Permeability of Wilcox shale and its effective pressure law
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abstract
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 m2 to 3 10-21 m2 as effective pressure Pe 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 Pe for Wilcox shale is given by k = k0 [1 - (Pe/P1)m]3, where P1 = 19.3 (1.6) MPa and m = 0.159 (0.007). The value of k0 for Wilcox shale is of the order of 10-17 m2 and may vary among samples by as much as 70%. Effective pressure is given in terms of the external confining pressure Pc and internal pore pressure Pp by Pe = Pc - Pp, where = 0.99 (0.06). While our measurements yield = 1 for shale with a clay content of 45%, others have reported values for clay-bearing sandstones that rise from 0.75 to 7.1 with increasing clay content (from 0 to 20%). The trends between and clay content revealed by these comparisons imply that the value of depends upon the relative distributions of compliant clay minerals and other stiffer minerals. These values of also suggest that effective pressures within interbedded sandstones and shales may differ, even at the same equilibrium Pc and Pp conditions. Copyright 2001 by the American Geophysical Union.
