The in situ stress field of the Cooper Basin and its implications for hot dry rock geothermal energy development

Abstract

Hot dry rock (HDR) geothermal energy resources are currently being investigated in the Cooper Basin, an area with a high geothermal gradient. The in situ stress field and natural fracture network are critical to the design of the subsurface circulation system in any HDR development. This study uses knowledge of the in situ stress field from petroleum–based data in the Cooper Basin to predict the likely stress conditions in basement granite bodies and thus determine the most probable fluid flow paths. In situ stress data indicate a relatively consistent maximum horizontal stress orientation of 1010 in the Cooper Basin and a transitional strike–slip to reverse faulting stress regime. Exploration for HDR geothermal energy should target areas with a high geothermal gradient and a reverse faulting stress regime. In a reverse faulting stress regime, shallowly dipping fractures have the highest permeability. Fluid flow is predicted to be focussed in the intermediate principal stress direction because this is the direction in which the elements of structural permeability intersect. Hence, in a reverse faulting stress regime in the Cooper Basin, fluid flow would be focussed in the minimum horizontal stress or north–south direction. Vertical conjugate shear fractures striking 073° and 133° have the greatest permeability in a strike–slip faulting stress regime where the intermediate principal stress and thus fluid flow is likely to be focussed vertically. Production and injection wells should be aligned in the direction of maximum fluid flow and configured to intersect the greatest number of permeable fractures.