Investigation Into Fracture Network Permeability Evolution And Geothermal Reservoir Deformation In Response To Hydraulic Stimulation, Utilizing Coupled Thermo-Hydro-Mechanical Modeling

Abstract

A series of hydraulic stimulations aimed at enhancing reservoir fracture-permeability were carried out in Desert Peak geothermal well 27-15 from September 2010 to March 2013. Fracturepermeability is controlled by the combined influence of induced thermal (T), hydrologic (H), and mechanical (M) processes, also known as THM. A hydro-mechanical (FLAC3D) model first, and a dual-porosity thermo-hydro-mechanical (TFReact) model second, are implemented to test if the proposed conceptual model is generally able to reproduce observations from the stimulation treatment. Numerical simulations are performed to determine: a) pore pressure diffusion and stress field modifications, b) development of mechanical deformation, and, above all c) relative impact of thermo-mechanical vs. shear deformation on reservoir permeability evolution. Both FLAC3D and TFReact coupled models display an evident cause-effect relation between stimulation of either the shallow or the extended intervals and shear deformation on a deep known structural feature, the STF ("Shearing Target Fault" - where most of the induced microseismicity is observed). Injection-induced thermo-mechanical and hydro-mechanical processes are found to govern permeability enhancement during stimulation of the shallow interval in well 27-15. Conversely, induced shear failure processes developing on a larger structural feature (STF) seem to control most of the permeability gain produced during the well 27-15 extended interval stimulation. Distinctive signatures at the well-head (pressure curve) are shown by the different permeability-change processes, and this is confirmed and matched by coupled THM simulation results.