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

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 m 2 , depending on flow direction relative to bedding, clay content (40–65%), and effective pressure P e (2–12 MPa). Permeability k is anisotropic at low P e ; measured k values for flow parallel to bedding at P e = 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 P e , 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 P e , following a cubic law of the form k = k 0 [1 − ( P e / P 1 ) m ] 3 , where k 0 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 P 1 (18–27 MPa) appears to be similar for all orientations and clay contents. Anisotropy and reductions in permeability with P e are attributed to the presence of crack‐like voids parallel to bedding and their closure upon loading, respectively.