Application of a New Mechanistic Parachor Model to Predict Dynamic Gas-Oil Miscibility in Reservoir Crude Oil-Solvent Systems

Abstract

Abstract 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. Introduction Interfacial tension is an important property for many processes such as enhanced oil recovery by gas injection and flow through porous media, and in mass and heat transfer phenomena. However, the experimental data on interfacial tension for complex fluid systems involving multicomponent phases are scarce. Therefore, there has long been a need for a simple and accurate computational model for prediction of interfacial tension in multicomponent hydrocarbon systems. Several models have been proposed for the calculation of interfacial tensions of simple fluids and mixtures in the past few decades. The most important among these models are the Parachor model [3, 4], the corresponding states theory [5], thermodynamic correlations [6] and the gradient theory [7]. While most of the thermodynamic properties refer to individual fluid phases, interfacial tension (IFT) is unique in the sense that it is a property of the interface between the phases. The IFT, being a property of interface, is strongly dependent on the compositions of fluid phases in contact, which in turn depend on the mass transfer interactions between the phases. The commonly occurring mass transfer mechanisms between the fluid phases to attain equilibrium are vaporization, condensation or a combination of the two. In the vaporizing drive mechanism, the vaporization of lighter components (C1 to C3) from the liquid (crude oil) to hydrocarbon vapor phase promotes the attainment of miscibility of the two phases. In condensing drive mechanism, the condensation of intermediate and heavy components (C4 to C8) from hydrocarbon gas to the crude oil is responsible for attaining miscibility between fluid phases. In combined condensation and vaporization drive mechanism, the simultaneous counter-directional mass transfer mechanisms, that is, vaporization of lighter components from crude oil to gas and condensation of intermediate and heavy components from gas to crude oil, are responsible for attaining miscibility of the phases. These mass transfer interactions affect the compositions of both phases and hence their interfacial tension. Therefore, the changes in IFT can be used to infer information on mass transfer interactions taking place prior to the attainment of fluid phase equilibrium and miscibility.