Molecular Simulation Of Adsorption Characteristics Of CO₂ And CH₄ In Nanopores

With the increasing energy crisis worldwide,shale gas has become a key resource to meet the growing energy demand in the future.At the same time,the huge CO₂ storage potential of the shale matrix can greatly reduce CO₂ emissions in the atmosphere.In this context,carbon dioxide storage and enhanced shale gas recovery（CS-EGR）project was proposed and gained widespread attention worldwide.Shale gas（mainly composed of CH₄）is stored in large quantities in the gaps of the shale matrix,and the gas in the adsorbed state accounts for more than 85%.Therefore,it is of great theoretical and practical significance to grasp the adsorption characteristics of CO₂ and CH₄ in the shale matrix.In this study,a heterogeneous surface nanopore model composed of graphene–Montmorillonite（MMT）was constructed by molecular dynamics（MD）simulation methods.The adsorption behavior of CO₂,CH₄ and their binary mixtures in shale pores has been studied,and the adsorption mechanism and the influence mechanism of the heterogeneous surface on the gas adsorption behavior are described from a microscopic perspective.In the simulation of the adsorption behavior of CO₂ and CH₄ pure gas,the temperature is fixed at 323K,the pore size is 4⁷ nm,and the pressure is 1.7²1.0MPa.Conclusions are as follows:（1）The heterogeneous surface has a great influence on the adsorption behavior of CO₂and CH₄.The density and adsorption amount of gas near graphene surface are significantly higher than that near MMT,showing that the adsorption capacity of the graphene surface is stronger than that of MMT surface,and the adsorption difference decreases with the increase of pore size and pressure;（2）The adsorption capacity of CO₂ is stronger than that of CH₄;（3）Increased pressure increases the interaction between gas and surfaces,and the interaction between CO₂ and MMT is significantly affected by the pore size.In the simulation of the competitive adsorption behavior of the CO₂/CH₄ binary mixture,the temperature is fixed at 323 K,the pore size is 3¹4 nm,the pressure is 0.7²3.0 MPa,and the molar ratio is 0.2¹.667.The conclusions are as follows:（1）The preferential adsorption characteristics of CO₂ are more obvious near graphene surface than near MMT surface;（2）The adsorption behavior near MMT surface lags behind that near graphene surface,especially in pores with pore diameters≤8 nm;（3）The adsorption behavior of CO₂ is obviously affected by the pore size.6 nm is regarded as the limit pore size.CO₂ reaches a sufficient adsorption state near graphene surface when the pore size reaches 6 nm;（4）The adsorption behavior near graphene is dominant in the pore;（5）The adsorption behavior of CH₄ in the mixture is mainly affected by CO₂ rather than pore size.The above results indicate that the heterogeneous surfaces in organic-inorganic pores have a crucial influence on the adsorption behavior of CO₂,CH₄ and their mixtures,while the asymmetry of gas density distribution and adsorption behavior have a decisive influence on the transport characteristics of gas.On this basis,the displacement behavior of CO₂ to CH₄ in the nanopores with heterogeneous surfaces was analyzed.The results in this paper will provide important data support and theoretical guidance for estimating shale gas reserves,analyzing the displacement characteristics of gas,and performing large-scale simulation of CO₂ enhanced shale mining.