Geophysical Inversion of 3D Seismic Data in Panax's Limestone Coast Geothermal Project to Determine Reservoir Porosity

Panax Geothermal Limited is developing a geothermal resource in South Australia, the Limestone Coast Project, which is a conventional “hot aquifer” reservoir in a deep sedimentary basin. The initial development is the Pretty Hill Formation sandstones in the Penola Trough of the Otway Basin. The area is an exploited gas basin with modern 3D seismic and excellent existing petroleum well data coverage. Wireline logging in the petroleum wells has already identified a low to moderate temperature geothermal resource, and reservoir quality is wellunderstood based on numerous well cores and wireline log data. This resource is currently at the development stage, the first known such resource in Australia. Salamander-1, the first production well on the project, is scheduled to be drilled in Panax’s Geothermal Exploration License 223 (GEL223), in the second half of 2009. The Limestone Coast Geothermal Project is designed to deliver the first proof that conventional geothermal resources within Australia’s sedimentary basins can be used to generate large amounts of competitively priced, zeroemission, base-load power. Innovation is required to optimize productivity. Panax worked with Schlumberger to derive data on the Pretty Hill Formation reservoir properties. This study aimed to generate a “porosity cube” based on the 3D seismic and nearby well data, to assist in planning the Salamander-1 location and predict the likely well productivity. The method used is genetic inversion of the 3D seismic (a patented Schlumberger product integrated within the Petrel 2009.1 seismic to simulation software). It produces a nonlinear multitrace operator that is applied to the seismic cube, and converts it into the property described by the logs used during the training phase. Wireline log sonic-derived porosities from six petroleum wells were used to train the data and invert the seismic cube to the porosity volume. The modeling indicated that relatively attractive reservoir qualities comparable to those measured in nearby gas wells could be expected at the proposed Salamander 1 location. This paper summarizes the results of this study together with the status of the geothermal development.