Numerical modeling of hydro-mechanical processes during hydraulic testing of pre-existing fractures at the Grimsel Test Site, Switzerland

This study presents a fully coupled hydro-mechanical framework for modeling hydraulic shearing in a mesoscale reservoir located at the Grimsel Test Site, Switzerland. The experiment was conducted on a ductile–brittle fault embedded in low-permeable granite. We observe that normal fracture opening increases flow channel recoverably, while fracture sliding locks asperities leading to a non-recoverable increase in flow. To couple these processes, we use a poro-elasto-plastic constitutive framework and employ a permeability function that depends on several parameters, such as dilation angle, in-situ stresses, residual aperture and maximum aperture. Our results capture the recorded pressure responses well, and indicate that the permeability changes by one order of magnitude during the experiment. • Numerical simulations of hydraulic shearing are carried out in the setting of the Grimsel test site, which features a fractured crystalline rock. • Permeability function reproducing normal opening and fracture sliding was implemented to capture the relationship between flow rate and injection pressure. • The application of stimulation significantly enhances recoverable permeability, while also causing a minor change in unrecoverable permeability. • The numerical framework effectively replicates the pressure responses both at the well and ten meters away.