Molecular simulation of the effect of CO₂/N₂ binary gas on methane adsorption in coal

The binary gas flooding technology (CO₂/N₂-ECBM) has become an important means of increasing the production of coalbed methane(CBM). It is of great significance to clarify the CO₂/N₂ competitive adsorption law in coal seams and its influence on the physical properties of coal seams. The molecular simulation software Materials Studio was used to establish a coal molecular model according to the actual temperature and pressure of the CBM block in Yanchuannan. On the basis of the Grand Canonical Monte Carlo(GCMC) method, the influence of each injection stage in the technology of CO₂/N₂ alternate displacement of CBM on the adsorption of CH₄ was studied, and the law of the influence of CO₂ and N₂ on the porosity and permeability of coal seams was clarified. The results show that during the CO₂ injection stage, the methane in the coal seam is rapidly desorbed, and due to the increase of the total amount of gas adsorbed in the coal seam, the coal matrix swelling effect enhances, which leads to a reduction in the pore volume of the coal. During the N₂ injection, the adsorption of CH₄ and CO₂ shows different degrees of reduction due to the partial pressure of N₂. The total adsorption amount of gas in coal molecules decreases rapidly when ω_N2/ω_CO2≤0.6, while it remains stable after the saturated adsorption of N₂. With the total amount of gas adsorption, the porosity and permeability of the coal seam show a rapid increase and then tend to be flat. In addition, the N₂ adsorption rate decreases rapidly after the ω_N2/ω_CO2>0.6, which makes the purity of the CH₄ lower in the produced gas, resulting in a large increase in the cost of later purification. Therefore, when ω_N2/ω_CO2=0.6, the amount of CH₄ desorption is the maximum and the coal porosity is relatively high, which is most conducive to CBM development.