Study On Mechanism And Feasibility Evaluation Of CO_ Near Miscible Displacement In F3 Oilfield Of CS Oilfield

Carbon dioxide (CO2) injection is a win-win situation for enhancing oil recovery and achieving greenhouse gas storage. It is difficult to achieve miscible CO2displacement physically in low permeability reservoir due to the purity of injected CO2, properties of crude oil, formation pressure and formation heterogeneity, etc. And near miscible CO2flooding is generally reached in the real reservoir conditions. Therefore, how to characterize phase behavior characteristics and percolation mechanism of the CO2near miscible displacement process is of great significance for CO2near miscible displacement optimization design and successful implementation.In this paper, CO2injection test area in the CS field FⅢ low-permeability reservoir was considered as the object in this research. Study was done regarding the problems of the low productivity, formation energy shortage and development difficult issues. Near miscible CO2displacement mechanism research was carried out here. Relevant content and results of the study was as follows:(1) The saturation pressure of original formation fluid, fluid density, volume factor and other parameters under the saturation pressure were obtained by the means of PVT phase behavior experiment. Winprop phase simulation software was applied to match the experimental data, which would meet the needs of the follow-up study.(2) The minimum miscible pressure (MMP) was obtained as26.62MPa, and the scope of the near miscible pressure was identified range22MPa to26.62MPa by the means of tubule experiment and formation oil CO2injection multistage contact pseudo-ternary phase diagrams.(3) Equilibrium model was carried to simulate forward and backward contact process of formation oil CO2injection. The changes of oil and gas composition, viscosity, density and interfacial tension with the increase in the number of contacts under the condition of immiscible and near miscible were obtained, revealing component mass transfer mechanism of condensate-evaporation dual function in the reservoir near miscible CO2displacement.(4) The influences of different development ways on oil displacement efficiency under near miscible pressure were obtained by the long core displacement experiment. Moreover, change rules of transition oil and gas composition, viscosity, density and interfacial tension after CO2injection under different miscible conditions were analyzed, revealing CO2near miscible displacement percolation characteristics in the porous media.(5) Numerical simulation model was established by means of geological modeling and numerical simulation software. On the basis of a history matching, the development effects of immiscible CO2injection, near miscible displacement and miscible displacement for CS FⅢ oilfield reservoir were comparatively evaluated, providing measures to avoid gas breakthrough during CO2near miscible displacement and effective theoretical foundation for CO2application in low permeability reservoirs.