Low-temperature oil displacement by enriched-gas or carbon-dioxide (CO2) can exhibit multiphase flow of three hydrocarbon phases; the oleic (L1), solvent-rich liquid (L2), and gaseous (V) phases. The L2 phase can play a significant role in these oil recovery processes. Recently, oil displacement by three hydrocarbon phases was explained on the basis of interphase mass transfer on phase transitions in multiphase flow. However, systematic investigation into the complex interplay between phase behavior and mobilities has been hindered by issues in multiphase compositional flow simulation, such as incorrect phase identification.

This paper presents the effect of relative permeability on oil displacement by three hydrocarbon phases. A new method for robust phase identification is developed and implemented in a 1D convective flow simulator with no volume change on mixing. The new method uses tie triangles and their normal unit vectors tabulated as part of the simulation input information. The extensions of the limiting tie triangles at the upper and lower critical endpoints (UCEP and LCEP) define three different regions in composition space; the super-UCEP, super-LCEP, and sub-CEP regions. The method can properly recognize five different two-phase regions surrounding the three-phase region; the two two-phase regions that are super-CEP, and the three different two-phase regions that originate with the corresponding edges of the three-phase region in the sub-CEP region. Multiphase behavior calculations are conducted rigorously by use of the Peng-Robinson equation of state with the van der Waals mixing rules during the flow simulation. Simulation case studies are presented with a quaternary model and the West Sak oil model with 15 components.

Results show that the phase-identification method developed in this research can correctly solve for phase identities in three-hydrocarbon-phase flow simulation. The method can quantify the relative location of the current overall composition to the three-phase region in composition space. Simulation results are analyzed by use of the distance parameters that describe interphase mass transfer on multiphase transitions in oil displacement. In the case study for the West Sak oil displacement, the analysis confirms that the miscibility level of oil displacement increases with increasing methane dilution. The effect of relative permeability diminishes as the miscibility level increases owing to methane dilution. The distance parameters can properly represent the interaction of phase behavior and mobilities since they are derived from mass conservation, not only from thermodynamic conditions.

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