Fluid phase behavior in reservoir and composition optimization for flue gas injection on condensate development

Flue gas injection is an approach for gas condensate reservoir development to maintain formation pressure and improve condensate recovery.And it is also an important measure to realize CO₂ storage and to have carbon dioxide emissions peak and achieve carbon neutrality on time respectively.The fluid phase behavior in the process of gas injection is complex and has a significant impact on the development effect.At present, most of the relevant studies focus on the influence of single gas injection such as CO₂ and N₂ on the phase state of condensate gas reservoir fluid.While flue gas is a mixture of the two, the compound influence of gas components with different proportions is not clear, and the optimization of flue gas components based on phase state also lacks corresponding basis.Based on PR equation of state and phase equilibrium theory, we analyzed the influence of injected gas on two-phase distribution and dew point pressure by drawing P-T phase diagram under the condition of a real gas condensate reservoir, studied the effects of injected gas (CO₂, N₂ and the mixture of them) on oil saturation and condensate recovery during retrograde condensation by simulated constant volume depletion test, and contrasted the miscibility of flue gas components driving condensate with the multiple-mixing-cell method to calculate minimum miscibility pressure.According to the phase characteristics analyzed, the development strategy is determined and the composition of flue gas is optimized.The results show that the injection of flue gas will increase the dew point pressure in the mixing zone, in which N₂ also increases the dew point pressure and CO₂ plays the opposite role; flue gas can evaporate condensate, among which CO₂ has the best evaporation effect; the miscibility of CO₂ is stronger than that of N₂.It is recommended to inject conventional flue gas when it is sufficient to keep the formation pressure higher than the dew point pressure, or to keep the CO₂ content in the flue gas above 50% when the flue gas is insufficient.