Constraining frictional stability-permeability evolution in rock fractures: A comparative study between granite and shale

Abstract

Large-scale fluid injection during hydraulic fracturing may cause seismic activity during deep geothermal and shale gas development. Constraining the relationship between frictional stability and permeability evolution of fractures is critical since it may provide an indicator to predict frictional behaviors based on monitoring hydraulic evolution. However, the relationship between frictional stability and permeability evolution is poorly understood, especially for different rocks. Hence, we conducted a series of velocity-stepping experiments to explore the shear slip stability of granite and shale fractures, the principal rocks for geothermal and shale gas reservoirs. The results indicate that with the increase of effective normal stress, the permeability and frictional stability (a-b) of both shale and granite fractures decrease. Shale exhibits a velocity-strengthening behavior, while granite exhibits a velocity-weakening behavior when a slip velocity change occurs. By calculating the permeability response parameters (Δk/k_trans), the results show that with the increase of fracture permeability, the possibility of seismicity induced by fluid injection increases, which provides an indicator as an early warning for potentially damaging seismic events. The experimental results of the permeability test also show that the fracture permeability increases slightly after the short slip. In contrast, the long-term slip will inevitably cause a further decrease in the permeability. This study improves our understanding of fracture permeability and slip stability.