Critical Points of Mixed Fluids and their Numerical Treatment

Abstract

The simulation of oil reservoirs based on compositional models requires efficient numerical methods for computing multiphase equilibria of mixed fluids. The number of coexisting phases e.g. liquid, vapour and aqua, depends on the composition of the fluid as well as the temperature and pressure. As these external conditions change, a given mixed fluid may reach a transition state where a new phase emerges or an old one disappears. Most of these so called saturation points do not correspond to local bifurcations in the governing equations. Rather we have a global exchange of stability as in the case of a double well potential whose two local minima have exactly the same energy. The proper numerical treatment of such transitions requires techniques that are familiar from the handling of degenerate constraints in optimization algorithms.