The Molecular Simulation Study On Adsorption And Diffusion In The Process Of Alkanes Displacement With Carbon Dioxide Injection On Illite And Calcite

Low permeability sandstone reservoirs have enormous reserves and the development characteristics of low permeability,low production and low pressure.Thus,the development is difficult,recovery rate is low and the benefit is poor.However,carbon dioxide injection has the dual advantages of supplementing formation energy and enhancing oil recovery and storing greenhouse gases.It is an important direction of international development in recent years.Compared to the micron grade pore throat diameter in conventional reservoirs,the low permeability oil and gas reservoirs mainly develop on large-scale nanoscale pore structure.The pore is small,channel is narrow,specific surface is large and the interaction between the fluid molecules and the pore is strong.Therefore,it is of great significance to study the interaction between the injected fluid molecules and porous media.One of the physical and chemical effects of low permeability sandstone reservoirs is adsorption,which leads to the difficulty of fluid flow.The second is diffusion,which leads to the difficulty of describing the relationship between the fluid and the injected fluid.The method of conventional oil and gas reservoir engineering mainly adopts nonlinear percolation theory to describe the interaction between fluid and medium,but is mainly studied from the macroscopic angle,but the reasons for these phenomena are still difficult to explain.In this paper,for low permeability sandstone reservoir and gas reservoir,the method of combining grand canonical Monte Carlo with molecular dynamics is used to study the adsorption and diffusion laws of single molecule and multi component in calcite and illite porous medium,the main minerals of low permeability sandstone,from the point of view of microscopic molecular action.In the simulation stage,the nano pore model of illite and calcite with different pore sizes are modeled according to the crystal database and the microstructure characteristics of minerals.The gas component of low permeability sandstone gas reservoir is methane.The oil and gas components of the low permeability sandstone reservoir are a mixture of alkanes,methane,n-butane and n-pentadecane,which is obtained by fitting the PVT data of a reservoir.Through the model building module and corresponding force field in the molecular simulation software,the all atom model of each fluid molecule is constructed.On this basis,42 groups of adsorption simulation and 40 groups of diffusion simulation are carried out.The results show that the adsorption capacity of CO₂ in the low permeability sandstone gas reservoir is abrupt at the depth of 1km.When the mole ratio of the components is not more than 2,the adsorption capacity of 2km is stable after the formation of 2km.However,when the molar ratio of the components is more than 2,the adsorption capacity increases with the stacking effect,and tends to be stable at 5km.The increase of the number of bulk CO₂ caused by the stacking effect can enhance the diffusion of the bulk molecules.When the effect is stronger than that of the adsorbed phase CO₂,the diffusion coefficient of CO₂ decreases.In the low permeability sandstone reservoir,with the increase of the formation pressure,the long chain alkane is preferentially adsorbed at low pressure,the adsorption capacity of the short chain alkane increases gradually and gradually replaces the long chain alkane.The adsorption capacity of CO₂ increased gradually,and the increasing trend gradually became slow due to the saturation of the adsorption phase and the accumulation effect.The diffusion coefficient of the molecules is increased first and then decreased by the influence of the surface diffusion and the diffusion of the bulk molecules,and the order of the molecular diffusion coefficient of the four fluids is negatively correlated with the order of the adsorption heat.In addition,compared to the two minerals,the adsorption ability of calcite to CO₂ and CH4 is stronger than that of illite,and their adsorption capacity of CO₂ is stronger than that of other fluid components.