Fracture Network Interpretation through High Resolution Velocity Models: Application to The Geysers Geothermal Field

Abstract

Steam at many geothermal fields including The Geysers is produced from a network of fractures in crystalline rocks. Some of these fracture networks have been created by injecting cool water into the hot rock while others are natural tectonic fractures associated with the nearby San Andreas Fault plate boundary. Due to very low permeability of the formation matrix in The Geysers reservoir, production depends on the presence of these natural or induced fractures. Hence, locating and characterizing fracture networks is of vital importance. During injection of water, newly created fractures induce microseismic events. A small number of triggered seismicity could also be created from fault failures. Although pinpointing the locations of microseismic events is a useful characterization tool, we go beyond the simple locations identification to characterize fractures more reliably. We apply tomographic inversion to the microseismic data to obtain high resolution compressional (P) and shear (S) wave seismic velocity volumes of the area of interest. We show how these velocity models can help us in characterization process. In addition, we demonstrate how P and S velocity volumes can be integrated with each other or with other data sets to derive additional reservoir property volumes. Such additional information can then be used to optimize injection schedule, improve the production rates or locating the potential zones for enhanced geothermal systems.