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 x 10(-22) to 3 x 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 Pe are attributed to the presence of crack-like voids parallel to bedding and their closure upon loading, respectively.