Permeability of Fault Rocks and Shale Caprocks: Where we Stand

Much published fault rock permeability data was measured using inappropriate laboratory conditions (low confining pressure and distilled water as the permeant). However, the results are reasonably consistent with theoretical predictions from simple clay-sand mixing models. This may suggest that the effects low confining pressures (increases permeability) and distilled water (reduces permeability) to a certain extent cancel each other out. The results from the mixing models suggest that the permeability of the sand end-member may have an important control on the permeability of fault rocks. This model helps explain the large scatter on Vclay-permeability cross plots but is not considered by existing fault seal algorithms. Techniques used in the shale gas industry for estimating permeability are not appropriate for use in measuring the permeability of shale caprocks. The lack of a rapid decline in permeability as a function of porosity of measurements made on low permeability shale core plugs as well as inconsistent relationships between pore-throat size and permeability, suggests perhaps that techniques using core plugs overestimate the permeability of low porosity (<15%) shales by several orders of magnitude. It is likely that this reflects the inability of current techniques to accurately measure permeabilities significantly below 1nD.