Reservoir characterisation of a low resistivity gas field: Otway Basin, South Australia

The Katnook, Haselgrove and Ladbroke Grove Fields of southeastern South Australia are characterised by a lack of resistivity contrast above and below known gas-water contacts, poor hole conditions, complex mineralogy and fresh formation water. A multidisciplinary review of all available data to characterise the Pretty Hill Sandstone reservoir accurately by integrating core, log and engineering data has enabled a comprehensive picture of reservoir heterogeneity and its influence on log response and well performance to be determined. The availability of extensive of core throughout the six wells has resulted in the accurate modelling of reservoir porosity and the derivation of a facies-dependent, quantitative permeability log which closely matches drill stem test and production test derived transmissibility (kh). Previous water saturation assumptions have been shown to be optimistic with Leverett J Function water saturation averaging 50-60 per cent through the reservoir. Detailed facies modelling from the cores extrapolated into areas with no core control has led to the derivation of a geological model which, when integrated into a three dimensional simulation, has resulted in calculated pressures within 1 per cent of those measured and has enabled the prediction of the pressure response from highly compartmentalised portions of the reservoir. Simulation-derived, material balance and volumetric original-gas-in-place for the Katnook Field now agree to within 5 per cent.The study has resulted in changes to previously accepted evaluation procedures for wells targeting the Pretty Hill Sandstone.