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Rock Fractures and Fluid Flow: Contemporary Understanding and Applications 1 Rock Fractures and Fluid Flow: Practical Problems Fractures are mechanical breaks in rocks; they originate from strains that arise from stress concentrations around flaws, heterogeneities, and physical discontinuities. They form in response to lithostatic, tectonic, and thermal stresses and high fluid pressures. They occur at a variety of scales, from microscopic to continental. Fractures are important in engineering, geotechnical, and hydrogeological practice because they provide pathways for fluid flow. Many economically significant petroleum, geothermal, and water supply reservoirs form in fractured rocks. Fracture systems control the dispersion of chemical contaminants into and through the subsurface. They also affect the stability of engineered structures and excavations. The application of fracture characterization and fluid flow analysis in engineering, geotechnical, and hydrogeological practice involves addressing three key questions: How can fractures that are significant hydraulic conductors or barriers be identified, located, and characterized? How do flow and transport occur in fracture systems? How can changes in fracture systems be predicted and controlled? These questions are discussed in turn below. 1. How can fractures that are significant hydraulic conductors or barriers be identified, located, and characterized? A fundamental step in understanding and predicting the behavior of fractures involves the identification and location of hydraulically significant fractures. Such fractures are conduits for fluid flow and are connected to other hydraulically conductive fractures to form systems or networks. Conductive fracture networks may include a large number of inter-
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Rock Fractures and Fluid Flow: Contemporary Understanding and Applications connected hydraulically active features or may be limited to a very small proportion of the total fractures in the rock mass. Sometimes it is necessary to locate the hydraulically active fractures explicitly. In other cases it may be sufficient to determine the types of patterns (i.e., regularly repeating geometrical arrangements of fractures) that the fractures form or their statistical properties (e.g., orientation and density) or to locate only the major fractures explicitly. The requirements vary from site to site and from application to application. Chapters 2 through 5 address methods for locating and characterizing hydraulically conductive fractures through a combination of geological mapping, geomechanical analysis, and geophysical and hydrological measurements. For instance, it may be possible to predict the locations of fractures at depth through an understanding of the processes that form fractures and the nature of the resulting fracture patterns. Chapter 2 addresses the development of fractures from a geological and geomechanical perspective and addresses the origin of fracture patterns as well as common fracture patterns found in various geological environments. Fractures represent concentrations of void space in rocks. The geometry of the void space affects both the flow properties and the physical properties of the rock mass, such as the elastic and electric properties. An understanding of how the void space geometry controls the fluid flow and geophysical properties of rock forms the foundation of geophysical methods used to detect fractures in the subsurface. These hydraulic and geophysical properties of fractures are reviewed in Chapter 3. Geophysical techniques can be used to locate and measure the properties of fractures in the subsurface. There have been tremendous recent improvements in subsurface fracture detection techniques. Geophysical methods are particularly useful for identifying large individual fractures as well as groups of closely spaced and interconnected fractures, sometimes referred to as fracture zones. These methods can be used to image large fractures and fracture zones, as discussed in Chapter 4. For example, seismic propagation is slower through fractures than through intact rock. This fact can be used to locate regions that have higher fracture densities and consequently regions that might also have higher hydraulic permeabilities. Because the relationship between geophysical properties of a rock mass and the hydrological properties of the fractures is not unique, it is usually necessary to make direct measurements of flow properties from wells drilled into the subsurface, as discussed in Chapter 5. Well tests and tracer tests can be used to characterize flow system geometry, including the size, density, orientation, and interconnectedness of hydraulically conductive fractures. Flow systems developed in fractures can have quite complex geometries. Special techniques have evolved to interpret test data from these systems. These techniques are discussed in Chapter 5. These flow properties are used in mathematical models of the flow system, as described in Chapter 6.
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Rock Fractures and Fluid Flow: Contemporary Understanding and Applications 2. How do flow and transport occur in fracture systems? Numerical models are used to obtain quantitative estimates of flow and transport behavior in fracture systems. The first step in the development of a numerical model is the construction of an appropriate conceptual model of the fracture system. The conceptual model is a physical model of the system that describes the main features of the geology and hydrology that control the flow and transport behavior of interest. A conceptual model incorporates an interpretation or schematization of reality that is the basis of mathematical calculations of behavior. Chapter 6 addresses the development of conceptual models from the information collected using the tools described in Chapters 2 through 5. Conceptual model development in fracture systems is an important but frequently undervalued step in numerical modeling. Particular difficulties arise in the determination of fracture geometry and flow physics because, as noted previously, fracture systems have complex geometries. In many cases more than one conceptual model can be constructed with available data. Indeed, most of the errors involved in predicting flow behavior with numerical models usually reflect deficiencies in the underlying conceptual models. A numerical model provides quantitative estimates of the flow and transport behavior of the system described by the conceptual model. The mathematical formulation of the numerical model is determined by the flow geometry specified in the conceptual model. A wide variety of fluid flow models have been developed to address the prediction of flow and transport in fracture systems. All methods subdivide the medium into a set of discrete conductive elements. The methods differ mainly in the way the conductive elements are defined (e.g., whether they represent single fractures or groups of fractures) and in the way the models are parameterized, that is, how the unknown parameters are estimated through field tests (e.g., whether unknown parameters are estimated based on statistical analysis of small-scale data or on interpretation of large-scale hydraulic tests). Several classes of numerical models, their relationships to conceptual models, and their applications to fracture flow systems are described in Chapter 6. Emphasis is on models that deal with the heterogeneous nature of fracture systems. Model parameterization also is addressed in Chapter 6. There is a strong dependence between field test design and model parameterization. For example, a numerical model of the distribution of fractures at a site requires the field collection of spatial statistics for parameters such as fracture density, orientation, size, and conductivity. A numerical model that explicitly specifies hydrogeological units such as fracture zones requires well tests to measure the hydraulic properties of those zones. Numerical models are usually developed to aid in management and design decisions. Two basic questions must be addressed before these models can be used effectively as a management or design tool. First, does the conceptual model provide an adequate characterization of the flow system? If not, it must be revised and reevaluated. Second, is the data base adequate to estimate the parameters in
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Rock Fractures and Fluid Flow: Contemporary Understanding and Applications the numerical model with sufficient precision to produce reliable predictions for the intended application? If not, additional field data must be collected. Development of a numerical model should be an iterative process. It begins with development of a conceptual model, which, in turn, is used to formulate a numerical model of the flow system. The parameters of the numerical model are determined through the collection and interpretation of field data, and the numerical model is used to predict the behavior of the flow system. The predicted behavior is compared to the actual behavior of the flow system as determined from field observations. If the predicted and observed behaviors do not match, the conceptual model is reevaluated and updated, and the process is repeated. 3. How can changes in fracture systems be predicted and controlled? The flow and transport behavior of fracture systems can be perturbed by natural processes or by the activities of people. For example, extraction of fluids from a fracture system can lower the fluid pressure, which will increase the effective stress (the stress transmitted directly from particle to particle) in the rock, thereby causing the fractures to close. Temperature-induced stresses can have similar effects. Changes in fluid chemistry can lead to either mineral precipitation or mineral dissolution on the fracture walls. In general, any change in the void geometry of a fracture system will alter the flow and transport behavior. Tools for predicting and controlling these changes are addressed in Chapter 7. In the final analysis there is no prescribed procedure that can be followed to address flow and transport in fracture systems. This report describes tools that can be used in the process of characterization, model building, prediction, and model refinement. These tools must be applied iteratively to determine if a flow and transport system has been characterized correctly. This iterative process must be designed to test improvements in the predictive capabilities of a numerical model. If the predictions improve with each iteration, confidence in the conceptual model as an accurate representation of the flow and transport system is enhanced. If the predictions do not improve, the conceptual model must be reexamined. Iterative procedures can be applied to many problems in the earth sciences. The tools associated with each step of this iterative process, as applied to fractures, are described in this report, but the report should not be viewed as a ''cookbook." Rather, the tools described here can be applied only in a manner appropriate to the specific geology and application under consideration. Case examples of the application of these tools are given in Chapter 8. PROBLEMS INVOLVING FRACTURES IN ENGINEERING PRACTICE The following section describe problems associated with the occurrence of fractures in underground reservoirs, in the isolation of hazardous wastes, in mining, and in rock excavations such as tunnels or caverns. In each of these
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Rock Fractures and Fluid Flow: Contemporary Understanding and Applications areas fluid flow in fractures represents opportunities or difficulties. Important resources (oil, gas, water) are produced through fractures. Waste and toxic substances can leak through fractures. Fluid flow in fractures can affect the structural stability of tunnels, mines, and dams. The discussions below give an overview of the applications that are most concerned with fluid flow in fractures. Reservoirs Important underground reservoirs include petroleum (oil and gas), geothermal, and water supply. In these reservoirs, fluids are frequently produced through fracture networks in rocks. Petroleum Reservoirs Fractures present both problems and opportunities for exploration and production from petroleum reservoirs. Many petroleum reservoirs form in highly fractured rocks, where fracture properties such as density and orientation are crucial to reservoir economics. In these cases the fractures are usually important because of permeability rather than porosity. Matrix porosity stores the hydrocarbons, and fractures provide permeable pathways for the transport of hydrocarbons to producing wells. The objective of hydrocarbon exploration in fractured reservoirs is to find areas of intense fracturing, or "sweet spots." These areas usually do not have a visible surface expression and must therefore be located by using remote sensing methods. Once the sweet spot is located, a second objective is to assess its size. Production can quickly exhaust hydrocarbons stored in the fractures themselves, so significant matrix porosity is necessary for favorable economics. It is important to know fracture orientation because orientation controls reservoir anisotropy. Subparallel fractures, for example, do not intersect; consequently, they form poorly interconnected networks. These fractures can be connected with directionally drilled boreholes oriented perpendicular to their traces. Oriented boreholes can greatly increase the efficiency of production from the reservoir. Even relatively small numbers of fractures can affect the propagation of seismic energy. Hence, seismic methods can be used to locate fracture zones in rocks that may act as reservoirs. Fractures can penetrate shale layers that otherwise act as hydrological barriers. These fractures can establish hydraulic continuity and, consequently, a near-hydrostatic pore pressure gradient across otherwise impermeable horizons. Fracture-controlled hydraulic connections can affect the development of a reservoir, even though the reservoir itself may not involve fractures in an important way. When producing from fracture-dominated reservoirs, it is important to understand fracture permeability and its dependence on pore pressure. Production of
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Rock Fractures and Fluid Flow: Contemporary Understanding and Applications fluids at economic rates from fractured reservoirs can lower in situ pore pressures, leading to closure of the fractures and a significant reduction in the effective permeability of the system. Artificial fractures can be created by hydrofracturing (fracturing by pumping water or other fluids and sand into a well at high pressure) in order to improve reservoir flow characteristics. Hydrofracture operations can consume a significant fraction (~25 percent) of the production budget of a well. Induced fractures can breach cap rocks or cause short-circuiting of fluids used to flood the reservoirs. Consequently, it is important to control and monitor hydrofracturing operations. Geothermal Reservoirs and Hot Dry Rock Most hydrothermal-geothermal systems are found in fractured rock masses. The success of field exploration and development efforts depends largely on locating major fractures, faults, or fracture zones that control subsurface fluid circulation. In contrast to petroleum reservoirs, geothermal systems are often located in low-porosity rock. Fractures provide conduits for fluid flow through the rock. These fluids extract the heat stored in the rock matrix. This heat can be "mined" by pumping these fluids to the surface through boreholes. Despite the importance of fractures to geothermal reservoir development, the technology for locating and characterizing fractures in hostile geothermal environments is not well developed. In hydrothermal reservoirs the rates at which steam can be produced or fluids can be reinjected are contingent on fractures that are both open and hydraulically connected. If fracture transmissivity (the rate at which fluid is transmitted through a fracture under a unit hydraulic gradient) is low, commercial operation may be uneconomical because of low fluid production or high costs of injecting waste fluids into the reservoir at high pressure. On the other hand, high fracture transmissivities from large, highly connected fractures can provide short circuits for recharge; in such cases, reinjected fluids will bypass the hot matrix rocks and will not absorb much heat. Fractures are very important for the successful exploitation of geothermal hot dry rock systems, which lack fluid plumbing systems like hydrothermal systems. Reservoirs in these systems are developed by creating fracture networks or extending existing networks to provide pathways for fluid circulation between wells. Reservoirs can be created by hydrofracturing, which requires knowledge of subsurface stresses, lithology, and the locations of natural fractures. Fracture properties can change during operation of the reservoir owing to fluid pressure changes, thermal cooling, and precipitation of minerals. Decreases in pressure during fluid extraction from the reservoir can cause fractures to close. Fluid injection into the reservoir, on the other hand, can produce pressure increases, causing fractures to open. Fracture behavior is hard to predict because the relationship between stress and permeability is complex and highly dependent
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Rock Fractures and Fluid Flow: Contemporary Understanding and Applications on temperature. Scaling, or precipitation of minerals in fractures, can occur if the fluid boils or cools during extraction. These precipitates plug and ultimately seal the fractures, preventing further operation of the reservoir. Thermal shrinkage during extraction of heat from the system, on the other hand, can lead to substantial increases in the conductance of the fracture network over the lifetime of the project. Fluid extraction also affects fractures on the periphery of the reservoir. Pressure increases or mineral precipitation can close or seal fractures at depth beneath the reservoir. Fractures on the periphery of the reservoir can be plugged by mineral precipitates as the reservoir cools during fluid extraction. These pressure and temperature effects can combine to make the reservoir self-sealing. Water Supply Reservoirs Water supply problems in fractured rocks are similar to those in petroleum or geothermal reservoirs. However, water supply projects usually do not command the financial resources available for petroleum or geothermal extraction. Water supply wells are frequently located by trial and error by drilling into fractures or fracture zones. Several wells may be drilled until a water-producing fracture or fracture zone is located. There is little cost-effective technology available for siting wells for optimum production. It is now more common for hydrofracture to be used as a method for stimulating water supply. Prediction of sustainable yields in fractured aquifers must account for the effect of fractures on the flow system. Calculation of sustainable yields requires an understanding of the water balance for the aquifer. Fractures may play an important role in this balance because they can control recharge and discharge from the aquifer. For example, subvertical fracture zones that outcrop in topographic lows may provide major recharge to the aquifer. Groundwater Contamination In relatively impermeable rock, fractures can form pathways for the migration of contaminants (toxic chemical or radioactive wastes) that are emplaced in or released to the subsurface environment. It is important to understand and predict where contaminants will migrate and if they will reenter the surface or near-surface environment. Toxic and Hazardous Wastes There will continue to be a need for the land disposal of toxic wastes, and fractured rock sites will be considered for disposal facilities. Improperly designed land disposal facilities can release contaminants into the subsurface through fractured rocks. Fracture networks control the dispersion of these contaminants
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Rock Fractures and Fluid Flow: Contemporary Understanding and Applications in the subsurface. Cost-effective site characterization is required to prevent subsurface contamination. However, such characterization is difficult, owing to the difficulty in determining the location of subsurface fractures and predicting the transport of contaminants through fracture systems. These characterization problems make it difficult to design monitoring and cleanup strategies. Some contaminants are injected into the subsurface through disposal wells. Typically, wastes are injected below an impermeable confining layer that provides a barrier to upward waste migration. Fractures in these confining layers can provide conduits for upward contaminant migration, potentially compromising the disposal system. The injection of waste into the subsurface at high pressure can artificially fracture the confining layer. These artificial fractures can also provide pathways for the upward migration of contaminants. The migration of acid mine drainage water through and out of underground mines is a related issue of concern. Acid mine drainage is produced by the interaction of water with sulfur, which is present as pyrite in mine wastes, to form sulfuric acid. Bacteria speed up the process. Acid mine drainage is a significant problem in eastern U.S. coal mining districts and in some western U.S. base metal mines. The migration of acid mine water in the subsurface is frequently controlled by fractures. A good understanding of fracture flow conditions is needed to limit the formation of acid mine water and to intercept it once it has formed. Aquifers, which are rock bodies that contain economically significant quantities of water, are particularly sensitive to contamination owing to the rapid and somewhat unpredictable movement of contaminants through fractures. Strategies designed to protect water supply wells usually employ a capture zone, the outer boundary of which is monitored for contamination. The boundary of the capture zone is usually defined as a surface from which the travel time to the well is a constant, for example, 10 years. In a relatively homogeneous porous medium, capture zone boundaries can be estimated by using numerical fluid flow models. In fractured systems the shape of the capture zone depends on the geometry of the fracture network. The shape may be very irregular and difficult to predict using numerical models. Usually, only modest resources are available for water well protection. Few field techniques are available for cost-effective site characterization in fractured rock. Numerical techniques used to model flow in porous rocks (i.e., equivalent continuum models; see Chapter 6) may be inadequate to predict the behavior of flow through fracture systems. Indeed, modeling techniques for demarcating capture zones in fracture systems are not in common use. Numerical techniques to model dilution, dispersion, adsorption, and decay of contaminants in the capture zone are not readily available for fractured rock. High-Level Nuclear Waste The term high-level nuclear waste refers to highly toxic and highly radioactive waste such as spent fuel from nuclear reactors. Throughout the world, geological
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Rock Fractures and Fluid Flow: Contemporary Understanding and Applications media are being considered as storage sites or repositories for the long-term or permanent disposal of high-level nuclear waste. Permanent repositories must isolate this waste for tens of thousands of years because of the very long half-lives of the radionuclides. The most commonly pursued repository design is a subsurface facility, in any of a variety of rock types, that contains waste stored, for example, in steel or concrete canisters. Other than human intrusion, groundwater is the only important mechanism for escape of radioactive waste from a repository. Fractures could play a key role in the movement of groundwater into a repository (see, e.g., National Research Council, 1992). If waste canisters are breached, fractures could control the transport of waste in groundwater from the repository into the environment. Several countries are considering construction of repositories in crystalline rock below the water table. These rocks generally have extremely low permeabilities, except where they are fractured. Fractures control fluid flow in these rocks. A series of experimental facilities have been established in crystalline rocks to study the feasibility of storing nuclear waste (see Chapter 8). At these facilities, it has been observed that a relatively small number of fracture zones or faults account for the majority of fluid flow. Consequently, the general concept for waste storage is to develop repositories in relatively unfractured rock in order to isolate the waste from conductive fracture zones. A key problem is to locate and characterize the major fracture zones without drilling exploratory holes, which are potential leakage pathways. Safety considerations usually require that canisters be located hundreds of meters from major fractures. Consequently, it is important to identify all major fractures or fracture zones in the vicinity of a repository. The relatively unfractured rock between fracture zones can have extremely low permeabilities. Although low permeability is attractive from a safety point of view, it makes the rock difficult to characterize in reasonable time periods. Depending on the particular design of a repository (e.g., how long the waste is allowed to cool before emplacement), the flow in fractures around the repository may be affected by heat generated by the waste or by two-phase flow owing to radiolysis (radiation-induced chemical decomposition), corrosion, or degassing of the groundwater entering the mined-out facility. For any rock type, fractures at depth must be studied and characterized from excavations in order to infer the nature of the rock mass around the potential repository. However, the excavation itself causes changes to the rock, including changes in stress state, saturation, and chemical conditions. To understand the fracture system based on information from excavations, it is also necessary to understand how the excavation changes the fractures. The United States is evaluating a potential repository above the water table in bedded volcanic tuffs at Yucca Mountain in southwestern Nevada (Figure 1.1). The tuffs contain cooling fractures (fractures formed by contraction of the rock during cooling), that tend to be confined to individual beds. A series of
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Rock Fractures and Fluid Flow: Contemporary Understanding and Applications FIGURE 1.1 Cross section through a potential repository site at Yucca Mountain, Nevada, showing geology and structure. From U.S. Department of Energy (1988). larger faults cut through these beds. The role of fractures in recharge, either as conduits or barriers to flow, is critical to understanding repository performance. Above the water table, larger fractures may be filled with gas, providing an effective barrier to fluid flow. On the other hand, fracture zones or faults may be the primary conduits for transport of water from the surface to the water table below the repository. Prediction of the behavior of the flow system around a repository requires conceptual models for flow in variably saturated, fractured rocks. Such understanding is as yet incomplete. Bedded salt formations are also under consideration as waste repositories. The Waste Isolation Pilot Project site in New Mexico has been studied extensively to assess its suitability as a repository for radioactive wastes generated from defense-related activities. The salt formations themselves are thought to be very impermeable, but there is concern that leakage could occur to overlying strata if the repository is breached. If this occurred, fractures in the overlying strata could control the subsequent transport of dissolved radionuclides. Consequently, it is important to understand the flow regime in these fractures. Fractures may also exist in the salt interbeds. Radiolysis and corrosion of the waste package could generate a significant amount of gas, producing pressure buildups that might open these fractures and establish leakage pathways. The Canadian program to develop a geological repository is focused at present on crystalline rocks at a site (yet to be determined) in the Canadian Shield, an area composed of crystalline rocks where the water table is at or close to the ground surface (Figure 1.2). The nuclear fuel waste is to be placed and sealed in engineered excavations 500 to 1,000 meters deep. Several engineered
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Rock Fractures and Fluid Flow: Contemporary Understanding and Applications FIGURE 1.2 Facility design at Atomic Energy of Canada Ltd.'s Underground Research Laboratory near Pinawa, Manitoba. From Everitt et al. (1994). and natural barriers will contribute to the isolation of the waste from the biosphere, including the properties of the waste; corrosion-resistant containers; engineered sealing systems for emplacement holes, excavations, and boreholes; and the properties of the surrounding rock and groundwater. It is expected that low rates of groundwater flow and geochemical retardation in a sparsely fractured repository will contribute to the overall containment design as required by existing regulatory standards. Fractures provide the most probable pathway to the biosphere for nuclear waste buried at depth. It is important to know, at least in a statistical sense, where these fractures are located and how they might provide fluid pathways over the life of the repository.
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Rock Fractures and Fluid Flow: Contemporary Understanding and Applications Mining Many problems in the mining industry relate directly to the flow of fluids through fractures in rock. A lack of understanding of fracture flow and the changes in flow conditions induced by mining (and, conversely, changes in mining conditions caused by altered fracture flow) can seriously impede mining operations. Mining activities most influenced by fracture flow include in situ leaching, mine waste disposal, underground mine dewatering, and structural stability. Mining by in situ leaching involves the circulation of dilute chemical solvents through an ore deposit to dissolve target metals. The solvents may be applied to the top of the ore body and allowed to seep through it by gravity. Alternatively, the solvents may be forced through the ore between a series of injection and recovery wells. The recovery wells pump the solutions containing dissolved metals to the surface. Fractures affect leaching operations in several ways. Fractures are important for developing and maintaining adequate capacity for the injection and recovery of leach solutions. New fractures may be induced by hydraulic fracturing to improve hydraulic conductivity. Fractures are also largely responsible for distributing leach solutions in the ore body. Flow through fractures may not leach effectively because it does not contact a high percentage of the target minerals. Changes in fracture geometry during leaching can profoundly change the effectiveness of leaching operations; a major concern is to prevent reprecipitation of minerals in the fractures, which seals them from further leaching action. Fractures may control the migration of leaching solutions to regions outside the leaching zone. Fracture flow may be important in restoring aquifers after leaching is completed, to satisfy environmental regulations. Structures Natural and Artificially Cut Slopes Fractures and fracture zones govern the stability of rock slopes. Most types of rock slope failures, such as translational/rotational sliding failures, toppling failures, and rock falls, can usually be associated with isolated fractures or fracture zones. Fractures represent zones of weakness and are therefore less resistant than intact rock to deformation and failure by shear and tensile stresses. The presence of water in the rock mass promotes failure by lowering effective stresses and increasing static hydraulic forces on potentially sliding blocks. This effect is somewhat more pronounced under transient conditions, when changes in seepage stresses occur. Transient conditions can occur during cutting of slopes. Most open-pit mine slope failures are caused by a combination of increasing water pressure and slope height. Water-induced pore pressures may also increase if the
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Rock Fractures and Fluid Flow: Contemporary Understanding and Applications fractures are blocked by ice, swelling of intact rock, or precipitation of minerals. Stress levels may also rise because of rainfall, and failure may follow heavy rain. Fractures in rock slopes have an increased susceptibility to weathering, which reduces shear and tensile resistance and may lead to blockage of water flow. Weathering of fractures may account for flatter slopes in fault zones than in the adjacent, less fractured rock. Dams and Surface Storage Reservoirs Fractures cause both foundation and slope problems that can affect the stability of dams, foundations, and surface storage reservoirs. Slope stability along reservoirs is affected by a rise in the water table level during the first reservoir filling and by subsequent water level changes during reservoir operations. Reservoir filling may also destabilize slopes downstream of the dam, owing to differences in water levels between the reservoir and the water table downstream. Slope instability or large deformations near the dam abutment can lead to dam failure. The load of a dam involves the actual foundation below the dam and the lateral abutments. In the case of an arch dam the abutments can carry loads similar in magnitude to those in the foundation. Hydraulic reservoir pressures exert significant loads parallel to the ground surface that can lead to opening of fractures upstream of the dam and closure of fractures downstream. This response produces shorter seepage pathways and higher water pressures near the ground surface downstream of the dam. These high water pressures lower the effective stresses, which can produce large ground deformations and lead to collapse of the dam. Fractures in dam foundations and abutments can also cause significant loss of water from the reservoir owing to increased permeability of the rock mass. Erosion of fault and fracture-filling materials under hydraulic gradients can cause leakage and possibly dam failure. Similarly, erosion of embankment soils into rock fractures can promote piping (formation of subsurface erosional conduits) in embankments. Underground Structures Water-filled fault zones and major fracture systems play an important role in the structural stability of underground openings such as tunnels and caverns. Most effects related to fractures concern the construction of underground openings and, to a lesser but not insignificant extent, its performance during operations. Fractures affect stability, deformation, and fluid flow into underground structures. Many underground sites must be dewatered before excavation can proceed. Water is pumped from wells arranged around the periphery of an underground opening or from interceptor wells above. Problems occur in fractured formations
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Rock Fractures and Fluid Flow: Contemporary Understanding and Applications because water influx is often dominated by high-conductivity fracture zones. Locating these fracture zones and positioning wells to dewater them is difficult. Even more difficult is the problem of predicting the ability of dewatering wells to lower the water table in fractured formations. Well placements are usually based on conditions of uniform flow, which may be inappropriate in fractured rock. Controlling water inflow can be a significant economic factor. Flow into tunnels through fractures can slow or delay construction. Construction conditions are often made more difficult by the flow of hot and/or chemically aggressive water into the tunnel. For example, during construction of the Simplon Tunnel through the Swiss-Italian Alps, inflows of cold water as high as about 18,000 liters per minute were encountered in fractured marble units. At another location in the same tunnel, hot water (46° C) discharged from fracture zones at rates up to about 6,300 liters per minute. Fractures may also serve as conduits for toxic (noxious) and explosive gases into tunnels and mines. Elevated water pressures reduce effective stresses around underground excavations, which can lead to rock deformation and instability. Particularly problematical are rock masses composed of alternating impermeable and permeable (fractured) zones. The impermeable zones act as dams, which can suddenly breach if they are excavated. Thus, tunneling through a fault zone can produce a sudden inrush of water and loss of support for the face. Occasionally, such inrushing can stop the tunneling work for a significant period of time. Fractures also have an effect on the support structure (lining) of underground excavations, which must accommodate both the rock load and water pressure. Support structures are usually designed to be watertight, which increases loads induced by water pressure. Many structures must be drained to reduce water pressure. The effects of drainage on support structures and on their interaction with the rock mass are not well understood. Drainage may induce mineral precipitation in fractures and artificial drainage conduits, which may lead to a blockage of flow and buildup of water pressures. Underground Fluid Storage and Transport Structures Tunnels and caverns may be used to store and transport gases and liquids. These stored fluids exert large internal pressures on the perimeters of underground structures. If the support structures fail or if no support exists, it is likely that the stored fluids will escape through fractures in the rock. Intensely fractured rocks, and rocks containing fractures with large apertures between the fracture walls (openings), are susceptible to deformation, producing large stresses that induce cracking in excavation linings. Stored fluids can escape through these cracks, and new fractures may be induced in surrounding rock masses through hydraulic fracturing. Hydraulic fracturing may also occur if no artificial support exists. Large induced pressures from groundwater lower the effective stresses in surrounding rock masses, which lowers their stability. If an
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Rock Fractures and Fluid Flow: Contemporary Understanding and Applications underground structure is near a slope at the surface, such conditions may destabilize the slope. Problems with fluid loss, lining, and slope stability can be largely avoided if positive pressure gradients are maintained from the groundwater outside these underground facilities. CONCLUSION Fractures are a significant component in many engineering problems, including reservoirs, contamination, and structures. A wide variety of tools are now in use and under development for characterizing, analyzing, and engineering fractures in rock.
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Rock Fractures and Fluid Flow: Contemporary Understanding and Applications APPENDIX 1.AFRACTURES IN THE GEYSERS FIELD The Geysers geothermal field in northern California produces more power than any other geothermal energy field in the world. The economics of The Geysers geothermal field are strongly affected by reservoir fractures. Steam productivity and drilling costs depend significantly on the number and characteristics of fractures in the producing wells. A good knowledge of the subsurface fracture network in the reservoir would increase the rate of success and reduce the cost of drilling. In The Geysers wells the steam feed zones correspond essentially to individual fractures or fracture zones in the so-called main graywacke and felsite units that compose the geothermal reservoir (Beall and Box, 1992; Thompson and Gunderson, 1992; Walters et al., 1992). Only a minor amount of steam is contributed by the rock matrix. The Geysers is a vapor-dominated geothermal system with low formation pressures. Like other vapor-dominated systems, the reservoir pressure is below hydrostatic (Carson and Lin, 1982; Evanoff et al., 1988). Wells at The Geysers are designed to intersect as many fractures in the reservoir as possible. In some cases, wells must be abandoned because they do not intersect a sufficient number of steam-producing fractures. For these wells, low production rates make it uneconomical to install and maintain the surface equipment needed to collect and transport steam to a power plant. The location and design of new production and injection wells at The Geysers could be optimized if the reservoir fracture network were better understood. In this and other geothermal fields it has been found that a costly and routinely encountered problem is lost circulation during drilling (Glowka et al., 1992). Lost circulation of drilling fluids occurs when the drillhole intersects a low-pressure fracture or fracture zone, which provides a pathway for flow of drilling fluid from the well. Lost circulation complicates and delays the drilling operation. The materials used to plug these low-pressure zones can damage the productivity of other feed zones. Prediction of the location and characteristics of the fractures in the reservoir would improve the design and construction of the wells and reduce the number of lost circulation zones.
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Rock Fractures and Fluid Flow: Contemporary Understanding and Applications APPENDIX 1.BSUPERFUND SITE: BYRON SALVAGE YARD During the 1960s, the Byron Salvage Yard, located in north-central Illinois, accepted industrial wastes and debris, including drums of electroplating solutions, oil sludges, cutting wheels, solvents, and scrap metal. The upper aquifer at this site consists of a fractured Paleozoic dolomite, overlain by Quaternary loess and till, with a somewhat noncontinuous shale aquitard that defines the aquifer base (Kay et al., 1989). Beneath the shale is a sandstone aquifer. Regional groundwater flow is toward the Rock River, approximately half a mile to the west and northwest. The upper dolomite aquifer contains elevated levels of volatile organics, metals, and cyanide. The U.S. Environmental Protection Agency, in cooperation with the Illinois Environmental Protection Agency and the U.S. Geological Survey, initiated a series of remedial investigations and feasibility studies starting in 1983. An integrated multiscale hydrogeological investigation has been conducted to characterize the nature of contaminant transport in the fractured dolomite (Paillet et al., 1993). There were three scales of investigation: (1) small-scale (borehole geophysical logs, natural gamma, acoustic televiewer, and caliper); (2) intermediate-scale (cross-correlated geophysical logs, electrical conductivity after fluid replacement, straddle-packer isolated slug tests, and high-resolution flowmeter logs); and (3) large-scale (topographic analysis, aerial photo lineament analysis, observation of water levels, and long-term pumping tests). These investigations indicate that the aquifer is behaving as a heterogeneous (stratified) anisotropic system. Near-vertical fractures connect with horizontal bedding planes to provide discrete flow zones. The greatest difficulty in the investigation was characterization of large-scale features such as major fractures, local faults, and caverns that are known to influence flow patterns, and ultimately contaminant transport, at the site. Several actions were taken to clean up the contamination at this site (U.S. Environmental Protection Agency, 1989). All waste sources were removed, and abandoned monitoring wells were plugged. The site was covered by a 30-cm-thick soil cover, that was graded and revegetated. An alternative drinking water supply was provided to the affected residences.
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Rock Fractures and Fluid Flow: Contemporary Understanding and Applications REFERENCES Beall, J. J., and J. T. Box, Jr. 1992. The nature of steam-bearing fractures in the south Geysers Reservoir. Special Report No. 17, Geothermal Resources Council. Davis, Calif., pp. 69–75. Carson, C. C., and Y. T. Lin. 1982. The impact of common problems in geothermal drilling and completion. Geothermal Resources Council Transactions, 6:195–198. Evanoff, J. I., S. W. Dunkle, and R. J. Crook. 1988. Field cementing practices help control lost circulation at The Geysers. Geothermal Resources Council Transactions, 12:37–40. Everitt, R. A., C. D. Martin, and P. M. Thompson. 1994. An approach to the underground characterization of a disposal vault in granite. Report No. AECL-10560. Atomic Energy of Canada Ltd., Whiteshell Laboratories, Pinawa, Manitoba, Canada. Glowka, D. A., D. M. Schafer, G. E. Loeppke, D. D. Scott, M. D. Wernig, and E. K. Wright. 1992. Lost circulation technology development status. Pp. 81–88 in Proceedings of DOE Geothermal Program Review X, March 24–26, San Francisco. CONF/920378. Washington, D.C.: U.S. Department of Energy. Kay, R. T., D. N. Olson, and B. J. Ryan. 1989. Hydrogeology and Results of Aquifer Tests in the Vicinity of a Hazardous Waste Disposal Site, Near Byron, Illinois. USGS Water Resources Investigation Report 98-4081, U.S. Geological Survey, Urbana, Ill . National Research Council. 1992. Groundwater at Yucca Mountain: How High Can It Rise? Washington, D.C.: National Academy Press. Paillet, F. L., R. T. Kay, D. Yeskis, and W. Pedler. 1993. Integrating well logs into a multiple-scale investigation of a fractured sedimentary aquifer. The Log Analyst, 34:41–57. Thompson, R. C., and R. P. Gunderson. 1992. The Orientation of Steam-bearing Fractures at The Geysers Geothermal Field. Special Report No. 17, Geothermal Resources Council. Davis, Calif., pp. 65–68. U.S. Department of Energy. 1988. Site Characteristics Plan Overview: Yucca Mountain Site. Washington, D.C.: U.S. Department of Energy. U.S. Environmental Protection Agency. 1989. Superfund Record of Decision, Byron Salvage Yard, Illinois, Third Remedial Action. EPA/ROD/RO5-89/-89, June, Washington, D.C. Walters, M., J. Sternfeld, J. R. Haizlip, A. Drenik, and J. Combs. 1992. A Vapor-Dominated High-Temperature Reservoir at The Geysers, California. Special Report No. 17, Geothermal Resources Council, Davis, Calif., pp. 77–86.
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