, D. D.; Soderlund, K. M.

2012-10-01

Europa’s chaos terrains may have formed above shallow water lenses formed by melting of the upper ice shell with ascending thermo-compositional plumes. A key factor in the creation of chaos terrain may be dramatic disruption and collapse of the ice lid above the forming melt lens along with potentially violent mixing upon its rupture; this is analogous to the collapse of terrestrial ice shelves in which massive ice bodies disintegrate in a few days. At Thera Macula, there is evidence for modification by water immediately external to the scarp that bounds the collapsed region. Since water runs either subaerially down hill or from high pressure to low when below or within ice, the swollen appearance of bands entering Thera Macula, which are uphill in terms of hydraulic and topographic gradients, raises the possibility that this steep scarp represents the place where the lens initially broke. As the ice lid ruptures, the overpressure within the lens may create sufficient pressure within the fracture to drive water through it, allowing water to escape into and modify surrounding terrain. Similar effects are seen when aquifers or subglacial water sources are tapped: water flows up the pipe until the pressure in the water body is relieved and the hydraulic “pressure head” in the pipe is lowered. We have modeled the hydraulic potential associated with a rupturing lens in order to investigate the range of parameters for overpressure, fracture width, and lid thickness that could produce such modification as is observed at Thera Macula. These place important constraints on the pressure within the lens and the energetics of a collapse event. These estimates may explain how ice masses within chaos are initially disrupted and provide a means for quantifying the vigor of surface-subsurface mixing that could be critical to Europa’s habitability.

Quantifying Dynamic Changes in Plantar Pressure Gradient in Diabetics with Peripheral Neuropathy.

PubMed

Lung, Chi-Wen; Hsiao-Wecksler, Elizabeth T; Burns, Stephanie; Lin, Fang; Jan, Yih-Kuen

2016-01-01

Diabetic foot ulcers remain one of the most serious complications of diabetes. Peak plantar pressure (PPP) and peak pressure gradient (PPG) during walking have been shown to be associated with the development of diabetic foot ulcers. To gain further insight into the mechanical etiology of diabetic foot ulcers, examination of the pressure gradient angle (PGA) has been recently proposed. The PGA quantifies directional variation or orientation of the pressure gradient during walking and provides a measure of whether pressure gradient patterns are concentrated or dispersed along the plantar surface. We hypothesized that diabetics at risk of foot ulceration would have smaller PGA in key plantar regions, suggesting less movement of the pressure gradient over time. A total of 27 participants were studied, including 19 diabetics with peripheral neuropathy and 8 non-diabetic control subjects. A foot pressure measurement system was used to measure plantar pressures during walking. PPP, PPG, and PGA were calculated for four foot regions - first toe (T1), first metatarsal head (M1), second metatarsal head (M2), and heel (HL). Consistent with prior studies, PPP and PPG were significantly larger in the diabetic group compared with non-diabetic controls in the T1 and M1 regions, but not M2 or HL. For example, PPP was 165% (P = 0.02) and PPG was 214% (P < 0.001) larger in T1. PGA was found to be significantly smaller in the diabetic group in T1 (46%, P = 0.04), suggesting a more concentrated pressure gradient pattern under the toe. The proposed PGA may improve our understanding of the role of pressure gradient on the risk of diabetic foot ulcers.

Experimental Simulations of Methane Gas Migration through Water-Saturated Sediment Cores

NASA Astrophysics Data System (ADS)

Choi, J.; Seol, Y.; Rosenbaum, E. J.

2010-12-01

Previous numerical simulations (Jaines and Juanes, 2009) showed that modes of gas migration would mainly be determined by grain size; capillary invasion preferably occurring in coarse-grained sediments vs. fracturing dominantly in fine-grained sediments. This study was intended to experimentally simulate preferential modes of gas migration in various water-saturated sediment cores. The cores compacted in the laboratory include a silica sand core (mean size of 180 μm), a silica silt core (1.7 μm), and a kaolin clay core (1.0 μm). Methane gas was injected into the core placed within an x-ray-transparent pressure vessel, which was under continuous x-ray computed tomography (CT) scanning with controlled radial (σr), axial (σa), and pore pressures (P). The CT image analysis reveals that, under the radial effective stress (σr') of 0.69 MPa and the axial effective stress (σa') of 1.31 MPa, fracturings by methane gas injection occur in both silt and clay cores. Fracturing initiates at the capillary pressure (Pc) of ~ 0.41 MPa and ~ 2.41 MPa for silt and clay cores, respectively. Fracturing appears as irregular fracture-networks consisting of nearly invisibly-fine multiple fractures, longitudinally-oriented round tube-shape conduits, or fine fractures branching off from the large conduits. However, for the sand core, only capillary invasion was observed at or above 0.034 MPa of capillary pressure under the confining pressure condition of σr' = 1.38 MPa and σa' = 2.62 MPa. Compared to the numerical predictions under similar confining pressure conditions, fracturing occurs with relatively larger grain sizes, which may result from lower grain-contact compression and friction caused by loose compaction and flexible lateral boundary employed in the experiment.

Experimental feasibility of investigating acoustic waves in Couette flow with entropy and pressure gradients

NASA Technical Reports Server (NTRS)

Parrott, Tony L.; Zorumski, William E.; Rawls, John W., Jr.

1990-01-01

The feasibility is discussed for an experimental program for studying the behavior of acoustic wave propagation in the presence of strong gradients of pressure, temperature, and flow. Theory suggests that gradients effects can be experimentally observed as resonant frequency shifts and mode shape changes in a waveguide. A convenient experimental geometry for such experiments is the annular region between two co-rotating cylinders. Radial temperature gradients in a spinning annulus can be generated by differentially heating the two cylinders via electromagnetic induction. Radial pressure gradients can be controlled by varying the cylinder spin rates. Present technology appears adequate to construct an apparatus to allow independent control of temperature and pressure gradients. A complicating feature of a more advanced experiment, involving flow gradients, is the requirement for independently controlled cylinder spin rates. Also, the boundary condition at annulus terminations must be such that flow gradients are minimally disturbed. The design and construction of an advanced apparatus to include flow gradients will require additional technology development.

Observations of wave-induced pore pressure gradients and bed level response on a surf zone sandbar

NASA Astrophysics Data System (ADS)

Anderson, Dylan; Cox, Dan; Mieras, Ryan; Puleo, Jack A.; Hsu, Tian-Jian

2017-06-01

Horizontal and vertical pressure gradients may be important physical mechanisms contributing to onshore sediment transport beneath steep, near-breaking waves in the surf zone. A barred beach was constructed in a large-scale laboratory wave flume with a fixed profile containing a mobile sediment layer on the crest of the sandbar. Horizontal and vertical pore pressure gradients were obtained by finite differences of measurements from an array of pressure transducers buried within the upper several centimeters of the bed. Colocated observations of erosion depth were made during asymmetric wave trials with wave heights between 0.10 and 0.98 m, consistently resulting in onshore sheet flow sediment transport. The pore pressure gradient vector within the bed exhibited temporal rotations during each wave cycle, directed predominantly upward under the trough and then rapidly rotating onshore and downward as the wavefront passed. The magnitude of the pore pressure gradient during each phase of rotation was correlated with local wave steepness and relative depth. Momentary bed failures as deep as 20 grain diameters were coincident with sharp increases in the onshore-directed pore pressure gradients, but occurred at horizontal pressure gradients less than theoretical critical values for initiation of the motion for compact beds. An expression combining the effects of both horizontal and vertical pore pressure gradients with bed shear stress and soil stability is used to determine that failure of the bed is initiated at nonnegligible values of both forces.