[1] Detailed observations of compaction bands exposed in the Aztec Sandstone of southeastern Nevada indicate that these thin, tabular, bounded features of localized porosity loss initiated at pervasive grain-scale flaws, which collapsed in response to compressive tectonic loading. From many of these Griffith-type flaws, an apparently self-sustaining progression of collapse propagated outward to form bands of compacted grains a few centimeters thick and tens of meters in planar extent. These compaction bands can be idealized as highly eccentric ellipsoidal bodies that have accommodated uniform uniaxial plastic strain parallel to their short dimension within a surrounding elastic material. They thus can be represented mechanically as contractile Eshelby inclusions, which generate near-tip compressive stress concentrations consistent with self-sustaining, in-plane propagation. The combination of extreme aspect ratio ($10 À4 ) and significant uniaxial plastic strain ($10%) also justifies an approximation of the bands as anticracks: sharp boundaries across which a continuous distribution of closing mode displacement discontinuity has been accommodated. This anticrack interpretation of compaction bands is analogous to that of pressure solution surfaces, except that porosity loss takes the place of material dissolution. We find that displacement discontinuity boundary element modeling of compaction bands as anticracks within a two-dimensional linear elastic continuum can accurately represent the perturbed external stress fields they induce.

Localized compaction in porous rocks is a recently recognized phenomenon that has been shown to reduce permeability dramatically. Consequently, the phenomenon is relevant to a variety of technologies involving fluid injection or withdrawal. This article summarizes current understanding of localized compaction and impediments to further progress. The article is based on discussions at a small workshop on localized compaction sponsored by the Office of Science, U. S. Department of Energy.

[1] Thin, tabular, low-porosity, low-permeability compaction bands form pervasive, subparallel, anastomosing arrays that extend over square kilometers of exposure in the Aztec Sandstone of southeastern Nevada, an exhumed analog for active aquifers and reservoirs. In order to examine the potential flow and transport effects of these band arrays at scales relevant to production and management, we performed a suite of simulations using an innovative discrete-feature modeling technique to capture the exact pattern of compaction bands mapped over some 150,000 m 2 of contiguous outcrop. Significant impacts related to the presence of the bands and their dominant trend are apparent: the average pressure drop required to drive flow between wells exceeds that for band-free sandstone by a factor of three and is 10% to 50% higher across the bands versus along them; reservoir production efficiency varies up to 56% for a typical five-spot well array, depending on its orientation relative to the dominant band trend; and contaminant transport away from a point source within an aquifer tends to channel along the bands, regardless of the regional gradient direction. We conclude that accounting for the flow effects of similar compaction-band arrays would prove essential for optimal management of those sandstone aquifers and reservoirs in which they occur.

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[1] The elastic strain energy released per unit advance of a compaction band in an infinite layer of thickness h is used to identify and assess quantities relevant to propagation of isolated compaction bands observed in outcrop. If the elastic moduli of the band and the surrounding host material are similar and the band is much thinner than the layer, the energy released is simply s + xh p where s + is the compressive stress far ahead of the band edge, xh is the thickness of the band and p is the uniaxial inelastic compactive strain in the band. Using representative values inferred from field data yields an energy release rate of 40 kJ/m 2 , which is roughly comparable with compaction energies inferred from axisymmetric compression tests on notched sandstone samples. This suggests that a critical value of the energy release rate may govern propagation, although the particular value is likely to depend on the rock type and details of the compaction process.