GEOMECHANICAL ANALYSIS OF THE GEOTHERMAL RESERVOIR AT SAN EMIDIO, NEVADA AND FRACTURE TOUGHNESS ANISOTROPY OF EGS COLLAB TESTBED ROCKS

Abstract

The WHOLESCALE (Water and Hole Observations Leverage Effective Stress Calculations and Lessen Expenses) project is aiming to simulate the spatial distribution and temporal evolution of stress throughout the geothermal system at San Emidio, Nevada, United States. Towards this goal, the stress state of a thermo-hydro-mechanical reservoir model is being constrained. Focal mechanisms recovered from microseismic events during a power plant shut down in 2016 were extensively analyzed through linear stress inversion methods to infer the insitu reservoir stress state. Additionally, other geophysical data including focal mechanisms from regional earthquakes, sickenlines, wellbore stress indicators observed in the surrounding region, and secular strain rate measurements were used to check consistencies with the in-situ reservoir stress state. From the estimates of in-situ reservoir stress, 78 different realizations of stress models were generated based on a range of maximum compressive horizontal stress (SHmax) azimuths and relative principal stress magnitudes. To investigate which stress model best describes the 2016 microseismicity, slip tendency analyses were performed using each of the 78 realizations of the stress models. Stress models with azimuths of SHmax ranging from North to N20°E and relative magnitudes of SV, SHmax, and Shmin describing a normal-transtensional regime produced the highest slip tendencies at where the microseismic events occurred and are therefore the best estimates of the in-situ reservoir stress state.