Viscosity Correlations for Compositional Reservoir Simulators

Abstract

Compositional dependence of viscosities of reservoir fluids, where in-situ inter-phase mass transfer dominates, warrants judicious selection of correlations to be used in compositional reservoir simulators. Two of the selection criteria are that the vapor and liquid phase viscosities approach each other in the vicinity of the critical point and they should be well behaved functions of pressure, temperature and composition. As vapor and liquid phase viscosity correlations are usually developed independent of each other these criteria are not often met for most of the reservoir fluids encountered. Some of the viscosity correlations are phase density dependent which further require a consistent set of vapor and liquid phase density correlations or a reliable equation of state to predict both the phase densities. Five sets of critical composition data are chosen for analysis. The choice is such so as to encompass reservoirs which may be candidates for compositional simulation and is also based on the mo lal average boiling points of the mixture. Each of these critical compositions are flashed along the critical isotherm at suitable pressure intervals, upto the critical point. Densities and viscosities are then computed for liquid and vapor phases at each pressure level using suitable correlations. Specific correlations are recommended for usage in phase viscosity computation, for each composition used in this study, supplementing the necessity of fine-tuning their parameters for specific reservoir fluids from laboratory derived fluid data. The suitability of phase density correlations, to be used in conjunction with viscosity correlations, is also discussed.