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

Abstract Low-porosity, low-permeability compaction bands (CBs) form a pervasive planar fabric throughout the upper 600 meters of the 1,400-meter-thick æolian Aztec sandstone of southeastern Nevada, an exhumed analog for aquifers and reservoirs of similar lithology. The existence of CBs, only recently recognized, raises issues of practical significance. How do they impact local permeability? Can they influence fluid flow and transport at production scales? When, how and why do they form? Can their presence, geometry and effects be forecast in the subsurface from sparse data? To address these questions, we applied a range of techniques, including: outcrop and thin-section observations and measurements; structural and tectonic analysis; scanning electron microscopy; detailed field mapping and aerial photograph interpretation; mechanical analysis and modeling; computational permeability estimation and fluid-flow simulation. Although derived entirely from the Aztec sandstone, our results offer a more general phenomenological understanding of CBs and their effects. Up to centimeters thick and hundreds of meters long, CBs comprise thin discs of uniaxial porosity-loss compaction that form anastomosing arrays generally symmetric to the maximum compression. Idealized as anticracks, CB propagation and pattern development can be modeled using a linear-elastic boundary element method. Mechanical interaction between CBs is inversely proportional to differential stress, such that connectivity within a fabric, and its directional impact on fluid flow, can be estimated from the stress state in which it formed, and vice versa. Computational estimation corroborates measured permeability reductions of 10 -3 inside CBs. At outcrop scales, this yields bulk permeability reductions up to 10 -2 and anisotropy up to 10 1 . Flow simulations performed on a detailed, 40-acre CB map reveal significant practical impacts: pumping and injection pressures increase three-fold; reservoir production efficiency depends on proper alignment of the well array to the CB fabric, and contaminant transport channels along the CBs regardless of the regional pressure gradient. We interpret CBs in the Aztec as resulting from the tectonic compression of a high- porosity, low-cohesion sandstone at modest confining pressures (
Acknowledgements Dissertations can be ephemeral beasts—mythic, elusive and notoriously difficult to capture on paper. Coaxing a good dissertation into publication has to be one of the more arduous, solitary journeys a person can make while so entirely swaddled in support. This volume represents the culmination of one such saga, and there are many people to thank for seeing me through the often dense fog to that distant, shining shore of “doneness.” First, I acknowledge the U.S. Department of Energy, Office of Basic Energy Sciences for its generous funding of my research and tenure at Stanford. Additional support was provided by the Stanford Rock Fracture Project. The Valley of Fire State Park in Nevada is a beautiful and convenient place to work—thanks to all the helpful, friendly staff there. My sincere thanks to the faculty who served on my committees and provided valuable guidance: Mark Zoback, Steve Graham, Amos Nur and, particularly, Lou Durlofsky and Atilla Aydin. Lou is great to work with—enthusiastic, incisive and human—and Atilla imbues every effort with dedication and principle. Thanks also to the administrative staff for their logistical support, especially Elaine Anderson—font of knowledge, candy and conversation. My gratitude also goes to Manika Prasad and Bob Jones for their tutelage in the rock physics lab and on the SEM, respectively. This truly has been a multidisciplinary effort, and I gratefully acknowledge all my co- authors and collaborators: John Rudnicki (Northwestern University), Lou Durlofsky, Mohammad Karimi-Fard, Jeffrey Chapin, Youngseuk Keehm, Tapan Mukerji, Gaurav Chopra and Ovunc Mutlu. Thanks also to David Lockner (U.S.G.S.), Bill Olsson and David Holcomb (Sandia National Labs) and Bezalel Haimson (Univ. of Wisconsin) for opening their minds and their laboratories to my experimental aspirations. No group deserves more credit than my fellow students and researchers, past and present. In particular, I acknowledge Eric Flodin, Phil Resor, Nick Davatzes, Jordan Muller, Juan Mauricio Florez, Frantz Maerten, Peter Eichhubl, Xavier Du Bernard, Fabrizio Agosta, Ian Mynatt, Ole Kaven, Ashley Griffith, Ghislain de Joussineau and Ramil Ahmadov. Special thanks to Brita Graham and Tricia Fiore for being “awesome” office mates, and to everyone for their humor, insights and precious computer skills. My good friend from Yale daze, John Childs, deserves an enormous thank you for donating his time and assistance in the field on three occasions over three years—all v
Page 1 and 2: STRUCTURAL GEOLOGY, PROPAGATION MEC
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 50 and 51: 9. Acknowledgements My sincere than
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
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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
Page 68 and 69:
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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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
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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
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168
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100 meters 5 meters NV UT CA AZ Par
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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
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The finite volume discretization fo
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isotropic) can lead to numerical di
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5. Simulations Three sets of 2-D si
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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
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Case 1 Case 1 Case 1 (a) (b) (c) In
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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
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Finally, there is the issue of fore
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200
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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
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Karimi-Fard, M., Durlofsky, L. J.,
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-, 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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