Permeability of Wilcox shale and its effective pressure law

The permeability of illite‐rich shale from the Wilcox formation has been measured as a function of effective pressure for bedding‐parallel flow of 1 M NaCl pore fluid. Permeability k decreases from ∼300×10 −21 m 2 to 3×10 −21 m 2 as effective pressure P e is increased from 3 to 12 MPa; these values confirm that shales form effective barriers to fluid transport in sedimentary strata over extended geologic times. The variation of k with P e for Wilcox shale is given by k = k 0 1 − ( P e / P 1 ) m 3 , where P 1 = 19.3 (±1.6) MPa and m = 0.159 (±0.007). The value of k 0 for Wilcox shale is of the order of 10 −17 m 2 and may vary among samples by as much as 70%. Effective pressure is given in terms of the external confining pressure P c and internal pore pressure P p by P e = P c − χ P p , where χ = 0.99 (±0.06). While our measurements yield χ = ∼1 for shale with a clay content of ∼45%, others have reported χ values for clay‐bearing sandstones that rise from ∼0.75 to 7.1 with increasing clay content (from 0 to 20%). The trends between χ and clay content revealed by these comparisons imply that the value of χ depends upon the relative distributions of compliant clay minerals and other stiffer minerals. These values of χ also suggest that effective pressures within interbedded sandstones and shales may differ, even at the same equilibrium P c and P p conditions.