Mineralogical characterization of coal samples relevant to coal bed methane porosity and permeability concerns

Lower than anticipated gas production rates in coal bed methane (CBM) operations are sometimes related to formation damage in which reduced permeability results from the interaction of bore and fracturing fluids with coal. This study assesses the potential for mineral recipitation to cause formation damage from a Permian coal seam in the Bowen Basin, eastern Australia, using geochemical modelling of coal mineralogy, formation fluid and bore fluid composition. The mineralogical composition of coal was assessed using petrography, X-ray iffraction, X-ray fluoresence and electron beam microanalysis. Geochemical modelling of ambient groundwater and drilling fluid interactions with coal samples was undertaken using Geochemist’s Work Bench(GWB). This modelling indicates that likely mineral precipitates/re-precipitates to adversely impact porosity and permeability include a range of clay, carbonate and sulphate minerals. Additionally, these interactions may induce alteration of precursor smectites to new species that reduce permeability. The resultant smectites also have a high propensity for expansion and dispersion in the presence of inappropriate drilling fluids. Precipitation, expansion and dispersion of these fine-grained minerals may potentially lead to large reductions in permeability, with profound impacts upon gas flow. Indications are that reduced permeability can be mitigated by suitable chemical matching of ground waters with drilling fluids.