Experimental Study On Oil-displacement Mechanism Of Alternate CO₂ And Modified Water In Low Permeability Reservoirs

Carbon dioxide and modified water alternate flooding is a combination of water flooding and gas flooding,a traditional technology that effectively improves oil and gas recovery.Alternate injection of carbon dioxide and water improves fluidity control and slows the trend of water and gas breakthrough.The combination of lighter carbon dioxide and heavier water improves the longitudinal displacement efficiency of the reservoir.This displacement method is widely used in foreign oil fields.In addition,the alternate injection of carbon dioxide and water can well control fluidity and stabilize the displacement front,thereby greatly improving the efficiency of macroscopic displacement.However,in order to further improve the development effect of water and carbon dioxide alternate flooding,based on sufficient literature data collection and research analysis,this paper pre-uses modified water instead of water,and in-depth explores and studies the effect of altered carbon dioxide flooding of modified water in low-permeability reservoirs on improving development effects and improving recovery ratio mechanisms,mainly completed the following tasks:Firstly,under acidic conditions?PH=5?,8 surfactants?CY active agent,lipopeptide active agent,Davis active agent,fluorocarbon active agent,carboxylate active agent,heavy alkylbenzene sulfonic acid?Salt active agent,petroleum sulfonate active agent,betaine active agent)and the interfacial tension of four kinds?betaine,Davis,heavy alkylbenzene petroleum sulfonate,petroleum sulfonate?reached the interfacial tension on the order of 10^-3m N/m,30 kinds of compound surfactants are compounded by combining 4 kinds of surfactants,and the optimal modified water formula system is finally selected as betaine 30%+heavy alkylbenzene by the determination of interfacial tension 70%of benzene petroleum sulfonate.Secondly,the fluid samples were retrieved on site to restore the configuration,and multiple sets of sand-filled pipe comparison experiments were performed under the conditions of reservoir formation temperature of 108?and formation pressure of 22MPa,different displacement media,modified water concentration,injection slug size,and alternate injection cycles,reasonably optimize the optimal scheme of carbon dioxide alternation flooding and optimal injection parameters.The results show that alternate flooding oil of modified water and carbon dioxide in low permeability reservoir have obvious effects on improving development,it is 22.62%more than simple water flooding,16.99%more than simple modified water flooding,10.42%more than simple carbon dioxide flooding,and 5.59%more oil recovery than water+carbon dioxide alternate flooding;The optimal injection parameters of modified water and carbon dioxide alternate flooding are 0.3%of the formula system concentration,the injection method is to alternately inject 0.1PV slugs of carbon dioxide and modified water,the total injection volume is about 0.6PV.At last,a comparison experiment of alternate flooding of carbon dioxide and modified water under the same experimental conditions was carried out by using a Beley core instead of a sand-filled pipe,and for a core 5 which is a nuclear magnetic resonance measurement,the micro oil displacement mechanism of different displacement media is analyzed.The results show that the trend of displacement effect is the same under the same displacement scheme of Beley core and sand-filled pipe.The average recovery rate of Bere core is 2.83%lower;nuclear magnetic resonance tests show that carbon dioxide displacement can greatly improve the oil displacement efficiency of micropores?about 47.43%?;alternate injection can further improve the oil displacement efficiency of micropores?about 70.6%?;modified water can improve oil displacement efficiency in large pores?about 56.47%?.The research results have deepened the understanding of carbon dioxide and modified water alternate flooding oil in low permeability reservoirs and enhanced oil recovery,it can provide certain theoretical guidance and technical support for the design of modified water and carbon dioxide alternate flooding schemes in low-permeability reservoirs,which is of great significance for guiding the production practice of low permeability reservoirs.