Oxidant stimulation to enhance coal seam permeability mechanism and application

The oxidant stimulation of coal seams aims to dissolve the coal organic matrix around the coal cleats by solvent oxidation for widening the cleat aperture and thus enhance coal seam permeability. The coal samples were collected from Bowen Basin, Australia, and examined with multiple techniques mainly including the pore measurement, permeability test, and computed tomography(CT) scanning to systematically elucidate the oxidant stimulation mechanisms and application prospect. The results reveal that the coal particles demonstrate dissolution, swelling, and breakage to a variable extent in different oxidants. Among them, Sodium Hypochlorite(NaClO) appears to show the most extensive oxidation that can effectively increase the coal porosity and enhance the artificial coal cleat aperture. The core flooding test approves the permeability enhancement after the oxidant flows through, especially the horizontal permeability, which is then elucidated by the micro-CT scanning showing that the NaClO can etch the coal cleat and meanwhile generate new pores and fractures after oxidation. The minerals deposited in the cleats have a negligible reaction with oxidants, but when they are removed by hydrochloric acid(HCl),the permeability after oxidation declines under confining pressure, although the oxidant can still increase the cleat aperture. This means the pristine minerals can perform as the proppants to maintain the integrity of the coal structures. The different lithotype coals separated from the same coal core exhibit different oxidation effects which are proposed to be controlled by their different pore structures rather than the differences in their molecular structures. Overall, the results in this study appear to illustrate that the oxidant stimulation as an alternative of the traditional coal seam stimulating methods can benefit the coal permeability and be conductive to the theory development with respect to coal oxidation from the level of coal molecular structures.