Permeability of Wilcox shale and its effective pressure law

The permeability of illite-rich shale from the Wilcox formation has been me asured as a function of effective pressure for bedding-parallel flow of 1 M NaCl pore fluid. Permeability k decreases from similar to 300 X 10(-21) m( 2) to 3 X 10(-21) m(2) as effective pressure PC is increased from 3 to 12 M Pa; these values confirm that shales form effective barriers to fluid trans port in sedimentary strata over extended geologic times. The variation of k with PC 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 Wi lcox 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 confinin g pressure P-c and internal pore pressure P-p by P-e = P-c - chiP(p) where chi = 0.99 (+/-0.06). While our measurements yield chi = similar to1 for sh ale with a clay content of similar to 45%, others have reported chi values for clay-bearing sandstones that rise from similar to0.75 to 7.1 with incre asing clay content (from 0 to 20%). The trends between chi and clay content revealed by these comparisons imply that the value of chi depends upon the relative distributions of compliant clay minerals and other stiffer minera ls. These values of chi also suggest that effective pressures within interb edded sandstones and shales may differ, even at the same equilibrium P-c an d P-p conditions.