A Numerical Model for the Effect of Permeability Change on Faulting During Fluid Injection

Slip and slip zone development during constant‐rate fluid injection into a permeable fault in an infinite, impermeable elastic rock is studied here numerically using a hydromechanical fracture model. The interplay of slip zone growth and fault dilatancy is assumed to be affected by linear slip weakening of frictional strength and slip‐induced dilation. The discretization error is minimized by using element sizes based on a mesh sensitivity study to obtain accurate slipping and pressurized lengths. Comparisons with published results for large and zero fault permeability cases demonstrate the validity of the numerical results. Dimensional analysis is used to identify five parameters that control slip development, including the slip‐induced dilatancy factor, initial fault permeability, the product of fluid viscosity and injection rate, the background shear to normal stress ratio, and the residual coefficient of friction. Conditions leading a quasi‐static stable slip to either continue as stable aseismic slip or accelerate are discussed. Acceleration of slip occurs on less permeable faults characterized by a hybrid tensile‐shear fracture subject to higher pressure, while prolonged aseismic slip occurs along a more permeable fault subject to lower pressure. During long‐duration aseismic slip the pressure varies slightly around the value that equalizes the fault shear strength and the background shear stress. A permeability perturbation in the form of a sinusoidal fault aperture distribution does not change the tendency of the slip to be slow and stable. Like preexisting permeability, slip‐induced dilation responses play a role in limiting pressure level and extending the slow slip period.