Molecular Simulation On Minimum Miscibility Pressure Of CO₂ Enhanced Oil Recovery And Oil Displacement In Nanopore

Extortionate dependence on foreign oil and the greenhouse effect caused by carbon dioxide emissions are two major problems that China is currently facing.CO₂ enhanced oil recovery(CO₂-EOR)has attracted extensive attention in the petrochemical field because it can enhance oil recovery and store carbon dioxide in stratum to reduce greenhouse effect at the same time.The minimum miscible pressure(MMP),as a parameter to distinguish miscible and immiscible flooding,is the basis for the selection of production pressure in the actual reservoir operation,and is one of the research emphases.The displacement of oil in nanopores and the factors affecting the displacement efficiency are also very important in the research of CO2-EOR.In this work,the MMP of multi-component crude oil is calculated by molecular dynamics(MD)simulation.Oil displacement in nanopores and the effects of pressure,alkanes filling rate,pore size and water content on displacement efficiency are also investigated.Firstly,the interfacial tension(IFT)of gas-oil interface under different pressure was calculated and the MMP was predicted by MD simulation based on the vanishing interface method.In the simulation of CO₂-crude oil system,the deviation between the predicted MMP and the experimental value is little.Furthermore,the effects of temperature,alkane chain length and asphaltene on the MMP were analyzed.Through analysis,the increase of temperature will lead to the increase of MMP,which is caused by the change of solubility of carbon dioxide in crude oil with the temperature changing.In a certain range,the MMP increases when the chain length of alkanes increases.Asphaltenes also increase the MMP between carbon dioxide and crude oil.Then we used non-equilibrium MD simulation to investigate the dynamic process of oil displacement in nanopores and the effects of displacement pressure,alkanes filling rate,pore size and water content on displacement efficiency.In the process of displacement,CO₂first displaces the alkanes on the wall and then displaces the alkanes in the middle area.Increasing pressure will improve displacement efficiency before reaching miscibility.Under low pressure,the increase of alkane filling rate will lead to higher displacement efficiency and speed,but the effect of alkanes filling rate decreases at higher pressure.The displacement efficiency of small pore is higher than that of large pore,but the displacement efficiency gap between different pore sizes will decrease at higher pressure.Water is more easily adsorbed on quartz surface than carbon dioxide.In addition,too much water content in the pores will lead to blockage of the pores and reduce the displacement efficiency.