Molecular dynamic simulation experimental design of CO2-water displacement tight oil process
[Objective]With the advancement of experimental teaching methods,computer simulation experimental methods have become a crucial component of university experimental courses.For colleges featuring petroleum engineering,computer simulation and simulation methods are indispensable parts of experimental teaching.Students can learn about various microdynamic processes in oil and gas reservoir development and important methods for conducting innovative practices through computer simulation and simulation experiments.[Methods]In China University of Petroleum(East China),a molecular dynamics simulation method was introduced into the experimental teaching of a modern experimental physics course,and a simulation experiment comparing the water injection,CO2 injection,and CO2-water injection displacement of tight oil processes was designed.Furthermore,we designed a 6 nm SiO2 nanopore throat with a surface modified with methyl groups to form hydrophobic surfaces and filled it with octane molecules to simulate a tight oil reservoir.The microdynamic displacement of tight oil in the nanopore processes of water injection,CO2 injection,and CO2-WAG injection was simulated.Subsequently,the flow behavior of gas,water,and oil during the oil displacement process was studied.The displacement efficiency,displacement rate,and threshold displacement pressure were analyzed on the basis of the resistance and microinteractions between different components.[Results]The simulation experimental results showed that(1)oil molecules had two occurrence states:adsorbed-state oil molecules near the rock surface and free-state oil molecules near the center of the nanopore throat.Compared to free-state oil molecules,adsorbed-state oil molecules were more difficult to drive out of the nanopore throat.(2)In CO2 and CO2-WAG injection,when CO2 molecules dissolved into the oil phase,there was competitive adsorption of CO2 molecules with the adsorbed-state oil molecules,making the adsorbed-state oil molecules more difficult to expel from the nanopore throat compared to water injection.(3)CO2 molecules could diffuse in the oil phase,thus,they could not displace the oil phase from the nanopore throat as a whole,resulting in lower oil displacement efficiency than water and CO2-WAG injection.Water injection and CO2-WAG injection did not have substantially different oil displacement efficiency.(4)The dissolution of CO2 into the oil phase reduced the overall volume of CO2 and oil,decreased the viscosity of the oil phase,and increased the fluidity of the oil phase,resulting in CO2-WAG injection having a higher oil displacement rate than water injection.(5)CO2 could reduce the interfacial tension between oil and water,effectively reducing the displacement pressure caused by the Jamin effect when water entered the nanopore throat.Therefore,the threshold displacement pressure of CO2-WAG injection was much smaller than that of water injection.[Conclusions]Our teaching practice has proven that through this experiment,students gain a deep understanding of the microprocesses of different displacement methods in tight oil reservoir development,as well as the influence of interactions between different molecules on the displacement process.They mastered the molecular dynamics simulation method for studying oil and gas reservoir development and laid a solid foundation for further innovative research.
molecular dynamicsCO2-WAG injectionthreshold displacement pressureoil and gas reservoir development
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