  Hydraulic stimulation of geothermal reservoirs in low-permeability basement
and crystalline igneous rock can enhance permeability by reactivation and shear
dilation of existing fractures. The process is characterized by interaction
between fluid flow, deformation, 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 hydraulic
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 hydromechanical 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 using our open-source software illustrate direct
and indirect hydraulic fault reactivation and corresponding permeability
enhancement. While conceptually simple, the examples illustrate the strong
hydromechanical couplings and the prospects of physics-based numerical models
in investigating the dynamics.
