Passive Seismic Monitoring: Mapping Enhanced Fracture Permeability

Aside from volcanic regions, most places in the world do not exhibit natural geological structures where temperature and rock-permeability meet the conditions required for a geothermal power production. The EGS (Enhanced Geothermal Systems) concept implies the creation of an artificial subsurface heat exchanger by enhancing the rock permeability in a spatially confined region. Such reservoir stimulation is achieved by injecting large volumes of fluid into the host rock under high pressures. Stimulation activities are frequently accompanied by induced seismicity and there exist a direct relationship between induced seismicity and hydraulic conductivity enhancement. A major challenge in EGS technology is an optimum positioning of the injection and production wells with respect to the stimulated zones. This is critically dependent on the spatial image of in situ hydraulic conductivity. In the current paper we present a new technology for determining 3-D images of enhanced reservoir conductivity based on the observed shear-displacement of the induced seismicity.