Investigations of Temporal Seismic Velocity Variations at The Geysers Geothermal Field

Abstract

In recent years, earthquakes of M > 3 have caused concern and objection from the public against the use of water injection to increase productivity at geothermal reservoirs worldwide. A planned expansion of the EGS site in Soultz-sous-forêt (France) was suspended after attempts to generate increased permeability through hydrofracturing of the reservoir rock generated an increase in seismicity. Similarly, production at the EGS site in Basel (Switzerland) was stopped after renewed seismicity caused concern and objection from the public in the city. Therefore, it becomes imperative to understand the relationship between these larger events and the induced stress changes in the medium if sustained generation of geothermal energy is the goal. However, while studies have been undertaken to study the relationship between water injection and induced micro-seismicity, no study has addressed the relationship between water injection or steam production and the effects on the local and regional stress field including the generation of larger events (M > 3). In our project, we attempt to understand the role of historical seismicity associated with past injection of water and/or production of steam at The Geysers geothermal reservoir. This paper will focus on the results of time-lapse tomographic imaging to evaluate and quantify temporal changes of the physical parameters in the reservoir. Seismic data collected by the Lawrence Berkeley National Lab (LBNL) seismic network from 2003 through present, were analyzed to delineate regions where compressional and shear-wave velocities indicate changes in reservoir properties. The applied inversion technique is based on 3-D double-difference tomographic imaging, which utilizes differential travel-times to improve the relative location of earthquakes and simultaneously estimates the velocity of reservoir rocks.