Comprehensive Evaluation Of Laboratorial Experiments For CO₂ Flooding Mechanism In YSL Low Permeability Reservoirs

YSL reservoir has been developed as "three-low-reservoir" for its low-permeability,low-yield and low-abundance.Due to its poor reservoir physical properties,it is difficult to establish effective displacement for water flooding or polymer flooding,and lead to problems of high water injection pressure,low efficiency of oil wells and difficulties of water injection.So it is very important to improve the development effect of low-permeability reservoirs by making proper development technology policies for YSL reservoir and drive the unused reserves economically and effectively.Based on the reservoir characteristics and development characteristics of YSL reservoir,the domestic and foreign literatures were investigated.According to the survey,we can see that gas injection can solve the dilemma of "difficult injection and unable to extract oil" in low-permeability reservoirs.Therefore,this paper took YSL reservoir as the research object,and focused on the CO2 flooding in low-permeability reservoirs,as well as it development feasibilities.From the viewpoint of oil-gas phase,this paper studied the miscibility mechanism of underground crude oil and revealed changes of fluid phase state characteristics under the primary geological condition,and it provides a theoretical basis for CO2 flooding to enhance oil recovery.Besides,based on the actual reservoir characteristics,this paper made experimental evaluations on the feasibility of CO2 injection.The main conclusions and understandings of this paper are as follows:(1)After injecting CO2 into formation fluid,its phase behavior shows that the crude oil belongs to black oil system,and the elastic expansion of formation fluid is small,which requires water injection or gas injection to maintain pressure development.The compatibility of injected gas with crude oil is good.The main mechanism of CO2 flooding is solubilization and viscosity reduction.The first contact miscibility pressure is 32.04MPa.(2)The minimum miscibility pressures measured by the tubule test is 21.12MPa respectively.The minimum miscibility pressure of primary contact of formation oil with CO2 measured by interfacial tension is 29.45MPa.So under formation conditions,miscibility can be achieved.(3)The results of micro-displacement mechanism and residual oil distribution show that small pore is developed in core,while large pore is locally developed.The proportion of pore diameter for fluid flow is small,but CO2 can enter smaller pore to displace crude oil.From the microscopic displacement effect we can see that the immiscible effect of CO2 after water flooding is not as good as that miscible displacement after water flooding.(4)The results of long-core displacement experiments show that CO2 injection can improve oil recovery on the basis of water flooding,but the immiscible displacement of CO2 is weaker than that of miscible displacement.Besides,post-soaking imbibition can further improve CO2 displacement effect.