Macromolecular insights into the influence of bituminous coal matrix deformation on CH4-N2 competitive adsorption and diffusion

Abstract

Competitive adsorption and two-component diffusion of CH4/N2 play a crucial role in the production of N2-enhanced coalbed methane (N2-ECBM). However, the complex production and geological conditions as well as the heterogeneity and anisotropy of the coal can easily result in different degrees of deformation of the coal. Therefore, the competing adsorption and diffusion properties of CH4/N2 in bituminous coal matrix under uniaxial tension and compression strains were studied by macromolecule simulation. The results show that the partial pressure effect of N2 increases the difficulty of CH4 adsorption. Compared to the compression strain, the tension strain has less influence on the gas adsorption potential energy, while the corresponding potential energy distribution has more overlap. Besides, the thermodynamic correction factor (Γ) is positive correlated with the bulk CH4 mole fraction (y(CH4)) and decreasing y(CH4) increases the effect of CH4 displacement by N2. On the other hand, compared to the pores before adsorption, the number of pore throats and smaller pores in the gas-containing coal matrix increases, and the compression strain reduces the flexibility of the coal molecule. More importantly, strain has a dual effect on adsorption-induced coal matrix swelling and gas diffusion, with CH4 diffusion having an inhibitory effect on N2 diffusion. Furthermore, the order of strain influence on the interactions between gases is N2-N2 > CH4-CH4 > CH4-N2 and CH4-N2 tends to cluster compared to pure gases. These results provide important clues for the optimal design of the N2-ECBM technology in deformed bituminous coal reservoirs.