Comparative Evaluation of a New MMP Determination Technique

Abstract A new experimental technique of vanishing interfacial tension (VIT) has been reported in recent literature for quick and cost-effective determination of gas-oil miscibility. However, this technique has been criticized due to the perceived absence of compositional path specification as well as for lack of the confirmation against standard gas-oil systems. In this paper, we address these concerns by conducting interfacial tension measurements at elevated pressures and temperatures in two standard gas-oil systems and at varying molar compositions of gas and oil in feed mixtures. The two standard gas-oil systems used are: CO2 against n-decane at 100°F, and CO2 against live decane consisting of 25 mole% methane, 30 mole% n-butane and 45 mole% n-decane at 160°F. In addition to the pendent drop technique, the capillary rise technique has been adapted and successfully used for low gas-oil interfacial tension measurements at elevated pressures and temperatures. Though gas-oil ratio was found to have an impact on mass transfer rates, the interfacial tension between gas and oil was unaffected as the fluid phases approached equilibrium. This indicates the compositional path independence of gas-oil interfacial tensions measured at near-equilibrium and miscibilities determined using the VIT technique. The minimum miscibility pressures determined using the VIT technique matched well (within 4-8%) with the reported slim-tube miscibilities for both the standard gas-oil systems used. This paper relates first- and multiple-contact miscibility development in gas injection displacement processes to laboratory gas-oil interfacial tension measurements. We also found that the dynamic behavior of gas-oil interfacial tension reflects the multi-stage contact between gas and oil that occurs in the reservoir displacement processes. Thus this experimental study demonstrates the indisputable interrelationship between interfacial tension and miscibility and hence encourages the wide use of VIT technique for rapid and cost-effective determination of minimum miscibility pressures and enrichments in improved oil recovery applications.