Seismic imaging for saturation conditions at the Geysers, California.

摘要

In-situ knowledge of saturation conditions at the Geysers is important for understanding the role of fluid injection in resource replenishment and to prospect for new drill sites. We are engaged in a three phase project to infer these properties from seismic imaging data. Phase I of the project is complete and we report on those results here. We are still collecting data on the other phases at the time of writing of this paper. Our method is to compute seismic compressional-wave velocity and attenuation images in terms of the geologic structure and fluid saturation. Our data consist of seismograms recorded from local earthquakes. Using compressional-wave arrival times, we invert for earthquake location, origin time, and velocity along a three-dimensional grid. In phase 1, to compute attenuation we used the initial pulse width of the compressional-wave. In the later phases, we intend to include attenuation parameters measured from spectral ratios and spectral matching. We find that the velocity structure correlates with known mapped geologic units, including a velocity high that is correlated with a felsite body at depth that is known from drilling. The dry steam reservoir, which is also known from drilling, is mostly correlated with low velocity. The Q (i.e. the inverse of attenuation) increases with depth from the surface to the top of the dry steam reservoir and decreases with depth within the reservoir. The decrease of Q with depth indicates that the saturation of the reservoir rock increases with depth.