On IFT Measurements to Estimate Minimum Miscibility Pressures

Abstract

Abstract Measurements of the interfacial tension of mixtures of a reservoir fluid and injection gas at various pressures have been proposed as an experimental method for predicting the minimum miscibility pressure (MMP) in an experiment called the vanishing interfacial tension (VIT) technique. In this paper we analyze the accuracy and reliability of the VIT approach based on phase equilibrium and slimtube experimental observations and equation of state calculations of the behavior of VIT experiments for the same systems. We consider 13 gas-oil systems for which phase equilibrium and density data and slimtube measurements of the MMP are available. We show that tuned equation of state characterizations using 15-components to represent the gas/oil systems yield calculations of phase compositions and densities and calculated MMPs that reproduce accurately the experimental observations. We assume that IFTs can be calculated with a parachor expression, and we simulate the behavior of a series of VIT experiments with different mixture compositions in the VIT cell. We show that compositions of mixtures created in the VIT cell are not, in general, critical mixtures and that calculated estimates of the MMP obtained by the VIT approach depend strongly on the composition of the mixture used in the experiment. We show also that those MMP estimates may or may not differ significantly from values obtained in slimtube displacements. Fortuitously chosen mixture compositions can result in VIT estimates that agree well with slimtube MMPs, while for other mixtures, the error of the estimates can be quite large. In particular, we show that errors in the VIT estimate of the MMP are often large for gas/oil systems for which the first contact miscibility pressure is much larger than the slimtube MMP. We conclude, therefore, that the VIT experiment is not a reliable single source of information regarding development of multicontact miscibility in multicomponent gas/oil displacements.