The rise of Hot Dry Rock (HDR) technology has reset the boundaries for what constitutes an exploitable geothermal resource. A new database of 5722 borehole temperatures, AUSTHERM03, has been assembled building upon the existing work of Somerville et al. (1994) through addition of newly available commercial open-file data. The application of more rigorous GIS techniques, including the use of new depth-to-basement and mean annual surface temperature coverages, has resulted in a more reliable extrapolation of this data to 5km depth. A new Australia-wide map of estimated temperature at 5km was created from the extrapolated data by interpolation using geostatistical kriging. This combination of new data and more sophisticated GIS processing has produced a more realistic map image that is characterised in areas of good data density by rounded, well constrained temperature anomalies. In most cases, thermal highs were found to be associated with areas of sedimentary basin cover. Geothermal resource analysis of this image indicates three major regions of prospectivity beneath the Cooper, McArthur and Carnarvon Basins.
Data available in AUSTHERM03 from 330 wells in the Cooper Basin of South Australia provided a unique opportunity to test the reliability of models for bottom of hole temperature recovery. Incorporated into a new database, the Cooper Basin Static Temperature or CBST, 61 of these wells proved suitable for use in thermal modelling. Included within these data are consecutive bottom of hole temperature (BHT) measurements taken shortly after drilling was completed. Also available are BHT from cement bond logs (CBL) that are recorded on average 482 days after drilling. These temperatures provide a good estimate of true formation temperature, forming the basis for comparison with model predictions. Four commonly used models of borehole thermal re-equilibration were tested: the Horner plot derived from Bullard (1947), the theoretical dual-media, zero-circulation cylindrical model of Cooper & Jones (1959), the empirical semi-log plot of Pitt (1986) and the exponential model of Nakaya (1953). On average, most models were found to under-estimate the true formation temperature. The magnitude of this bias was found to be dependent upon assumptions implicit in each model. In most cases model prediction accuracy was improved to within around 5% of the CBL when at least one perturbed BHT was recorded >20 hours after the end of drilling. Of the models tested, the theoretical Horner plot was found to have the best combination of accuracy, utility and predictability.
A subset of 306 Cooper Basin wells the BHT recorded in AUSTHERM03 were corrected using the Horner Plot. Two images of temperature distribution at 5km depth were constructed for these wells, one using the perturbed AUSTHERM03 data, the other the corrected values. While the effects of the drilling temperature suppression were evident on the uncorrected image the overall anomaly structure was found to be largely undisturbed, confirming the utility of the AUSTHERM03 image for geothermal exploration.
Clearly visible within Cooper Basin temperature images is a strong bullseye temperature anomaly. Previously recognised by Somerville et al. this anomaly is believed to be a product of high heat producing granites buried beneath low conductivity strata and is currently the site of Habanero, Australia’s first geothermal resource development. A geological and geochemical study of drill cuttings derived from Habanero 1, the first deep geothermal well drilled on this site, revealed the presence of a fractured, fractionated I-type granite body at depth. Similar in composition to many better known granite bodies around Australia, this data provides the basis for a geological model of an Australian-style geothermal resource that is of direct relevance to future HDR exploration.