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

Figure A. Cover photo that accompanied publication of Chapter 6 in AAPG Bulletin (Sternlof et al., 2004). This view shows steeply east-dipping compaction bands in the bleached middle Aztec sandstone, just above the red, iron oxide-stained lower Aztec. Dark Paleozoic rocks of the over-riding Cretaceous Muddy Mountain Thrust loom in the background. These compaction bands are approximately 1 cm thick and 100 m long. xii
Introduction Some brittle structures in rock act to enhance and channel fluid flow, others to impede and compartmentalize it. Commonly, structures of both types interact with and overprint each other to form complex fabrics that exert a dominating influence on flow in subsurface aquifers and reservoirs, even those endowed with high intrinsic permeability such as porous sandstone. Understanding the distribution, geometry and flow properties of these structural fabrics is therefore vital to optimal resource production and management. Unfortunately, data on the attributes of these fabrics—whether from 3-D seismic surveys, borehole geophysical logs, pump tests and production data, or core analysis—are generally limited and may be virtually nonexistent for small-scale structures. One approach to addressing this problem is to study exhumed analog aquifers and reservoirs, where structures and fabrics can easily be observed (at least in 2-D), sampled and analyzed. The ultimate goal of such studies is to develop insights and tools for forecasting the presence and influence of structures in the subsurface based on limited hard data. That has been the theme of an ongoing, decade-long research project undertaken by David Pollard and Atilla Aydin with funding from the U.S. Department of Energy, Office of Basic Energy Sciences—Structural Heterogeneities and Paleo Fluid Flow in an Analog Sandstone Reservoir. My thesis work fits into this larger research initiative, just as my topic—deformation bands and, more specifically, compaction bands—fits into the framework of structures that did demonstrably impact paleo fluid flow in the Aztec sandstone and are now breathtakingly exposed in the aptly named Valley of Fire of southeastern Nevada. My effort follows three earlier Stanford dissertations that also took shape from the colorful outcrops of the Valley of Fire: Fluid flow and chemical alteration in fractured sandstone (W.L. Taylor, 1999); Structure and hydraulics of brittle faults in sandstone (R.D. Myers, 1999); and Structural evolution, petrophysics and large-scale permeability of faults in sandstone, Valley of Fire, Nevada (E.A. Flodin, 2003). This dissertation focuses on the oldest structural fabric components present in the Aztec sandstone—compaction bands—and endeavors to comprehend what they are, why they formed, and how they influence bulk permeability and fluid flow. In essence, I take a broad look at a specific structural phenomenon, one which physical logic dictates is 1
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 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
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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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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
Page 226:
Wong, T. F., David, C., and Zhu, W.
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