Simulation Study On Oil Displacement By Supercritical Fluid And Modified Nano-SiO₂ Particles

The importance of oil in various industries of the national economy is self-evident.In order to meet the increasing oil demand every year,while looking for alternative energy sources,improving the recovery rate of existing oil fields has become one of the important methods to solve the problem.At present,it has been developed to the tertiary oil recovery stage.For increasing oil displacement efficiency to improve crude oil recovery rate,it is mainly in the choice of oil displacement agent.At present,there are mainly surfactants,alkalis and polymers.Overall,the recovery rate has increased to varying degrees,but there is still a lot of oil left in the formation.And after going through these oil displacing agents,the formation environment is more complicated,and the secondary pollution is also intensified.Therefore,the search for new efficient and environmentally friendly oil displacing agents has also become an urgent problem to be solved.Supercritical fluid（SCF）and Nano-silica（Nano-SiO₂）particles have attracted attention for their environmentally friendly,economical and other excellent properties.In this paper,these two types of oil displacement agents are selected for oil displacement simulation experiments,and their oil displacement effect and microscopic explore and analyze the mechanism.1. Establishing a rock surface model,and conducting a balanced simulation of the effects of supercritical carbon dioxide（scCO₂）and supercritical nitrogen（sc N₂）on the oil droplets on the rock surface.The research results found that two types of SCF can selectively extract the saturated alkanes in oil droplets.In addition,by calculating the radial distribution function of asphaltene molecules in the oil droplets,it was found that scCO₂ changed the arrangement of molecules inside the crude oil and asphaltene molecules tended to distribute on the surface of the rock layer;By calculating the contact angle of oil droplets,it is found that scCO₂ increases the contact angle of oil droplets,while sc N₂ is the opposite.At its root,the presence of SCF changes the interaction energy between the oil droplets and the rock formation.2. Using the non-equilibrium molecular dynamics（NEMD）research method,scCO₂ and sc N₂ were used to conduct oil displacement experiments on crude oil in nanopores.The results show that the oil displacement process can be described by a three-stage mechanism.In the early stage,the water film was ruptured and the alkane was dissolved.In the later stage,the difference between the two was found.scCO₂can make the oil phase separate from the surface of the rock layer and successfully drive the crude oil away from the nanopore,while sc N₂ cannot.Analysis of its internal factors,scCO₂ can greatly reduce the interaction between crude oil and nanopores,and reduce the viscosity of crude oil,so as to successfully displace;in contrast,sc N₂,even if there is an external force,it can not effectively reduce the interaction between the oil phase and nanopores and promote oil phase.3. Through the NEMD research method,water is used as the basic fluid,and Nano-SiO₂ particles and methyl-modified Nano-SiO₂ particles（Nano-SiO₂-CH₃）are used to simulate oil displacement.Both types of nanoparticles are assembled.In addition,Nano-SiO₂ particles have basically no effect on crude oil,while Nano-SiO₂-CH₃ can temporarily close the nanopores and push the oil phase under the action of water flow.When the pores are reopened,the oil phase no longer moves.4. In order to reduce the deposition of Nano-SiO₂-CH₃ particles,a small amount of scCO₂ was used as the basic fluid and placed in water for oil displacement.The study found that the expected effect can be achieved and the crude oil can be successfully displaced.The presence of scCO₂ enhances the interaction between NPs and the oil phase thereby promoting the movement of crude oil.