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$Drilling-induced tensile wall-fractures - Stanford University$

9. Acknowledgements My sincere thanks go to John Childs for his able and entertaining assistance in the field, and to David Pollard for many thought-provoking discussions. Thanks also to the Valley of Fire State Park staff for their indulgence. This work was funded by the U.S. Department of Energy, Office of Basic Energy Science, Geosciences Research Program under grant DE-FG03-94ER14462, awarded to Pollard and Atilla Aydin as principal investigators. Additional support was provided by Stanford Rock Fracture Project. 38
1. Abstract Chapter 2 Anticrack-inclusion model for compaction bands in sandstone Detailed observations of compaction bands exposed in the Aztec sandstone of southeastern Nevada indicate that these thin, tabular, bounded features of localized porosity loss initiated at pervasive grain-scale flaws, which collapsed in response to compressive tectonic loading. From many of these Griffith-type flaws, an apparently self- sustaining progression of collapse propagated outward to form bands of compacted grains a few centimeters thick and tens of meters in planar extent. These compaction bands can be idealized as highly eccentric ellipsoidal bodies that have accommodated uniform uniaxial plastic strain parallel to their short dimension within a surrounding elastic material. They thus can be represented mechanically as contractile Eshelby inclusions, which generate near-tip compressive stress concentrations consistent with self-sustaining, in-plane propagation. The combination of extreme aspect ratio (>10 -4 ) and significant uniaxial plastic strain (~10%) also justifies an approximation of the bands as anticracks— sharp boundaries across which a continuous distribution of closing-mode displacement discontinuity has been accommodated. This anticrack interpretation of compaction bands is analogous to that of pressure solution surfaces, except that porosity loss takes the place of material dissolution. We find that displacement discontinuity boundary element method modeling of compaction bands as anticracks within a two-dimensional linear elastic continuum can accurately represent the perturbed external stress fields they induce. 2. Introduction Compaction bands represent one kinematic end member of the suite of structures known collectively as deformation bands, which also includes shear and dilation bands (Antonellini et al., 1994; Aydin, 1978; Du Bernard, 2002; Mollema and Antonellini, 1996). Due to the internal loss of porosity and permeability they accommodate, deformation bands can significantly affect bulk sandstone permeability when present as pervasive arrays, with important implications for the management of both groundwater and hydrocarbon resources (Sternlof et al., 2004). 39
Page 1 and 2: STRUCTURAL GEOLOGY, PROPAGATION MEC
Page 4 and 5: Abstract Low-porosity, low-permeabi
Page 6 and 7: delivered with fortitude, humor, su
Page 8 and 9: Chapter 3—Energy-release model of
Page 10 and 11: List of Illustrations Figure A. Cov
Page 12 and 13: Figure A. Cover photo that accompan
Page 14 and 15: likely present in subsurface sandst
Page 16 and 17: hooking-tip interactions—using th
Page 18 and 19: and sandstone, my co-authors—Moha
Page 20 and 21: the hard data from which accurate p
Page 22 and 23: Although the Aztec sandstone experi
Page 24 and 25: Waterpocket Fault 36 o 26‘ N 0 1
Page 26 and 27: Cenozoic Mesozoic Paleozoic Quatern
Page 28 and 29: 3.3. Deformation The Aztec also has
Page 30 and 31: There also are relatively high-angl
Page 32 and 33: (e) (f) (c) 500µm compaction band
Page 34 and 35: dihedral angle of 80° or more, and
Page 36 and 37: 5. Compaction band orientations Ori
Page 38 and 39: n = 20 M n = 20 P n = 22 B n = 20 R
Page 40 and 41: were encountered, giving the dihedr
Page 42 and 43: Eichhubl et al., 2004) did not lend
Page 44 and 45: P (a) (c) S P S Figure 1.10. Stereo
Page 46 and 47: possible, and use these to better c
Page 48 and 49: 1.5(ρgz) (b) WEST Willow Tank Uppe
Page 52 and 53: The term compaction band (CB) was c
Page 54 and 55: Pollard, 2002). The particular util
Page 56 and 57: Hue-based image analysis using MATL
Page 58 and 59: a direct genetic relationship (Hill
Page 60 and 61: Compaction band fin Depositional be
Page 62 and 63: suggests—that to first approximat
Page 64 and 65: Porosity Porosity 0.3 0.25 0.2 0.15
Page 66 and 67: pore-clogging clay—due presumably
Page 68 and 69: 500 µm Figure 2.9. Electron backsc
Page 70 and 71: (a) (b) σ 3 σ 1 x 3 x 1 x 1 (c) u
Page 72 and 73: 6. Elastic properties Despite a lon
Page 74 and 75: Comparison of the two approaches es
Page 76 and 77: term in (6a) and (6c) begins to dom
Page 78 and 79: MPa MPa 80 70 60 50 40 30 20 10 0 0
Page 80 and 81: 2002) and use a BEM approach (Crouc
Page 82 and 83: MPa 10 4 10 3 10 2 10 1 10 −5 10
Page 84 and 85: concentration of quartz plasticity
Page 86 and 87: conducted on well-cemented sandston
Page 88 and 89: ~ 62 m compaction band trend 500µm
Page 90 and 91: Sternlof et al. (2005) have suggest
Page 92 and 93: 2005). It consists of a long (infin
Page 94 and 95: ⎡ ∆ ⎤ ⎧ p 1 ∆ ⎛ M ⎞
Page 96 and 97: which point the inelastic strain ha
Page 98 and 99: they can exert significant effects
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interact is inversely proportional
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(a) (b) (c) (d) Figure 4.3. Typical
Page 104 and 105:
Viewed individually, CB traces tend
Page 106 and 107:
(a) (b) (c) (d) (e) (f) Figure 4.7.
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1973; Mardon, 1988; Peck et al., 19
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undamaged host rock. That CBs canno
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0.5 0.4 0.3 0.2 0.1 0.5 0.4 0.3 0.2
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Normalized stress magnitude 3 2.5 2
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helps to explain why the oblique ap
Page 118 and 119:
and ts = G·ds + H·dn (2) where tn
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plastic compaction, as suggested by
Page 122 and 123:
To determine the sensitivity of pro
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segments will self-correct to provi
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6.3. Approaching tip interactions A
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0.2 0.15 0.1 0.05 0 −0.05 −0.1
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0.2 0.15 0.1 0.05 0 −0.05 −0.1
Page 132 and 133:
the hooking patterns commonly obser
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(a) (b) (c) σ1 compaction band max
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σ 3 σ 2 σ 1 Figure 4.24. Schemat
Page 138 and 139:
NV UT CA AZ Park Road Map Detail Pa
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compaction band Figure 5.2. Typical
Page 142 and 143:
3. Computational method The methodo
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compaction band A 5 mm A‘ compact
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4. Application to the Aztec sandsto
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Permeability (mD) 10 4 10 3 10 2 10
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B‘ A‘ A c f m c m c f c f c f c
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equivalent of the Aztec sandstone,
Page 154 and 155:
effective permeability represents a
Page 156 and 157:
NV UT CA AZ Park Road Map Detail Pa
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Commonly from ~1 mm to ~1.5 cm in t
Page 160 and 161:
Although systematic arrays of DBs p
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to 2 m, with both sets in a cross-h
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y identical blocks all subject to t
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contiguous areas. This type of char
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(0,b) n 3 (0,0) f 2 n 2 n 1 (a,0) (
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6.1. Parallel Effective permeabilit
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By contrast, band-parallel effectiv
Page 174 and 175:
Relative Effective Permeability 1.0
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(a) (b) (c) 0 meters 3 = 10-2 kb /k
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Though grossly systematic, anastomo
Page 180 and 181:
168
Page 182 and 183:
100 meters 5 meters NV UT CA AZ Par
Page 184 and 185:
compaction band trend 500µm Figure
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Cover Outcrop Band N 100 meters 20
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(CBs) and the matrix rock in all si
Page 190 and 191:
The finite volume discretization fo
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isotropic) can lead to numerical di
Page 194 and 195:
5. Simulations Three sets of 2-D si
Page 196 and 197:
Normalized ∆P ∆P ratio (n/p) 4.
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P P P P I I P P Case 1 P P Case 2 I
Page 200 and 201:
Case 1 Case 1 Case 1 (a) (b) (c) In
Page 202 and 203:
5.2.2. Discussion The elliptical pa
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5.3.1. Results With the regional gr
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(a) (b) Leak 200 meters Regional gr
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anticipated sustainable pumping rat
Page 210 and 211:
Finally, there is the issue of fore
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200
Page 214 and 215:
Bakke, S., and Øren, P. E., 1997,
Page 216 and 217:
Carpenter, D. G., and Carpenter, J.
Page 218 and 219:
Eichhubl, P., Taylor, W. L., Pollar
Page 220 and 221:
Karimi-Fard, M., Durlofsky, L. J.,
Page 222 and 223:
-, 1987, Fracture from a straight c
Page 224 and 225:
Shipton, Z. K., and Cowie, P. A., 2
Page 226:
Wong, T. F., David, C., and Zhu, W.
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