Molecular simulation of the adsorption and diffusion characteristics of CH4 in coal subjected to N-Methylpyrrolidone treatment

Chemical stimulation of the coal reservoir is a potential method for enhancing coalbed methane production. N-Methylpyrrolidone (NMP) solvent extraction is a promising chemical stimulation method. To evaluate the effects of NMP treatment on the adsorption and diffusion characteristics of CH4 in coal, the coal molecular model was established based on elemental analysis analysis, solid-state 13C nuclear magnetic resonance (13C NMR) and X-ray photoelectron spectroscopy (XPS). Then, the adsorption and diffusion characteristics of methane were simulated using Materials Studio software and the distribution of methane molecules around functional groups was studied. The simulation results show that: under the action of NMP solvent, the saturated adsorption ca-pacity of methane molecules was reduced by 41.6%, and the diffusion capacity was increased by 1.72 times. Additionally, methane molecules were mainly enriched around hydroxyl groups and thiophenes, and adding NMP reduced the methane enrichment around functional groups. These show that NMP treatment can promote the diffusion of methane from coal matrix. Hydroxyl and thiophenes affect the diffusion of methane molecules. This study helps to understand the effect of NMP solvent on the adsorption and diffusion mechanism of coal methane from a molecular perspective.