Application of high-resolution borehole image analysis in conjunction with wellbore geophysical log data to improve coal characterisation in Permian coal measures, Bowen Basin, Australia

Abstract

Coal reservoirs are unconventional in the sense that the scale of vertical and lateral variability is high when compared to conventional sandstone reservoirs. This requires higher resolution data (metres to decimetres to sub-microns levels) for calculation of reservoir properties. Rock core is the best, but it's expensive to obtain and destroyed during analysis. Recent advances in geophysical imaging of the wellbore provide "pseudo-core" from which to characterise the reservoir, but require advanced processing techniques coupled with ground-truthing to make them a reliable alternative.This project sought to develop methods for advanced coal seam gas reservoir characterisation using wellbore geophysical data and image logs, and to use this information to interpret coal seam gas reservoir properties, in particular coal lithotype, in the context of geological development of the Bowen Basin.The first problem solved during the research was a conversion of borehole electrical images into a microresistivity curve. This procedure allowed the researcher to make a step from qualitative analysis of borehole images to the quantitative one. The resulting microresistivity curve was added to the wireline suite for geostatistical cluster analysis and the development of electrofacies. Thus, the resulting wireline suite consisted of gamma ray, bulk density, photo-electric factor, image-based microresistivity and deep laterolog wireline data.Electrofacies analysis produced a coal lithotype profile which was validated by the results of other studies such as mm scale brightness profile logging and maceral content analysis. Implementation of the microresistivity curve method enhanced the resolution of coal profiling to decimetres level, while photo-electric curve allowed distinguishing between vitrinite-and inertinite-rich low density coal.The results of electrofacies analysis were obtained for 26 wells spread over the Northern Bowen Basin area and the distribution of the coal electrofacies were analysed for both stratigraphical and geographical trends. It was observed that the inertinite-rich coal interpreted from electrofacies analysis increases towards the top of Late Permian coal and towards the north of the Northern Bowen Basin area. These results were consistent with previously observed trends defined by petrographic analysis. 3 Overall, this research project has resulted in the development of a new method for coal characterisation which goes beyond simple cut off values for density, and allows coal lithotype advanced characterisation based on high-resolution wireline borehole data.This method was tested in a case study where stratigraphical and geographical distributions of coal lithotypes were generated over the Northern Bowen Basin area.4