Interpretation of Reservoir Creation Process at Cooper Basin by Microseismic Multiplet Analysis

Abstract

Development of a commercial power plant using HDR reservoir is underway at Cooper Basin, Australia since 2003 by Geodynamics Ltd. Researchers in Tohoku University have collected microseismic events from the first stimulation at Cooper Basin in 2003 in cooperation with Geodynamics and Japanese researchers. Extension and macroscopic structure of the stimulated zone have been successfully estimated by the previous mapping of the microseismic events. In this paper, the authors describe results from clustering and re-location of microseismic multiplets, which are groups of microseismic events with highly similar waveforms despite of different origin time and hypocenter. The events were re-clustered under new criterion for multiplet identification considering source mechanism and earth transfer function. The multiplets were then sub-clustered by their focal mechanisms. Integrated interpretation on the multiplets using spatio-temporal distribution of hypocenter, fault plane solutions (FPS), and critical stress state for shear slip was made in this study. It has been revealed that most of the multiplet events occurred on sub-horizontal fracture. Multiplet events which may origin from sub vertical fracture connecting subhorizontal fractures were also found, Relationship of microseismic multiplets to events with larger magnitude was also investigated. Results from this analysis supported our previous conclusion that large events occurred on horizontal fracture.