Fracture permeability evolution during shearing: The role of shear-induced dilation

The evolution of fracture permeability during shearing is crucial in defining the impact of hydraulic stimulation in geothermal and hydrocarbon reservoirs and in describing earthquake mechanisms in induced seismicity. We report laboratory measurement of fracture permeability evolution associated with transient increases in shear velocity during frictional shearing. Core plugs of Westerly granite are saw-cut to form a smooth axial fracture that is subsequently roughened to simulate a natural fracture with controlled surface topography. Fluid-flow experiments are conducted in a triaxial pressure vessel to independently apply confining pressure, pore pressure, and shear velocity, and to concurrently monitor the evolution of fracture permeability, k, during experiments. We conduct velocity stepping experiments, where shear-induced dilation is expected to occur in response to shearing velocity increase [e.g., Samuelson et al., 2009], under a range of conditions: background shearing rate of ~1 mm/s with steps to ~5, ~10, and ~15 micron/sec at confining stresses of 1, 3, and 6 MPa. We find that, regardless of confining stress, the trend of permeability-versus-displacement clearly varies in response to an increase in shearing velocity. In most cases, the k values following a velocity increase are larger than would be expected by extrapolating the permeability-displacement trend at lower velocity. We interpret this discrepancy in k as being caused by shear-induced dilation. In other words, shear-induced dilation may have a role in incrementing fracture permeability. This concept is probed by linking permeability evolution to concepts of dilation and wear recovered from rate-state characterizations of frictional behavior.