Analysis of stress-dependent permeability in nonorthogonal flow and deformation fields

The stress-dependent permeability of porous-fractured media is examined where principal stresses do not coincide with the principal permeabilities. This condition is the norm, and may arise when either flow is controlled at the local level due to the presence of inclined bedding partings or oblique fractures, or as a result of the evolving loading environment. Permeability response is controlled by shear and normal stiffnesses of fractures, frictional dilation coefficients, skeletal and grain modulii, initial permeabilities and stress state. For parameters representative of intact and fractured rocks, hydrostatic loading modes are shown to have the greatest effect in the pre-failure regime. Shear dilation effects are small, primarily controlled by the selected magnitudes of shear stiffnesses and dilation coefficients. The resulting stress-permeability relationships, which cover both fractured and intact media, are examined in a numerical study of fluid flow injected across the diameter of a cylindrical core with inclined fabric, subjected to various loading configurations. This is used to produce relationships that allow one to reduce flow test data in non-standard specimen geometries, where effective stress changes are simultaneously applied. These results confirm the significant impact of inclination of the rock fabric with respect to both flow and loading geometry on the evolving permeability field.