Solving stiff mass transfers in compositional multiphase flow models: Numerical stability and spurious solutions

This paper points out two numerical problems linked to the resolution of co mpositional multiphase flow models for porous media with the finite-volume technique. In particular, we consider fluid mixtures featuring fast mass tr ansfers between the phases, hence stiff. In this context, we show how the c omputation of mass exchange kinetics can be expensive and that erroneous sa turation front locations arise. A numerical splitting method is developed w hich is proven to be stable with advection-type time steps, whatever the st iffness of the mass transfer. The erroneous front location problem is illus trated and shown to be intrinsically linked to the numerical diffusion. If we assume that the fluids are in thermodynamical equilibrium, we find that spurious solutions can be avoided by deriving and solving a new uncoupled h yperbolic equation for the saturation.