PERMEABILITY EVOLUTION DURING PROGRESSIVE DEVELOPMENT OF DEFORMATION BANDS IN HIGH POROSITY SILICLASTIC SANDSTONES

A common feature of major faults in high porosity sandstones is the presence of deformation bands in close proximity. Field evidence shows that these develop in a sequential way, accommodating displacement by increasing the number of bands. This pattern has been confirmed in air-dry rock samples under controlled laboratory conditions. In this paper we examine the evolution of permeability associated with this structural evolution. Sandstone cores 100 mm in diameter were deformed to varying ultimate strains and mean stresses in a triaxial cell at a constant axial strain rate and room temperature. The steady-state liquid permeability and volumetric strain of the bulk sample was measured as a function of axial strain. Deformed samples were examined microscopically to examine the style of deformation and quantify the number of deformation bands. The micro-structural observations were used to develop a quantitative model for the permeability evolution of the bulk sample. The permeability of all our samples followed a three-stage strain-dependent evolution. The first stage is a linear decrease prior to sample failure, associated with poro-elastic compaction. Secondly, there was a transient increase associated with dynamic stress drop, interpreted to be a suction pump effect due to rapid dilatant slip rather than real permeability change. This phase was followed by a systematic quasi-static decrease in permeability during slip displacement on the fault, despite overall sample dilatancy. Micro-structural studies confirm that post-failure strain is accommodated by progressive formation of new bands, leading to complex fault zone geometry, where fault zone width and the number of bands increase linearly with slip displacement. These observations have been used to develop a new model for permeability evolution during the sequential formation of deformation bands, validated by the data to a high degree of statistical significance. The model parameters vary with confining pressure, rock type and pore fluid, allowing prediction of fault permeability and sealing potential as a function of burial depth and total slip displacement.