Earthquake Science Center Seminars
Seismic Imaging of Fluid Distribution in The Geysers Geothermal Reservoir
Three-dimensional Vp/Vs-ratio structure is presented for The Geysers geothermal field using seismic travel-time data. The study is based on 32,000 events recorded in 2011 and represents the highest resolution seismic imaging campaign at The Geysers to date. The results indicate low Vp/Vs-ratios in the central section of The Geysers within and below the current reservoir. The extent of the Vp/Vs anomaly deceases with increasing depth. Spatial correlation with micro-seismicity, used as a proxy for subsurface water flow, indicates the following. Swarms of seismicity correlate well with areas of high and intermediate Vp/Vs estimates, while regions of low Vp/Vs estimates appear almost aseismic. This result supports past observations that high and low Vp/Vs-ratios are related to water and gas saturated zones, respectively. In addition, the correlation of seismicity to intermediate Vp/Vs-ratios is supportive of the fact that the process of water flashing to steam requires four times more energy than the initial heating of the injected water to the flashing point. Because this energy is dawn from the reservoir rock, the associated cooling of the rock generates more contraction and thus seismic events than water being heated towards the flashing point. The consequences are the presence of some events in regions saturated with water, most events in regions of water flashing to steam (low steam saturation) and almost no events in regions of high steam concentrations where the water has already been converted to steam. We also investigate spatio-temporal changes in Vp/Vs between 2005 and 2011. The temporal changes include increases in Vp/Vs that correlate with the locations of the largest injection wells in the central and south-east Geysers and decreases in Vp/Vs that are confined to the north and north-west Geysers and correlate with the high-temperature reservoir thus indicating sustained boiling of water to steam. Spatial correlation of Vp/Vs to larger magnitude events indicates that most M>4 events are located along boundaries that delineate the transition zone from water to steam. Because the majority of the M>4 events recorded since 1992 have a statistically significant isotropic component, it is conceivable that the isotropic part of the source rupture processes is initiated by first, mode 1 cracks and second, a sudden volume change associated with water flashing to steam.
Roland Gritto, Array Information Technology
May 14, 2014