Evolution of thermodynamic behaviors of methane adsorption in variable migration pores of coals with different deformations
Ming Li, Kun Zhang, Zhaoping Meng, Baolong Wang
Abstract
Coals subjected to deformation exhibit altered gas dynamic behaviors, which can influence gas production. To explore the variations in pore structure and thermodynamic properties of CH 4 within different coal structures, N 2 , CO 2 , and CH 4 adsorption experiments were conducted on coal samples. The results indicate that with increasing coal damage, the CH 4 adsorption capacity rises from 31.85 cm 3 /g to 38.91 cm 3 /g, while SSAs and pore volume increase from 2.63 m 2 /g to 88.83 m 2 /g and from 0.002 cm 3 /g to 0.043 cm 3 /g, respectively. Ultra-micropores and micropores are primarily developed during the deformation process. The increase in SSAs in highly deformed coals results in a rise in Henry’s coefficient from 0.37 to 0.57, as well as an increase in the absolute values of surface potential and Gibbs free energy from 0.39 to 6.0 J/g and from 2.83 to 5.0 kJ/mol, respectively. These changes indicate an enhanced CH 4 adsorption affinity from intact to mylonitic coal. However, elevated temperatures cause these three parameters to decrease by approximately half, which is unfavorable for CH 4 adsorption in coals. The disorder degree of coal reservoirs increases during tectonic deformation, mainly resulting in a decrease in entropy loss from 22.33 KJ/mol/K to 15.87 KJ/mol/K. After coal damage, adsorption sites with high adsorption potential and increased adsorption space are created, accompanied by an enhancement in adsorption capacity. CO 2 in coal exhibits higher adsorption potential and space compared to CH 4 , suggesting that CO 2 has superior adsorption capacity over CH 4 .