Fault slip in hydraulic stimulation of geothermal reservoirs: Governing mechanisms and process-structure interaction

Hydraulic stimulation of geothermal reservoirs in basement or crystalline igneous rock can enhance permeability by reactivation and shear-dilation of existing fractures. The process is characterized by interaction between fluid flow and the fractured structure of the formation. The flow is highly affected by the fracture network, which in turn is deformed because of hydromechanical stress changes caused by the fluid injection. This process-structure interaction is decisive for the outcome of a stimulation and, in analysis of governing mechanisms, physics-based modeling has potential to complement field and experimental data. Here, we show how recently developed simulation technology is a valuable tool to understand governing mechanisms of hydro-mechanical coupled processes and the reactivation and deformation of faults. The methodology fully couples flow in faults and matrix with poroelastic matrix deformation and a contact mechanics model for the faults, including dilation because of slip. Key elements are high aspect ratios of faults and strong nonlinearities in highly coupled governing equations. Example simulations illustrate direct and indirect hydraulic fault reactivation and corresponding permeability enhancement.