structural geology, propagation mechanics and - Stanford School of ...

1. Abstract
Chapter 6
Permeability effects of deformation band arrays in sandstone
We apply established numerical modeling techniques to the calculation of effective
permeability for porous sandstone containing systematic arrays of low permeability
deformation bands (DBs). The numerical method, derived from homogenization theory,
can produce effective permeability results for characteristic DB geometries, and provides
a quantitative framework with which to expand these effects to the reservoir-simulation
scale from spatially limited data. The method is demonstrated in two dimensions for each
of three characteristic DB patterns—parallel, cross-hatch and anastomosing—exposed in
the Aztec sandstone at the Valley of Fire, Nevada, which provides an excellent exhumed
analog for active sandstone reservoirs. Our analysis indicates that these extensive,
systematic DB patterns can reduce effective permeability by as much as two orders of
magnitude at scales relevant to reservoir production, while inducing similar magnitudes
of permeability anisotropy. The potential permeability impacts of DB arrays on reservoir
production thus rival those routinely attributed to depositional heterogeneity—e.g.
bedding and shale streaks. We suggest therefore that properly accounting for the
aggregate effects of DBs where they occur could be as important to optimal reservoir
simulation and production management in sandstone as accounting for sedimentary
architecture.
2. Introduction
Sedimentary and structural heterogeneities both large (e.g. shale lenses, seismically
detectable faults) and small (e.g. bedding, fractures) have been shown to profoundly
influence effective permeability in sandstone. Small features, however, cannot explicitly
be modeled in standard flow simulations using coarse (10 to 100 m) grid blocks. For
porous, granular materials like sandstone, the simplifying assumption of isotropic
background permeability is commonly assumed, with perhaps an anisotropy related to
bedding. To more realistically account for the aggregate influence of small-scale features,
an effective permeability must be calculated and assigned to each simulation block. This
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