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

Finally, there is the issue of forecasting where volumetrically extensive compaction and deformation band arrays are most likely to occur. This is an active area of research that to date has emphasized discrete deformation band style faults in sandstone (e.g. Aydin and Johnson, 1983, Shipton and Cowie, 2001; Davatzes et al., 2005) over the distributed style of tectonic fabric formed by CBs in the Aztec (Sternlof et al., 2005). Much of the research into both types of bands has focused on their occurrence in Mesozoic æolian sandstones of the southwestern U.S., although deformation bands are increasingly being identified in a wide variety of porous, granular geologic materials ranging from nonwelded ignimbrites (Wilson et al., 2003) to unlithified Holocence beach sands (Cashman and Cashman, 2000). Compaction bands, however, have yet to be positively identified outside of the Navajo (Mollema and Antonellini, 1996) and Aztec (Sternlof et al., 2005) sandstones. As of this writing therefore, the most that can be said is that CB arrays such as exposed in the Aztec might reasonably be suspected in any high- porosity sandstone that has been subjected to a differential tectonic stress prior to substantial lithification. 7. Conclusions In this study, we performed flow modeling using a state-of-the-art discrete-feature, finite-volume simulator applied to a CB data set of unprecedented detail and scale, which was derived from a real geological system that serves as an exhumed analog for active sandstone aquifers and reservoirs. Our results demonstrate the potential for CB arrays, such as abundantly exposed in the Aztec sandstone at the Valley of Fire, Nevada, to exert profound effects on subsurface flow at scales relevant to aquifer and reservoir modeling and production. Specifically, we found that: • The CBs as mapped cause an average three-fold increase in the pressure drop required to maintain a given flow rate between two wells, as compared to CB-free sandstone; • There is a mild directional aspect to this pressure effect—an anisotropy—with ∆P across the dominant band trend exceeding that along the trend by an average of 25%; and 198
• The tendency for transported fluids to channel preferentially along the dominant band trend can overpower the effects of regional pressure gradients, both natural (contaminant plume scenarios) and induced (reservoir production scenarios). Compaction bands, and other types of deformation bands with similar flow characteristics, almost certainly exist unrecognized in many sandstone aquifers and reservoirs. Reliable borehole geophysical techniques for detecting these types of structures in the subsurface and assessing their gross geometry, density, connectivity and petrophysical characteristics are needed. Nonetheless, our modeling suggests that simple steps based on even limited data can mitigate the impact of CB arrays. While not an exhaustive study, the character and magnitude of the flow and transport effects modeled here demonstrate that accounting for such band fabrics could prove essential to the optimal management of sandstone aquifers and reservoirs in which they occur. 8. Acknowledgements Our thanks go to John Childs for his acute and cheerful assistance in the field. Primary funding for this work was provided by the U.S. Department of Energy, Office of Basic Energy Sciences under grant DE-FG03-94ER14462 awarded to David Pollard and Atilla Aydin at Stanford University. Additional support was provided by the Stanford Reservoir Simulation Consortium (SUPRI-B) and the Stanford Rock Fracture Project (RFP). 199
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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
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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
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NV UT CA AZ Park Road Map Detail Pa
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compaction band Figure 5.2. Typical
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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,
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effective permeability represents a
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NV UT CA AZ Park Road Map Detail Pa
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Commonly from ~1 mm to ~1.5 cm in t
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Page 170 and 171: 6.1. Parallel Effective permeabilit
Page 172 and 173: By contrast, band-parallel effectiv
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Page 176 and 177: (a) (b) (c) 0 meters 3 = 10-2 kb /k
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Page 182 and 183: 100 meters 5 meters NV UT CA AZ Par
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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
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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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