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 100oF, and CO2 against live decane consisting of 25 mole% methane, 30 mole% n-butane and 45 mole% n-decane at 160oF. 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.

1. Introduction
1.1 Current EOR Scenario in the US

Nearly about 377 billion barrels of crude oil remains trapped after primary recovery and secondary water floods in depleted oil fields of United States. This enormous known to exist oil resource base in depleted oil fields shows the potential prospect for enhanced oil recovery (EOR) processes in the US, especially in the present scenario of high oil prices. Currently in the US, 48% of EOR production comes from gas injection and the rest from steam injection in heavy oil fields [1]. Stosur et al. [2] studied EOR developments and their future potential in the US and concluded that miscible CO2 gas injection is slowly becoming popular and will continue to grow faster than other EOR methods. This changing mix of EOR scenario in the US is clearly depicted in Figures 1 and 2. In Figure 1, the total EOR production and the percent of total EOR production from gas injection are plotted for the past two decades. Similarly, the gas injection EOR production and the percent of gas injection EOR production from miscible CO2 gas injection are plotted for the last two decades in Figure 2. As can be seen from Figure 1, the percent of gas injection share in total EOR production has steadily increased from 18% in 1984 to about 48% in 2004. From Figure 2, the growth of miscible CO2 gas injection can be seen when compared to other conventional gas injection EOR processes. The CO2 miscible gas injection is contributing major portion of gas injection EOR production and its share in gas injection EOR has steadily increased from 38% in 1984 to about 65% in 2004. Thus, it can be concluded that currently miscible CO2 gas injection process has become the most popular EOR process for light oil reservoirs in the US. Similar trends can be observed even in the World EOR scenario [1].

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