Techniques for improving the petrophysical evaluation of the Patchawarra formation in the Toolachee Field, Cooper Basin, South Australia

Abstract

The Toolachee Field is the largest liquid-rich gas field in the southern Cooper Basin, South Australia. Due to the extensive depletion of the field, many challenges exist to exploit the remaining reserves in this area. The objectives of this thesis are: to improve the current formation evaluation methods to explore the bypassed potential gas pays in existing wells, and to investigate the relationship between log-derived porosity and formation pressure in the Patchawarra Formation in the Toolachee Field. Conventional log-derived lithology overlooks most of thin sandstone beds, leading to underestimation by up to 35 per cent of gross sandstone in each well through the Patchawarra Formation. A new method of log analysis for thin bedded shaly sandstone reservoirs has been successfully applied to commonly available log suites. Without the need for high resolution devices, the thin bed method is able to detect beds as thin as 6 inches. The method gives a more correct picture of reservoir rock distribution, indicating more potential pay than conventional log analysis. Further studies may be focused on petrophysical characteristics and hydrocarbon productivity of thin bedded sandstones in the Patchawarra Formation. Existing information shows that there is an association between low sonic porosities and low formation pressures within the Patchawarra Formation. Combination of published experimental studies and the available data suggested that the anomalously low log-derived porosities observed in pressure depleted reservoirs are caused by the effect of reduced pore pressure on sonic tool response. The conventional method of porosity calculation from sonic log data in the Cooper Basin may lead to underestimation of the actual porosity of the reservoir rocks in a pressure depleted reservoir. Based on the available data an empirical equation has been derived which corrects the sonic log readings for the effects of formation pressure variation. This equation provides an approximate correction for the conventional sonic porosity in partially depleted reservoirs in the study area. A detailed multidiciplinary study is required to derive a more correct correction. The techniques developed in this thesis offer two routes toward maximising the production performance of the reservoirs in the Toolachee Field. These techniques are also likely to be useful in other gas fields within the Cooper Basin and indeed may find application in other basins.