Geothermal Systems Assessment - Identification and Mitigation of EGS Exploration Risk

Abstract

The conventional view in Australia (and many other places) is that achieving optimal temperature is the critical risk in exploration for conductive geothermal resources. This view has often resulted in other areas of risk being overlooked. A Geothermal Systems Assessment (GSA) investigates and ranks four largely independent critical risk areas—heat source, thermal/hydraulic insulation, reservoir potential, and working fluid. These four factors determine the probability of encountering an economic conductive geothermal resource. The GSA approach is synonymous with risk methodologies used in Petroleum Systems Analyses.

Heat source risk can be quantified via a detailed assessment of surface heat flow. Developments in hardware and software tools and methodologies have resulted in a rapid increase in the number of reliable heat flow measurements in Australia, and the quantification of uncertainty in existing measure-ments.

Thermal insulation can be routinely and rapidly measured on core or cuttings samples using new electronic steady state thermal conductivity meters. Combining these data with precision surface heat flow measurements allows the prediction of temperature distribution at depth in one, two or three dimensions.

Reservoir risk assessment for 'classic' Engineered Geothermal Systems (EGS) requires detailed measurement and modeling of tectonic stresses, fracture densities/orientations and rock strength. Increasing interest in Deeply Buried Sedimentary Aquifer (DBSA) systems for geothermal exploitation in Australia suggests that forward geological modeling techniques such as seismic sequence stratigraphy may reduce reservoir risk.

Water availability and quality risk, whilst largely a developmental issue, can impact on exploration decisions and needs to be considered at an early stage of planning.