Modeling And Simulation Of CH₄ Displacement By CO₂ From Different Rank Coals

With the gradual establishment and improvement of China’s"carbon sink"market,it has become one of the important options to improve the economic benefit and development effect of coalbed methane wells by carrying out relevant research on molecular level of carbon dioxide flooding coalbed methane,improving the mechanism of coalbed methane displacement process,realizing the injection and storage of carbon dioxide in coal reservoirs,reducing the carbon dioxide content in the atmosphere and improving the recovery ratio of coalbed methane.Through molecular modeling,the molecular model of coal with different coal qualities can be constructed.At the same time,through the simulation of methane flooding with carbon dioxide based on coal macromolecules,the process and mechanism of carbon dioxide flooding coalbed methane can be understood from the molecular level,and the theoretical understanding of carbon dioxide flooding coalbed methane can be deepened.In this paper,the macromolecular structural units of fat coal（R_O≈1.27%）,lean coal（R_O≈1.96%）and lean coal（R_O≈2.04%）are mainly constructed at the molecular level.the molecular chemical formulas of fat coal,lean coal and lean coal are C235H207N3O6S2,C199H139O3N3S2 and c1883 respectivelyEnergy optimization was carried out on the initial structural units of different rank coal macromolecules,including molecular mechanics optimization and molecular dynamics optimization,to obtain the model with the lowest energy,and then the lattice optimization of the model was carried out.It was found that the density of the macromolecular microcrystalline model of fat coal and lean coal was 1.3 g/cm³,the density of the macromolecular microcrystalline model of lean coal was 1.35 g/cm³,and the unit cell sizes of coal were 16.04（?）,15.08（?）and 14.80（?）respectively.Saturation adsorption simulation of coal macromolecules of different rank was carried out by using the sorption module in Materials Studio,and the results showed that the actual saturated adsorption capacity of CH₄,CO₂ and H₂O by the macromolecular structure model of fat coal was34.7 cm³/g,62.46 cm³/g and 166.56 cm³/g.Saturated adsorption capacity of coal macromolecular structure model of lean coal is 25.05 cm³/g,83.50 cm³/g and 152.68 cm³/g respectively.The saturated adsorption capacity of the macromolecular structure model of lean coal is 26.49 cm³/g,52.98 cm³/g and 150.11 cm³/g,respectively.Univariate,binary and ternary isothermal adsorption simulation of CH₄,CO₂ and H₂O by coal macromolecules was carried out.The results showed that the competitive adsorption capacity of rich coal,lean coal and lean coal in the univariate and multivariate adsorption of CH₄,CO₂ and H₂O showed the advantage of CH₄<CO₂<H₂O.Temperature has obvious influence on the three adsorbate molecules,and the adsorption capacity of the three adsorbate molecules decreases gradually with the increase of temperature in one-dimensional isothermal adsorption simulation.As far as pressure sensitivity is concerned,CH₄ and CO₂ are more sensitive to pressure changes than H₂O adsorption,while H₂O adsorption requires higher pressure and is more affected by pressure.