Integrating Microseismic Multiplet and Source Parameter Analyses to Define EGS Reservoir Structure: Cooper Basin, Australia

Abstract

A listed company, Geodynamics Ltd, has been developing a commercial-scale power plant using an artificially created EGS reservoir at a site in the Cooper Basin, Australia since 2003. Researchers in Tohoku University have collected microseismic events from the first stimulation at the Cooper Basin site in 2003 in cooperation with Geodynamics and Japanese researchers. The extent and macroscopic structures of the stimulated zone have been successfully estimated in previous studies by the authors. However, details of the internal structure of the reservoir had not been well delineated mainly because of uncertainties in microseismic mapping. In this study we have made an integrated analysis of microseismic multiplets and source parameters to define structures, especially sub-vertical structures, which hydraulically connect dominant sub-parallel fractures. Polarization at the onset of P waves at each station and the amplitude ratio of P and S waves (P/S ratio) are used to determine possible conjugate pairs of fractures where a microseismic event occurred. The Coulomb criterion for shear slip and the hydraulic pumping record were then used to determine which fracture in the potential conjugate pair slipped during/after the stimulation. We discuss the resolved structures inside the stimulated zone and their creation process at the Cooper Basin site showing the spatio-temporal distribution of hypocenters determined by a double difference high resolution mapping technique.