Brittle versus ductile deformation as the main control on the transport properties of low-porosity anhydrite rocks

An experimental approach has been taken to study the prefailure fluid flow properties of borehole samples of low-porosity anhydrites and the evolution of permeability (k) during deformation leading to brittle and ductile failure. The permeability measured under hydrostatic stress conditions prior to loading at room temperature is generally low (k = 10(-21) - 10(-19) m(2)) and displays an anisotropy and pressure sensitivity controlled by grain size and fabric orientation (foliation). Triaxial loading test results show that the brittle-ductile transition occurs for effective pressure P-e < 20 MPa and is almost independent of fabric orientation and grain size. All samples, whether deforming in a brittle (localized deformation) or ductile (distributed deformation) mode, show dilatancy after an initial phase of compaction. During loading, the k starts to increase prior to the phase of sample dilation and before the yield stress is attained. The k rise is characterized by an upward concave trend, prior to localized deformation (brittle failure), and by a downward concave trend, during distributed deformation (ductile failure). The k increase prior to brittle failure is about 1 order of magnitude higher than during ductile failure. We interpret the different shape of the k curve as due to the observed different degrees of fracture connectivity (widespread development of intragranular and intergranular fractures) reached during brittle (low) and ductile (high) deformation, respectively. Our experimental results imply that for low-porosity rocks the mode of failure, controlled by P-e, has an overwhelming effect on the evolution of permeability, compared to other factors such as grain size and fabric