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Induced Microearthquakes And Seismicity Permeability Relationships In Fractures
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Book Synopsis Induced Microearthquakes and Seismicity-Permeability Relationships in Fractures by : Yi Fang
Download or read book Induced Microearthquakes and Seismicity-Permeability Relationships in Fractures written by Yi Fang and published by . This book was released on 2017 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Permeability evolves monotonically for smooth fractures but in a fluctuating pattern for highly roughened fractures. A higher roughness is likely to result in alternating compaction and dilation during shearing. Significant permeability damage may occur for rough samples when asperities are highly worn with wear products blocking fluid pathways. (4) There is no obvious correlation between permeability evolution and frictional behavior for rough fracture samples when fractures are subject to sudden sliding velocity change. Implications of our lab-scale experimental results suggest that characterization of fracture geometry would be beneficial for better understanding and managing induced seismicity and permeability development. In shale reservoir stimulation, fractures are propped to increase the permeability of the formation. On the other hand, the proppants may also influence the frictional strength of fractures. Thus, in the appendix, we explore the evolution of friction and permeability of a propped fracture using shearing-concurrent measurements of permeability during constant velocity shearing experiments. We observe that (1) the frictional response is mainly controlled by the normal stress and proppant thickness. High normal stress results in the crushing of proppant particles although this change in particle size distribution has almost no impact on the frictional response of the proppant-fracture system. The depth of shearing-concurrent striations on fracture surfaces suggests that the magnitude of proppant embedment is controlled by the applied normal stress. Moreover, under high normal stress, the reduced friction implies that shear slip is more likely to occur on propped fractures in deeper reservoirs. The increase in the number of proppant layers, from mono-layer to triple-layers, significantly increases the friction of the propped fracture due to the interlocking of the particles and jamming, suggesting that high proppant density during emplacement would help stabilize the fractures during injection. (2) Permeability of the propped fracture is mainly controlled by the magnitude of the normal stress, the proppant thickness, and the proppant size. Permeability of the propped fracture decreases during shearing due to proppant particle crushing and related clogging. Compared to the multi-layered specimen, the mono-layer case which has fewer displacement degrees-of-freedom exhibits the smallest initial permeability due to proppant embedment. Proppants are prone to crushing if the shear loading evolves concurrently with the normal loading. These combined conclusions suggest that the use of high-density proppants not only provides high hydraulic conductivity for hydrocarbon production but may also help to mitigate the risk of induced seismicity.
Book Synopsis Fluid-Induced Seismicity by : S. A. Shapiro
Download or read book Fluid-Induced Seismicity written by S. A. Shapiro and published by Cambridge University Press. This book was released on 2015-04-23 with total page 299 pages. Available in PDF, EPUB and Kindle. Book excerpt: This book provides a quantitative introduction to the physics, application, interpretation, and hazard aspects of fluid-induced seismicity, focussing on spatio-temporal dynamics. Including many real data examples, this is a valuable reference for researchers and graduate students of geophysics, geomechanics and petrophysics, and a practical guide for petroleum geoscientists and engineers.
Book Synopsis Co-Evolution of Fracture Permeability and Friction by : Elif Yildirim
Download or read book Co-Evolution of Fracture Permeability and Friction written by Elif Yildirim and published by . This book was released on 2018 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: In the following, we examine fracture permeabilityseismicity relationships in two separate studies. These combined studies separately elucidate (1) the role of preslip frictional healing, and (2) the combined effect of dynamic stressing and fluid-pressure-induced shear deformation on the evolution of fracture permeability and friction throughout the phases of the seismic cycle.Fracture permeability is a dynamic property under conditions of varying stress and responds to fluid overpressures applied during hydraulic stimulation. We use samples from the SIGMA-V site (Sanford Underground Research Facility (SURF), SD) to measure the co-evolution of fracture permeability and friction throughout phases of the seismic cycle. This is accomplished via slide-hold-slide and pore pressure stepping experiments completed in double direct shear. Fracture reactivation results in permeability enhancement only after sufficiently long interseismic repose periods. The magnitude of permeability increase from each reactivation, following the long hold periods, is critically dependent on the degree of fracture healing achieved in each pre-slip hold period. Shear dilation and permeability enhancement only results following a threshold repose period. Permeability enhances continuously with each pressure step with the highest permeability increase rate being with the first reactivation event. Our study establishes a direct linkage between fracture permeability and friction evolution throughout the seismic cycle and hydraulic shear, which applies across different fracture surface roughnesses.Mechanisms controlling fracture permeability enhancement during dynamic stressing remain unresolved. We probe these mechanisms through a series of fluid pressure pulse reactivations on saw-cut fractures in impermeable rock samples confined under in situ stresses. Each spiked pore fluid pressure pulse returns to the background control pore pressure while the evolution of fracture permeability and friction are continuously monitored. Peak magnitudes of the pore pressure pulses are successively incremented to both exclude and then include shear reactivation. Fracture permeability is shown to increase, both in the absence and then presence of reactivation by shear slip. Fracture permeability enhancement is permanent in the short-term despite the transient nature of the pressure pulses. The initiation of injection-induced slip significantly magnifies permeability increase over that due to changes in normal stress alone. The shear-induced permeability increase is apparent with a short delay after the first observed shear slip. Differentiation between the contribution of shear dilation and normal stress-only related processes, including unclogging and asperity damage and reseating, is apparent with a major slope change in permeability increase. Permeability increase scales with pore pressure amplitude and permeability increment scales with the amount of pre-stimulation sealing. This sealing and unsealing behavior is systematic and reversible. Enhanced permeability eventually returning to the pre-stimulated value over the long-term once the effective stress perturbations cease.
Book Synopsis Induced Seismicity Potential in Energy Technologies by : National Research Council
Download or read book Induced Seismicity Potential in Energy Technologies written by National Research Council and published by National Academies Press. This book was released on 2013-08-14 with total page 238 pages. Available in PDF, EPUB and Kindle. Book excerpt: In the past several years, some energy technologies that inject or extract fluid from the Earth, such as oil and gas development and geothermal energy development, have been found or suspected to cause seismic events, drawing heightened public attention. Although only a very small fraction of injection and extraction activities among the hundreds of thousands of energy development sites in the United States have induced seismicity at levels noticeable to the public, understanding the potential for inducing felt seismic events and for limiting their occurrence and impacts is desirable for state and federal agencies, industry, and the public at large. To better understand, limit, and respond to induced seismic events, work is needed to build robust prediction models, to assess potential hazards, and to help relevant agencies coordinate to address them. Induced Seismicity Potential in Energy Technologies identifies gaps in knowledge and research needed to advance the understanding of induced seismicity; identify gaps in induced seismic hazard assessment methodologies and the research to close those gaps; and assess options for steps toward best practices with regard to energy development and induced seismicity potential.
Book Synopsis Evolution of Permeability and Induced Seismicity Within Fractured Reservoirs by : Ghazal Izadi
Download or read book Evolution of Permeability and Induced Seismicity Within Fractured Reservoirs written by Ghazal Izadi and published by . This book was released on 2012 with total page 228 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Book Synopsis The Mechanism of Induced Seismicity by : Cezar I. Trifu
Download or read book The Mechanism of Induced Seismicity written by Cezar I. Trifu and published by Birkhäuser. This book was released on 2012-12-06 with total page 632 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Book Synopsis Unconventional Reservoir Geomechanics by : Mark D. Zoback
Download or read book Unconventional Reservoir Geomechanics written by Mark D. Zoback and published by Cambridge University Press. This book was released on 2019-05-16 with total page 495 pages. Available in PDF, EPUB and Kindle. Book excerpt: A comprehensive overview of the key geologic, geomechanical and engineering principles that govern the development of unconventional oil and gas reservoirs. Covering hydrocarbon-bearing formations, horizontal drilling, reservoir seismology and environmental impacts, this is an invaluable resource for geologists, geophysicists and reservoir engineers.
Book Synopsis Crustal Permeability by : Tom Gleeson
Download or read book Crustal Permeability written by Tom Gleeson and published by John Wiley & Sons. This book was released on 2016-11-30 with total page 557 pages. Available in PDF, EPUB and Kindle. Book excerpt: Permeability is the primary control on fluid flow in the Earth’s crust and is key to a surprisingly wide range of geological processes, because it controls the advection of heat and solutes and the generation of anomalous pore pressures. The practical importance of permeability – and the potential for large, dynamic changes in permeability – is highlighted by ongoing issues associated with hydraulic fracturing for hydrocarbon production (“fracking”), enhanced geothermal systems, and geologic carbon sequestration. Although there are thousands of research papers on crustal permeability, this is the first book-length treatment. This book bridges the historical dichotomy between the hydrogeologic perspective of permeability as a static material property and the perspective of other Earth scientists who have long recognized permeability as a dynamic parameter that changes in response to tectonism, fluid production, and geochemical reactions.
Book Synopsis Geomechanical Studies of the Barnett Shale, Texas, USA by : John Peter Vermylen
Download or read book Geomechanical Studies of the Barnett Shale, Texas, USA written by John Peter Vermylen and published by Stanford University. This book was released on 2011 with total page 143 pages. Available in PDF, EPUB and Kindle. Book excerpt: This thesis presents five studies of a gas shale reservoir using diverse methodologies to investigate geomechanical and transport properties that are important across the full reservoir lifecycle. Using the Barnett shale as a case study, we investigated adsorption, permeability, geomechanics, microseismicity, and stress evolution in two different study areas. The main goals of this thesis can be divided into two parts: first, to investigate how flow properties evolve with changes in stress and gas species, and second, to understand how the interactions between stress, fractures, and microseismicity control the creation of a permeable reservoir volume during hydraulic fracturing. In Chapter 2, we present results from adsorption and permeability experiments conducted on Barnett shale rock samples. We found Langmuir-type adsorption of CH4 and N2 at magnitudes consistent with previous studies of the Barnett shale. Three of our samples demonstrated BET-type adsorption of CO2, in contrast to all previous studies on CO2 adsorption in gas shales, which found Langmuir-adsorption. At low pressures (600 psi), we found preferential adsorption of CO2 over CH4 ranging from 3.6x to 5.5x. While our measurements were conducted at low pressures (up to 1500 psi), when our model fits are extrapolated to reservoir pressures they reach similar adsorption magnitudes as have been found in previous studies. At these high reservoir pressures, the very large preferential adsorption of CO2 over CH4 (up to 5-10x) suggests a significant potential for CO2 storage in gas shales like the Barnett if practical problems of injectivity and matrix transport can be overcome. We successfully measured permeability versus effective stress on two intact Barnett shale samples. We measured permeability effective stress coefficients less than 1 on both samples, invalidating our hypothesis that there might be throughgoing flow paths within the soft, porous organic kerogen that would lead the permeability effective stress coefficient to be greater than 1. The results suggest that microcracks are likely the dominant flow paths at these scales. In Chapter 3, we present integrated geological, geophysical, and geomechanical data in order to characterize the rock properties in our Barnett shale study area and to model the stress state in the reservoir before hydraulic fracturing occurred. Five parallel, horizontal wells were drilled in the study area and then fractured using three different techniques. We used the well logs from a vertical pilot well and a horizontal well to constrain the stress state in the reservoir. While there was some variation along the length of the well, we were able to determine a best fit stress state of Pp = 0.48 psi/ft, Sv = 1.1 psi/ft, SHmax = 0.73 psi/ft, and Shmin = 0.68 psi/ft. Applying this stress state to the mapped natural fractures indicates that there is significant potential for induced shear slip on natural fracture planes in this region of the Barnett, particularly close to the main hydraulic fracture where the pore pressure increase during hydraulic fracturing is likely to be very high. In Chapter 4, we present new techniques to quantify the robustness of hydraulic fracturing in gas shale reservoirs. The case study we analyzed involves five parallel horizontal wells in the Barnett shale with 51 frac stages. To investigate the numbers, sizes, and types of microearthquakes initiated during each frac stage, we created Gutenberg-Richter-type magnitude distribution plots to see if the size of events follows the characteristic scaling relationship found in natural earthquakes. We found that slickwater fracturing does generate a log-linear distribution of microearthquakes, but that it creates proportionally more small events than natural earthquake sources. Finding considerable variability in the generation of microearthquakes, we used the magnitude analysis as a proxy for the "robustness" of the stimulation of a given stage. We found that the conventionally fractured well and the two alternately fractured wells ("zipperfracs") were more effective than the simultaneously fractured wells ("simulfracs") in generating microearthquakes. We also found that the later stages of fracturing a given well were more successful in generating microearthquakes than the early stages. In Chapter 5, we present estimates of stress evolution in our study reservoir through analysis of the instantaneous shut-in pressure (ISIP) at the end of each stage. The ISIP increased stage by stage for all wells, but the simulfrac wells showed the greatest increase and the zipperfrac wells the least. We modeled the stress increase in the reservoir with a simple sequence of 2-D cracks along the length of the well. When using a spacing of one crack per stage, the modeled stress increase was nearly identical to the measured stress increase in the zipperfrac wells. When using three cracks per stage, the modeled final stage stress magnitude matched the measured final stage stress magnitude from the simulfrac wells, but the rate of stress increase in the simulfrac wells was much more gradual than the model predicted. To further investigate the causes of these ISIP trends, we began numerical flow and stress analysis to more realistically model the processes in the reservoir. One of our hypotheses was that the shorter total time needed to complete all the stages of the simulfrac wells was the cause of the greater ISIP increase compared to the zipperfrac wells. The microseismic activity level measured in Chapter 4 also correlates with total length of injection, suggesting leak off into the reservoir encouraged shear failure. Numerical modeling using the coupled FEM and flow software GEOSIM was able to model some cumulative stress increase the reservoir, but the full trend was not replicated. Further work to model field observations of hydraulic fracturing will enhance our understanding of the impact that hydraulic fracturing and stress change have on fracture creation and permeability enhancement in gas shales.
Book Synopsis Studying Physical Properties of Deformed Intact and Fractured Rocks by Micro-scale Hydro-mechanical-seismicity Model by : Samin Raziperchikolaee
Download or read book Studying Physical Properties of Deformed Intact and Fractured Rocks by Micro-scale Hydro-mechanical-seismicity Model written by Samin Raziperchikolaee and published by . This book was released on 2014 with total page 212 pages. Available in PDF, EPUB and Kindle. Book excerpt: The pore pressure variation in an underground formation during hydraulic stimulation of low permeability formations or CO2 sequestration into saline aquifers can induce microseismicity due to fracture generation or pre-existing fracture activation. While the analysis of microseismic data mainly focuses on mapping the location of fractures, the seismic waves generated by the microseismic events also contain information for understanding of fracture mechanisms based on microseismic source analysis. We developed a micro-scale geomechanics, fluid-flow and seismic model that can predict transport and seismic source behavior during rock failure. This model features the incorporation of microseismic source analysis in fractured and intact rock transport properties during possible rock damage and failure. The modeling method considers comprehensive grains and cements interaction through a bonded-particle-model. As a result of grain deformation and microcrack development in the rock sample, forces and displacements in the grains involved in the bond breakage are measured to determine seismic moment tensor. In addition, geometric description of the complex pore structure is regenerated to predict fluid flow behavior of fractured samples. Numerical experiments are conducted for different intact and fractured digital rock samples, representing various mechanical behaviors of rocks and fracture surface properties, to consider their roles on seismic and transport properties of rocks during deformation. Studying rock deformation in detail provides an opportunity to understand the relationship between source mechanism of microseismic events and transport properties of damaged rocks to have a better characterizing of fluid flow behavior in subsurface formations.
Book Synopsis Fracture Permeability and Seismic Wave Scattering--Poroelastic Linear-slip Interface Model for Heterogeneous Fractures by :
Download or read book Fracture Permeability and Seismic Wave Scattering--Poroelastic Linear-slip Interface Model for Heterogeneous Fractures written by and published by . This book was released on 2009 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Schoenberg's Linear-slip Interface (LSI) model for single, compliant, viscoelastic fractures has been extended to poroelastic fractures for predicting seismic wave scattering. However, this extended model results in no impact of the in-plane fracture permeability on the scattering. Recently, we proposed a variant of the LSI model considering the heterogeneity in the in-plane fracture properties. This modified model considers wave-induced, fracture-parallel fluid flow induced by passing seismic waves. The research discussed in this paper applies this new LSI model to heterogeneous fractures to examine when and how the permeability of a fracture is reflected in the scattering of seismic waves. From numerical simulations, we conclude that the heterogeneity in the fracture properties is essential for the scattering of seismic waves to be sensitive to the permeability of a fracture.
Book Synopsis Rock Fractures and Fluid Flow by : National Research Council
Download or read book Rock Fractures and Fluid Flow written by National Research Council and published by National Academies Press. This book was released on 1996-08-27 with total page 568 pages. Available in PDF, EPUB and Kindle. Book excerpt: Scientific understanding of fluid flow in rock fracturesâ€"a process underlying contemporary earth science problems from the search for petroleum to the controversy over nuclear waste storageâ€"has grown significantly in the past 20 years. This volume presents a comprehensive report on the state of the field, with an interdisciplinary viewpoint, case studies of fracture sites, illustrations, conclusions, and research recommendations. The book addresses these questions: How can fractures that are significant hydraulic conductors be identified, located, and characterized? How do flow and transport occur in fracture systems? How can changes in fracture systems be predicted and controlled? Among other topics, the committee provides a geomechanical understanding of fracture formation, reviews methods for detecting subsurface fractures, and looks at the use of hydraulic and tracer tests to investigate fluid flow. The volume examines the state of conceptual and mathematical modeling, and it provides a useful framework for understanding the complexity of fracture changes that occur during fluid pumping and other engineering practices. With a practical and multidisciplinary outlook, this volume will be welcomed by geologists, petroleum geologists, geoengineers, geophysicists, hydrologists, researchers, educators and students in these fields, and public officials involved in geological projects.
Book Synopsis Seismicity Caused by Mines, Fluid Injections, Reservoirs, and Oil Extraction by : Shahriar Talebi
Download or read book Seismicity Caused by Mines, Fluid Injections, Reservoirs, and Oil Extraction written by Shahriar Talebi and published by Birkhäuser. This book was released on 2012-12-06 with total page 230 pages. Available in PDF, EPUB and Kindle. Book excerpt: The perturbation of the earth by mankind causes earthquakes in a variety of situations. This phenomenon continues to be a major concern to engineers and scientists concerned with the mitigation of the consequences of this seismicity, as well as better understanding the processes at its origin. The present volume contains twelve papers from six countries, dealing with observations of triggered and induced seismicity in four continents. The reported cases include seismicity due to hard-rock mines, coal mines, underground research facilities for nuclear waste disposal, water injections, reservoirs, acquifers and oil fields. This volume provides case studies of previously unavailable observations of this phenomenon, investigations of the cause and source mechanism of seismic events, studies of source location distributions, determinations of seismic source parameters, cases of the use of such parameters in assessing rockburst hazard in mines, and measurements of velocity an attenuation properties of rock masses. The present collection of papers provides an excellent indication of the current state of the art and new developments in this area of research.
Book Synopsis Earth’s Crust and Its Evolution by : Mualla Cengiz
Download or read book Earth’s Crust and Its Evolution written by Mualla Cengiz and published by BoD – Books on Demand. This book was released on 2022-10-12 with total page 210 pages. Available in PDF, EPUB and Kindle. Book excerpt: Despite decades of study, fundamental aspects of the development of the Earth’s crust remain enigmatic. This book presents geophysical and geological studies obtained from different tectonic structures and geological time intervals. It contains three sections: “Crustal Evolution and Tectonic Problems”, “Geophysical Methods in Geological Applications” and “Seismic Forecasting, Seismotectonics and Geodynamic Evolution of the Himalayan Belt”. Chapters address such topics as the evolution of tectonic structures of Earth, how geophysical and geological data can be used for modelling this evolution, and the geodynamic processes in the Earth’s crust with the present tectonic activity.
Book Synopsis Hydraulic Fracturing and Induced Seismicity by : Mohammad Hossein Azad
Download or read book Hydraulic Fracturing and Induced Seismicity written by Mohammad Hossein Azad and published by . This book was released on 2017 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: The injection of fluids into the ground is undertaken in a number of engineering practices including hydraulic fracturing, liquid waste disposal, CO2 sequestration and extraction of geothermal heat (EGS). Study of mechanisms for slip on faults/natural fractures due to hydraulic fracturing is of great interest for the energy resource industry in view of efficiency of hydraulic fracturing, seismic monitoring and seismicity related safety hazards. In this thesis, semi-analytical approaches are applied to investigate physical processes associated with hydraulic fracturing induced seismicity and to develop a deeper understanding of the problem. The primary focus is on the modeling of initiation and growth of slip on a pre-existing fault/fracture due to interaction with a single propagating hydraulic (tensile mode) fracture. The first part of the thesis is concerned with the relationship between hydraulic fracturing injection into a fault and the possibility of a seismic slip. The results show that the nucleation of dynamic slip on a fault with slip-weakening friction is only weakly dependent on the magnitude of the stress perturbation ahead of the propagating hydraulic fracture (HF), or the HF propagation regime, and is mainly controlled by the hydraulic fracture length (i.e., the size of the fully unloaded fault segment at a given time). The growth of the fault slipping patch remains stable when the background shear stress is smaller than the residual fault strength under ambient conditions. Otherwise, nucleation of dynamic slip takes place when the hydraulic fracture grows to the critical size, which is vanishingly small for critically-stressed faults (i.e., when the background stress approaches the fault peak strength) and is diverging when the stability boundary is approached. In addition, no dynamic slip transients are predicted when background shear stress is less than the residual fault strength. In the second part of the thesis we examine the possibility of a microseismic slip on a natural fracture as a result of poromechanical interactions with an advancing hydraulic fracture. Nucleation of slip on the frictional fracture approached by the hydraulic fracture depends on the state of in-situ stresses, hydraulic fracture pressure, angle of approach and friction of the fracture. Slip instability, consistent with field observations, occurs on the critically stressed and favorably oriented strike-slip fracture. Nucleation takes place as the slipping patch reaches the critical length. Finally, we have studied nucleation of slip on a natural fracture crossed by a propagating hydraulic fracture. Slip initiates along the edge crack as the frictional strength drops due to increase in pore pressure by diffusion of fluid from the hydraulic fracture.
Book Synopsis Thermo-Hydro-Mechanical Coupling in Fractured Rock by : Hans-Joachim Kümpel
Download or read book Thermo-Hydro-Mechanical Coupling in Fractured Rock written by Hans-Joachim Kümpel and published by Birkhäuser. This book was released on 2012-12-06 with total page 355 pages. Available in PDF, EPUB and Kindle. Book excerpt: (4). The next three papers extend these views by taking a closer look on parameters that govern hydraulic diffusivity in sandstones and other types of rocks. Specific targets addressed are the influence of differential stress on permeability (5), imaging of the fracture geometry (6), and pressure induced variations in the pore geometry (7). Contributions no. 8 to 10 cover investigations of permeability-porosity relationships during rock evolution (8), of the formation, propagation, and roughness of fractures in a plexi-glass block (9), and pressure oscillation effects of two-phase flow under controlled conditions (10). The subsequent four articles focus on diverse modeling approaches. Issues considered are how the geometry and the mechanical behavior of fractures can be characterized by mathematical expressions (11), how the evolution of permeability in a microcracking rock can be expressed by an analytical model (12), deviations from the cubic law for a fracture of varying aperture (13), and the numerical simulation of scale effects in flow through fractures (14). Three further papers refer to in situ observations, being related to topics as the assessment of in situ permeability from the spatio temporal distribution of an aftershock sequence (15), to the scale dependence of hydraulic pathways in crystalline rock (16), and to the significance of pore pressure - stress coupling in deep tunnels and galleries (17).
Book Synopsis Mechanical and Hydraulic Properties of Rocks Related to Induced Seismicity by : Paul A. Witherspoon
Download or read book Mechanical and Hydraulic Properties of Rocks Related to Induced Seismicity written by Paul A. Witherspoon and published by . This book was released on 1976 with total page 49 pages. Available in PDF, EPUB and Kindle. Book excerpt: The mechanical and hydraulic properties of fractured rocks are considered with regard to the role they play in induced seismicity. In many cases, the mechanical properties of fractures determine the stability of a rock mass. The problems of sampling and testing these rock discontinuities and interpreting their non-linear behavior are reviewed. Stick slip has been proposed as the failure mechanism in earthquake events. Because of the complex interactions that are inherent in the mechanical behavior of fractured rocks, there seems to be no simple way to combine the deformation characteristics of several sets of fractures when there are significant perturbations of existing conditions. Thus, the more important fractures must be treated as individual components in the rock mass. In considering the hydraulic properties, it has been customary to treat a fracture as a parallel-plate conduit and a number of mathematical models of fracture systems have adopted this approach. Non-steady flow in fractured systems has usually been based on a two-porosity model, which assumes the primary (intergranular) porosity contributes only to storage and the secondary (fracture) porosity contributes only to the overall conductivity. Using such a model, it has been found that the time required to achieve quasi-steady state flow in a fractured reservoir is one or two orders of magnitude greater than it is in a homogeneous system. In essentially all of this work, the assumption has generally been made that the fractures are rigid. However, it is clear from a review of the mechanical and hydraulic properties that not only are fractures easily deformed but they constitute the main flow paths in many rock masses. This means that one must consider the interaction of mechanical and hydraulic effects. A considerable amount of laboratory and field data is now available that clearly demonstrates this stress-flow behavior. Two approaches have been used in attempting to numerically model such behavior: (1) continuum models and (2) discrete models. The continuum approach only needs information as to average values of fracture spacing and material properties. But because of the inherent complexity of fractured rock masses and the corresponding decrease in symmetry, it is difficult to develop an equivalent continuum that will simulate the behavior of the entire system. The discrete approach, on the other hand, requires details of the fracture geometry and material properties of both fractures and rock matrix. The difficulty in obtaining such information has been considered a serious limitation of discrete models, but improved borehole techniques can enable one to obtain the necessary data, at least in shallow systems. The possibility of extending these methods to deeper fracture systems needs more investigation. Such data must be considered when deciding whether to use a continuum or discrete model to represent the interaction of rock and fluid forces in a fractured rock system, especially with regard to the problem of induced seismicity. When one is attempting to alter the pressure distribution in a fault zone by injection or withdrawal of fluids, the extent to which this can be achieved will be controlled in large measure by the behavior of the fractures that communicate with the borehole. Since this is essentially a point phenomena, i.e. the changes will propagate from a relatively small region around the borehole, the use of a discrete model would appear to be preferable.
