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Seismic Analysis of Reservoir Conditions for Inducing Seismicity at the San Emidio Geothermal Field, Nevada, USA

At the San Emidio geothermal field, Nevada, a substantial increase in microseismic activity during a power plant shutdown (i.e., cessation of all production and injection activities) was observed in December 2016 by a local seismic network with more than 1,300 vertical component nodal instruments. Here, we present our seismic analysis of the 2016 dataset, including locating microseismic events (MEs), P-wave velocity (Vp) tomography, focal mechanism (FM) inversion and stress inversion, to investigate material properties, distribution of existing faults and local stress state in the reservoir for understanding the mechanisms for inducing MEs during plant shutdown. The Vp model shows large lateral variations, with main structural features that are consistent with normal faults dipping westward. Two low-Vp zones (LVZs) to the west of the surface trace of the main fault and near some operational wells are imaged at depths of ~0.2-1.2 km below land surface. The northern LVZ is closer to two production wells and the southern one is closer to four injection wells. Most MEs occurred within or surrounding the northern LVZ. FM results show diverse faulting regimes, dominated by normal faulting. Stress inversion using high-quality FMs yields a maximum compressive stress axis plunging nearly vertically toward the northeast. The intermediate and minimum compressive stress axes are both nearly close to horizontal toward the WSW and SSE, respectively. Orientations of ME hypocenters and FMs show that a majority of MEs may occur on a large-scale fault and/or some small-scale faults/fractures within the LVZ, suggesting that the activation of faults/fractures due to pore pressure increases caused by the cessation of pumping triggered some of the MEs. Modeling pressure changes due to pumping cessation suggests fluid pressure increases of ~25-50 kPa at the hypocenters of MEs, which are predominantly near shutdown production wells. The work presented herein has been funded in part by the Office of Energy Efficiency and Renewable Energy, U.S. Department of Energy, under Award Numbers DE-EE0007698 and DE-EE0009032.

Session: De-risking Deep Geothermal Projects: Geophysical Monitoring and Forecast Modeling Advances I

Type: Oral

Room: Grand B

Date: 4/20/2022

Presentation Time: 08:30 AM Pacific

Presenting Author: Hao Guo

Student Presenter: No

Additional Authors