Electrical Resistivity Measurements in Coal: assessment of coal-bed methane content, reserves and coal permeability

Coal-bed methane (CBM), also referred to as Coal seam gas (CSG), relates to the production of methane from coal beds by drilling wells, hence lowering formation pressure, and triggering methane release. While the potential of this resource is significant, the assessment of the quantity and the producibility of methane from coal seams is highly variable. For this reason the objective of this work is to investigate the assessment of gas content, gas-in-place and coal permeability through petrophysical of analysis and by gaining a better understanding of coal bulk properties,. In this study 17 cored production wells were analysed from the Walloon Sub-group coal seams fairway in the Surat Basin in Queensland Australia, which is today the most ambitious investment in CSG worldwide. A total of 2374 coal beds were investigated to understand how the nature of the different coal lithotypes are reflected in core analysis, wireline logs measurements and DST test results, and how they affect coal quality, and control gas content, fracture development and reservoir permeability. High-resolution studies involving fine scale are required to estimate volumes and CSG formation evaluation turns to the interpretation of standard wireline tools readings in hundreds of coal seam wells. Nevertheless, the heterogeneous thin-bedded nature of coal seams, together with the fact that methane within coal is mainly stored by adsorption, create several difficulties in wireline log petrophysical analysis. Consequently core description is used to validate the combination of the density log with the shallow focused electric and induction resistivity measurements, benefitting the recognition and thickness estimation of thin coal beds and coal laminae rich mudstones. This observation, and a refined coal quality and gas content estimation methodology, are presented and tested against previously published workflows and provide an improved and tested strategy for petrophysical analysis of CSG.