Experimental Study On EOR By Secondary Foamy Oil Promotion System For Ultra-heavy Oil Reservoirs

The existence of foamy oil phenomenon in the depletion development process of ultraheavy oil reservoirs leads to a high recovery factor in the stage,but the depletion development has no energy supplement,the formation pressure keeps declining,the foamy oil gradually disappears,and the development effect becomes worse.Cold production methods such as conventional gas flooding and water flooding are difficult to drive,natural gas reinjection is difficult to dissolve and easy to come off,so it is difficult to achieve effective oil displacement,and thermal recovery technology is difficult to achieve significant economic benefits.According to the gas flow characteristics of natural energy exploitation of foamy oil and gasbearing oil exploitation,the secondary foamy oil promotion system composed of oil-soluble viscosity reducer,gas and foaming liquid can form secondary foamy oil in crude oil by oilsoluble viscosity reducer slug and gas foam,which can greatly prolong the time of gas retention in ultra-heavy oil,enhance the elastic energy of gas-containing ultra-heavy oil,reduce the dynamic viscosity of ultra-heavy oil and prolong the production time.However,the applicability of secondary foamy oil promotion system for ultra-heavy oil reservoirs and the mechanism of enhanced oil recovery need to be further studied.In this paper,on the basis of fully investigating the literature,firstly,the oil-soluble viscosifier was optimized.Then,the interfacial tension of ultra-heavy oil and gas with surfactant was studied by using high temperature and high pressure interfacial tensiometer.The stability of foamy oil generated by secondary foamy oil promotion system was studied by high temperature and high pressure foamy oil stability experimental equipment.Then,4 sets of tests,water alternating gas flooding and methane flooding and naphtha flooding and secondary foamy oil promotion system flooding were performed using the specially designed seepage experiment device.By comparing the oil recovery factor,gas-oil ratio,oil recovery rate and the foamy oil phenomenon captured in the window verified the feasibility of the secondary foamy oil promotion system flooding process;Finally,the secondary foamy oil promotion system flooding,and the factors such as injection timing,surfactant concentration,injection mode and gas-liquid ratio on the development effect of secondary foamy oil promotion system were studied,and the mechanism of EOR enhancement by secondary foamy oil was revealed.The results show that the viscosity reduction effect of viscosity reducer naphtha on ultraheavy oil increases with the increment of addition amount.With the improve of temperature and storage time,the viscosity reduction rate showed a downward trend.The larger the pressure is,the smaller the oil-gas interfacial tension is.The oil-gas interfacial tension reduces first and then increases as the improve of surfactant concentration.The higher concentration of surfactant should be used under high oil saturation,which is more conducive to reducing oilgas interfacial tension.The stability of secondary foamy oil improves first and then reduces with the increment of oil content.Improving surfactant concentration is conducive to improving the stability and expansion of foamy oil.Compared with nitrogen,the foamy oil generated by methane has stronger stability.The secondary foamy oil injection system can effectively generate secondary foamy oil with highly dispersed bubbles in crude oil,and the ultimate recovery is 26.4%,23.74% and 17.4% higher than that of naphtha flooding,methane flooding and WAG flooding,which is an effective method to improve the recovery rate of ultra-heavy oil reservoirs.The optimal injection timing of secondary foamy oil injection system is when reservoir pressure drops to between bubble point pressure and quasi-bubble point pressure.The effect of simultaneous injection of methane gas and foaming liquid is better than that of alternating injection.The surfactant concentration should be increased as much as possible under economic conditions,and there is an optimal gas-liquid ratio.