Study On Aaptability Ealuation Of CO₂ Fooding In Extra Low Permeability Reservoir Of Bei Block In Daqing Oilfield

The research object of this thesis is the Bei Block of Daqing Oilfield,which is a reservoir mainly with extra-low permeability（1～10 m D）,and the area of Bei Block is about 4.5 km².The block is rich in oil and gas resources,but due to the poor physical properties of the reservoir.There are problems such as large starting pressure of water flooding,difficulty of water injection diffusion,small effective area of the well,and low recovery rate.CO₂ flooding technology is used for such problems,but the current CO₂flooding technology has insufficient understanding of the dissolution mechanism,unclear influencing factors and poor field effect.So it is necessary to carry out CO₂ flooding adaptability evaluation research.In this thesis,firstly,we use PVT experimental method to clarify the dissolution law of CO₂ and reveal the interaction mechanism between CO₂ and oil,water and oil-water mixture.Then,we use indoor long core displacement experiment to carry out experimental research on the main control factors of CO₂ flooding to improve the recovery rate.Such as temperature,pressure and permeability.Finally,based on the mine data,we combine the conclusion of the influence factors of the previous CO₂ flooding to evaluate the adaptability of the test area.CMG reservoir model is established based on geological,development,fluid and other dynamic and static data.On the basis of reserves fitting and history matching,the wells in the test area are classified and provided with optimization plan based on the results of previous experiments combined with field production data.The current CO₂ flooding recovery enhancement potential of the test area is evaluated.The dissolved amount of CO₂ in formation oil（61.64 mol%）is 26 times higher than that in formation water（2.41 mol%）under the formation pressure of 17.6 MPa,indicating that formation water（or injection water）can seriously affect the dissolution effect of CO₂in the reservoir.The polar difference analysis method shows that the main and secondary factors affecting the CO₂ flooding to improve the recovery rate in order are permeability,temperature,water content,gas injection,and pressure.Based on this,the adaptation evaluation of reservoir in Bei block of Daqing oilfield,the best permeability of Zone 3 is4.9 Md.According to the oilfield records,it is still in the stage of gasless oil recovery.Combined with the results of gas injection experiment,it is judged that the recovery rate is less affected by the gas-oil ratio.The maximum effective thickness of the reservoir is63.80 m,and the second largest available reserve is 292×10⁴ t.The potential for CO₂ gas injection development is large,so the block selected for this CO₂ gas injection test is Zone3.The gas injection wells in the Zone 3 are categorized and adjusted.The annual injection level of type I is small（0.008 PV/a）,and the oil recovery rate increases year by year.So the gas injection should be increased.The annual injection level of type II is stable（0.01PV/a）,and the oil recovery rate decreases year by year.So the gas injection should be reasonable.The annual injection level of type III is high（0.02 PV/a）,and the oil recovery rate decreases year by year.So the gas injection should be controlled.The optimization results of gas injection scheme for this block show that the best scheme is gas-water slug ratio of 1:1 and gas-water alternation period of WAG21（180d gas and 90d water）,which has the best effect of comprehensive recovery improvement under the conditions of this parameter.The geological model of reservoir in Zone 3,after the adjustment of WAG,has 98.97×10⁴ t of gas injection and 38.96×10⁴ t of gas production in 15 years,which can realize 38.42×10⁴ t of oil production increase.The research results of this thesis will provide important theoretical support and technical support for the promotion of CO₂flooding technology in Bei Block of Daqing Oilfield.