Study On Dynamic Miscibility And Optimization Of Produced Gas Recycling In CO₂ Flooding Process

CO₂ miscible flooding is a very effective EOR method.The most important condition to achieve miscible displacement is that the reservoir pressure is higher than the minimum miscible pressure?MMP?of oil-CO₂,so as to achieve higher displacement efficiency.Currently there are many methods to get MMP including experiment methods and simulation methods,but most of these methods are just focusing on the statistic test of the miscibility pressure without considering the influence of oil compostion change,so it's necessary to study the dynamic change of MMP in oil reservoir.In order to reduce MMP,the requirements for the purity of the injected CO₂ is high?such as CO₂ content>95%?,this is challenge to the disposal of CO₂flooding produced gas.Considering the hige costs of separating CO₂ and natural gas,reinjecting the produced gas directly or mixing with pure CO₂ are cost efficient methods,so it's necessary to evaluate the influence of gas reinjection on miscibility and EOR.This paper is aimed at the low permeability CO₂ flooding demonstration block in Jilin oilfield.The experiment and reservoir simulation are applied to study the oil property and interaction with different gas?CO₂,CH₄,N₂?.The MMP is determined through slim tube experiment,reservoir simulation method,empirical formula method and PVT calculation method.The conceptual model of CO₂ flooding is built to study the composition transfer,interfacial tension,viscosity change in CO₂ flooding.The dynamic change of MMP due to pressure decline,gas injection volume,pressure and temperature is analysed.The geological model of target field is built to study the gas breakthrough characteristics of production well.Moreover the reinjection standard is determined.Different reinjection plans including direct reinjection,mixing reinjection and WAG are presented.The results show CO₂ has strong extraction ability,which makes the gas flooding front rich in light hydrocarbon components such as C2-C20.The interfacial tension of oil and gas decreases,which is favorable for the mixing of CO₂ with oil.However,with the increase of gas injection volume,the content of heavy composition increased and MMP increased in the remaining oil due to the continuous extraction of light components by CO₂.Therefore,it is difficult to achieve the miscible state again after gas flooding.Compared with high displacement efficiency?>80%?of laboratory displacing experiment,the comprehensive gas injection recovery obtained from the actual reservoir simulation is very low,28.42%for C2-C3,25.11%for CO₂,20.87%for CH₄ and 19.33%for N₂.This is due to the strong heterogeneity of actual reservoir,which leads to the early gas channeling,and reduces the efficiency and comprehensive recovery of the injected gas.Therefore,on the basis of maintaining a certain degree of miscibility between crude oil and injected gas,effective control of gas channeling and increasing wave efficiency are the key to improve the recovery of CO₂ flooding.The simulation results show that the difference between produced gas direct reinjection and mixing reinjection is small?range from23.98%-25.11%?,so the direct reinjection of produced gas is recommended in target reservoir,there is no need to use expensive and complex CO₂ separation processes.In order to further improve the recovery of CO₂ flooding,the method of expanding the volume of wave is considered.The results show that the oil recovery can be improved obviously by WAG.Compared with the pure CO₂ flooding,the comprehensive recovery of WAG can be over29.10%.