CO2 flooding is one of the most promising EOR methods. Due to the complicated compositional change during the displacement process, the effect of CO2 injection on phase behavior of oil and gas system is still not clear so far. It can great limits the forecast of development effect and composition optimization of injection gas. In this paper, taken an offshore oilfield of China as an example, based on compositional grouping, PVT test and slim tube test, phase behavior of oil and gas system during impure CO2 flooding was studied by PVT Sim 20. Results show that under reservoir conditions, the thickness of oil layer can greatly influence the composition distribution of the reservoir fluids. Thus, the phase behavior can be significantly different. For the initial stage of CO2 flooding, two-phase region of the phase diagram continuously extends. The cricondenbar increases obviously and the cricondentherm decreases. Critical point moves towards top left. With the CO2 flooding proceeding, the two-phase region begins to shrink when the gas oil ratio achieves 2:1. The cricondenbar increases slightly and the cricondentherm decreases significantly. Moreover, with the mixing of contaminated gases, the two-phase region enlarges and the cricondenbar increases greatly. However, the cricondentherm changes slightly. For pure CO2, the saturation pressures for different oil gas ratio are all lower than reservoir pressure, thus, it can achieve miscible flooding. Comparatively, for impure CO2, it becomes more and more difficult to fully miscible with oil. In sum, the CO2 displacement is the synthetic action of dissolution, dispersion, expansion, extraction, and viscosity reduction. Macroscopically, it is shown as the dynamic change of phase behavior of CO2-oil system.

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