Hydrocarbon and non-hydrocarbon gas injection are among the most effective methods to enhance oil recovery. A key design parameter in any gas injection project is the minimum miscibility pressure (MMP). MMP can be measured in the laboratory with slim tube test, rising bubble apparatus and vanishing-interfacial-tension. Although laboratory measurements are more reliable, their high cost and longer period of time to get a few data sets made them less advantageous. There are numerous analytical, numerical, and empirical approaches devised to determine the MMP through studying thermodynamic behavior of reservoir and injection fluids in a given reservoir condition. Some numerical approaches need additional fluid and rock interaction properties such as relative permeability and capillary pressure.

Conceptually, miscibility between injected gas and reservoir oil can occur through three multi-contact mechanisms, namely condensing drive mechanism (CDM), vaporizing drive mechanism (VDM) and both condensing and vaporizing (CV) drive mechanism. Some models such as single-cell algorithm usually give overestimated MMP values since the model is based on the assumption that the drive mechanism is dictated only by CDM or VDM. In most cases, as we will see from the case studies of this research and reported by others as well, miscibility develops through both condensing and vaporizing (CV) driving mechanisms. Multiple mixing cell approach is a very effective way to handle CV drive mechanism. In this study, we modify the algorithm proposed by Ahmadi and Johns (2008), by including additional checking criterion. In spite of increased computational time overhead, our added checking criterion is found to be very useful to crosscheck the validity of the determined MMP, hence improve the reliability.

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