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Rock Fractures and Fluid Flow: Contemporary Understanding and Applications 8 Case Histories The heterogeneity and complexity of flow paths in fractured rocks make field studies very difficult. As a result, there are relatively few field study sites where the distribution and character of fractured rocks have been described in detail. These sites are an extremely valuable scientific resource for a number of reasons. First, field testing and verification of various fracture characterization methods and data analysis techniques require sites where these methods can be developed, applied, and evaluated. Case history studies from such well-documented field sites are useful for demonstrating the application of specific techniques. These studies illustrate how different fracture characterization techniques can be applied to radioactive waste repository siting or water resource development, for example. The careful and thorough documentation of fractures, geomechanical properties of fractured rocks at various scales, and the patterns of tracer dispersal through fractures provide insights into how large-scale geological structure and tectonic history relate to the details of fracture properties and fracture distribution as identified in boreholes, core samples, and outcrops. A list of some of the better-documented sites where fractured rocks have been studied is provided in Table 8.1. The table lists the locations of the sites, the rock types involved, the depths of investigation, and the primary applications for which the studies were intended. The table does not describe all sites in existence but does provide a representative sample of sites where work has been carried out, and it gives a reasonably complete series of examples of the various fracture characterization techniques that can be applied in the field. The application supporting the majority of the long-term, large-scale fracture studies is high-level radioactive waste repository siting in North American and Europe. Most of
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Rock Fractures and Fluid Flow: Contemporary Understanding and Applications TABLE 8.1 Summary of Fracture Study Sites Site Location Rock Type Application Maximum Depth (m) Reference Underground Research Laboratory Southeast Manitoba, Canada Granite Radioactive waste disposal 500 Martin (1990); Everitt et al. (1990); Paillet (1991) Mirror Lake White Mountains, New Hampshire Schist Contaminant transport 200 Shapiro and Hsieh (1994); Paillet and Kapucu (1989); Morganwalp and Aronson (1994) Hanford Columbia River Plateau, Central Washington Basalt Radioactive waste disposal 1,000 Kim and McCabe (1984); Paillet and Kim (1987) Oracle Santa Catalina Mountains, South-Central Arizona Granite Radioactive waste disposal 100 Hsieh et al. (1983) University of Waterloo Clarkson, Ontario, Canada Dolomitic shale Contaminant transport 20 Paillet et al. (1992) Stripa Sweden Granite Radioactive waste disposal 1,000 Olsson (1992); Nelson et al. (1982) Grimsel Crystalline Alps, Switzerland Granite Radioactive waste disposal 1,000 Martel and Peterson (1991) Red Gate Woods Argonne, Northeast Illinois Dolomite Contaminant dispersal 100 Robinson et al. (1993); Nicholas and Healy (1988); Silliman and Robinson (1989) Fenton Hill North-Central New Mexico Granite Geothermal 1,000 Robinson and Tester (1984); Fehler (1989); Block et al. (1994)
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Rock Fractures and Fluid Flow: Contemporary Understanding and Applications Site Location Rock Type Application Maximum Depth (m) Reference Antrim Shale Eastern Michigan Shale Natural gas resources development 300 Turpening (1984) Multiwell Experiment Northwest Colorado Siltstone Natural gas resources development 2,000 Lorenz and Finley (1991); Lorenz et al. (1989) Nevada Test Site Southwest Nevada Tuff Radioactive waste disposal 1,000 Geldon (1993); Nelson (1993) The Geysers Field North-Central California Metamorphic Geothermal 3,000 Geothermal Resources; Council (1992); Oppenheimer (1986) Finnsjön Sweden Granite Radioactive waste disposal 500 Andersson et al. (1991); Gustaffson and Andersson (1991) East Bull Lake South-Central Ontario, Canada Gabbro Radioactive waste disposal 500 Paillet and Hess (1986); Kamineni et al. (1987); Ticknor et al. (1989) Aitkokan Southwest Ontario Granite Radioactive waste disposal 1,000 Paillet and Hess (1987); Stone and Kamineni (1982); Kamineni and Bonardi (1983) Niagara Falls Northwest New York Dolomite Contaminant dispersal 200 Yager (1993) Pierre Shale Central South Dakota Shale Waste disposal 100 Bredehoeft et al. (1983); Neuzil (1993) Travis Peak Southeast Texas Sandstone Gas production 3,000 Laubach (1988); Laubach et al. (1988) Waste Isolation Pilot Project Southeastern New Mexico Dolomite and anhydrite Radioactive waste disposal 1,000 Davies et al. (1991); Beauheim (1988)
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Rock Fractures and Fluid Flow: Contemporary Understanding and Applications Site Location Rock Type Application Maximum Depth (m) Reference Apache Leap Central Arizona Tuff Radioactive waste disposal 800 Basset et al. (1994, 1996) Wake/Chatham Central North Carolina Sandstone and shale Radioactive waste disposal 120 Chem-Nuclear Systems (1993) Sellafield Northwestern Coast of England Basalt and sediments Radioactive waste disposal 2,000 Michie (1992)
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Rock Fractures and Fluid Flow: Contemporary Understanding and Applications these studies deal with crystalline rocks. The sites listed in Table 8.1 are too numerous to be described in detail in this report. Each entry is associated with one or two key references to provide the most efficient introduction to the literature. A few representative sites are discussed in detail in the following sections. CASE HISTORY I. U.S. GEOLOGICAL SURVEY FRACTURED ROCK RESEARCH SITE NEAR MIRROR LAKE, NEW HAMPSHIRE The U.S. Geological Survey is conducting research on fluid flow and solute transport in fractured rock at a site near Mirror Lake in central New Hampshire (Winter, 1984; Shapiro and Hsieh, 1991). Started in 1990, this study aims to (1) develop and assess field methods for characterizing fluid flow and solute transport in fractured rocks; (2) develop a multidisciplinary approach that uses geological, geophysical, geochemical, and hydrological information for data interpretation and model building; and (3) establish a site for long-term monitoring. The discussion below summarizes the preliminary results of this ongoing study. Additional information can be found in an overview paper by Shapiro and Hsieh (1995) and in a series of papers edited by Morganwalp and Aronson (1995). Mirror Lake lies at the lower end of the Hubbard Brook valley in the southern White Mountains of New Hampshire. The surface area that drains into Mirror Lake occupies 0.85 km2 of mountainous terrain, which varies in altitude from 213 m at the lakes surface to 481 m at the top of the drainage divide. The bedrock is a sillimanite-grade schist extensively intruded by granite, pegmatite, and lesser amounts of lamprophyre. It is covered by 0 to 55 m of glacial drift. Outcrops are few; the largest exposure of bedrock occurs where a highway cuts through a small hill. Here, four subvertical surfaces, exposed by road construction, and one subhorizontal surface, cleared by glaciation, provide approximately 8,000 m2 of exposed rock for mapping and studying fractures and geology. The roadcut shows a complex distribution of rock types. The schist is multiply folded. The granitic intrusions occur as dikes, irregular pods, and anastomosing fingers, ranging in width from centimeters to meters. Pegmatite and basalt dikes cross-cut both the schist and granite. Investigations at the Mirror Lake site are proceeding at two scales: the 100-m scale and the kilometer scale. The 100-m-scale investigations focus on several subregions, each occupying an area of approximately 100 × 100 m. The goal is to characterize in detail the fracture geometry and the hydraulic and transport properties to a depth of about 80 m. The kilometer-scale investigations cover approximately 1 km2 (Figure 8.1), including the entire surface area that drains into Mirror Lake. The site investigations are proceeding outward from the vicinity of the lake in a systematic fashion, and some of the most recently drilled bedrock boreholes are located in areas beyond the actual surface watershed. The goal is to characterize the large-scale movement of groundwater to a depth of about 250 m.
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Rock Fractures and Fluid Flow: Contemporary Understanding and Applications FIGURE 8.1 The Mirror Lake, New Hampshire, study area, showing the location of individual bedrock well and the two well fields. The larger of the two squares represents the FSE borehole array, and the smaller represents the CO array. From Paillet and Kapucu (1989). 100-m-Scale Investigation The 100-m-scale investigations use many of the tools described in Chapter 2 (fracture mapping), Chapter 4 (fracture detection by geophysical methods), and Chapter 5 (hydraulic and tracer tests). Surficial mapping of fractures is carried out at the highway roadcut. For subsurface investigations, two well fields (Forest Service East, or FSE, and Camp Osceloa, or CO) have been established (Figure 8.1). In the following discussion of characterization techniques used at the Mirror Lake site, the reader can find a detailed description of the fracture mapping, geophysical method, and hydraulic tracer test methods in Chapters 2, 4, and 5. At the highway roadcut, fractures were mapped by the ''pavement method" developed by Barton and Larson (1985) and described by Barton and Hsieh (1989). This method consists of (1) making a detailed map of the fractures on an exposed rock surface (pavement); (2) measuring the orientation, surface roughness, aperture, mineralization, and trace length of each fracture; and
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Rock Fractures and Fluid Flow: Contemporary Understanding and Applications (3) measuring the connectivity, density, and scaling characteristics of the fracture network. Results suggest a correlation between fracturing and rock type. The granite is more densely fractured with shorter and more planar fractures. The schist has fewer and less planar fractures. Connectivity of the fracture network at Mirror Lake is low compared to fractures mapped in volcanic tuff, quartz diorite, limestone, and sandstone at other sites. The low connectivity at the Mirror Lake site suggests that fluid moves through highly tortuous paths in the bedrock. In a 100 × 100 m area adjacent to the CO well field, directional soundings using direct current electricity and refracted seismic waves were carried out to determine the predominant strikes of near-vertical fractures in the bedrock, which underlies 3 to 10 m of glacial drift. Analyses yield predominant strikes of N 30° E from the electrical sounding and N 22° E from the seismic survey. These orientations agree closely with the predominant strike of 30° determined from fractures mapped at the highway roadcut. The agreement suggests that, where overburden is thin (e.g., less than 10 m), directional sounding can be an effective method for determining the predominant strikes of near-vertical fractures. At the FSE well field west of Mirror Lake, 13 wells were drilled in a 120 × 80 m area (Figure 8.2). Drill cuttings and downhole video camera images show that the wells penetrate varying thicknesses of schist, granite, and pegmatite but that there is little to no apparent correlation in the distribution of rock types in neighboring wells. Borehole televiewer logs show that, between depths of 20 m (bedrock surface) and 80 m, each well intersects 20 to 60 fractures. With a few exceptions, these fractures do not project from one well to another (Hardin et al., 1987; Paillet, 1993). In each well, water-producing fractures were determined by single-borehole flowmeter surveys and single-borehole packer tests. The results show that one to three fractures in each well together produce more than 90 percent of the water when the well is pumped. The remaining fractures are less transmissive by two to five orders of magnitude. These findings suggest that bedrock underlying the FSE well field contains a small number of highly transmissive fractures within a larger network of less transmissive fractures. The geometry and interconnectivity of the highly transmissive fractures were examined by cross-hole flowmeter survey (pumping one well and measuring vertical velocity in an observation well), vertical seismic profiling, seismic and electromagnetic tomography, multiple-borehole hydraulic tests, and converging-flow tracer tests. These field data are still under analysis, but a conceptual picture of the fracture network is emerging. The highly transmissive fractures appear to form local clusters; each fracture cluster occupies a near-horizontal, tabular-shaped volume several meters thick and extends laterally a distance of 10 to 40 meters. These clusters are connected to each other via a network of less transmissive fractures. Figure 8.3 illustrates the inferred locations of four highly transmissive fracture clusters, marked A through D, in the vertical section between wells FSE1 and FSE6.
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Rock Fractures and Fluid Flow: Contemporary Understanding and Applications FIGURE 8.2 Locations of the 13 boreholes in the Forest Service East (FSE) well field at the Mirror Lake site. See Figure 8.1 for location of the FSE well field in the Mirror Lake area. From Hsieh and Shapiro (1994). Preliminary analyses of the geophysical tomography results suggest that these techniques are extremely valuable for tracing the high-transmissivity zones between wells. For example, in the vertical section between wells FSE1 and FSE4 (14 m apart), fracture cluster B was detected by seismic and electromagnetic tomography and also by vertical seismic profiling. The low-velocity region in the electromagnetic tomogram (Figure 8.4) agrees closely with the highly transmissive fractures identified by flowmeter survey and hydraulic testing. At greater separation distances between wells, however, the tomogram becomes more fuzzy, owing to decreasing signal strength at the receivers. There are also instances in which a low-velocity zone in a tomogram does not correlate with a high-transmissivity fracture zone, possibly because of heterogeneities in rock properties. Therefore, hydraulic tests are needed to interpret the tomography results. Difference tomography (comparing tomographs made before and after injecting
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Rock Fractures and Fluid Flow: Contemporary Understanding and Applications FIGURE 8.3 Vertical cross section between wells FSE1 and FSE6 at the Forest Service East well field. Four clusters of highly permeable fractures labeled A, B, C, and D occur in the less permeable fractured rocks. Borehole packers are shown in black. Modified from Shapiro and Hsieh (1994). electrically conductive fluid into a fracture zone; Andersson et al., 1989) could also help in resolving ambiguities. The highly transmissive fracture clusters in the FSE well field exert a strong influence on multiple-borehole hydraulic tests. To prevent hydraulic communication through the wells, fracture clusters in each well are isolated from one another by packers, as illustrated by Figure 8.3. During pumping, the drawdown behavior is different from that in a homogeneous aquifer. If two packer-isolated intervals straddle the same fracture cluster, the drawdowns in the two intervals tend to be nearly identical. In contrast, if two packer-isolated intervals straddle different fracture clusters, the drawdowns are significantly different. To analyze these tests, analytical methods are generally not suitable because they are based on oversimplified assumptions. Instead, a conventional porousmedium type numerical model is used to simulate the highly transmissive fracture clusters as high-permeability zones and the surrounding network of less transmissive fractures as low-permeability zones. Preliminary analyses yield transmissivities in the range of 10-5 to 10-4 m2/s for the highly transmissive fracture clusters, and an equivalent hydraulic conductivity of about 10-7 m/s for the surrounding rock mass.
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Rock Fractures and Fluid Flow: Contemporary Understanding and Applications FIGURE 8.4 Electromagnetic velocity tomogram in vertical section between wells FSE1 and FSE4. From Hsieh et al. (1993). Kilometer-Scale Investigations Compared to the 100-m scale investigation, kilometer-scale investigations are less detailed for practical reasons. Drilling a dense network of wells (e.g., on a square grid of 50-m spacing) throughout the entire 1-km2 study area is too expensive. In fact, such a dense network of wells might be undesirable. If left open, the wells could alter the natural flow of groundwater by connecting previously unconnected fractures. Another constraint on the kilometer-scale investigation is that many of the tools described in Chapters 4 and 5 provide information on small volumes of rock. Fracture detection methods (such as borehole logging and cross-hole tomography) are typically limited to less than 100 m of penetration. Hydraulic and tracer tests also are impractical. Response to pumping becomes undetectable beyond a few hundred meters from the pumping well, and tracer movement over a kilometer may take many years. Therefore, kilometer-scale investigations aim to characterize the large-scale flow of groundwater while neglecting small-scale details. The kilometer-scale investigation monitors the response of the groundwater system to natural perturbations and long-term human disturbances. For example,
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Rock Fractures and Fluid Flow: Contemporary Understanding and Applications seasonal and long-term variations in infiltration to the groundwater system cause fluctuations in hydraulic heads. By monitoring the recharge and discharge of groundwater and the temporal and spatial variations in hydraulic head, it may be possible to infer hydraulic properties on the kilometer scale. Collection of groundwater samples for chemical analysis is another method for kilometer-scale investigations. Recent advances in the detection of human-made chemicals such as chlorofluorocarbons (used as refrigerants and aerosol propellants) and the parent-daughter isotopes tritium and helium-3 (produced from atmospheric testing of thermonuclear devices) have made it possible to determine the ages of shallow groundwaters (Busenberg and Plummer, 1992; Solomon et al., 1992). Knowledge of the spatial distribution of groundwater ages can help identify flow paths. As groundwater flows from recharge to discharge areas, its chemical composition evolves as the water reacts with the rock. Understanding this chemical evolution can help determine groundwater velocity. Hydrological monitoring in the study area includes precipitation measurements at two locations, streamflow and lake discharge measurements using flumes, various meteorological measurements for evaporation calculations, and the construction of 14 well sites for hydraulic head monitoring and groundwater sampling. Each site consists of a well drilled into bedrock with packers and piezometers installed at different depths. Multiple packers are installed in the bedrock portions of the wells to allow hydraulic head measurements at different depths. The packers also prevent hydraulic communication between fractures through the wellbore. Over 30 piezometers, screened at the water table, are installed throughout the study area to monitor the position of the water table in the glacial drift. Hydrological properties on the kilometer scale are inferred from modeling studies. As a base case, the bedrock and glacial drift are each represented as a layer of porous medium. Each layer has a homogeneous and isotropic hydraulic conductivity. Calibration of this model to match observed hydraulic heads and stream discharges yields a bedrock hydraulic conductivity of 4 × 10-7 m/s. This value is close to the average hydraulic conductivity of 3 × 10-7 m/s determined from over 100 single-borehole packer tests conducted at the 14 well sites. The near agreement suggests that, at the Mirror Lake site, large-scale hydraulic conductivity can be inferred from a statistical average of many small-scale measurements. The chlorofluorocarbon and tritium-helium-3 methods were used to determine the ages of water samples collected from packer-isolated intervals in bedrock wells and from a select number of piezometers in the glacial drift. For this purpose, groundwater age is defined as the time between water infiltration into the saturated groundwater system and water collection for analysis. Both methods yield similar ages. Most of the samples are less than 45 years old. However, the spatial distribution of groundwater ages suggests that flow paths are highly complex. Figure 8.5 shows the distribution of groundwater ages in a vertical section on a hillslope through wells R1, TR2, and T1. If the land has a uniform
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Rock Fractures and Fluid Flow: Contemporary Understanding and Applications FIGURE 8.20 Cross section through fracture zones 2 and 2.5, with the subvertical "room 209 fracture." The Geysers geothermal field is located in the Coast Ranges province of central California. Because of the difficulty in obtaining geophysical soundings in this rugged and geologically complex terrain, models of The Geysers geothermal reservoir have been developed mostly from surface geological and structural investigations and from detailed study of borehole cuttings and cores. Surface investigations reveal a series of northwest-trending, steeply dipping, strike-slip faults superimposed on previously faulted and folded terrain. Reservoir rocks consist of blueshist- and greenshist-grade metasedimentary rocks of the Franciscan assemblage intruded by a large felsic pluton that appears genetically related to late Tertiary and Quaternary surficial rhyolites of the Clear Lake volcanic field, which lies just northeast of the geothermal reservoir (McLaughlin and Donnelly-Nolan, 1981). The reservoir itself is located beneath relatively impermeable caprocks and is developed in both the pluton and overlying Franciscan metagraywackes and argillites (Figure 8.21). The intrusive "felsite" is at least 1.3 million years ago (Schriener and Suemnicht, 1981; Dalrymple, 1992). The reservoir is believed to have developed in the graywacke because of its intrinsic brittleness and high susceptibility to fracturing and because of hydrothermal dissolution of Franciscan calcite, aragonite, and other minerals that were only
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Rock Fractures and Fluid Flow: Contemporary Understanding and Applications FIGURE 8.21 Sketch map (top) and schematic cross sections (middle and bottom) of the geothermal reservoir at The Geysers geothermal field. From Thompson (1992). partially filled by late-stage secondary phases (Gunderson, 1990; Hulen et al., 1992). The heat source for the geothermal system is believed to be from felsite intrusions beneath the reservoir (Hebein, 1985; Walters et al., 1988). Surface geophysical measurements yield some information about the nature of the geothermal reservoir and underlying rocks, but the complexity of the terrain and geological environment have made these measurements very difficult to interpret. Gravity measurements indicate a pair of negative anomalies associ-
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Rock Fractures and Fluid Flow: Contemporary Understanding and Applications ated with the reservoir (Chapman, 1978; Chapman et al., 1981; Isherwood, 1981). The larger and presumably deeper anomaly is centered northeast of the field and is believed to be associated with a magmatic body at depth below the Clear Lake volcanic field. A smaller, shallower gravity low (the "production low") is apparently associated with the geothermal reservoir itself. The local density deficiency is attributed to a combination of effects, including fluid withdrawal, the presence of steam in the reservoir, geochemical dissolution of minerals, and the presence of relatively less dense minerals in reservoir rocks (Denlinger, 1979; Denlinger and Kovach, 1981). Aeromagnetic surveys generally confirm the structure indicated by the gravity data and help further define the lateral limits of the reservoir. Surface resistivity measurements have done little more than confirm the separation of the subsurface environment into three layers: basement, reservoir, and cap rock (Keller and Jacobson, 1983; Keller et al., 1984). Passive seismic surveys have been especially useful in defining reservoir properties, based on both identification of source areas for microseismic events and characterization of reservoir volumes through which such seismic waves pass (Iyer et al., 1979; Majer and McEvilly, 1979). The seismic source area maps indicate the location of a magma chamber at a depth of several kilometers to the northeast. The elevated level of seismic activity may be associated with fluid withdrawal from the reservoir (Young and Ward, 1981). Seismic activity in the reservoir provides an important constraint on geomechanical models of the reservoir. The most frequently cited mechanism for the generation of this activity is the response of the fractured rock to compression as steam is withdrawn (Hamilton and Muffler, 1972; Majer and McEvilly, 1979). Most recent studies indicate that microseismic activity is closely associated with both injection and withdrawal of fluids (Majer et al., 1988; Stark, 1990). Active seismic surveys have been very difficult to carry out and have done little more than confirm the stratigraphy and faults inferred from drilling. Coupling of source energy to the ground surface has been a continuing problem. Most effective surface seismic surveys have used the Vibroseis method, which was designed to improve seismic sounding in such terrains (Denlinger and Kovach, 1981). Even with this method, most energy is apparently scattered in the reservoir volume. The few weak deep reflections probably represent the top of the basement. However, the microseismic monitoring technique has been much more effective in delineating the properties of reservoir rocks in part because the energy source is well coupled to the rock mass. These surveys indicate that the developed reservoir volume is associated with relatively low Vp/Vs ratios (ratios of compressional to shear velocity). This implies a reduced value for Poisson's ratio of reservoir rocks from which fluids have been withdrawn (O'Connell and Johnson, 1991). A similar result was obtained in vertical seismic profile studies reported by Majer et al. (1988). Geochemical investigations generally form a major part of geothermal reservoir studies, and this is certainly true of The Geysers. Patterns of geochemical
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Rock Fractures and Fluid Flow: Contemporary Understanding and Applications alteration of reservoir rocks and minerals deposited in fracture fillings indicate that the reservoir has evolved from a liquid-dominated to a vapor-dominated system (Sternfeld, 1989). This has apparently occurred in part because the relatively impermeable caprock and sealed margins of the system inhibited recharge of the reservoir (White et al., 1971). Much of the geochemical and thermal history of the reservoir and caprock is based on interpretation of fluid inclusions in core samples and the compositions, textures, and paragenesis of minerals deposited in fractures, breccias, and dissolution cavities (Walters et al., 1988; Hulen et al., 1991). This result has important consequences for fracture studies. The complex evolution of a geothermal reservoir is important in evaluating the coupling between flow, temperature, stress, and fluid chemistry. Depending on location, the caprock is also a consequence of stratigraphy and the interaction of stress, temperature, and mineral deposition. Geomechanical investigations have also contributed to the study of the reservoir. Much of this work centers on definition of the geomechanical nature of the reservoir and is concerned with questions about the effects of structural control on the lateral continuity of permeable zones and the flow of steam toward production wells. Fault and fracture orientation may be the primary determinant of flow in the reservoir (Thompson and Gunderson, 1989; Beall and Box, 1992). Several models have been proposed for the generation of open fractures in the reservoir, including wrench faulting of blocks and opening of near-vertical fractures and faults in the direction of minimum horizontal stress (Oppenheimer, 1986; Thompson and Gunderson, 1989; Nielson and Brown, 1990). At least some oriented cores indicate that the strike of fractures is perpendicular to the present direction of the least principal stress (Nielson and Brown, 1990). Other data indicate a strong lithological control on fracture generation or preservation; in some cores, graywacke beds are fractured, but intervening argillite beds are not (Sternfeld, 1989; Hulen et al., 1991). Production data indicate that there is horizontal continuity between producing wells. Fracture generation and opening mechanisms need to account for this horizontal continuity as well as the presence of conduits for the upward convection of fluids. Observations of fractures, veins, and the texture of core samples have further contributed to an understanding of the source and movement of fluids in the reservoir. A double-porosity framework has been applied to the reservoir (Williamson, 1990). Major flow conduits are assumed to be fractures, faults, and brecciated zones, although only a single example of a major steam conduit has been recovered from core (Gunderson, 1990). The bulk of fluid reserves in the reservoir is stored in "matrix" porosity, where the matrix refers to everything besides the main fluid conduits. Detailed core examination reveals that the matrix porosity consists of open microfractures, dissolution voids from leaching of calcite and aragonite, and vuggy hydrothermal veinlets (Gunderson, 1990; Hulen et al., 1992). Much of the reservoir production apparently comes from water
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Rock Fractures and Fluid Flow: Contemporary Understanding and Applications adsorbed on the surfaces of minerals lining pore spaces and veins (Barker et al., 1992). One of the most effective methods for investigating the flow of water and steam along fractures in the reservoir is production testing and tracer studies. Fluids injected into the reservoir appear to preferentially follow planes perpendicular to the direction of the least principal stress (Thompson and Gunderson, 1989). In the past, tritium and deuterium from power plant injectate have been used as tracers in attempting to follow the path of injected water from injection to production wells (Gulati et al., 1978). These tracers are difficult to interpret because relatively high detection limits are required and the effects of vapor fractionation on the tracers in the reservoir are unknown. Recent studies have used halogenated alkenes, which fractionate almost exclusively into steam and have extremely low detection limits (Adams et al., 1991a,b). Vapor-phase tracers have been detected in production wells within days of injection, indicating horizontal velocities in the reservoir as large as 1 km/day (Adams et al., 1991a,b). The path traveled by the first few percent of steam generated from injection water appears to be the same as that indicated for the injection water using deuterium tracers. The extremely short travel times indicate that flow takes place along major fractures and faults, rather than through the "matrix" porosity of the bulk of the reservoir rock. These results demonstrate the way in which various lines of investigation can be used to constrain one of the most complicated problems related to fracture flow—delineation and modeling of flow in geothermal reservoirs. The difficulty in obtaining measurements in a complex geological environment, the hostile environment of boreholes, and the multivariate nature of two-phase flow in fractured media combine to make such studies extremely difficult to carry out. One of the most interesting aspects of fracture studies in geothermal reservoirs is the interaction of temperature, stress, and geochemistry in controlling flow. Stress and temperature determine mechanical properties, and temperature and geochemistry determine where mineral dissolution and growth occur. These interrelationships allow for many possible kinds of behavior. One of the most important examples is the generation of a caprock. If these zones of sealed fractures were in fact formed during the evolution of geothermal systems, the fracture infilling clearly influences the distribution of temperature in the reservoir as well as the geometry of convective flow. The number of independent variables in such geomechanical investigations requires that the full array of potential measurements be applied. This overview of investigations at The Geysers geothermal area provides an example of the techniques that can be applied to these difficult investigations and the various models that can be developed in attempting to constrain a multivariate, two-phase, dual-porosity fracture flow problem where reservoir porosity and permeability are a time-varying function of temperature, pressure, and in situ stress conditions.
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Representative terms from entire chapter: