On Interfacial-Tension Measurements to Estimate Minimum Miscibility Pressures

Summary Measurements of the interfacial1 tension (IFT) 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 referred to as the vanishing-IFT (VIT) technique. In this paper, we analyze the accuracy and reliability of the VIT approach using phase equilibrium and slimtube experimental observations and equation-of-state (EOS) 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 EOS characterizations using 15 components to represent the gas/oil systems yield calculations of phase compositions and densities and calculated MMPs that reproduce the experimental observations accurately. 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 slim-tube displacements. Fortuitously chosen mixture compositions can result in VIT-experiment 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-technique estimate of the MMP are often large for gas/oil systems for which the first-contact miscibility pressure (FCMP) is much larger than the slimtube MMP. We conclude, therefore, that the VIT experiment is not a reliable single source of information regarding the development of multicontact miscibility in multicomponent gas/oil displacements.