Fracture Characterization at The Geysers Geothermal Field using Time Lapse Velocity Modeling, Fractal Analysis, and Microseismic Monitoring

Abstract

Temporal change in velocities and the cluster centers of microseismic events, along with the fractal based self-similarity of related patterns, may be correlated with changes in fracture networks. An extensive fracture network with large spatial distribution is required to create and sustain the enhanced geothermal system so that injectant can be heated and transported to the production zone. Since microseismic events at geothermal ?elds may be attributable to pore-pressure increase, temperature changes, volumetric change due to ?uid withdrawal/injection and chemical alteration of fracture surfaces. The occurrence and movement of microseismic events may be correlated with the migratory fracture network in a hot dry rock system, and may express fracture connectivity, aiding in accurate mapping of the fracture network. In this study we explore the relationship between microseismic cluster transit and temporal change of the compressional velocity model at The Geysers. Fracture propagation or ?uid movement within the fracture network could be examined by considering the contribution of fractured area on velocity model heterogeneity and source mechanisms of microseismic events. Here, we introduce three methods for characterizing fractures to accurately locate the boundaries of the connected fracture network. These methods are based on fractal analysis, fuzzy clustering and velocity modeling.