Modification effect of supercritical CO2 on coal microstructure and its influence on CH4/CO2 adsorption and diffusion

During the process of injecting CO₂ into coal seams to increase the production of coalbed methane (CBM), the CO₂ will present a supercritical state due to the increase in depth. Supercritical CO₂, with its high diffusivity characteristic of gases and strong dissolving capacity typical of liquids, possesses exceptional extraction properties capable of substantially modifying the physicochemical characteristics of coal, consequently impacting the adsorption and diffusion behaviors of coalbed methane (CBM). In this paper, the effect of modification treatment on the physicochemical structure of coal samples has been analyzed through supercritical CO₂ modification experiments using elemental analysis, low-temperature nitrogen adsorption, FTIR, and ¹³C-NMR. Meanwhile, the molecular model of coal before and after modification is constructed, and molecular dynamics simulation has been used to investigate the effect of modification on the adsorption and diffusion properties of CO₂/CH₄ adsorption. The results show that the supercritical CO₂ modification led to the shortening of the length of the fatty chains and the increase of the degree of condensation of the aromatic structure in coal samples, which contributed to a more organized coal structure. Supercritical CO₂ has little effect on the pore morphology of the coal body, promotes the formation of smaller pores, and improves the porosity. The adsorption capacity of the modified coals for CH₄ and CO₂ increases significantly, and CO₂ still dominates in the competitive adsorption process. In addition, supercritical CO₂ modification greatly enhances the diffusion performance of CH₄ and CO₂ in coal, especially the diffusion effect of CO₂ is more significant.