Seismic Imaging of Spatial Heterogeneity at the Geysers Geothermal Reservoir Using a Cost-Effective Dense Seismic Network

Abstract

In this paper, we present results of high-resolution seismic imaging using a cost-effective dense seismic network to image spatial heterogeneity of the reservoir structure and flow paths at The Geysers geothermal reservoir in northern California, USA. The project employs 91 seismic sensors, spaced at approximately 500 m over a 5 km x 5 km study area. Seismic data for more than 17,000 earthquakes, over a period of one year, were acquired and automatically processed for P- and S-wave phase arrival times. The data were subsequently inverted using a joint inversion approach to image the spatial heterogeneity of the reservoir including the 3D P- and S-wave velocity structure and hypocenter locations. The resulting tomographic images are appraised by integration into The Geysers’ 3D reservoir model and by spatial correlation to injection and production wells. Spatial correlation of P-wave velocity images to water injection and steam productions wells reveal higher velocities below injection wells, likely due to higher water saturation, and lower velocities in the vicinity of steam-producing well, like due to the presence of steam in the surrounding reservoir rocks. The spatial correlation of steam to production wells is highlighted in images of the Vp/Vs-ratio, which show decreased estimates in the vicinity of a group of steam production wells. This suggest that Vp/Vs-ratio is a useful parameter to estimate the spatial distribution of steam in the reservoir.