Energy effects and correlation analysis of methane adsorption for vacuum layer structure by coal molecules under the influence of water molecules

• Built two models called “VL-OWM-CM and VLWM-CM” and analyzed adsorption differences. • At 1 nm vacuum layer with 20 MPa pressure showed “adsorption depression” phenomenon. • Methane diffuses slower in VL-OWM-CM than VLWM-CM, thus harder to desorb. • Characteristic energy at peak maximum of energy distribution is principal component. Coalbed methane (CBM) is a vital energy resource and efficient extraction reduces greenhouse gas emissions. Water in coal seams significantly influences methane adsorption and release the part energy, but the energy effect and correlation of methane adsorption under microscopic conditions have not been fully explored. This manuscript employed molecular simulation method to construct two models called “vacuum layer-original water molecules-coal molecules” (VL-OWM-CM) model and “vacuum layer water molecules-coal molecules” (VLWM-CM) model, which investigated their methane adsorption differences and analyzed energy effects in methane adsorption (focusing on thermal energies, characteristic energies at the maximum energy distribution peak and activation energies). By elucidating the relevant parameters, reveal their significant via principal component analysis (PCA) and Pearson correlation. The results showed that non-monotonic “adsorption depression” occurred at 20 MPa under 1 nm vacuum layer, driven by repulsive forces and competition for adsorption sites. VL-OWM-CM model exhibited higher methane adsorption capacities. Similarly, energies of corresponding Henry’s contents, average isosteric heat, characteristic energies at the maximum energy distribution peak and activation energies were also all more obvious, indicating enhanced adsorption strength and weakened desorption. Notably, with heat attenuation at 6 %-8% water molecular content versus 2 %-6%. Correlation analysis revealed that characteristic energies at the maximum energy distribution peak were the most significant in both models. These energies positively correlatde with average isosteric heat and Henry’s constant-related energies but negatively correlate with activation energies. According to reveal the energy micro mechanism of gas–water-coal interaction, this study provides a theoretical basis for optimizing CBM storage and methane recovery.