Interfacial tension (IFT), being an important thermodynamic property of the interface between two fluid phases in contact, can be used for phase behavior characterization, interpretation of fluid phase equilibria and miscibility. In order to examine these multitudes of roles played by interfacial tension, gas-oil IFT measurements were carried out for Rainbow Keg River (RKR) and Terra Nova reservoir fluids at elevated pressures and temperatures [1, 2]. A computerized drop shape analysis technique was used to fit the actual drop profiles of pendent oil drops with the iterative solution of the Laplace capillary equation for gas-oil IFT determination. These measurements have enabled the development of a new gas-oil miscibility determination technique called the vanishing interfacial tension (VIT) technique [1, 2].

In this paper, we propose a mass transfer enhanced mechanistic Parachor model to predict the gas-oil IFT at reservoir conditions and to identify the governing mechanism of mass transfer to attain fluid phase equilibria. The proposed model incorporates the ratio of diffusivities between the fluid phases raised to an exponent to account for mass transfer effects. The proposed model resulted in good IFT predictions as well as indicating that vaporization of light hydrocarbon components from crude oil to gas phase is the governing mass transfer mechanism for the attainment of fluid phase equilibria in these two reservoir fluids. The sensitivity studies on model results indicate that the provision of a single experimental IFT measurement is sufficient for accurate IFT predictions at other conditions from the model. Regression equation has been developed relating the exponent in the mechanistic model to the normalized solute compositions present in either of the two fluid phases for RKR and Terra Nova reservoir fluids. This regression equation, if generalized, using more reservoir crude oil-solvent systems, can be used for a-priori prediction of exponent in the mechanistic model by simply knowing the reservoir fluid compositions, without the need for experimental measurements. The use of diffusion coefficients in the mechanistic model indicates dynamic nature of IFT, thus enabling the use of model predictions to determine dynamic gas-oil miscibility in gas injection EOR projects.

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