Monitoring reservoir processes at The Geysers geothermal field: joint time-lapse acquisition and cooperative joint inversion of passive seismic and magnetotelluric data

Abstract To monitor time-lapse changes in reservoir water and steam concentrations, as well as subsurface flow in The Geysers geothermal field in California, we implemented a multi-physics geophysical imaging approach. This approach utilized continuously monitored microseismic events coupled with repeat magnetotelluric (MT) observations. Travel-time data from microseismic events recorded at surface stations were collected over an area of approximately 75 km 2 continuously from 2018 to 2023, alongside three repeat MT surveys conducted in 2021, 2022, and 2023. The resulting seismic and MT data were inverted both separately and jointly to generate time-lapse images of geophysical properties that correspond to temporal changes in water and steam saturation within the reservoir, as well as flow paths and barriers. By correlating the geophysical images with known water injection and steam production volumes, we were able to calibrate the data and gain confidence in the results, which can now be applied throughout the reservoir where borehole data are unavailable. The joint imaging results, combined with reservoir data derived from borehole observations, allow for the interpretation of temporal changes in the reservoir. This enables an operator to optimize reservoir management and its drilling program.