The Utah FORGE 2022 hydraulic stimulations: tensile hydraulic fractures or/and fluid-induced dilatant shear ruptures?

ABSTRACT: We investigate two hydraulic stimulation stages performed in April 2022 at the Utah FORGE enhanced geothermal system test site using analytical solutions and numerical modeling of fluid-induced tensile and shear rupture problems. These stimulation stages differ mainly by the fracturing fluid's viscosity used. They show a similar pressure response but significantly different spatial and temporal distributions of induced microseismicity. We first estimate the aseismic rupture area from the recorded and located microseismic events assumed to be associated with the macroscopic aseismic rupture growth. We examine how the rupture area develops with increasing injected volume and compare these observations against theoretical scaling relations for both hydraulic fractures and fluid-induced frictional shear ruptures. We find out that the stimulation stage using a more viscous-fluid corresponds most likely to a viscosity dominated hydraulic fracture exhibiting post shut-in growth. For the stage using a less viscous-fluid, we find that either a tensile hydraulic fracture or a frictional shear rupture model can equally explain the observations. We confirm these results by performing numerical simulations of these two stages using a 3D axisymmetric mixed-mode fully coupled hydro-mechanical solver. This solver notably has allowed us to model one stage as a hydraulic fracture and the other either as a hydraulic fracture or as a dilatant shear rupture.