Construction of a Coal Molecular Model Based on Molecular Simulation and the Effect of Moisture Content on Methane Adsorption

Low-rank coalbed methane is an important resource of unconventional natural gas in China. Restricted by reservoir water-bearing conditions, the adsorption and occurrence laws of coalbed methane are complex, which limits the efficient development of resources. Taking low-rank coal from the Jimei Block 14 in the Erlian Basin as the research object, the coal macromolecule and periodic unit cell models were constructed; combined with elemental analysis, 13C-NMR and XPS test results, the pore structure was characterized, and the grand canonical Monte Carlo (GCMC) method was adopted to simulate methane adsorption under different moisture contents. The results show that the coal sample has a low degree of aromatic condensation and is rich in oxygen-containing functional groups such as hydroxyl and carboxyl groups. The equilibrium density of the model is 1.25 g/cm3, and the pores are dominated by ultramicropores smaller than 1.5 Å. The increase in moisture content significantly inhibits methane adsorption. When the moisture content is 2%, the saturated adsorption capacity decreases by 49.5% compared with dry coal. The decrease in methane adsorption capacity may be associated with competitive adsorption between water and methane at polar functional groups and the possible restriction of accessible pore space by water molecules. This result can provide a theoretical reference for the development of low-rank coalbed methane in the study area.