Permeability enhancement and coal fines removal through oxidation treatment in coalbed methane reservoirs

This study utilized anthracite coal samples from Zhijin Block and applied NaClO as the oxidant to investigate the effects of oxidation treatment on permeability enhancement and coal fines migration. Subsequent to oxidation treatment, the coal samples demonstrated an average increase of 42.17% in the equivalent hydraulic aperture (EHA). This treatment also resulted in the expansion of fluid flow channels, contributing to an average permeability enhancement of 108.85% in the coal samples. Resulting from the non-uniform dissolution of the oxidant on the fracture surface of coal, self-propping fractures were formed, effectively maintaining the conductivity of fractures. The oxidation treatment converted the wettability of fracture surface from hydrophilic to hydrophobic. This transformation proved advantageous for the dewatering process within the fractures. Moreover, even under high confining stress, the 5 wt% NaClO solution still exhibited a favorable permeability enhancement effect on coal. The oxidation treatment resulted in a particle size degradation rate of 40.38%. Considering the properties and particle size distribution characteristics of both the coal fracture surface and coal fines, the affirmative impact of oxidation treatment on coal fines removal was analyzed based on the extended DLVO theory. The calculated results showed that the adhesion force between coal fines and fracture surface was weakened. Coupled with the effect of fracture widening, the outcome was a notable 26.99% increase in the quantity of coal fines that migrated with the fluid under the drag force after oxidation treatment. The research results presented in this paper provide another option for enhancing permeability of coalbed methane reservoirs, which has important reference significance for the broader economic development of such reservoirs.