Alterations in Coal Permeability and Adsorption Capacity Post Oxidation Treatment: Insights into Enhancing Coalbed Methane Production

Abstract

Abstract Organic matter dissolution by oxidants can enhance the coal permeability. However, as coalbed methane (CBM) is predominantly reserved in the adsorbed state, the organic matter dissolution changes the adsorption–desorption equilibrium, thus affecting the CBM production. Current studies have not sufficiently addressed the combined variations in permeability and adsorption capacity after oxidation, which are critical for enhancing CBM production via oxidation. Experiments on coal permeability, molecular structures, adsorption capacity, wettability, and pores were conducted. The results demonstrated that new pores and fractures were formed after oxidation, as evidenced by an average increase in porosity of 15.57%, leading to an average increase of 161.16% in the initial permeability (k0, at atmospheric pressure). However, the increased pore size and oxygenated functional group content lead to a decreasing contact angle (CA) and specific surface area (SSA), ultimately causing an average reduction in adsorption capacity of 72.20%. Changes in k0/CA were negatively correlated with moisture content (Mad) + ash yield (Ad) and positively correlated with fat/oxygenated functional group (FA/OFG). The decreasing CA indicates significantly enhanced hydrophilicity, inhibiting depressurization by drainage, and negatively affecting CBM production. The change in k0/the change in CA after oxidation generally exceeds 10 when Mad + Ad < 14% and FA/OFG > 0.5, which is favorable for CBM production enhancement under similar treatment conditions in this study. These findings determine the coals to be more suitable for oxidation with equal treatment and offer insights for assessing the CBM production enhancement potential via oxidation.