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 − χPp, 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 Pc and Pp conditions.
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have