Application of a New Technique to Optimize Injection Gas Composition For the Rainbow Keg River F Pool Miscible Flood

Abstract

Abstract A new experimental technique, called the Vanishing Interfacial Tension (VIT) technique, has been successfully utilized in this study to optimize solvent composition for the miscible gas injection project being considered for the Rainbow Keg River F Pool. The VIT technique consists of measuring the interfacial tension between the injected gas phase and the reservoir crude oil at the temperature of the particular oil containing formation and at varying pressures and/or enrichment levels of the gas phase. These interfacial tension measurements are carried out by computer digitization of the image of the profiles of sessile and pendant drops of crude oil enclosed in a surrounding medium of the injection gas. By fitting these experimental drop profiles with the iterative solution of the Laplace capillary equation, the value of the interfacial tension is obtained at each pressure or enrichment level. By making a plot of the interfacial tension against the independent variable (either pressure or enrichment), accurate values of the minimum miscibility pressure (MMP) and/or the minimum miscibility composition (MMC) are obtained by extrapolation to zero interfacial tension. This paper describes the new VIT technique and provides experimental evidence for its validity by comparing the VIT-MMP with slim-tube-MMP. Each MMP determination using the VIT technique was accomplished within 4 – 6 hours, while 4 – 6 weeks were required for the slim-tube technique. The paper also describes the application of the VIT technique to the problem of optimizing the injection gas composition at the Rainbow Keg River F Pool miscible flood. For this field application of the technique, live reservoir crude oil as well as lean and rich gas mixtures, matching field gases in composition, were used in the experiments. Gas/oil interfacial tension measurements were made in "first contact" as well as "equilibrium" modes. The optimized solvent composition was then verified by a single slim-tube test and compared with equation of state predictions. It has been demonstrated through this study that the new VIT technique is a reliable, accurate and fast technique for cost effective determination of MMPs and optimum injection gas compositions. Introduction Hydrocarbon miscible flooding is by far the most important conventional oil EOR process in Canada. It accounts for 83% of the 164,600 b/d of Canadian conventional oil EOR production(1). Furthermore, it is worth noting that most of the 104,300 b/d EOR production increase in 1992 (over that in 1990) in the US was also due to miscible projects based on both CO2 and hydrocarbon gases(1). Further improvements in the economics of miscible flooding could be accomplished by minimizing the cost of solvent blends and the cost of their injection. These can be achieved by (1) optimizing the solvent composition so that minimum enrichment of the dry gas by the C2+ fraction would yield cheaper solvents for injection and enable increased sales of the natural gas liquids, (2) increasing the accuracy of determination of minimum miscibility pressures for a given solvent composition and reservoir temperature which, in turn, would enable operation at near miscible conditions resulting in considerable compression cost savings.