Friction-permeability relationships for reservoir caprocks

There is wide concern that pressurized CO₂ fluid has a potential to induce seismicity in the impermeable caprocks that overlie CO₂ injection formations. However, the possible impact of induced seismicity on sustainable CO₂ containment from geological CO₂ sequestration remains unclear because the earthquakes play a significant but mysterious role in influencing the integrity of the caprocks by hypothesized interrelated friction-permeability interaction processes: (1) the earthquakes may occur seismically (i.e., frictionally unstable), enhancing the permeability of faults instantly and leading to potential breaching and loss of inventory; or (2) the earthquakes may occur aseismically (i.e., frictionally stable), closing the aperture of faults and reducing permeability through creep. In this study, we explore these processes through experiments in which we measure the frictional parameters and hydraulic properties using Green River shale sample as an analogue caprock candidate. We observe that fracture permeability declines during shearing while the increased sliding velocity reduces the rate of decline. The physics of these observed behaviors are explored via parametric study and surface measurement of fractures, showing that both permeability and frictional strength are correlated to the fracture asperity evolution that is controlled by the sliding velocity and fracture material. Through the velocity step, the velocity strengthening behavior is observed for Green River shale, suggesting that for Green River shale, only aseismic slip would occur at a low sliding velocity during which the permeability would decrease.