Study On Oil Displacement Mechanism And Influencing Factors Of CO2-WAG In Heterogeneous Reservoir

Water-alternating-gas（WAG）can effectively control the displacement front and improve the sweep efficiency while giving full play to the advantages of CO₂.There are great differences in reservoir physical properties and dominant seepage channels in heterogeneous reservoirs.Therefore,different reservoir conditions and gas injection parameters have a great impact on the displacement effect of water gas alternate drive.In this thesis,the effects of heterogeneity and water gas alternation parameters are systematically studied from the macro and micro perspectives through indoor experiments and numerical simulation,and the seepage mechanism is analyzed.Firstly,through the experiment of CO₂-WAG after parallel long core water flooding,the oil displacement characteristics under different wag parameters are studied.The experiment shows that CO₂-WAG can effectively alleviate the negative effects caused by strong heterogeneity.Compared with different injection parameters,the oil recovery ratio is the highest when the permeability ratio is 10,the gas-water ratio is 1:3 and the slug size is 0.05pv.Secondly,through nuclear magnetic resonance technology,the characteristics of CO₂ gas water alternate flooding under interlayer heterogeneity and micro pore heterogeneity and the changes of micro pores before and after CO₂ displacement are quantitatively characterized.Experiments show that CO₂-WAG can effectively drive pores with radius less than 2μm under strong interlayer heterogeneity.In view of micro heterogeneity,the recovery rate of single peak core is the highest.CO₂ flooding has a certain damage effect on the core,some large pores are blocked and become small pores,and the heterogeneity becomes stronger.Finally,using numerical simulation method,taking Changqing H block reservoir as the research object,the WAG efficiency of different vertical heterogeneous reservoir models and actual reservoir models is evaluated,and the best development parameters are optimized.For vertical heterogeneous reservoir,positive rhythm reservoir has the highest recovery ratio.The injection parameters of the geological model of Changqing H block are optimized.The results show that the overall oil recovery ratio and economic applicability are high when the gas water ratio is 1:3,the gas injection speed is 4500 m³/D,the alternating period is 75 days,and the water cut of transfer injection is less than 60%.