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

pore-clogging clay—due presumably to groundwater filtering by the compacted band coupled with mechanically enhanced feldspar degradation—at least partially explains the greater resistance to weathering exhibited by CBs, as well as their strong impact on fluid flow. That clay remains the primary cement in the weakly indurated Aztec today further suggests that cementation was light to nonexistent during CB formation. Six sets of backscatter images were collected at locations ranging from the middle to the tip of CB-A. Each set comprises images taken from inside and immediately outside the band within the same depositional layer(s). Porosity-reduction volume loss related to CB formation at these locations ranged from 9.4% to 11.7% (Figure 2.8). These results suggest that mechanical compaction throughout the band is uniform at about 10%, regardless of the local variations in sandstone porosity through which it propagated. Cathodoluminescence imaging indicates that redistribution of quartz via pressure solution is volumetrically insignificant and has not appreciably contributed to the spatial distribution of porosity now observed. This is not surprising given the shallow burial history of the study area (< 2 km), which, for any reasonable estimate of geothermal gradient, would have made quartz pressure solution an extremely slow to nonexistent process. Cracking of quartz grains at point contacts, due both to overburden and tectonic loading, is pervasive throughout the thin sections. Wholesale micro-fracture- accommodated plastic deformation of quartz grains, however, comprises the dominant micro-structural characteristic of the band, and is the obvious mechanism by which granular rearrangement and porosity loss compaction was accommodated (Figure 2.9). Roughly half of the quartz grains within the band exhibit plasticity, creating the hint of a mild shape fabric of deformed grains elongated parallel to the band trace (i.e. orthogonal to the inferred direction of maximum compression). Far more striking is the relative absence of granular disaggregation, despite often intense micro-fracturing and distortion. Taken together, these observations corroborate the field interpretation of the band as representing nearly pure uniaxial compaction with little or no shear, and suggest a stable, quasi-static process of plastic collapse. Isolated examples of grain plasticity and collapse can also be found outside the band (Figure 2.10). We interpret these as incipient CBs, representing an early stage of 54
0.30 0.25 0.20 0.15 0.10 Porosity 0.35 0.05 0.00 0.1 0.3 0.6 1.8 4.2 8.7 Distance from tip (m) Figure 2.8. Clustered bar chart showing the distribution of porosity inside (middle bar) and immediately outside (left bar) the compaction band with distance from the tip (clay included as porosity. The difference between (right bar) gives the relative volume strain attributable to mechanical compaction at each location. This compaction strain is remarkably uniform throughout the band at about 10%. Figure 2.10. Electron backscatter image of localized quartz-grain plasticity and collapse (interlocked grains at center of image) that is not part of a well-developed compaction band. Similar grain-scale features appear scattered throughout the sandstone and can be interpreted as incipient compaction bands. Black is porosity, medium gray is quartz, white is feldspar. 55
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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
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
Page 62 and 63: suggests—that to first approximat
Page 64 and 65: Porosity Porosity 0.3 0.25 0.2 0.15
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
Page 100 and 101: interact is inversely proportional
Page 102 and 103: (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.
Page 108 and 109: 1973; Mardon, 1988; Peck et al., 19
Page 110 and 111: undamaged host rock. That CBs canno
Page 112 and 113: 0.5 0.4 0.3 0.2 0.1 0.5 0.4 0.3 0.2
Page 114 and 115: 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,
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Carpenter, D. G., and Carpenter, J.
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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
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Shipton, Z. K., and Cowie, P. A., 2
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Wong, T. F., David, C., and Zhu, W.
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