Study On Enhancing Recovery Factor By CO₂ Injection In Fuyu Oil Layer In Punan Oil Field

It is abound of geologic reserves of low permeability fields in China, Which is about 26.9% of land proven reserves. It is difficult to exploit these reservoirs, and which are being mainly developed by flooding and depleting at present. Some low permeability oil fields are so difficult to develop that they can't be developed normally or produced simply. Those reservoirs developed by depletion type of drive are very low in recovery factor because of no energy supplement. Therefore, it is a puzzle how to develop low permeability reservoirs economically and efficiently.There are many examples of development of low permeability reservoirs abroad, especially by gas injection. The statistical data shows that, gas injection is the second important method of EOR abroad. The technique of gas injection develops slowly in China, but with the proportion and reserves of low permeability reservoirs are becoming more and more, there are many oil fields have implemented feasibility assessments and field tests. For example, Pubei oil field in Tuha has been flooding by hydrocarbon gas for 3 years and obtained favorable results; Jing'an oil field in Changqing has been successful in natural gas flooding and acquired some experience; Fumin oil field has launched CO₂ Huff-Puff and gas drive tests successfully, and acquired obvious economic benefit; The CO₂ Huff-Puff tests in Liaohe oil field and CO₂ drive tests in Daqing periphery oil fields are being carried out. Therefore, Gas injection(especially CO₂) for EOR has the enormous applied potential as a gordian technique for tertiary oil recovery of low permeability oil fields.In this paper, based on the comprehension of the physical and chemical properties of CO₂, a series of experiments were carried out including expansion experiment,CO₂ flooding and CO₂ Huff-Puff laboratory experiment. The results indicate that, CO₂ could make formation crude expanded,thined down and decrease the boundary tension between CO₂ with oil after injecting CO₂; The relationships of injection pressure with recovery ratio,oil exchange ratio and production gas-oil rate were researched, the higher injection pressure, the higher ultimate recovery, and it could be more than waterflooding's to 22.90%～54.33%; considering the recovery ratio,oil exchange ratio and production gas-oil rate etc, it is concluded that there are the best value of soak time,injection volume and injection pressure through CO₂ Huff-Puff, it is not benefit to visbreaking and expanding if the soak time too short, and deplete producing energy if too long; High pressure is benefit for CO₂ to intrude into reservoir deep, however, it could drive formation crude around the injection well to reservoir deep and affect Huff-Puff effect.