Computational Investigation of THMC Effects on Transmissivity Evolution During Selected Injection Phases at the Desert Peak EGS Project, NV.

Abstract

The low-flow-rate injection phase of an Engineered Geothermal System (EGS) experiment in Desert Peak well 27-15 produced increased injectivity at wellhead pressures less than the minimum principal stress, consistent with hydraulically induced mechanical shear failure in the surrounding rock. We use statistical fracture analysis and hydro-mechanical modeling to simulate the observed pressure response during this shear stimulation, to explore one possible conceptual framework for the overall Desert Peak EGS experiment. This is part of a long-term study to simulate the complete Desert Peak EGS stimulation, including both shearing and hydraulic fracturing (tensile) failure. Discrete fracture network simulations, based on fracture/fault attributes measured downhole and at the surface, were used to derive equivalent permeability tensors for comparison with preferred fluid migration directions observed in hydraulic and tracer tests. FLAC3D, a hydro-mechanical simulator, was used to investigate changes in stress and displacement according to a Mohr-Coulomb frictional model subjected to perturbations in pore pressure. Although almost all of the seismicity observed during the EGS stimulation occurred during the high-flow-rate tensile stimulation phase, we use this seismicity to illuminate the geometry of largescale geologic structures that could also have served as preferential flow paths during shear stimulation. This analysis shows that conditions for shear failure during the low-flow-rate shear stimulation could occur in locations consistent with locations of microseismicity seen during the tensile phase of the EGS experiment, providing a possible hydrologic connection between EGS well 27-15 and injection/production wells further south-southwest. This FLAC3D hydro-mechanical model will next be coupled to TOUGHREACT to investigate the nearfield evolution of reservoir transmissivity associated with thermal, hydraulic, mechanical and chemical processes during all phases of the Desert Peak EGS stimulation.