Enhanced Microseismicity During Production Pumping Cessation at the San Emidio Geothermal Field (Nevada, USA) in December 2016.

Tectonic activity, geothermal fluids, and microseismic events (MSEs) tend to occur in similar locations as a result of spatiotemporal changes in the subsurface stress state. To quantify this association, we analyze data from a dense seismic array deployed at the San Emidio geothermal field, Nevada for 1 week in December 2016 to coincide with a 19.45‐hr shutdown of all injection and production pumping operations. 123 MSEs were detected, of which 101 occurred during the shutdown. The spatial association of the MSEs with the production wells suggests a causal relationship between the production cessation and the MSEs. Here we performed a detailed analysis to investigate reservoir material properties, distribution of seismically activated faults, and local stress state. We determined the hypocenters, magnitudes, and focal mechanisms for the MSEs, P‐wave tomographic velocity model, and local stress tensor. The results show that most MSEs occurred near the production wells. Magnitudes fall between −2.2 and 0.0 with larger events located closer to the production wells. Most MSEs occurred within a westward‐dipping normal fault zone in the reservoir associated with anomalously low P‐wave velocity values. The focal mechanism and stress inversion results show predominantly normal faulting with the maximum horizontal stress oriented north‐south. We suggest that the MSEs during shutdown were triggered on pre‐existing, small‐scale, critically stressed fault patches in the reservoir as the pore pressure increased around the production wells when the production pumping ceased. We interpret the larger MSE magnitudes closer to the production wells as a result of higher pore pressure increase. Plain Language Summary: Microseismic events (MSEs) associated with production and injection of oil, gas, and water have been observed in many locations. In December 2016, a seismic array with 1,302 instruments was deployed at the geothermal field at San Emidio, Nevada, USA to coincide with a 19.45‐hr time interval when all pumping operations were temporarily shut down. The seismic network detected 123 MSEs, of which 101 occurred during the shutdown. To understand the physical mechanism driving these MSEs, we perform a detailed seismic and stress analysis with the seismic data collected at San Emidio in 2016. Our analysis includes estimating the hypocentral locations, magnitudes, and focal mechanisms of the microseismic events, forming 3‐dimensional images of P‐wave velocity via seismic tomography, and calculating the orientations of the principal axes of the local stress tensor. Our results show that most MSEs occurred within a normal fault zone in a fluid‐filled reservoir near the production wells. We suggest that the temporary cessation of production pumping increased the pore pressure, decreased the effective stress on the pre‐existing, critically stressed fault patches and fractures, and thereby enhanced the microseismicity. Key Points: We performed a detailed seismic analysis of microseismicity during a short, planned pumping cessation at the San Emidio geothermal fieldThe microseismicity likely occurred on critically stressed faults and fractures due to pore pressure increase when production pumping ceasedThere may be a positive correlation between pore pressure increase and microseismic event magnitude [ABSTRACT FROM AUTHOR]