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

1.5(ρgz) (b) WEST Willow Tank Upper Aztec (zone of interest) Lower Aztec Triassic strata (a) σ1 ~ 270/38 traction-free ground surface detachment 0 m 200 m 800 m 3,600 m 36 km 9 km 5 km 0 km ρgz σ3 ~ 024/27 σ2 ~ 138/39 z EAST 2(ρgz) Figure 1.11. Paleostress orientations and tectonic interpretation for the Aztec sandstone during compaction band formation. (a) Equal area, lower hemisphere stereographic projection of the approximate inferred principal paleostress orientations (σ1 > σ2 > σ3, compression positive). (b) Simplified 2-D linear elastic, finite element model (using Abaqus®) of one tectonic scenario that could produce the inferred orientation of the maximum compressive paleostress (σ1). In this formulation, we consider a homogeneous, isotropic, linear elastic block representing an east-west cross section through the Mesozoic strata of the study area, which is orthogonal to the mean orientation of the dominant CB set. This block, the hanging wall of a horizontal thrust, is subjected to gravitational forces and unbalanced lateral forces that push it quasi-statically westward along a rigid detachment, which resists the motion by generating opposing shear tractions. The resulting trajectories of σ1 induced a few km from the eastern edge of the block and within the CB-genic zone of the upper Aztec (tick marks) roughly coincide with the inferred orientation (z is depth, g is gravity, ρ is density, each grid block is 200 m by 1,000 m). 36
7. Concluding observations The data and analyses presented above indicate that CBs formed as anticracks in the Aztec while it comprised a shallowly buried, weakly lithified sandstone aquifer subjected to regional compression associated with the earliest phases of Sevier tectonism in the area. Interpreted as such, the presence of two orthogonal, coeval and coexisting sets of CBs can be used to constrain the 3-D paleostress state in which they formed, with the dominant set orthogonal to the maximum compressive stress (σ1) and the secondary set orthogonal to intermediate compressive stress (σ2). Furthermore, the mutually cross- cutting presence of both sets in the same outcrop suggests that σ1 ≈ σ2. That the inferred paleostress orientations depart radically from classical Andersonian expectations can reasonably be interpreted as evidence of as yet unrecognized regional structures, specifically a relatively shallow detachment along which a coherent structural block containing the Aztec now exposed in the Valley of Fire moved relatively westward. Based on an average spacing between CBs of the dominant, north-south trending set of 0.7 m, and an average band thickness of 1 cm representing 10% uniaxial compaction (~1.1 mm), the Aztec sandstone experienced less than 0.2% total shortening as the result of CB formation. Even assuming an average band spacing of only 10 cm hardly bumps the shortening past 1%. If homogeneously distributed, dropping overall porosity from 25% to 24%, such a mild deformation event would be of no consequence to fluid flow. The highly localized, yet pervasively interconnected nature of the CB fabric formed, however, constitutes a considerable network of low-permeability baffles to fluid flow (CB permeability ~0.1% of the inherent sandstone permeability) that has been shown capable of exerting profound effects at scales of practical interest (Sternlof et al., 2004; Sternlof et al., 2006). The analysis presented here of compaction bands as indicators of paleostress in and tectonic structure around the exhumed analog aquifer/reservoir that is the Aztec sandstone can also be applied in reverse. That is, given independent knowledge of the tectonic stress and material history of a sandstone similar to the Aztec, reasonable forecasts of the possible presence and gross geometry of CBs in the subsurface can be made. Such insights could prove highly valuable in the efficient development aquifers and reservoirs. 37
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 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
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
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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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