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Suggested Citation:"8 Case Histories." National Research Council. 1996. Rock Fractures and Fluid Flow: Contemporary Understanding and Applications. Washington, DC: The National Academies Press. doi: 10.17226/2309.
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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

Suggested Citation:"8 Case Histories." National Research Council. 1996. Rock Fractures and Fluid Flow: Contemporary Understanding and Applications. Washington, DC: The National Academies Press. doi: 10.17226/2309.
×

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)

Suggested Citation:"8 Case Histories." National Research Council. 1996. Rock Fractures and Fluid Flow: Contemporary Understanding and Applications. Washington, DC: The National Academies Press. doi: 10.17226/2309.
×

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)

Suggested Citation:"8 Case Histories." National Research Council. 1996. Rock Fractures and Fluid Flow: Contemporary Understanding and Applications. Washington, DC: The National Academies Press. doi: 10.17226/2309.
×

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)

Suggested Citation:"8 Case Histories." National Research Council. 1996. Rock Fractures and Fluid Flow: Contemporary Understanding and Applications. Washington, DC: The National Academies Press. doi: 10.17226/2309.
×

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.

Suggested Citation:"8 Case Histories." National Research Council. 1996. Rock Fractures and Fluid Flow: Contemporary Understanding and Applications. Washington, DC: The National Academies Press. doi: 10.17226/2309.
×

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

Suggested Citation:"8 Case Histories." National Research Council. 1996. Rock Fractures and Fluid Flow: Contemporary Understanding and Applications. Washington, DC: The National Academies Press. doi: 10.17226/2309.
×

(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.

Suggested Citation:"8 Case Histories." National Research Council. 1996. Rock Fractures and Fluid Flow: Contemporary Understanding and Applications. Washington, DC: The National Academies Press. doi: 10.17226/2309.
×

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

Suggested Citation:"8 Case Histories." National Research Council. 1996. Rock Fractures and Fluid Flow: Contemporary Understanding and Applications. Washington, DC: The National Academies Press. doi: 10.17226/2309.
×

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.

Suggested Citation:"8 Case Histories." National Research Council. 1996. Rock Fractures and Fluid Flow: Contemporary Understanding and Applications. Washington, DC: The National Academies Press. doi: 10.17226/2309.
×

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,

Suggested Citation:"8 Case Histories." National Research Council. 1996. Rock Fractures and Fluid Flow: Contemporary Understanding and Applications. Washington, DC: The National Academies Press. doi: 10.17226/2309.
×

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

Suggested Citation:"8 Case Histories." National Research Council. 1996. Rock Fractures and Fluid Flow: Contemporary Understanding and Applications. Washington, DC: The National Academies Press. doi: 10.17226/2309.
×

FIGURE 8.5 Groundwater ages (in years) determined from CFC-12 concentrations at the Mirror Lake study site. From Shapiro and Hsieh (1994).

slope and the subsurface has a uniform hydraulic conductivity, the flow lines should be similar to those illustrated in Figure 8.6. In the vertical direction, ages should increase with depth. Along any flow line, groundwater should be younger near the recharge area (at higher elevations) and older near the discharge area (at lower elevations). In contrast, the observed age distribution in Figure 8.5 is more complex. Younger water is found at several deeper locations and close to

FIGURE 8.6 Expected flow lines for the cross section shown in Figure 8.5, assuming a uniform hillslope and uniform hydraulic conductivity.

Suggested Citation:"8 Case Histories." National Research Council. 1996. Rock Fractures and Fluid Flow: Contemporary Understanding and Applications. Washington, DC: The National Academies Press. doi: 10.17226/2309.
×

an anticipated discharge area. These findings suggest that hillslope topography and bedrock heterogeneity at this site strongly influence groundwater flow paths.

Groundwater samples were also analyzed for major ions, dissolved gases, and a variety of stable and radioactive isotopes. An interesting finding from these analyses is an apparent correlation between alkalinity and groundwater age. Figure 8.7 shows that alkalinity appears to increase with age. That is, water in the glacial till is younger and has a lower alkalinity, whereas water in the bedrock is older and has a higher alkalinity. The higher alkalinity of bedrock water is almost entirely due to the presence of bicarbonate ions (HCO3-). Carbon isotope analyses suggest that bicarbonate ions are derived from the dissolution of carbonate minerals such as calcite. However, there is no evidence of calcite on fracture surfaces. Instead, samples of granite from outcrops and cores were found to contain small amounts (approximately one weight percent) of calcite in the rock matrix. This finding suggests that calcite dissolution occurs inside the rock matrix, releasing bicarbonate ions. The bicarbonate ions diffuse from the rock matrix into the fractures, causing an increase in groundwater alkalinity. Because older groundwater has been flowing through fractures for a longer time, it should be higher in alkalinity. This relationship should hold until the alkalinity reaches equilibrium with respect to calcite in the groundwater.

FIGURE 8.7 Plot of groundwater age versus alkalinity of groundwater samples from the Mirror Lake site. From Shapiro and Hsieh (1994).

Suggested Citation:"8 Case Histories." National Research Council. 1996. Rock Fractures and Fluid Flow: Contemporary Understanding and Applications. Washington, DC: The National Academies Press. doi: 10.17226/2309.
×

The importance of matrix diffusion in the evolution of groundwater chemistry is supported by laboratory measurements of rock porosity and diffusion coefficients. Porosities of 32 intact granite samples average 1.5 percent. To measure diffusion in the matrix, a granite sample was soaked in a solution of cesium-137. After 101 days, the cesium-137 was found to have penetrated the granite to a depth of approximately 7 mm. This suggests that over tens of years matrix diffusion is an important mechanism for chemical transport between a fracture and the rock matrix. The calculated effective diffusion coefficient for cesium-137 in the granite matrix is approximately 6 × 10-13 m2/s.

To explore the relationships between alkalinity, groundwater age, and groundwater velocity, a simple model was developed to simulate bicarbonate transport. In the model a flow path in the bedrock is represented by a fracture bounded by intact rock. Groundwater enters the fracture with low alkalinity, characteristic of water in the glacial drift. As the water moves along the fracture, its alkalinity increases because of the incoming flux of bicarbonate ions from the rock matrix (Figure 8.8). The relationship between alkalinity and age is controlled by dissolution of calcite and diffusion of bicarbonate ions in the matrix and the velocity of groundwater in the fracture. Assuming the diffusion coefficient is known from laboratory measurements, the groundwater velocity can be estimated by adjusting its value until the modeled bicarbonate concentration and groundwater age match the measured values for groundwater samples (Figure 8.7). Based on this approach, preliminary analysis suggests that groundwater velocities in the bedrock vary between 10-3 and 10-2 m/day.

Discussion

Research at the Mirror Lake site clearly demonstrates the need for an interdisciplinary approach to fractured rock characterization. At the same time, multiple

FIGURE 8.8 Schematic illustration of a simple flow model used to estimate fluid velocities at the Mirror Lake site. The flow path is from left to right in the fracture. Bicarbonate (HCO3/-) ions diffuse into the fracture from the surrounding rock matrix. From Shapiro and Hsieh (1994).

Suggested Citation:"8 Case Histories." National Research Council. 1996. Rock Fractures and Fluid Flow: Contemporary Understanding and Applications. Washington, DC: The National Academies Press. doi: 10.17226/2309.
×

investigation efforts must be coordinated. Detailed studies on the 100-m scale may require the explicit identification and characterization of major (highly transmissive) fractures. Combining fracture detection methods with hydraulic and tracer testing yields a promising approach to accomplishing this objective. Knowledge gained from fracture mapping provides a sound basis for making inferences and for data interpretation. For kilometer-scale investigations, long-term monitoring, groundwater age dating, and geochemical analyses are useful and deserve greater exploitation. The identification of flow paths through a heterogeneous rock environment remains a challenge.

CASE HISTORY II. THE SITE CHARACTERIZATION AND VALIDATION PROJECT: STRIPA MINE, SWEDEN

The Site Characterization and Validation (SCV) Project was performed as a part of the Organization for Economic Cooperation and Development/Nuclear Energy Association's International Stripa Project from 1986 to 1992. The objectives of the project were to test the predictive capabilities of newly developed radar and seismic characterization methods and numerical groundwater models. A basic experiment was designed to predict the distribution of water flow and tracer transport through a volume of granitic rock before and after excavation of a subhorizontal drift (the validation drift) and to compare these predictions with actual field measurements.

A multidisciplinary characterization program was implemented at the SCV site. Because the site was located several hundred meters below the ground surface, all investigations were performed from drifts and boreholes drilled from drifts. The dimensions of the investigated volume were approximately 150 × 150 × 50 m.

The fractures in the drifts adjacent to the SCV site were mapped along scanlines. Maps of the drift walls were made in selected locations. Detailed maps were also made to study the variability in fracturing in fracture zones intersected by several drifts. All boreholes were mapped and oriented by identifying reference fractures from TV logging. The fracture mapping program provided data on fracture orientations, trace lengths, termination modes, and spacing.

Cross-hole and single-hole radar measurements were made to determine the orientation and extent of fracture zones at the site. The directional borehole radar system developed for the project proved particularly useful because it provided data on the orientation of fracture zones based on measurements in a single borehole (see Chapter 4). Radar difference tomography also was used to show how saline tracer injected in a borehole became distributed in the rock mass as it traversed three survey planes.

Seismic techniques were used successfully to determine the orientation and extent of fracture zones. The seismic program included both cross-hole reflection and tomography measurements. The reflection measurements provided the best

Suggested Citation:"8 Case Histories." National Research Council. 1996. Rock Fractures and Fluid Flow: Contemporary Understanding and Applications. Washington, DC: The National Academies Press. doi: 10.17226/2309.
×

data for characterization of the fracture zones. The success of the seismic method was largely due to the application of the Image Space Transform, a novel processing technique developed for the project (Cosma et al., 1991).

To obtain in situ data on the physical properties of the rock in the vicinity of the boreholes, the following logs were run: borehole deviation, sonic velocity, single-point resistance, normal resistivity, caliper, temperature, borehole fluid conductivity, natural gamma radiation, and neutron porosity. The sonic velocity, single-point resistance, and normal resistivity were found to be useful in identifying fractures and fracture zones.

Initially, single-borehole testing was done to provide data on transmissivity and head along the boreholes. Equipment was developed to ensure that reliable information could be collected in the mine environment in reasonable times. The system was built around a multiple-packer probe that allowed rapid testing of permeable features with high spatial resolution. Single-borehole testing was followed by cross-hole testing to define hydraulic properties of the fracture zones on the scale of the site (» 100 m). An important aspect of the cross-hole testing was that it provided a check on the hydraulic properties of the fracture zones identified by using other geophysical techniques. The hydraulic program also included monitoring of head in more than 50 locations across the site. This monitoring provided data on the hydraulic responses to various activities in the mine that could be used to characterize hydraulic connections across the site.

Groundwater samples were taken during hydraulic testing and analyzed for major constituents. The analysis showed that there were three types of groundwater present. These were classified as shallow, mixed, and deep. The groundwater was also found to contain about 3 percent of dissolved gas by volume (at standard temperature and pressure), mainly nitrogen.

An important aspect of groundwater flow through fractures is the effect of stress on fracture transmissivity. Flow through fractures under different stress loads was studied on several samples and in one in situ test. This yielded stress-permeability relationships that were used for modeling studies. Measurements were made, using the overcoring method with a tool called the CSRIO Hollow Inclusion Cell, to determine in situ stresses. At the level of the validation drift, the maximum principal stress was oriented parallel to the drift (i.e., NNW-SSE). It is interesting to note that almost all of the water inflow to the drift was through a single fracture perpendicular to the maximum principal stress.

Characterization of the SCV site was made in several stages. Initial data collection was followed by data interpretation and predictive modeling. Additional boreholes were then drilled to check the predictions based on the initial data set. These new data were then used to refine the conceptual model of the site and groundwater flow predictions. Finally, the predictions were checked by a series of dedicated experiments.

To provide for an adequate description of groundwater flow through the site, the key issue for the characterization work was to identify important flow paths.

Suggested Citation:"8 Case Histories." National Research Council. 1996. Rock Fractures and Fluid Flow: Contemporary Understanding and Applications. Washington, DC: The National Academies Press. doi: 10.17226/2309.
×

In fractured rock environments, fracture zones are normally identified as important permeable hydraulic units; this was the working assumption at the onset of the SCV Project. However, the locations, widths, and extents of fracture zones are commonly defined by expert judgment. This can, in many cases, impose a number of problems, as the opinions of experts may vary, and the facts behind a given opinion may be obscure or poorly documented.

An attempt to circumvent this problem and to arrive at a more objective definition of what constitutes a fracture zone was made during the project (Olsson, 1992). A fracture zone index was defined in order to address the following issues:

  • Is a binary division of the rock mass into ''fracture zones" and "average-fractured rock" appropriate?

  • Is there an objective method of identifying a fracture zone, and can it be used to define the boundaries of a zone?

  • Is the arrived-at procedure for fracture zone identifications appropriate for a hydraulic description of the site?

For characterization of a rock volume deep below the ground surface, it is common to base a binary representation of the rock mass on physical properties measured in the vicinity of the boreholes. Hence, the location and width of "fracture zones" can be defined where they intersect boreholes. The extent and geometry of the zones at larger distances from the boreholes can then be probed by using remote sensing methods.

A subset of the data, including normal resistivity, sonic velocity, hydraulic conductivity, coated (and presumably open) fractures, and single-hole radar reflections, was selected for identification of the fracture zones using principal component analysis. First, logarithms were taken of the normal resistivity, sonic velocity, and hydraulic conductivity data. The data were then normalized by subtracting the mean value and dividing by the standard deviation for each parameter. A matrix of correlation coefficients was formed, and the eigenvectors were found for that matrix. Each eigenvector represents a weighting of the data, and new parameters (principal components) were produced by multiplying an eigenvector by the normalized data values. The parameter associated with the largest eigenvalue should represent the most important characteristic of the rock.

For the SCV site the parameter associated with the largest eigenvalue was expected to represent fracturing of the rock. This parameter is referred to as the fracture zone index (FZI). There is essentially only one rock type at the site. Consequently, all observed anomalies in rock properties are caused by fracturing or faulting.

The usefulness of a binary representation of the rock mass can be determined from the frequency distribution of the FZI. Based on the skewed frequency

Suggested Citation:"8 Case Histories." National Research Council. 1996. Rock Fractures and Fluid Flow: Contemporary Understanding and Applications. Washington, DC: The National Academies Press. doi: 10.17226/2309.
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distribution of the FZI (Figure 8.9), it is justifiable to use a binary description of the rock mass, where average-fractured rock is represented by FZI less than 2 and fracture zones are represented by FZI greater than 2. Using this index, the points in the boreholes that were considered to represent the occurrence of fracture zones could be defined.

The FZI compresses the information from single-hole investigations into a single parameter that describes the most significant properties of the rock (see Figure 8.10). It simplifies interpretation because it allows a single parameter to be used for identification of the anomalous sections in boreholes. Because FZI has been obtained through a quantitative and well-defined procedure, it provides an objective means of classifying the rock into the two classes, averagely fractured rock and fracture zones.

The FZI is also considered to be better for identifying hydraulically significant features than single-hole hydraulic conductivity data alone. The basic reason is that single-hole hydraulic tests yield parameters that are applicable only in a very small volume surrounding the borehole. In the fractured rock at Stripa, hydraulic properties vary by more than an order of magnitude over small distances. Hence, a weighted parameter that incorporates several types of data should be less

FIGURE 8.9 Frequency distribution of FZI (principal component 1). Values for the tail of the distribution (FZI > 2) are designated as "fracture zones," while values less than 2 are designated as "average rock." From Olsson (1992).

Suggested Citation:"8 Case Histories." National Research Council. 1996. Rock Fractures and Fluid Flow: Contemporary Understanding and Applications. Washington, DC: The National Academies Press. doi: 10.17226/2309.
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FIGURE 8.10 Composite log of the FZI and the single-hole logs used to construct it. The letters at top (H1, Hb, I, and B) indicate major zones correlated between the boreholes. From Olsson (1992).

sensitive to small-scale variations in the rock mass and better for defining the hydraulically important features. In the definition of the FZI, hydraulic conductivity is included as just one of several measurements, and the weighting is determined by the data set itself.

Suggested Citation:"8 Case Histories." National Research Council. 1996. Rock Fractures and Fluid Flow: Contemporary Understanding and Applications. Washington, DC: The National Academies Press. doi: 10.17226/2309.
×

Based on this concept of a binary representation of the rock mass, a procedure was defined for constructing a conceptual model of the site. The procedure is based on identification of fracture zone locations in the boreholes using the FZI and finding the extent of the zones through the use of remote sensing techniques (i.e., radar and seismic techniques). The hydrogeological significance of the geometric model thus obtained was then determined by cross-hole hydraulic testing, which also yielded data on the hydraulic properties of the zones. Further checking of the consistency of the conceptual model was made by comparison with geological and geochemical data. This procedure is iterative and produces lists of identified features, as well as lists of inconsistencies and unexplained anomalies. The procedure is outlined graphically in Figure 8.11.

FIGURE 8.11 Outline of procedure used for construction of the conceptual model of the SCV site. From Olsson (1992).

Suggested Citation:"8 Case Histories." National Research Council. 1996. Rock Fractures and Fluid Flow: Contemporary Understanding and Applications. Washington, DC: The National Academies Press. doi: 10.17226/2309.
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The conceptual model for the SCV site was found to be consistent with field and test data. Major hydraulic responses were confined to the identified fracture zones, and there were few anomalies in the data that could not be explained. At the site, 80 to 90 percent of the flow was through these fracture zones, as evidenced both by single-hole and cross-hole hydraulic tests. Flow in the fractured rock was dominated by a small fraction of the identified features. Flow in the fracture zones was concentrated in one or two fractures in the zones, and the transmissivity distribution in these fractures was heterogeneous. The hydraulic transmissivity in the fracture zones varied by one to two orders of magnitude over a distance of a meter. Of the fractures in the averagely fractured rock, only a few were found to be transmissive.

Much effort went toward on numerical modeling of groundwater flow and solute transport at the site. Several different models were used. Most included stochastic representations of the permeable features in the rock mass. The conceptual model described above, which provides a deterministic representation of major flow paths, cannot adequately represent the heterogeneity of flow through a fractured rock mass. To achieve more realistic descriptions of the flow system, discrete fracture models were developed and tested. However, to achieve reasonable agreement between predicted and observed flow distributions, it was necessary to include the fracture zones explicitly in the stochastic fracture models.

The SCV Project demonstrated that fracture zones are the dominant groundwater pathways at Stripa and suggested that this may be a common situation in fractured crystalline rock. This finding is consistent with investigations at many other sites in crystalline rock. Work at this site also demonstrated that fracture zones need to be included explicitly in groundwater flow and transport models in crystalline rock. In the SCV Project, procedures were outlined for a quantitative and objective definition of fracture zones. The project demonstrated the capability of radar and seismic techniques to correctly describe the geometry of these zones. It is also evident that the application of these techniques is a prerequisite for constructing a reliable conceptual model for a site. Cross-hole tests should be used to verify the hydraulic significance of geophysically identified fracture zones and to quantify their hydraulic properties. The refined representation of flow heterogeneity requires stochastic modeling techniques. This project demonstrated that data required for stochastic modeling could be collected with a reasonable effort and that discrete fracture network models provide predictions of flow and transport that are in good agreement with observations.

CASE HISTORY III. HYDROCARBON PRODUCTION FROM FRACTURED SEDIMENTARY ROCKS: MULTIWELL EXPERIMENT SITE

The U.S. Department of Energy developed the Multiwell Experiment (MWX) site in order to perform detailed experiments on all aspects of low-permeability

Suggested Citation:"8 Case Histories." National Research Council. 1996. Rock Fractures and Fluid Flow: Contemporary Understanding and Applications. Washington, DC: The National Academies Press. doi: 10.17226/2309.
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natural gas reservoir evaluation, stimulation, and production (Spencer and Keighin, 1984; Finley and Lorenz, 1987; Lorenz and Finley, 1991). Natural and stimulated fractures are expected to be the primary source of production in these relatively "tight" formations. The MWX site is located in the Piceance Basin of Colorado, about 14 km west-southwest of the town of Rifle. The rocks of interest are primarily sandstones, siltstones, shales, mudstones, and coals of the upper Cretaceous Mesaverde group. At MWX, these strata occur at depths between 1,200 and 2,500 m. The reservoirs in the bottom 250 m of the section consist of marine strandplain sandstones (fossil beach sediments); the overlying rocks are deltaic and fluvial in origin.

The MWX site consists of three closely spaced wells (spacings of 30 to 67 m), from which over 1,200 m of core has been taken; about one-third of the core is oriented (Lorenz, 1990). Testing at the site consisted of detailed in situ stress measurements, single-well drawdown and buildup tests, multiwell interference tests, tracer injections, stimulation experiments, and poststimulation production tests. Detailed core analyses and multiple log runs also were performed. Subsequent to MWX, the Department of Energy conducted a follow-up test, named the Slant-Hole Completion Test (SHCT), with the objective of using directional drilling technology to intercept the natural fractures and enhance production. Several hundred feet of core provided additional valuable information about the natural fractures at this site. Primary information about the natural fractures has been derived from the abundant core at this site (Lorenz et al., 1989; Lorenz and Finley, 1991). Two basic types of fractures have been found at the MWX site: extensional fractures in the sandstone and siltstones and shear-type features in the mudstones and shales. Many of the shear-type fractures in the mudstones appear to be dewatering features or other planes of weakness that have accommodated some shear offset and thus display slickenlines. These fractures do not appear to be important for gas production.

The extensional fractures are part of a regional fracture pattern, with essentially all of the fractures being vertical and oriented about N 70° W. The extensional fractures, some of which are incompletely cemented, are the primary production sites from these tight sands; the matrix rocks have submicrodarcy permeability, and gas flow from the matrix is not economic. The degree of fracturing is highly depth dependent. There are one to two orders of magnitude more fractures present in core at depths of 1,675 to 1,890 m than from depths greater than 1,980 m. Televiewers were run in these wells to identify fractures, but the high mud weights required to control abnormal formation pressures made them useless for fracture identification. Formation microscanners and televiewers with variable-frequency and focused transducers were not available in the early 1980s when these wells were drilled and cased.

Extensive outcrops of correlative strata exist on the east and west sides of the Piceance Basin, and these have provided ancillary information on the fracture

Suggested Citation:"8 Case Histories." National Research Council. 1996. Rock Fractures and Fluid Flow: Contemporary Understanding and Applications. Washington, DC: The National Academies Press. doi: 10.17226/2309.
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systems. Figure 8.12 shows a plan view of fractures found in outcrop sandstone and the projection of those fractures into the subsurface, where they would be intersected by boreholes. Clearly, the small orthogonal fractures, which are not seen in core, are relief fractures. The predominant regional extensional fractures are unidirectional, subparallel, and poorly interconnected. Outcrops have also provided data on fracture spacing, length, and height, although these data are possibly affected by relief. The SHCT directional core, however, provides direct evidence of fracture spacings in the subsurface, yielding two populations of fractures, one widely spaced population (1.2 to 2.1 m) and a second population with a spacing of a few centimeters. Spacing is not related to bed thickness in any obvious way.

Field testing of the productive capacity of the fracture systems was performed in eight different intervals of the section (Lorenz, 1989). In the marine sandstones, single-well drawdown/buildup tests yielded permeabilities of 0.15 md and 400 md in two separate intervals. For comparison, in situ matrix permeabilities in

FIGURE 8.12 Plan view illustration of fractures from a sandstone outcrop at the Multi-well Experiment site. The subsurface view shows real data. From Lorenz and Finley (1991).

Suggested Citation:"8 Case Histories." National Research Council. 1996. Rock Fractures and Fluid Flow: Contemporary Understanding and Applications. Washington, DC: The National Academies Press. doi: 10.17226/2309.
×

these zones were only about 0.2 d. Interference tests showed that horizontal permeability anisotropies were on the order of 100:1 owing to the unidirectional nature of the fracture system. Production tests showed that the natural fracture systems are highly stress sensitive. By decreasing the reservoir pressure below a critical value (typically about 6.9 MPa at this site), the production from the well could be almost totally stopped because the decrease in pressure created higher effective confining stresses that physically closed the fractures.

In the fluvial/deltaic sandstones, tests were conducted in six different lenticular reservoirs. Single-well drawdown/buildup tests yielded system total permeabilities of 12 to 50 d; matrix permeabilities measured in core were 0.1 to 2 d. Figure 8.13 shows a comparison of system permeabilities for various intervals compared to rock matrix permeabilities. Interference tests were conducted in five nonmarine reservoirs, but interference was detected in only one. Tracer injections were conducted in two reservoirs, but only minimal amounts of the tracers were detected in the offset wells, and they were detected in an almost random pattern relative to the pump cycles. The interference patterns suggested permeability anisotropies of 30:1 to 50:1 for most of these reservoirs. Fracture systems in these reservoirs were also stress sensitive, and stimulation experiments showed that they were easily damaged by fracturing fluids. Studies of outcrops of these reservoirs showed that the fractures were limited by lithological variations in the sand bodies, resulting in compartmentalized fracture systems of limited extent, with minimal connections across compartments.

Laboratory experiments on plugs containing fractures were performed for several samples. Mudstone fractures (mostly unmineralized planes of weakness) showed a rapidly decreasing, irreversible loss in conductivity with increasing

FIGURE 8.13 Comparison of system permeabilities to rock matrix permabilities from various intervals in the Multiwell Experiment site. Modified from Lorenz et al. (1989).

Suggested Citation:"8 Case Histories." National Research Council. 1996. Rock Fractures and Fluid Flow: Contemporary Understanding and Applications. Washington, DC: The National Academies Press. doi: 10.17226/2309.
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stress. Conductivities of fractures in sandstone were also sensitive to changes in stress, but conductivity loss was reversible. One sandstone fracture, however, showed no stress sensitivity whatsoever.

In summary, the natural fractures were found to be the gas production sites in tight sandstone reservoirs. The fractures are unidirectional, of limited extent, and stress sensitive. They are also easily damaged by drilling and completion fluids. Correlation of fracture data from core, outcrop, and various well tests was necessary to define the fracture system and its response to drilling, completion, and production activities.

CASE HISTORY IV. INVESTIGATING THE ANATOMY OF A LOW-DIPPING FRACTURE ZONE IN CRYSTALLINE ROCKS: UNDERGROUND RESEARCH LABORATORY, MANITOBA

In-depth studies of a single large-scale fracture zone are very rare in the literature, and there are relatively few such studies where the results show precisely how groundwater flow through individual fractures relates to the geometry and movement of a fracture zone. One of the most complete studies is the investigation of a fracture zone intersected by a shaft constructed at an Atomic Energy of Canada Limited (AECL) research site on the Canadian Shield. Investigations pertaining to the safety and feasibility of the concept of spent nuclear fuel disposal in plutonic rocks are being conducted at this site at AECL's Underground Research Laboratory (URL). The main working levels of the URL are at depths of 240 and 420 m (240 and 420 levels) with shaft stations at 130 and 300 m (Figure 8.14). Access to the 240-m level is provided by a 2.8 × 4.9 m timber-framed shaft and to the 420-m level by a 4.6-m-diameter circular shaft. Bored (1.83-m-diameter) raises between the surface and the 240 level and between the 240 and 420 levels provide ventilation and alternative access. This section discusses the results obtained from the intensive study of a fracture zone intersected by the URL shaft at about 250 m in depth.

The URL is excavated in the Archean granite of the Lac Du Bonnet Batholith, approximately 120 km northeast of Winnipeg, Manitoba, at the western edge of the Canadian Shield (Figure 8.14). The rocks of the batholith crystallized at a depth 10 to 16 km, approximately 2,670 million years ago, near the close of the regional deformation, which affected the surrounding metavolcanics, metasediments, and gneisses (Everitt et al., 1990). Apart from autointrusive dikes and foliations, there are no significant deformational features in the batholith. The existing fracture network was largely created in the early Proterozoic during cooling and crystallization of the batholith roof zone and in response to ambient regional stresses. Portions of this fracture network were open (reactivated) during regional peneplanation, deposition and then removal of phanerozoic sediments, and subsequent glaciation and deglaciation. However, no new fracture systems are believed to have been formed by these processes (Everitt et al., 1990).

Suggested Citation:"8 Case Histories." National Research Council. 1996. Rock Fractures and Fluid Flow: Contemporary Understanding and Applications. Washington, DC: The National Academies Press. doi: 10.17226/2309.
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FIGURE 8.14 Location and layout of the Underground Research Laboratory. The location of the Lac Du Bonnet batholith is shaded on the map. On the right are fracture zones 3, 2.5, and 2.

The URL is located near the southern contact of the batholith with the surrounding gneiss. The distribution of xenoliths and deuteric alteration indicates that the present topographic surface is close to the original roof zone of the batholith. The roof zone is marked by shallow-dipping compositional layering (Everitt et al., 1990). The URL access shafts (Figure 8.15) provide a cross section of the roof zone. The geology and fracture distributions in the vicinity of the URL site were extensively investigated by surface and borehole geophysical techniques (Soonawala, 1983, 1984; Wong et al., 1983; Paillet, 1991). These studies showed that the structural geology and hydrogeology of the portion of the batholith surrounding the URL is dominated by a series of southeastward dipping fracture zones (Davison, 1984). Three major low-dipping fracture zones and associated splays were identified at the URL during surface-based drilling and shaft construction. These fracture zones are parallel the shallow-dipping layering of the batholith roof and are generally confined to xenolithic zones or their margins.

Suggested Citation:"8 Case Histories." National Research Council. 1996. Rock Fractures and Fluid Flow: Contemporary Understanding and Applications. Washington, DC: The National Academies Press. doi: 10.17226/2309.
×

FIGURE 8.15 Fracture zones encountered by the URL shaft and their relationship to large-scale distribution of fractures at the URL site. Adapted from Everitt and Brown (1996).

Suggested Citation:"8 Case Histories." National Research Council. 1996. Rock Fractures and Fluid Flow: Contemporary Understanding and Applications. Washington, DC: The National Academies Press. doi: 10.17226/2309.
×

Fracture Zone 2 (FZ2, the primary fracture zone discussed here; Figure 8.15) is the dominant member of the low-dipping fault group. Fracture Zone 3 (FZ3) is similar but has less displacement, whereas Fracture Zone 2.5 (FZ2.5) is a splay between these two large fracture zones. A fourth fracture zone (FZ1) is not encountered in the excavations and most boreholes and is not described here. Subvertical fractures are ubiquitous above FZ2.5. Between FZ2.5 and FZ2 they are confined to the fault margins, and they are absent below FZ2 (Everitt et al., 1990; Everitt and Brown, 1996). In general, the fracture zones comprise several chloritic slip surfaces, cataclasite horizon(s), and a variety of smaller-scale fractures and associated alterations extending into the hanging wall and, to a lesser extent, the footwall. The cataclasites consist of recrystallized fault rubble cemented by a fine-grained chlorite-carbonate matrix and are cross-cut by the chloritic slip surfaces, minor fractures, and seams of soft clay-goethite gouge. This assemblage is in varying degrees of groundwater-induced decomposition.

FZ2, FZ2.5, and FZ3 differ in the degree of complexity of their internal fracture patterns, and the extent of fracturing alteration into the adjacent rock. Fracture patterns become simpler, and the extent of fracturing and of alteration is more restricted, with increasing depth. FZ2, the deepest fracture zone intersected by the excavations, comprises a relatively simple system of conjugate shear and extension fractures (the cataclasite zone/chloritic fractures and the antithetic hematite-filled fractures, respectively). Displacement appears to have been dipslip only, with the overlying block moving 7.3 m to the northwest.

The fracture patterns for FZ2.5 and FZ3 are dominated by the same general arrangement of the major slip surfaces, but additional low-dipping and subvertical fracture sets are present. Overall, their geometry suggests two conjugate systems, superimposed to give orthorhombic symmetry, as described by Davis (1984). Reverse dip-slip (up to 1-m throws) dominates in these zones, but strike-slip and oblique-slip lineations also are present. The fracture zones divide the rock mass into a number of tabular-to-wedge-shaped blocks. These blocks are cross-cut by one or more sets of subvertical fractures, the pattern and frequency of which vary from one block (or fracture domain) to the next. The factors influencing the pattern of intrablock fracturing include overall distance from the ground surface, proximity to the bounding faults, and local rock type. The subvertical fractures become less frequent, less continuous, and simpler in pattern with increasing depth. They also become increasingly confined to the immediate margins of the fault zones or to lithological heterogeneities such as dikes. The most prominent set of subvertical fractures parallels the strike of the thrust faults. However, fracture sets oblique or perpendicular to this direction are common above FZ3. Variations in the structure of the fracture zones, and in the fracture domains between them, are illustrated by using the model depicted in Figure 8.15. The northeast face of the model is normal to the strike of the fracture zones as seen in the area of the excavations. FZ2 forms an arcuate outcrop pattern along the south and west sides of the model.

Suggested Citation:"8 Case Histories." National Research Council. 1996. Rock Fractures and Fluid Flow: Contemporary Understanding and Applications. Washington, DC: The National Academies Press. doi: 10.17226/2309.
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In the block above FZ2, the present-day maximum principal stress is oriented northeast-southwest, parallel to the dominant fracture set and the strike of FZ2. In the shaded area below FZ2, subvertical fracturing is rare or absent, and the maximum principal stress is oriented northwest-southeast, perpendicular to the strike of the thrust fault. The geometry of the thrust faults suggests they were formed when the regional stress field was oriented such that the plane containing the maximum and intermediate principal stresses was subhorizontal, with the former aligned in the northwest-southeast direction. This stress field is believed to be associated with plate accretion on the margins of the Superior craton during the late Archean/early Proterozoic (Everitt et al., 1990). In the case of FZ2, the simple conjugate system of fractures suggests that strain accommodated by fracturing was largely two dimensional. In the case of FZ2.5 and FZ3, however, the orthorhombic pattern of low-dipping major and minor fractures suggests that brittle strain was three dimensional (Davis, 1984). This difference is seen as a consequence of FZ2.5 and FZ3 being ''piggybacked" on FZ2. As such, strike and oblique slip in FZ2.5 and FZ3 are seen as a natural accommodation to displacement on the underlying and dominant thrust fault (FZ2). The subvertical fracture sets are seen as extensional intrablock fracturing that was initiated by geometric flexing and general expansion of the thrust plates in the late Archean to early Proterozoic. The plane containing the maximum and intermediate principal stresses was still subhorizontal, but the local maximum principal stress axis was reoriented and is now aligned northeast-southwest. Reactivation and extension of some fractures likely occurred during Paleozoic transgression, during subsequent removal of the Paleozoic cover, and during repeated continental glaciations. The decreasing frequency, extent, and complexity of subvertical fracturing with depth from the surface are seen as a consequence of both the stacking of the thrust plates and the distance from the surface. The greatest and most varied "flexing" and fracturing would occur in the uppermost blocks. In a single fracture domain the pattern and frequency of subvertical fracturing reflect the distance from, and configuration of, the underlying thrust fault. Between FZ2 and FZ2.5, for example, the pattern of subvertical fractures varies from unimodal to bimodal (orthogonal) as the wedge of rock between FZ2 and FZ2.5 thins to the south. Similar variations are seen in the complexity and preferred orientations of fracturing above FZ2.5, as the plane of FZ3 curves from northeast to north striking.

Hydrogeological studies including single-hole straddle-packer tests and large-scale multiple-borehole hydraulic pressure interference tests conducted before, during, and after shaft construction revealed complex local and regional-scale patterns of permeability in the fracture zones (e.g., Davison and Kozak, 1988; Everitt et al., 1990). In FZ2, permeabilities range over six orders of magnitude, with high and low permeabilities appearing to form distinctive channels at the site scale (Figure 8.16). The prominent northeast-trending transmissivity channel is believed to coincide with the intersection of this fault with FZ2.5. The other channels apparently result from other factors, some of which include

Suggested Citation:"8 Case Histories." National Research Council. 1996. Rock Fractures and Fluid Flow: Contemporary Understanding and Applications. Washington, DC: The National Academies Press. doi: 10.17226/2309.
×

FIGURE 8.16 Hydraulic conductivity variations in FZ2. Modified from Davison and Kozak (1988).

structural controls and hydrogeochemical phenomena, such as the precipitation of different minerals in fillings owing to the mixing of groundwaters with dissimilar chemistries in the fault. In the area of the 240 level, a well-defined isolated region of high transmissivity and low storage is located in the fault immediately northwest of the shaft (Figure 8.17). This region is surrounded entirely by extremely low permeability conditions and has very limited hydraulic communication, with a much more extensive region of high permeability and high storage to the north and west.

These variations in permeability are accompanied by:

  • Flexures in the fault zone, generalized here by structure contours representing the "middle" of the central cataclasite horizon.

  • "Anomalies" in the rock-type map of the fault (Figure 8.18); the fracture zone is largely confined to a xenolithic horizon (area 1 in Figure 8.18), but to the west and northwest the zone changes in orientation such that it cross-cuts the layering to intersect the neighboring or gneiss granites.

Suggested Citation:"8 Case Histories." National Research Council. 1996. Rock Fractures and Fluid Flow: Contemporary Understanding and Applications. Washington, DC: The National Academies Press. doi: 10.17226/2309.
×

FIGURE 8.17 Hydraulic conductivity variations in FZ2 in the area of 240 Level. Modified from Davison and Kozak (1988).

  • Occurrence of core disking in this area that represents locally high in situ stresses adjacent to the fault zone and by variations in the in situ stress normal to the fault zone (Figure 8.19) (in situ stress data from Martin et al., 1990).

It is concluded that the variations in the character and permeability of FZ2, and the variations in the stress magnitudes, are the direct result of undulations in the fault surface. As shown in Figure 8.20, movement on any undulating surface can be expected to result in dilational gaps, restraining bends, fault-bounded structural wedges (such as that between FZ2 and FZ1.9) and secondary subvertical fractures in the fault-bounded blocks. Variations in relative permeability in the fracture zone are reflected by corresponding variations in the thickness of the alteration halo. This correlation is a useful one because it serves as a qualitative indicator of historic flow variation, which in turn has practical application in the layout of characterization drilling.

The subvertical fracture shown in Figure 8.20 is a wedge-shaped zone of fractures that begins at the base of FZ2.5 and narrows downward until it terminates at the 240 level, about 35 m above FZ2. It parallels the strike of FZ2 and is known to extend 35 m vertically and at least 105 m horizontally. This fracture is interpreted as having formed in response to flexing of the fault block owing

Suggested Citation:"8 Case Histories." National Research Council. 1996. Rock Fractures and Fluid Flow: Contemporary Understanding and Applications. Washington, DC: The National Academies Press. doi: 10.17226/2309.
×

FIGURE 8.18 Map of litho-structural domains crossed by FZ2.

to the change in dip angle of the fault directly beneath it. Such flexing would have led, at least locally, to a reorientation of the principal stresses. The maximum principal stresses below and above the fault zone are perpendicular and parallel, respectively, to the strike of the thrust fault (Everitt et al., 1990). The stress field above the thrust fault is oriented such that the subvertical fractures in this area are open and conductive. Extensive efforts to characterize the geology, hydrogeology, and geomechanical characteristics of this major thrust fault have led to the following conclusions:

  • Complex patterns of permeability exist in FZ2 at the scale of the site and at the scale of the excavations. These patterns include channels of high or low permeability that alternate along the strike of the fault.

  • The variations in permeability appear to correlate with undulations in the plane of the fracture zone, which in turn correlate with dilational gaps (the high-conductivity channels), restraining bends (the areas of core disking and high normal stresses), and fault-bounded structural wedges and secondary fractures in the fault-bounded blocks.

These interpretations are based on the compilation of geological, hydrogeological, and geomechanical data and emphasize the need for an integrated multi-

Suggested Citation:"8 Case Histories." National Research Council. 1996. Rock Fractures and Fluid Flow: Contemporary Understanding and Applications. Washington, DC: The National Academies Press. doi: 10.17226/2309.
×

FIGURE 8.19 Areas of disking and measured normal stresses. From Martin et al. (1990).

disciplinary approach to characterizing permeability variations in a fractured medium.

CASE HISTORY V. FRACTURE STUDIES IN A GEOTHERMAL RESERVOIR: THE GEYSERS GEOTHERMAL FIELD, CALIFORNIA

The Geysers geothermal field in central California (Figure 8.21) is one of the best-known geothermal reservoirs in North America and one where steam production is associated with fractures and faults in otherwise low-permeability metasedimentary and hypabyssal plutonic rocks. This field is one of the most thoroughly studied geothermal reservoirs in the world. However, the characteristics and hydraulic properties of fractures are only partially understood at even this well-studied site for a variety of reasons common to most geothermal study sites: (1) complexity of the local geology; (2) difficulty of geological mapping and geophysical soundings in a deeply weathered and rugged terrain; (3) problems in obtaining well logs and other measurements in hostile borehole environments; and (4) difficulties in modeling two-phase flow in heterogeneous, dual-porosity reservoirs. Despite these difficulties, the results from ongoing studies at The Geysers provide examples of how geological, geochemical, geophysical, and reservoir modeling techniques can be applied to one of the most difficult problems in fracture hydrology.

Suggested Citation:"8 Case Histories." National Research Council. 1996. Rock Fractures and Fluid Flow: Contemporary Understanding and Applications. Washington, DC: The National Academies Press. doi: 10.17226/2309.
×

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

Suggested Citation:"8 Case Histories." National Research Council. 1996. Rock Fractures and Fluid Flow: Contemporary Understanding and Applications. Washington, DC: The National Academies Press. doi: 10.17226/2309.
×

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-

Suggested Citation:"8 Case Histories." National Research Council. 1996. Rock Fractures and Fluid Flow: Contemporary Understanding and Applications. Washington, DC: The National Academies Press. doi: 10.17226/2309.
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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

Suggested Citation:"8 Case Histories." National Research Council. 1996. Rock Fractures and Fluid Flow: Contemporary Understanding and Applications. Washington, DC: The National Academies Press. doi: 10.17226/2309.
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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

Suggested Citation:"8 Case Histories." National Research Council. 1996. Rock Fractures and Fluid Flow: Contemporary Understanding and Applications. Washington, DC: The National Academies Press. doi: 10.17226/2309.
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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.

Suggested Citation:"8 Case Histories." National Research Council. 1996. Rock Fractures and Fluid Flow: Contemporary Understanding and Applications. Washington, DC: The National Academies Press. doi: 10.17226/2309.
×

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Scientific understanding of fluid flow in rock fractures—a process underlying contemporary earth science problems from the search for petroleum to the controversy over nuclear waste storage—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.

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