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 compositional multiphase flow models for porous media with the finite‐volume technique. In particular, we consider fluid mixtures featuring fast mass transfers between the phases, hence stiff. In this context, we show how the computation of mass exchange kinetics can be expensive and that erroneous saturation front locations arise. A numerical splitting method is developed which is proven to be stable with advection‐type time steps, whatever the stiffness of the mass transfer. The erroneous front location problem is illustrated 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 hyperbolic equation for the saturation. This revised version was published online in July 2006 with corrections to the Cover Date.