Molecular Dynamics Simulation of CH 4 Displacement through Different Sequential Injections of CO 2 /N 2

As a clean energy source, coalbed methane (CBM) produces almost no exhaust gas after combustion, and its extraction and efficient utilization play a key role in supporting sustainable development. Therefore, molecular dynamics simulations were used to research the diffusion of CH 4 in coal after injecting CO 2 /N 2 in different sequences and to clarify the efficiency of CBM extraction under different injection sequences, so as to contribute to sustainable development. The results show that the adsorption amounts of CO 2 and N 2 in different injection sequences are obviously different. To narrow the gap between the two injection amounts, the injection pressure of N 2 can be appropriately increased and that of CO 2 can be reduced, or N 2 can be injected preferentially instead of CO 2 . When CO 2 is injected first, the interaction energy between CH 4 and coal is stronger and increases slightly with displacement time as a whole. The interaction energy curve of the N 2 injection decreases, and the displacement effect becomes worse and worse. From the diffusion and relative concentration distribution of CH 4 , it can be seen that the diffusion of CH 4 molecules outside the grain cell is more obvious when N 2 is injected first. In terms of the number of CH 4 molecules diffusing outside the crystal cell, it is less when CO 2 is injected first than when N 2 is injected first. The average value of the velocity distribution of CH 4 increases slightly when CO 2 is injected first and decreases significantly when N 2 is injected first, but the average value is overall higher for N 2 injection first. From the difference in diffusion coefficients before and after the gas injection, it can be seen that the decrease in permeability due to the expansion of the coal matrix by CO 2 is more obvious than the increase in permeability due to the contraction of the coal matrix by N 2 .