Effect of Small Molecule Organic Matter on CH₄ Adsorption and Desorption Characteristics in Coal

Small molecule organic matter significantly influences methane adsorption and desorption in coal. To understand how these molecules affect methane capacity, this study investigated CO₂ injection's microscopic effects on methane recovery. Models incorporated three abundant small molecules at varying compositions and concentrations. Density functional theory (DFT) and molecular dynamics simulations analyzed their impact on methane adsorption. Results show 2,3-2-methylnaphthalene binds methane most strongly, while alkanes exhibit the weakest adsorption. Variations in these molecules' composition and concentration obstruct or block coal pores, reducing methane adsorption capacity. Methane adsorption also decreases with rising temperature and higher concentrations of small organic molecules. Correspondingly, the isosteric heat of methane adsorption declines with temperature and is sensitive to small molecule concentration. Injecting gas (like CO₂) into coal rich in small organic molecules effectively enhances methane recovery. However, efficiency declines with increasing alkane concentrations, suggesting that extending injection duration could improve outcomes. These findings provide crucial insights into the microscopic mechanisms of coal methane adsorption, aiding optimization of coalbed methane extraction and mine gas hazard mitigation.