Molecular simulation of adsorption and diffusion behavior of CH4/CO2 in coals of different ranks under stress loading

Stress loading can markedly alter the gas adsorption–diffusion behavior in coal and affect the molecular-scale stability of gas-bearing coal systems. In this study, four coal molecular models representing lignite (LJZ), long-flame coal (DLT), coking coal (LHBX), and anthracite (YM) were constructed based on experimental characterization data, including elemental analysis, X-ray photoelectron spectroscopy (XPS), and solid-state carbon-13 nuclear magnetic resonance ( 13 C NMR). Molecular simulations were performed to examine the adsorption behavior, energy evolution, and diffusion properties of CH 4 and CO 2 in coals of different ranks under stress loading. The effects of coalification degree and isotropic compressive stress on the structural response of gas-bearing coal models were analyzed. The molecular formulas of the LJZ, DLT, LHBX, and YM macromolecular models were C 124 H 99 O 39 N, C 127 H 87 O 26 N, C 148 H 88 O 10 N 2 , and C 150 H 65 O 11 NS, respectively. External stress reduced the pore free volume, pore surface area, and porosity of the coal models, which increased the model density. With increasing compressive stress, the adsorption behavior of CH 4 and CO 2 in the coal models exhibited a “decrease–increase–decrease” trend. Compared with CO 2 , CH 4 exhibited greater sensitivity to stress loading. Under stress loading, the self-diffusion coefficients of CH 4 in the four coal samples decreased by 64.52%, 56.00%, 63.83%, and 46.40%, respectively, whereas those of CO 2 decreased by 51.66%, 51.39%, 59.68%, and 39.38%, respectively. Under identical conditions, CO 2 exhibited stronger adsorption and diffusion capacities than CH 4 . With increasing coalification degree, the gas adsorption capacity increased gradually, whereas the gas diffusion capacity followed a U-shaped trend. The compressive stress was positively correlated with the bulk modulus (K Hill ) and shear modulus (G Hill ) of the coal models but negatively correlated with compressibility. The cohesive energy density (CED) of the gas-bearing coal models increased progressively with increasing stress and coal rank. These findings provide molecular-level reference information for understanding the CH 4 /CO 2 adsorption–diffusion behavior in stress-affected coal reservoirs with different deformation degrees.