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

compaction band trend 500µm Figure 7.2. Typical outcrop pattern of sub-parallel CB fins in the Aztec sandstone (top). Backscatter electron images from just outside (bottom left) and inside (bottom right) a CB—white is hematite, light gray is feldspar, medium gray is quartz, dark gray is clay, black is porosity. Porosity drops from >20% in the sandstone to 1,000 mD (sandstone) to
out in positive relief as distinct fins (Sternlof et al., 2005; Mollema and Antonellini, 1996) (Figure 7.2). Millimeters to centimeters thick and tens of meters in planar extent, CBs represent a kinematic end member of the suite of structures known collectively as deformation bands, which also includes shear and dilation bands (Aydin, 1978; Antonellini et al., 1994; Mollema and Antonellini, 1996; Du Bernard et al., 2002). The petrophysical changes accommodating mechanical compaction inside CBs, in conjunction with subsequent preferential cementation, can reduce porosity within the bands to a few percent. Decreased porosity, and corresponding reductions in pore throat diameter and connectivity, can in turn reduce saturated permeability within the bands by 1 to 4 orders of magnitude relative to that of the surrounding host rock (Pittman, 1981; Freeman, 1990; Antonellini and Aydin, 1994; Crawford, 1998; Gibson, 1998; Taylor and Pollard, 2000; Sigda and Wilson, 2003). Permeability anisotropy due to depositional bedding in the Aztec and Navajo sandstones is generally less than a factor of two (Freeman, 1990; Antonellini and Aydin, 1994; Flodin et al., 2005), minimal when compared to the reduced permeability represented by the CBs. There is no data to suggest that significant anisotropy related to bedding persists inside the bands, and no reason to expect that the consequences of any such remnant anisotropy would be significant. In the analyses to follow, we therefore make the simplifying assumption of homogeneous, isotropic permeability for both the sandstone and the CBs. 3. Mapping The fins of CBs cropping out in the Aztec sandstone are plainly visible in low-altitude aerial photographs. A high-resolution (3,000 dpi) digital scan made from one such standard (23 cm x 23 cm) aerial photo color negative reveals an extensive exposure of nearly continuous Aztec outcrops at a scale of 1:2,400 (Figure 7.1). This scan captured a level of detail down to the grain of the negative, producing a digital image with a pixel dimension relative to outcrop of about 2 cm. Loading this image into Adobe Illustrator® and enlarging it, we were able to recognize and trace individual cm-thick CBs (or in many cases, closely space clusters of CBs) for tens to hundreds of meters (Figure 7.1, inset photos). This photo mapping effort was guided by our familiarity with the appearance of CBs in outcrop and was checked extensively in the field. 173
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STRUCTURAL GEOLOGY, PROPAGATION MEC
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Abstract Low-porosity, low-permeabi
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delivered with fortitude, humor, su
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Chapter 3—Energy-release model of
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List of Illustrations Figure A. Cov
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Figure A. Cover photo that accompan
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likely present in subsurface sandst
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hooking-tip interactions—using th
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and sandstone, my co-authors—Moha
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the hard data from which accurate p
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Although the Aztec sandstone experi
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Waterpocket Fault 36 o 26‘ N 0 1
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Cenozoic Mesozoic Paleozoic Quatern
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3.3. Deformation The Aztec also has
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There also are relatively high-angl
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(e) (f) (c) 500µm compaction band
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dihedral angle of 80° or more, and
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5. Compaction band orientations Ori
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n = 20 M n = 20 P n = 22 B n = 20 R
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were encountered, giving the dihedr
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Eichhubl et al., 2004) did not lend
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P (a) (c) S P S Figure 1.10. Stereo
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possible, and use these to better c
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1.5(ρgz) (b) WEST Willow Tank Uppe
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9. Acknowledgements My sincere than
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The term compaction band (CB) was c
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Pollard, 2002). The particular util
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Hue-based image analysis using MATL
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a direct genetic relationship (Hill
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Compaction band fin Depositional be
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suggests—that to first approximat
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Porosity Porosity 0.3 0.25 0.2 0.15
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pore-clogging clay—due presumably
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500 µm Figure 2.9. Electron backsc
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(a) (b) σ 3 σ 1 x 3 x 1 x 1 (c) u
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6. Elastic properties Despite a lon
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Comparison of the two approaches es
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term in (6a) and (6c) begins to dom
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MPa MPa 80 70 60 50 40 30 20 10 0 0
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2002) and use a BEM approach (Crouc
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MPa 10 4 10 3 10 2 10 1 10 −5 10
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concentration of quartz plasticity
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conducted on well-cemented sandston
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~ 62 m compaction band trend 500µm
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Sternlof et al. (2005) have suggest
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2005). It consists of a long (infin
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⎡ ∆ ⎤ ⎧ p 1 ∆ ⎛ M ⎞
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which point the inelastic strain ha
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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
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Viewed individually, CB traces tend
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(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
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and ts = G·ds + H·dn (2) where tn
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plastic compaction, as suggested by
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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
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the hooking patterns commonly obser
Page 134 and 135: (a) (b) (c) σ1 compaction band max
Page 136 and 137: σ 3 σ 2 σ 1 Figure 4.24. Schemat
Page 138 and 139: NV UT CA AZ Park Road Map Detail Pa
Page 140 and 141: compaction band Figure 5.2. Typical
Page 142 and 143: 3. Computational method The methodo
Page 144 and 145: compaction band A 5 mm A‘ compact
Page 146 and 147: 4. Application to the Aztec sandsto
Page 148 and 149: Permeability (mD) 10 4 10 3 10 2 10
Page 150 and 151: B‘ A‘ A c f m c m c f c f c f c
Page 152 and 153: 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
Page 158 and 159: Commonly from ~1 mm to ~1.5 cm in t
Page 160 and 161: Although systematic arrays of DBs p
Page 162 and 163: to 2 m, with both sets in a cross-h
Page 164 and 165: y identical blocks all subject to t
Page 166 and 167: contiguous areas. This type of char
Page 168 and 169: (0,b) n 3 (0,0) f 2 n 2 n 1 (a,0) (
Page 170 and 171: 6.1. Parallel Effective permeabilit
Page 172 and 173: By contrast, band-parallel effectiv
Page 174 and 175: Relative Effective Permeability 1.0
Page 176 and 177: (a) (b) (c) 0 meters 3 = 10-2 kb /k
Page 178 and 179: Though grossly systematic, anastomo
Page 180 and 181: 168
Page 182 and 183: 100 meters 5 meters NV UT CA AZ Par
Page 186 and 187: Cover Outcrop Band N 100 meters 20
Page 188 and 189: (CBs) and the matrix rock in all si
Page 190 and 191: The finite volume discretization fo
Page 192 and 193: 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.
Page 198 and 199: 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
Page 204 and 205: 5.3.1. Results With the regional gr
Page 206 and 207: (a) (b) Leak 200 meters Regional gr
Page 208 and 209: anticipated sustainable pumping rat
Page 210 and 211: Finally, there is the issue of fore
Page 212 and 213: 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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