Shear induced permeability evolution is a crucial mechanism in understanding changes in the subsurface hydraulic system driven by natural seismic events and in the stimulation for subsurface energy recovery. We investigate permeability response during aseismic-through-seismic shear deformation on saw-cut fractures of varying low-order roughness. We report shear-instability-permeability direct shear experiments on Westerly granite in a triaxial pressure cell that concurrently and continuously measures friction and permeability during multiple sequential slide and hold events. We observe two clearly different responses in permeability evolution representing end-of-spectrum behaviors. These are (i) shear induced permeability reduction which is dominant in the initial stage of shear slip to a shear offset of several millimeters and (ii) gradual permeability enhancement with further shear offset. When the sample is static (experimental hold period), permeability continuously decreases and follows a power law decay with time. We observe that, with a given (~1mm) shear displacement, the normalized permeability enhancement Δ q/qinitial is greater after a longer hold period. This observation implies that the permeability of natural pre-existing faults that are locked over long durations can be significantly enhanced by shear deformations. The result suggests that permeability can be engineered in reservoirs by reactivated shear deformation and also suggests that the well-documented seismicity-associated permeability increase in natural hydraulic systems (e.g., Elkhoury et. al., 2006) may also be contributed to by systematic shear deformations.
Understanding the mechanism of fracture permeability is an important scientific and engineering challenge. Natural seismic activity is observed to cause transient changes in the natural hydraulic system (Elkhoury et. al., 2006; Manga et. al., 2011). Also, it is observed that shear slip on pre-existing faults may strongly influence the stimulation of shale gas reservoirs (Zoback et. al., 2012). The evolution of permeability in fractures under static loads has been discussed as driven by both mechanical and fluid driven processes. Pressure solution may drive continuous reduction of fracture aperture (Polak et. al., 2003; Yasuhara et. al., 2003) or and free-face etching may increase permeability. Conversely, pore pressure perturbations have been observed to instantaneously enhance fracture permeability (Elkhoury et. al., 2011; Candela et. al., 2014). In the following, we investigate the direct effect of dynamic shear deformation on fracture permeability in multiple slide and hold motions.
