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8 Case Histories
Pages 455-498

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From page 455...
... 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.
From page 456...
... 456 Cal .~ V)
From page 457...
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From page 459...
... 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~.
From page 460...
... 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)
From page 461...
... The low connectivity at the Mirror Lake site suggests that fluid moves through highly tortuous paths in the bedrock. In a 100 X 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.
From page 462...
... well field at the Mirror Lake site. See Figure 8.1 for location of the FSE well field in the Mirror Lake area.
From page 463...
... 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 1O-s 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.
From page 464...
... 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)
From page 465...
... This value is close to the average hydraulic conductivity of 3 X 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.
From page 466...
... FIGURE 8.5 Groundwater ages (in years) determined from CFC-12 concentrations at the Mirror Lake study site.
From page 467...
... . BEDROCK WELL o DRIFT PIEZOMETER 0 10 20 30 40 50 GROU ND-WATER AG E, IN YEARS FIGURE 8.7 Plot of groundwater age versus alkalinity of groundwater samples from the Mirror Lake site.
From page 468...
... Based on this approach, preliminary analysis suggests that groundwater velocities in the bedrock vary between 10-3 and 10-2 mJday. Discussion Research at the Mirror Lake site clearly demonstrates the need for an interdisciplinary approach to fractured rock characterization.
From page 469...
... 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.
From page 470...
... 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.
From page 471...
... 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?
From page 472...
... 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.
From page 473...
... 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.
From page 474...
... . 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.
From page 475...
... 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.
From page 476...
... 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.
From page 477...
... 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)
From page 478...
... o -0 O WELL TEST o -OCORE (RESTORED STATE) o 100 1000 MICROOARCIES FIGURE 8.13 Comparison of system permeabilities to rock matrix permabilities from various intervals in the Multiwell Experiment site.
From page 479...
... 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.
From page 480...
... These studies showed that the structural geology and hydrogeology of the portion of the batholith surrounding the URL is dominated by a series of southeastwarddipping 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.
From page 481...
... _~ . :Ftacture Zoses "d Dom~ns 'tA" Oracle Domain A "3" Fracture Zone 3 "B" ~Eraculre DOmBiDi B "2~5" Fracture Zone 25 "C" Fracture Domain C "2w Fracture Zone 2 '~D'' F~tme Domain D 1.9" Fracture Zonel.9 FIGURE 8.15 Fracture zones encountered by the URL shaft and their relationship to large-scale distribution of fractures at the URL site.
From page 482...
... 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)
From page 483...
... 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 regionalscale patterns of permeability in the fracture zones (e.g., Davison and Kozak, 1988; Everitt et al., 1990~.
From page 484...
... 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.
From page 485...
... 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.
From page 486...
... 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~.
From page 487...
... CASE HISTORY V FRACTURE STUDIES IN A GEOTHERMAL RESERVOIR: THE GEYSERS GEOTHERMAL FIELD, CALIFORNIA The Geysers geothermal field in central California (Figure 8.21)
From page 488...
... by\ . 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.
From page 489...
... and schematic cross sections (middle and bottom) of the geothermal reservoir at The Geysers geothermal field.
From page 490...
... 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)
From page 491...
... 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.
From page 492...
... 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.
From page 493...
... 1981. A reservoir assessment of The Geysers geothermal field.
From page 494...
... USGS Water Resources Investigations Report 92-4016, U.S. Geological Survey, Reston, Va.
From page 495...
... 1994. Hydraulic characteristics of fractured bedrock underlying the FSE well field at the Mirror Lake site, Grafton County, New Hampshire.
From page 496...
... USGS Water Resources Investigations Report 91-4034, U.S. Geological Survey, Reston, Va.
From page 497...
... USGS Water Resources Investigation Report 87-4154, U.S. Geological Survey, Reston, Va., 36 pp.
From page 498...
... USGS Water Resources Investigations Report 84-4266, U.S. Geological Survey, Reston, Va., 61 pp.


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