Permeability of illite‐bearing shale: 1. Anisotropy and effects of clay content and loading

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Abstract
Permeability of illite‐rich shale recovered from the Wilcox formation and saturated with 1 M NaCl solution varies from 3 × 10−22 to 3 × 10−19 m2, depending on flow direction relative to bedding, clay content (40–65%), and effective pressure Pe (2–12 MPa). Permeability k is anisotropic at low Pe; measured k values for flow parallel to bedding at Pe = 3 MPa exceed those for flow perpendicular to bedding by a factor of 10, both for low clay content (LC) and high clay content (HC) samples. With increasing Pe, k becomes increasingly isotropic, showing little directional dependence at 10–12 MPa. Permeability depends on clay content; k measured for LC samples exceed those of HC samples by a factor of 5. Permeability decreases irreversibly with the application of Pe, following a cubic law of the form k = k0 [1 − (Pe/P1)m]3, where k0 varies over 3 orders of magnitude, depending on orientation and clay content, m is dependent only on orientation (equal to 0.166 for bedding‐parallel flow and 0.52 for flow across bedding), and P1 (18–27 MPa) appears to be similar for all orientations and clay contents. Anisotropy and reductions in permeability with Pe are attributed to the presence of crack‐like voids parallel to bedding and their closure upon loading, respectively.