Permeability is a key controlling factor for gas migration in coal seam reservoirs and has invariably been the barrier to economically viable gas production. A large variety of completion techniques have been utilised in coal bed methane (CBM) production with varying degrees of success. Low permeability stimulation treatments primarily consist of hydraulic fracturing and multi-lateral horizontal wells. High stress, cleating and low Young's Modulus in coal can lead to complex and inefficient fractures compared to conventional reservoirs. The creation and transport of fines due to shear failure can reduce the fracture conductivity and thus coal permeability. The unique structure of coal may offer opportunities for the development of new methods for creating gas flow paths for low permeability coals.
Coal seams contain upwards of approximately 10 wt.% mineral matter with a significant proportion contained within the natural cleat system. This study has involved characterization of the cleat system in a core sample from the central Bowen Basin. It was found that the volume of cleats was around 0.52% with around 73% of the cleats mineralized. LTA-XRD analysis found the minerals in the whole coal to be quartz, kaolinite, clays, calcite, apatite and bassanite. SEM-EDS analysis found that calcite and kaolinite dominate the mineralology of the cleats. The removal of these minerals is thought to create additional gas flow paths whilst maintaining the structural integrity of the coal. Furthermore, given that coal is reactive toward oxidants there is the possibility to undergo controlled reaction of the macromolecular coal structure to further enhance permeability.
Batch leaching experiments with HCl were effective at dissolving the Ca-bearing minerals. Of the oxidants studied, potassium permanganate appeared to break down the coals structure, producing a significant level of organics in solution Both hydrogen peroxide and Fenton's reagent didn't yield organics in solution although a slight mass loss was found, indicating that oxidation may have occurred with CO2 produced as the reaction product. Permeability tests were performed on a Bowen Basin core sample with HCl solutions. An increase of 7 wt% was observed, however, permeability was found to decrease again on account of the confining pressure exceeding the cleat pressure.