Molecular Simulation Studies Of Supercritical CO₂ Swelling Oil And Reducing Oil/Water Interfacial Tension

With the increase in the global demand and consumption of oil and gas,the exploitation of unconventional petroleum resources attracts considerable attention.However,the great development of unconventional resources petroleum resources has been limited due to its complex reservoir conditions.Researches indicate that the injection of carbon dioxide(CO2)into unconventional reservoirs not only swells oil but also make the oil/water interfacial tension reduce,thus leading to the decrease of oil viscosity and the growth of enhanced oil recovery(EOR).Currently,there are few studies on CO2 enhancing oil recovery and the interactive mechanism between CO2 and crude oil is still not clear,which greatly limits the application of CO2 in EOR.Therefore,the molecular dynamics(MD)simulation is performed to investigate the microscopic behavior and mechanism of CO2 swelling oil and reducing the oil/water interfacial tension under different conditions.This study will provide theoretical guidance for the CO2 EOR and the application of CO2 in the exploitation of unconventional reservoir.The results from CO2 swelling oil indicate that CO2 in supercritical state plays a dominant role in the volume swelling of CO2-alkane systems,and the increase in pressure and simplification of alkane molecular structure and the reduction in temperature are of benefit to the volume swelling of CO2-alkane systems.The observed microprocess of volume swelling shows that the increases in the molecular stretch and the average distance between alkane molecules result in the increases of CO2 solubility in oil and the volume swelling.The calculated interaction energies in CO2-alkane system indicate that the interaction between CO2 and alkane molecules is responsible for the volume swelling of system,and the dispersion interaction plays a dominant impact on the volume swelling of CO2-alkane system.The simulated results of oil/water interfacial system indicate that the interfacial tension reduces with the increase of temperature and the decrease of pressure.The influences of supercritical CO2 on the water/oil interface shows that the interfacial tension reduces with the increase of the mole fraction of CO2 dissolved in oil.CO2 molecules prefer to accumulate at the interface,and the contact surface of oil and water becomes corrugated and the interfacial thickness increases as the CO2 mole fraction increases.In the vicinity of the interface,oil molecules prefer to be perpendicular to the interface at high CO2 composition.CO2 displays amphipathic “surface-active” similar to surfactant at water-oil interface and its accumulation is driven by the IFT difference between water-decane and water-CO2.In the interfacial region,the water-CO2 interaction and the oil-CO2 interaction,enhancing with the increase in the CO2 mole fraction,are comparable,which provides the mechanism for lowering the interfacial tension.The diffusion coefficient profile shows the mobility of the system enhances with the addition of CO2.This is particularly important to impel CO2 interact actively with water and cause the decrease in the interfacial tension.In addition,the effects of temperature and pressure on the oil/CO2/water interfacial system indicate that the increase of temperature and decrease of pressure are favorable to the reduction of interfacial tension.