Molecular Dynamics Study Of CO₂-oil Systems And Their Mixtures Confined Within Nanopores In CO₂-EOR

CO₂ enhanced oil recovery?CO₂-EOR?has gained great attention worldwide for increasing oil production and reducing greenhouse emissions,which involves complex multiphase systems,like CO₂-oil system and CO₂-oil-rock system.Understanding the phase properties and interfacial properties are propitious to apply the CO₂-EOR project.In this work,we studied the minimum miscibility pressures?MMPs?of CO₂-oil system and the structural properties of CO₂-oil mixtures confined within nanopores using molecular dynamics?MD?simulations.Fristly,we used MD simulations to calculate the MMP,which is the key factor to determine the possibility of CO₂ miscible-phase displacement.We estimated the MMP of CO₂-oil system by obtaining interfacial tensions?IFTs?based on the vanishing interfacial tension?VIT?technique.To get accurate IFTs,modified Lorentz-Berthelot rules were developed for all simulations.Frist,we performed simulations for CO₂+n-hexane binary system,CO₂+n-hexane+n-decane ternary system and complex CO₂+crude oil system to calculate the MMPs.MMPs predicted by MD simulation show good agreement with previous experimental data,which documents MD simulation is a rapid alternative way with good accuracy to estimate the MMP.Then,the effects of injected gas components,oil components and reservoir temperature on the MMP were investigated.The results demonstrate that:1)methane and N₂ will increase the MMP of CO₂-oil system significantly;2)the effects of a certain oil component on the MMP depend on the properties of crude oil,generally speaking,heavy oil components will increase the MMP while light oil components decrease it;3)temperature will increase the MMP of CO₂-oil system,while it will decrease the MMP of N₂-oil system,which is due to their different solubility in oil.Secondly,the structural properties of CO₂+oil confined within nanopores of quartz,calcite,illite,montmorillonite and kaolinite were investigated.The main conclusions are as follows:1)the adsorption strength of rocks to n-decane has the order from strong to weak as follows:calcite>kaolinite>montmorillonite>quartz>illite,which is in line with the results of interaction energy between rocks and n-decane and the orientation distributions of n-decane in the adsorbed layer.2)The structure of rock surfaces influences the fluid distributions,the strong adsorption of calcite is due to the strong interactions between Ca in calcite and n-decane,and the flexible H atoms on the surface of quartz and kaolinite make it more uniform for n-decane distribution.3)The pore size,the filling fraction of alkanes and CO₂ also have impacts on the properties of n-decane.For n-decane,when the nanopore size is bigger than about 3 nm,the adsorption layers will no longer change;when the filling fraction of n-decane decreasing,the density far from the walls decreases,and then the density of adsorption layer decreases;and it can be found that the CO₂ adsorption layers are closer to the rock surface than the alkane's,and the amount of n-decane in the adsorption layer decreases with the increase of pressure,which reveals good displacement effect.