Molecular Simulation Of Adsorption Of Methane/Carbon Dioxide In The Pore Structure Of Rectorite

Shale gas is a new and widely distributed unconventional gas,which is mainly composed of methane and is widely recognized as clean energy and efficient energy in the world.The occurrence state of shale gas includes adsorbed state,dissociation state and dissolved state,especially the absorbed state.Some scholars have shown that clay minerals and organic matter are the main adsorption substances in shale.Because of the largest and most widely distributed,illite/smectite mixed layer is an important part of shale gas reservoir.Due to the special crystal structure and complex mixing ratio of the illite/smectite mixed layer,there is no suitable crystal model to reflect the real structure of illite/smectite mixed layer.Rectorite is a special kind of the illite/smectite mixed layer minerals,it is composed by a mixture of montmorillonite and illite layers in a ratio of 1:1,which provides an ideal crystal model to explore illite/smectite mixed layer minerals.In the study of the adsorption properties of methane,domestic and foreign scholars mainly focus on experimental analysis.However,clay minerals and organic matter often form complexes,it can makes errors and irrationality only through experimental research.In this paper,rectorite adsorption methane and carbon dioxide are studied by means of molecular simulation in the micro perspective.This paper provides some theoretical guidance for the development of shale gas resources.In this article,through Material Studio simulation software,2 nm,4 nm,6 nm aperture of rectorite pore structure were build.Molecular mechanics,Monte Carlo and molecular dynamics were combined to explore the adsorption behavior of methane and carbon dioxide of rectorite at different burial depths.The results are as follows:1)The adsorption capacity of methane and carbon dioxide showed increase first,then decrease with the increase of the buried depth of the different aperture rectorite model.The large aperture rectorite model has a large adsorption capacity of methane.Under the condition of 2km buried depth,the method of carbon dioxide substituted methane is more effect.The adsorption of methane is physical adsorption,and the small aperture rectorite has stronger adsorption effect on methane.2)The adsorption of methane molecules in the pore of the rectorite is mainly concentrated in the near of wall of rectorite.As the temperature and pressure increase,the methane molecules gradually migrate to the pore structure.When methane and carbon dioxide are competing for adsorption,methane and carbon dioxide are stratified,concentrated on different side of the pore structure.3)The accumulation of methane in the pore model of the rectorite is mainly influenced by the adsorption of the tetrahedral layer and pore cation.The oxygen and silicon on the tetrahedral sheet are strongly adsorbed to methane.With the increase of the pore diameter,the adsorption of methane is decreased in rectorite.The addition of carbon dioxide reduces the adsorption effect of cationic ions on methane,and the adsorption of methane is increased in the tetrahedral layer.4)The self-diffusion coefficient of methane molecules increases with the increase of burial depth.With the addition of carbon dioxide,the self-diffusion coefficient of methane is obviously increased,and the method of carbon dioxide displacing methane to improve recovery is feasible.