Solubility, Miscibility and their Relation to Interfacial Tension for Application in Reservoir Gas-Oil Systems

Abstract

Abstract Miscible CO2 gas injection is becoming the most popular enhanced oil recovery process for light oil reservoirs in the world. An accurate determination of miscibility conditions is essential to improve the economics of these gas injection projects. The terms, miscibility and solubility, are widely used in phase behavior studies of ternary fluid systems. The distinction between these two terms appears to be still hazy, leading to their synonymous use in some quarters. Furthermore, the relation of these two properties with interfacial tension has largely remained unexplored. Recently, a new experimental technique of Vanishing Interfacial Tension (VIT) has been reported in the literature, relating miscibility with interfacial tension for gas-oil systems. Therefore, the objective of this study is to correlate miscibility and solubility with interfacial tension and to investigate the applicability of the new VIT technique to determine miscibility compositions in ternary fluid systems. For this purpose, a standard ternary liquid system of benzene, ethanol and water was chosen since their phase behavior and solubility data were readily available. The interfacial tension of benzene in aqueous ethanol at various ethanol enrichments was measured in pendant drop mode, using the Drop Shape Analysis (DSA) technique. The experimental results indicate the applicability of VIT technique to determine miscibility conditions for ternary liquid systems as well. Comparison of IFT measurements with solubility data showed a strong mutual relationship between these two properties, in addition to demonstrating a clear distinction between solubility and miscibility. This study has identified the need to use pre-equilibrated solutions for IFT measurements in soluble regions in order to eliminate compositional effects on IFT. The interfacial tension appears to be independent of solvent-oil ratio in feed, provided pre-equilibrated solutions are used during experimentation. The conceptual extension of these experimental insights to gas-oil systems at reservoir conditions would be of immense use in determining miscibility conditions for gas injection EOR projects. Introduction Currently, miscible CO2 gas injection has become the most popular enhanced oil recovery process for light oil reservoirs in the United States. Flue gas is slowly becoming a potential source of CO2. For economical process design of these gas injection projects, an accurate laboratory determination of fluid-fluid miscibility conditions is essential. The primarily available experimental methods to evaluate fluid-fluid miscibility under reservoir conditions are the slim-tube displacement, the rising bubble apparatus and the pressure composition diagrams. However, recently the new Vanishing Interfacial Tension (VIT) technique has been reported for experimental determination of miscibility conditions in gas-oil systems [1 - 3]. This technique relies on the concept that at miscibility, the interfacial tension between the fluids must become zero due to the absence of an interface. In this method, the interfacial tension between the fluids is measured at reservoir temperature at varying pressures or enrichment levels of gas phase. The minimum miscibility pressure (MMP) or minimum miscibility enrichment (MME) is then determined by extrapolating the plot of interfacial tension, against pressure or enrichment, to zero interfacial tension. None of the other previously mentioned experimental techniques provides such direct and quantitative information on interfacial tension. In addition to being quantitative in nature, this new VIT technique is quite rapid as well as cost effective. The VIT technique has been applied to evaluate gas-oil miscibility in two field gas injection projects [1 - 3].