A "Volcanoseismic" Approach to Geothermal Exploration and Reservoir Monitoring: Olkaria, Kenya and Casa Diablo, USA

Abstract

We have been able to map the size, depth, and fracture density of possible shallow geothermal energy sources by analyzing microearthquake data in volcanically active areas. In this volcanoseismic approach to exploration and monitoring, we search for seismicity gaps, seismic velocity ratios, S-wave attenuation and splitting, reflected arrivals, and converted waves. Our analysis of shear wave split data for fracture density in the Mammoth field show two potentially fractured areas that are potential targets for drilling exploratory wells. Vp/Vs ratios in two Olkaria fields can be related to reservoir fluid phases. This ratio is 1.58 in the East Production Field and corresponds to a decrease in P-wave velocity in the area with low-pressure steam saturation in the reservoir. It is 1.71 in the high-pressure liquid-saturated North East Field. We have compared the ratios with down hole pressure, temperature and fluid chemical change measurements. These data show that Vp/Vs values depend on reservoir fluid phases and can be useful tools for reservoir monitoring.