An analysis of the vanishing interfacial tension technique for determination of minimum miscibility pressure

Observations of interfacial tensions for mixtures of a gas and oil have been proposed as a way to measure the minimum miscibility pressure for displacement of a multicomponent oil by a gas mixture in a porous medium. The experimental approach known as the “vanishing interfacial tension technique” (VIT) is to measure interfacial tensions for a gas–oil mixture at a sequence of pressures. The estimate of the minimum miscibility pressure (MMP) is taken to be the pressure at which the interfacial tension extrapolates to zero when plotted against pressure. In this paper, the behavior observed in the VIT experiment is analyzed by performing phase equilibrium calculations with an equation of state and calculations of interfacial tensions using a parachor approach. The analysis shows that the VIT estimate of the MMP depends strongly on the overall composition of the gas–oil mixture used in the VIT experiment. Comparison of the estimates obtained for systems with three and four components and for a multicomponent crude oil system with MMPs calculated from solutions of the differential equations that describe the interactions of flow and phase equilibria indicates that the VIT approach can give estimates of the MMP that differ substantially from the MMP that would be observed in a displacement experiment. For some choices of the composition of the fluid mixture in the cell and the compositions of the oil and gas, however, it can also give estimates that are reasonably close to the value that would be obtained in a slim tube displacement experiment. However, the overall composition that gives the smallest error in the MMP estimate and the magnitude of the error cannot be determined from the VIT experiment alone. The uncertainty in the VIT estimate of the MMP arises from a fundamental limitation of the experiment: it investigates mixture compositions that are linear combinations of the initial oil and injection gas that are quite different from the critical mixture that forms at the MMP in a gas–oil displacement in a porous medium. The results indicate that the VIT approach to determine the MMP for multicomponent gas–oil displacements should be used with caution given the potential for significant errors in the resulting estimate of the MMP.