Molecular Dynamics Study On The Micro-mechanism Of CO₂ Enhanced Oil And Gas Recovery

Carbon capture,utilization and storage(CCUS)technology has emerged as a way to reduce CO₂ emissions while optimizing energy structure and ensuring energy security.Among the many CCUS technologies,CO₂ enhanced oil and gas recovery(CO₂-EOR/EGR)technology has been receiving more and more attention,because it can increase the oil and gas recovery while permanently storing greenhouse gases underground,which is expected to solve the problem of oil and gas shortage and CO₂ emission in China.However,unconventional reservoirs have complex geological structures and contain a large number of nano-structures.At the nano-scale,the laws of adsorption and flow of oil and gas are unknown and clearly different from the macroscale,such as extremely low permeability and flow disobeying Darcy's law.Therefore,studying the adsorption of crude oil in nanopores and the mechanism of CO₂-EOR/EGR is the theoretical basis for the development of this technology.In this dissertation,molecular dynamics(MD)simulations was used to study the mechanism of CO₂-EOR/EGR,including the interfacial properties of oil and CO₂,the adsorption of oil and gas in nanopores and competitive adsorption.Firstly,for the two-phase system of CO₂ and alkanes,modified Lorentz-Berthelot mixing rule in MD simulation have been used to accurately describe the interface structure and calculate the interface properties,such as CO₂ solubility,phase density,interfacial tension,and diffusion coefficient.Vanishing interfacial tension method has been used to predict the minimum miscibility pressure(MMP).All simulation results are close to the experimental values,suggesting that MD simulation is an effective method for calculating interface properties.At the same time,it was found that the dissolution of CO₂ improves the diffusion of alkanes and facilitates its flow.Accurate simulation and microscopic understanding of the two-phase system have laid the foundation for the study of CO₂-EOR/EGR mechanism in nanopores.The mechanism of CO₂-EOR in the quartz mesopores(>2 nm)has been investigated by MD simulation.Due to the stronger interaction with quartz,CO₂ can displace the alkanes from the rock surface into the channel,transforming it from the adsorbed state to the bulk state,which is conducive to oil recovery.By analyzing the orientation parameters of the molecules in the nanopore,it is possible to distinguish adsorbed and free alkanes,so as to calculate the oil displacement efficiency of CO₂ and find a suitable CO₂ injection ratio.In addition,the diffusion coefficients of alkanes in the channel are also calculated and used to comprehensively consider the optimal injection ratio of CO₂.Then,CO₂-EOR in quartz nanopores(<2 nm)has been further explored.Due to the strong interaction between the two side walls,the adsorption structure of the alkanes in the nanopores changes drastically with the pore size in this ultra-confinement condition,which results in oscillation of the density and diffusion of alkanes in the nanopores.After the injection of CO₂,it was found that the smaller the pore diameter,the higher the oil displacement efficiency.Oil displacement efficiency increases with increasing pressure,and an inflection point appears near the MMP.By analyzing the change of the adsorption structure from micropores to mesopores,a method for calculating the alkane adsorption amount and oil displacement efficiency in nanopores has been summarized.For this displacement process,a mathematical model is derived based on Fick's law,and the results are in good agreement with the MD simulation results,which shows that the displacement process is dominated by diffusionFinally,the adsorption and desorption of shale gas in kerogen has been studied by MD simulation.In nano-porous media,the adsorption capacity of methane exhibits an exponential decay law with increasing pore size.Due to the existence of the throat structure,adsorbed methane cannot desorb kerogen after the pressure drops,resulting sorption hysteresis.The twice-percolation method has been proposed to quickly predict the degree of sorption hysteresis in kerogen.The results are consistent with MD simulations,and the speed is 1-2 orders of magnitude faster.The study of the CH₄ sorption hysteresis in kerogen also gives us a preliminary understanding of the adsorption law in complex nano-porous media.