Molecular Simulation Study Of Supercritical CO₂ Adsorption On Clay Minerals In Shale

Embedding carbon dioxide in shale gas reservoir is one of the most potential strategies to reduce greenhouse gas content in the atmosphere.Carbon dioxide exists as supercritical state in the reservoir,but at present,the research on the adsorption of carbon dioxide in shale is mostly based on physical experiments,which can not meet the actual conditions of high temperature and pressure in the reservoir.As a result,the adsorption theory needs to be further improved.In this paper,based on the crystal cell structure parameters,the molecular models of smectite,mixed layer illite-smectite and illite are established.The adsorption and diffusion behavior of carbon dioxide in clay minerals at high temperature and high pressure was studied by using the GCMC simulation and MD method,and the influencing factors of adsorption were analyzed.The adaptability of the classical adsorption theory was also investigated.To sum up,we have the following understandings:(1)By analyzing the data of absolute adsorption capacity,excess adsorption capacity and adsorption phase density of carbon dioxide in clay minerals,this paper discusses the characteristics of supercritical adsorption of carbon dioxide,analyzes the influence of temperature,pressure and clay mineral pore size on adsorption behavior,and provides theoretical guidance for the optimization of carbon dioxide geological burial site.(2)According to the snapshot and density profile of carbon dioxide molecules in clay minerals,the spatial distribution and adsorption characteristics of carbon dioxide molecules are determined.(3)Combined with the results of molecular simulation,such as the characteristic parameters of carbon dioxide adsorption,the radial distribution function of between carbon dioxide molecules and interlayer cations,and the density profile,the influence of clay mineral types on carbon dioxide adsorption is analyzed,and the influence of interlayer cation types,isomorphic substitution and other factors on the adsorption capacity is clarified.(4)Based on the basic theories of molecular dynamics and molecular thermodynamics,the changes of adsorption heat,interaction force,van der Waals force and electrostatic force between carbon dioxide molecule and clay mineral slit model are studied to reveal the micro adsorption nature of carbon dioxide on shale.(5)Langmuir single-layer adsorption model,Freundlich empirical formula,D-R microporous filling model and BET multi-layer model are selected to analyze the fitting degree of theoretical model and molecular simulation results,and to investigate the adaptability of the classical adsorption model in the supercritical adsorption of carbon dioxide on clay minerals in shale.