Molecular Simulation of Diffusion and Competitive Sorption of CH4 and CO2 in Shale Nanopores

Based on the competitive adsorption of CH4 and CO2, molecular dynamics (MD) and Grand Canonical Monte Carlo (GCMC) are used to study the mechanism of increasing shale gas recovery by carbon dioxide injection. The influence of different factors on the diffusion capacity of CH4 in unrestricted space and restricted space is analyzed, and the diffusion capacities of CH4 and CO2 in shale nanopores are compared. Under different conditions, the difference in the sorption amount and sorption heat of CH4 and CO2 is studied, and the displacement simulation of CH4 and CO2 is conducted. The obtained results show that the diffusion capacity of CH4 in restricted space is much smaller than that in unrestricted space, and the difference between them is related to temperature, pressure, and pore size. When pressure exceeds 10 MPa, the difference gradually decreases. The diffusion capacity of CO2 is weaker than that of CH4 under the same conditions, which contributes to the retention of CO2. There is a competitive sorption relationship between CH4 and CO2. The sorption amount and sorption heat of CH4 and CO2 are affected by the combination of pressure, temperature, density ratio, pore size, and mineral type. The adsorption capacity of CO2 is much higher than that of CH4. When sorption conditions are more favorable, the adsorption difference between CH4 and CO2 will become larger. In shale nanopores, CO2 can replace CH4 that is adsorbed on the pore surface to improve shale gas recovery.