Rock Fractures and Fluid Flow Contemporary Understanding and Applications (1996) / Chapter Skim
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2 Physical Characteristics of Fractures and Fracture Patterns
2 Physical Characteristics of Fractures and Fracture Patterns
Pages 29-102
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From page 29... ...
This includes stress concentration around material flaws and other physical discontinuities as well as broad variations in the stress field. It is this heterogeneity of stress that controls the initiation and propagation of individual fractures and the localization and clustering of the fracture systems.
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From page 30... ...
. Pressure solution surfaces, also referred to as stylolites, are known as anticracks in which the sense of the displacement discontinuity is opposite that of dilating fractures or mode I fractures (Fletcher and Pollard, 1981~.
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Joints, faults, and pressure solution surfaces filled by minerals are known as veins, seams, and filled pull-aparts. The mineral fillings have important consequences for fluid flow because they may alter the flow properties of the fractured rock.
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This mismatch may produce open channels that are potential pathways for fluid flow. Joints and faults are fundamentally different in terms of their associated stress fields (Pollard and Segall, 1987~.
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This analysis showed that the fractures formed as the granite was emplaced or shortly thereafter, suggesting that they formed as the granite cooled. The Sierran fractures are nearly parallel to one another, quite unlike the polygonal fracture patterns that develop as a lava flow
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Mechanical analyses can help relate fracture patterns to the causative geological processes when direct information on the timing of fracturing is absent. For example, Delaney et al.
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From page 35... ...
used these relationships to infer the stresses responsible for natural fracture patterns in folded rock layers. In some cases, regional distributions of joint and fault patterns mimic the trends of mountain belts, suggesting a causative relationship between the formation of fracture systems and a particular tectonic event responsible for the mountain belt.
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Furthermore, tensile stresses can be produced even under hydrostatically compressive loading systems (Figure 2.7a, b) if the internal fluid pressure, P
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6 FIGURE 2.6 (a) A circular hole of radius a in a plate subjected to uniaxial compression (~0)
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6 FIGURE 2.7 (a) A circular hole of a radius of a in a plate subjected to hydrostatic compression (~0)
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(2.1) Here, Km is the stress intensity factor, that is, a measure of the intensity of the stress concentration, which depends on the applied load and fracture geometry, and is known as Kr, Kit, and Kill, for modes I, II, and III, respectively.
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Regional stress fields are generally insufficient to initiate fractures. As a result, heterogeneous stress concentrations are generally necessary for fracture propagation.
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From page 41... ...
For example, if the geometry and dimensions of a lava bed or regional strike-slip fault are known, the geometry of accompanying fractures can be inferred. Geological and mechanical analyses together can identify stress concentrations in the earth's crust, describe the nature of the stress concentrations, and yield useful descriptions of the fracture distributions.
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From page 42... ...
It should be noted, however, that the task of predicting subsurface fractures is more difficult in complexly deformed areas and, obviously, when information about the regional structures is limited. FRACTURE PROPAGATION AND INTERNAL STRUCTURES The stress field around the fracture tip controls fracture propagation (Figure 2.8)
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(c) A series of drawings showing the development of small faults in granitic rocks failed in compression tests.
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There is a great need to understand the micromechanical processes of faulting in a host of common geological media (e.g., unconsolidated sediments, shale, sandstone, limestone, granite) , the localization of deformation into zones, and possible effects of deformation on fluid flow.
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For more 1 Composite Joint Surface Silt stone | l Shale FIGURE 2.13 A composite joint surface in siltstone multilayers separated by thin shale laminae. From Helgeson and Aydin (1991~.
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in the granitic rocks of the Sierra Nevada, California. The joint trace-length frequency distribution is of the form
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FIGURE 2.15 (a) Map of a joint set in granitic rocks of the Sierra Nevada, California.
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Fracture sets can be described by the areal and vertical extent, the spacing or density of individual fractures, and the orientation distribution. This information can be used to assess the physical connections between individual fractures, which influences fluid flow.
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(b) Joint spacing in different rock types.
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0 50 100 150 (fl Time (min) FIGURE 2.17 Variations of joint spacing as a function of (a)
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~ ',- , - FAWT Bout FAULT / An/ _ ~ ~ ~ ~ _G ~ ~ ''~ | SOLVAY" DIVA '/ '~ ~ /// - CO-ED ~ ///~, ,,,,,,/,/,,,, - /, FIGURE 2.18 Schematic illustration of the development of strike-slip faults and strikeslip fault zones in granitic rocks of the Sierra Nevada, California. From Martel (1990~.
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Large crustal-scale stopovers, however, may provide rather heterogeneous fluid flow pathways because of uneven internal fracture distribution. There has been significant progress in understanding the mechanics of fault interactions in the past 15 years.
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PHYSICAL CHARACTERISTICS OF FRACTURES AND FRACTURE PATTERNS 53 GLASS J crack , 25 mm GRANITIC ROCK vein 25 cm T 250 m _ dike MANCOS SHALE OCEANIC CRUST . _~~ / T ridge 2.51km FIGURE 2.19 Overlapping geometry of en-echelon dilating fractures at various scales.
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From page 54... ...
. It has been shown by field observation and field experiments that interaction zones can, in fact, contribute significantly to fluid flow (Sibson, 1981; Aydin et al., 1990; Martel and Peterson, 1991; see Chapter 89.
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The calcite fillings at the extensional steps and pressure solution cleavages in the contractional quadrants are consistent with the stress field shown in b. Friction on the faults is assumed to be 0.6.
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From page 56... ...
are actually fracture zones. These zones are important for fluid flow because of their great extent, connectivity, and conductivity and will be described in detail in this section.
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PHYSICAL CHARACTERISTICS OF FRACTURES AND FRACTURE PATTERNS 57 it, ~ FIGURE 2.22 Two joint zones in the Entrada Sandstone, Arches National Park, Utah. Photographs by A
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Because the contribution of fracture zones to the fluid flow in fractured rocks may be substantial. A better understanding of this subject is needed.
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Variations in the overall shape of a fault zone contribute to markedly heterogeneous stress fields along a fault zone (e.g., Segall and Pollard, 1980; Bilham and King, 1989) , and this heterogeneity leads to a nonuniform fracture distribution.
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Geological Survey, 1984~. Because macroscopic planes of weakness are inherent in most rock masses, they strongly influence how fault zones develop.
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The clay cake faults develop as a series of fractures form and then coalesce (Riedel, 1929; Wilcox et al., 1973~; the resulting pattern resembles the fracture pattern observed along many natural faults (Skempton, 1966; Tchalenko, 1970; Wilcox et al., 1973~. These findings collectively suggest that the grain-scale structure and Ethology of rocks influences fault zone formation.
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From page 62... ...
For example, large-scale field tests (e.g., Black et al., 1990) demonstrate that the average hydraulic conductivity of fault zones in granitic rocks is three or four orders of magnitude higher than that in unfaulted granite and can approach the hydraulic conductivity of fine-grained sandstone.
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From page 63... ...
.. : ~=x.' '~ stage 1 Same 2 So 3 FIGURE 2.28 Development of fault zones in granitic rocks.
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(b) A natural fault zone mapped in the field.
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(i-j) Polygonal fracture patterns in mud (from Birkeland and Larsen, 1989)
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From page 66... ...
The size of hexagons and the spacing of joints are determined primarily by the rate of temperature change (DeGraff and Aydin, 1993~. Because the top and bottom of a flow cool simultaneously but at different rates, joint spacing is different in the fractures formed by propagation from the top surface downward than in those formed by propagation from the bottom surface upward.
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From page 67... ...
presumably related to the spatial and temporal inhomogeneities in the stress field during the development of fracture systems. In Figure 2.32b, which shows a fracture network in the Arches National Park, intersecting relationships suggest a progression of fracturing from the bottom right side to the top left side of the photograph.
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From page 68... ...
C FIGURE 2.32 (a) Joint domains on the flanks of a salt anticline in the Arches National Park, Utah.
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(b) A more complicated fracture evolution in which new joints initiate at the lower lateral fronts of the previous joints.
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One of the crucial tasks remaining for better understanding of multiplejoint patterns is the possible causes for changes in the orientation of joint sets to produce distinctive joint domains. A survey of fracture patterns in various sandstone formations of the Colorado Plateau and experimental simulation of the formation and geometric characteristics of fracture domains indicate that fracture domain boundaries provide the best fracture connectivity.
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From page 71... ...
Normal faults are commonly segmented like other fracture types and have complicated interaction features (Schwartz and Coppersmith, 1984; Wu and Bruhn, 1992; Antonellini,
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(b) A normal fault pattern produced by the distinct element model simulating the boundary conditions in the sand box experiment above.
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. It has been estimated that the velocity of channelized fluid flow along thrust faults in accretionary environments is three to four orders of magnitude faster than the flow through the matrix sediments.
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(1988) suggested that fractures in the fault zones dilate episodically under high fluid pressure, producing an interconnected network responsible for fluid transport.
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(f) Map showing normal faults situated in a thrust sheet bounded by two thrust faults.
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(a) Extensional step with connecting normal faults.
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The hierarchical similarities provide a rationale for extrapolating fracture pattern data from one scale, such as the scale of a borehole or outcrop, to another, such as that of a hydrocarbon reservoir. Fracture data generally are scaled up: details of fracture patterns are best resolved in small rock masses, yet the behavior of large rock masses is usually of greater interest.
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From page 79... ...
Ionic odd ter',oen - ' roe Godly deformed ~ ~ To '\ °1 101> ~ FIGURE 2.41 Geological map of the Greater San Francisco Bay Area (compiled by Aydin and Page, 1984) showing major strike-slip faults and their associated structures Fracture patterns typically have been described by using fractals in terms of the fractal dimension D, which quantifies the degree to which curves or surfaces fill space over a range of scales.
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Not only can fracture trace lengths distributed according to a power law (e.g., Figure 2.15) be described with a constant fractal dimension (Gillespie et al., 1993)
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From page 81... ...
Natural fractures have inhomogeneous distributions of densities and apertures, even along a single fracture. Fractures with the same fluid flow properties along their entire lengths, or large blocks of unfractured matrix between broadly spaced faults, probably do not exist (Nelson, 1987~.
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From page 82... ...
In (B) each individual data set could be described well by a single common fractal dimension, yet the fractal dimension of the entire data collection is not constant.
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From page 83... ...
It is also clear that a better conceptual understanding of the geology, physics, and hydraulics of major fracture types is essential for a meaningful representation of fracture networks. The spatial and temporal variations of fracture geometry in terms of the state of stress, fracture type, rock type, and scale pose many problems that need to be addressed in order to build useful models for simulating fluid flow in fractured media (see Chapter 6 for further discussion of this issue)
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From page 84... ...
Panlally Cemented tractor" FIGURE 2.A1 Mechanisms promoting natural fracture permeability at depth.
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........................... FIGURE 2.A2 Diagenetically modified natural fractures.
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. Fracture Characteristics Recognizing the existence of mineral cements and their influence on natural fracture permeability is not as easy as it might initially appear.
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From page 87... ...
and Laubach (1988~. Influence of Diagenesis on Fracture Behavior Diagenetic modifications of natural fractures influence the way a fracture system will respond to changing fluid flow conditions.
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From page 88... ...
One approach to making inferences about the nature of fluid flow in rocks that cannot be seen is to be able to gain information about the fracture pattern from understanding how the fractures formed. Studies of fracture formation in areas where the fracture pattern is exposed can help in this regard.
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From page 89... ...
~n:35 \ Fracture trace 0 40ft ~T O lOm 40 80 120 Fraetu~re length tt Covered or not mapped FIGURE 2.B 1 Fractures formed during basin subsidence, illustrating evenly distributed, poorly interconnected fractures. Map of fracture traces on bedding plane, Little Coal Creek (Hill)
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From page 90... ...
Exclusion of short fractures from this map introduces a significant bias; many small fractures both within and between swarms cannot be shown at this scale. Local measurements and large-scale fracture trace maps show that, locally, small fractures (centimeters to millimeters long)
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From page 91... ...
~ Bedding-plane trace do 30 E -20 . -10 _ o 0- 20- 40- 60- 80- 10~ 120- 140- 160- 180 Fracture strike FIGURE 2.B2 East-striking fractures formed during thrusting, illustrating fracture swarms.
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From page 92... ...
. Fault zone fluids originally derived from the country rock are maintained at high pore pressure by compaction of fault zone materials during shearing.
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Geological Society of America Bulletin, 95(11)
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Geological Society of America Bulletin, 66:241-256.
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From page 95... ...
Geological Society of America Bulletin, 99:605-617. DeGraff, J
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From page 96... ...
1980. Heat flow and energetics of the San Andreas fault zone.
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1994. Description and interpretation of natural fracture patterns in sandstones of the Frontier Formation along the Hogsback, Southwestern Wyoming.
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1989. Inferring paleostress from natural fracture patterns: a new method.
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From page 99... ...
Fault behavior and characteristic earthquakes; examples from the Wasatch and San Andreas fault zones. Journal of Geophysical Research, 89:5681-5698.
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From page 100... ...
Geological Society of America Bulletin, 70:379-382. Stearns, W., and M
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1964. Microjointing in basement, middle Rocky Mountains of Montana and Wyoming: Geological Society of America Bulletin, 75:287-306.
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