Carbon dioxide flooding is considered to be one of the most effective enhanced oil recovery methods for the light oil reservoirs. Depending on the operating pressure, the process might be miscible or immiscible. Minimum miscibility pressure (MMP) is the most important parameter for assessing the applicability of any miscible gas flood for an oil reservoir. The miscibility condition is determined by conducting displacement tests at various pressures and the recovery is expected to improve as the displacement pressure increases, and then stays almost constant above the MMP. In this study, the MMP of pure carbon dioxide and an Iranian oil reservoir is estimated using slim-tube test. The experiment is performed under constant CO2 injection rate anddifferent displacement pressures above the bubble point pressure; input and output pressures are continually monitored during the test; the effluent liquid is flashed to atmospheric conditions and produced gases are passed through gas chromatography for compositional analysis. Volumetric oil recovery is measured, and oil recovery is plotted against the injection pressure at a 1.2 pore volume of the injected fluid. The results show that the oil recovery is more than 90% of original oil in place for all the pressures with a constant slope above the bubble point pressure. Therefore, MMP is considered to be equal to the bubble point pressure. In addition to the plot of oil recovery versus pressure, miscibility is also confirmed by the effluent gas composition, slim-tube pressure drop, and visual observations of phase behavior. The single phase high pressure fluid at the outlet of slim-tube, the small pressure drop across it, and no methane bank in the composition of effluent gas indicate that the miscibility is completely achieved. The experimental results are validated by modeling the test using a fully compositional simulator. The effect of varying CO2 mole fraction in the injected fluid is also investigated. The results show that an increase in CO2 mole fraction of the injected gas significantly decreases MMP.
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