Displacement Mechanism Of Shale Oil By Supercritical Carbon Dioxide From Molecular Dynamics Simulations

At present,China is facing the pressure of increasing energy demand and the difficulty of conventional oil reservoir exploitation.Shale oil could effectively alleviate the energy pressure as its huge reserves.The injection of supercritical carbon dioxide?scCO₂?could effectively improve the shale oil recovery.At the same time,the injection of CO₂ could cause the asphaltene precipitation because of the asphaltene component in the shale oil reservoir.But,the microscopic mechanism of the CO₂ displacing shale oil is not clear,and there are few researches on the asphaltene precipitation in process of oil recovery,which limited the application of scCO₂.Therefore,molecular dynamics simulation method was used to observe the process of scCO₂ displacing shale oil and asphaltene deposition.And the mechanisms of shale displaced and asphaltene deposition were proposed,which can be help to exportation of scCO₂ enhancing oil recovery.The results of CO₂ displacement showed that:the injected scCO₂ preferentially adsorbed in proximity of the surface and form layering structures due to hydrogen bonds interactions between CO₂ and-OH groups.The adsorbed dodecane were separated partly from the surface by scCO₂,as a result the competitive adsorption between CO₂ and dodecane,and thus enhancing the mobility of oil.In the CO₂displacement front,interfacial tension?IFT?reduction and dodecane swelling enhanced the mobilization of dodecane molecules,which played the crucial role in the CO₂ EOR process.The downstream dodecane were found to aggregate and pack tightly because of the CO₂ driving,alkane molecules tangle and oil swelling.The morphology of meniscus changed in the order convex-concave-CO₂ entrainment with the increase of the injection rate.The results of asphaltene deposition showed that:the mechanism of surface deposition,entrainment and stripping are included in the asphaltene deposition.The injected CO₂ could extract the heptane and toluene in crude oil,and destroy the balance of the crude oil system,which lead to the solubility of asphaltene decreasing.In addition there were two different mechanisms of asphaltene deposition.Part of the asphaltene monomer could form aggregations and then adsorbed onto the rock surface.As well as asphaltene monomer was first deposited on the surface of rock,which adsorbed the other free asphaltene monomer in crude oil,then the deposition bodies on the rock wall was formed.The injection rate of CO₂ had an effect on the asphaltene precipitation process.With the increase of CO₂ injection rate,the asphaltene deposition decreased.When the injection rate was increased to 10m/s,the original asphaltene deposition on the rock surface could be peeled off,and migrated along with the CO₂.