Molecular Simulation Study on the Diffusion Mechanism of Fluid in Nanopores of Illite in Shale Gas Reservoir

Through the combination of grand canonical Monte Carlo (GCMC) and molecular dynamics (MD), the diffusion properties of CO2/CH4 mixture in pore structure of illite that is the clay mineral in shale reservoir are studied. The simulated results show that the variation tendency of the self-diffusion coefficient of CO2 can be well explained through the mechanism of surface diffusion and Fick diffusion, that is, the diffusion of fluid is mainly influenced by surface diffusion in the micro-pore of shale reservoir and surface diffusion coefficient increases with the increase of formation temperature, formation pressure and number of adsorbed gas molecules that take part in the diffusion process. In the diffusion process that takes the function of surface adsorption and adsorption desorption into consideration, there is a reciprocal relationship between the influence of surface diffusion and Fick diffusion on the diffusion coefficient of CO2. Meanwhile, through the analysis of the radial distribution function (RDF) between CO2 and oxygen atom on the surface of illite's pore structure and the concentration profile (CP) of CO2, the mathematical model of surface diffusion and the interrelationship of surface diffusion and Fick diffusion are validated further. Through the study of this model, the diffusion mechanism of fluid and the related mathematical model in micro-pore of shale reservoir can be proved with the influence of adsorption considered from the point of view of molecular simulation, which can provide a certain theoretical basis for the efficient development of shale gas reservoir.