Geothermal Systems Assessment: Understanding Risks in Geothermal Exploration in Australia

The conventional emphasis on temperature risks as the key aspect of geothermal exploration has resulted in other areas of risk being overlooked. This paper identifi es four critical risk areas which must be considered when undertaking a holistic Geothermal Systems Assessment (GSA) of an area, and discusses some parameters involved in the assessment of these risks. The probability of encountering an economic geothermal resource is largely controlled by four, largely independent, geological factors—heat fl ow (conductive and advective), thermal resistance (insulation), reservoir characteristics (including prevailing stress regime) and access to a working fl uid (water or steam). This approach is synonymous with risk methodologies used in Petroleum Systems Analyses. Temperature risks cannot be accurately assessed without a detailed understanding of rock thermal resistance and heat fl ow. The use of geothermal gradient methodologies can introduce signifi cant errors in assessing the temperature potential of a region. Temperature risk can be further complicated by data quality issues including the use of non-equilibrated and uncorrected bottom hole temperatures. Reservoir risks for Engineered Geothermal Systems (EGS) cannot be accurately assessed without detailed measurement and modelling of tectonic stresses and rock properties. An increasing emphasis by some companies on Deeply Buried Sedimentary Aquifer (DBSA) systems in geothermal exploration in Australia suggests that reservoir forward modelling techniques such as seismic sequence stratigraphy may have a signifi cant impact in quantifying risks. Water access risk, whilst largely a developmental issue, can impact on exploration decisions and needs to be considered at an early stage of planning.