Molecular Simulation On Competitive Adsorption And Diffusion Of Shale Gas

The main components of shale are inorganic minerals and organic kerogen.In order to simulate the real shale structure,this paper starts with the most basic shale organic kerogen model and gradually proposes methylnaphthalene-quartz shale slit model and kerogen-quartz slit model.The grand canonical Monto Carlo method was used to analyze the competitive adsorption characteristics of methane and water and different gas molecules(like carbon dioxide and nitrogen).The molecular dynamics was used to study the methane molecules diffusion and transport characteristics in different shale models.In the organic kerogen II-d model,two major pore types were found in different kerogen II-D models.The presence of moisture significantly reduces the volume fraction of accessible pores.This is the main reason for the decrease in methane adsorption on the kerogen II-d model.As the water content increases,the adsorption amount of methane,isothermal adsorption heat and diffusion coefficient decrease linearly.A temperature of 338 K and a moisture content of2.4 wt% are considered to be a relatively cost-effective shale gas development environment.In addition,we have established a shale flow model to study the flow flux of methane molecules.Studies have shown that maintaining the external low pressure is more conducive to the migration and flow of methane molecules,which will greatly improve the mining efficiency.In the methylnaphthalene-quartz simplified shale slit model,the adsorption state of gases transfers from single adsorption layer to four adsorption layers.The order of interaction between gas and shale is carbon dioxide > methane > nitrogen,which means that carbon dioxide is the best injection gas.Studies have shown that the optimal development conditions for carbon and nitrogen flooding for shale gas are 30 ? and injection pressure > 5MPa.In the kerogen-quartz shale slit model,the constructed and proposed III series model agrees with the experimental data.The order of adsorption of methane molecules in the three kerogen slit models was kerogen III-C> kerogen II-d> kerogen III-B.The order of the overall diffusion coefficient of methane in the three kerogen slit models was kerogen II-d> kerogen III-C> kerogen III-B.The diffusion coefficients of methane molecules in different regions are ranked as slit region > surface region > matrix region.