Molecular Dynamics Simulation-based Study On The Reduction Of Miscible Pressure In CO₂ Displacement

In the oil and gas field development industry,the use of CO₂ displacement extraction technology is a common method of oil recovery.It can not only improve the recovery of crude oil,but also solve the problem of CO₂ sequestration.However,in field applications,many reservoirs have difficulty in achieving a miscible phase with CO₂.Therefore,how to effectively reduce the miscible phase pressure between CO₂ and crude oil is a difficult area for recovery enhancement research.Based on this,a series of molecular dynamics（MD）models were developed in this study to investigate the effect of amphiphilic surfactants on the interfacial tension between CO₂ and crude oil,and then to obtain the laws affecting the minimum miscible pressure of CO₂ miscible displacement.Then,a model for the CO₂-n-decane system was developed to calculate the minimum miscible pressure,and on this basis,the effects of injection gas component,crude oil component and reservoir temperature on the minimum miscible pressure were investigated.Finally,a model of CO₂-surfactant-n-decane system was developed to study the effect of surface active agent on the minimum miscible phase pressure.In the molecular simulation of CO₂ gas,the density calculated in this paper matches the experimental value and it is found that the supercritical CO₂ at low density is close to the nature of a gas and at high density is close to the nature of a liquid.In the simulation of the CO₂-n-decane system,the minimum miscible pressure of CO₂ and n-decane is obtained by using the indirect calculation of the interface disappearance method,and the simulation results are close to the experimental values.Secondly,it was found that the addition of CH₄ gas significantly increased the minimum miscible pressure of CO₂-n-decane;the effect of crude oil components on the minimum miscible pressure depended on the nature of the crude oil,in general,heavy oil components would increase the minimum miscible pressure,while light oil components would decrease the minimum miscible pressure;the increase in temperature would increase the minimum miscible pressure of the CO₂-n-decane system.The results of the molecular simulation study of the CO₂-surfactant-n-decane system showed that both surfactants were able to reduce the minimum miscible phase pressure by around 16%.Then,the mechanism of action of the two surfactants was explored,and it was found that the main mechanism of promoting CO₂-n-decane miscibility was to reduce the interfacial tension between CO₂ and n-decane,increase the interaction force between CO₂molecules and n-decane molecules,thus enabling CO₂ and n-decane molecules to come together more closely,increasing the dissolved amount of CO₂ in n-decane,and ultimately reducing the minimum miscible phase pressure,facilitating the miscible phase of both gas and liquid phases.The best results in reducing the miscible phase pressure were achieved when the surfactant fatty alcohol polyoxyalkylenes had a degree of pro-CO₂ polymerisation of 6 and a chain length of 12at the lipophilic end.Finally,the effect of the fatty alcohol polyoxy propylene ether-n-pentyl alcohol combination system on the mixing pressure was simulated and it was found that the reduction of the minimum miscible pressure was higher than that of the system with only the addition of fatty alcohol polyoxy propylene ether,indicating that the selected reagent had a better effect on promoting miscibility.