The use of foam for mobility control is a promising mean to improve sweep efficiency in EOR. Experimental studies discovered that foam exhibits three different states (weak foam, intermediate foam, and strong foam). The intermediate foam state is found to be unstable in the lab whereas the weak- and strong-foam states are stable.
The model of Kam (Colloids Surfaces A, 2008) is the only mechanistic foam model that can fit a variety of steady-state experimental data including multiple steady states. This model is modified from a previous mechanistic foam model to resolve the intrinsic instability of the strong-foam state. Simple finite-difference simulations have found that an arbitrary perturbation grows for the unstable intermediate foam but diminishes for the strong- and weak-foam states.
The issue of the stability of foam states, especially the strong-foam state, is a serious concern in application of foam in EOR. Instabilities may rule out one or more states and consequently have considerable effect on reservoir sweep efficiency and injection pressure.
Here, for the first time the stability of the various foam states is investigated by a semi-analytical stability-analysis method. We demonstrate the instability of most intermediate states, consistent with the laboratory observations. However, fine analysis reveals a slight instability of the strong-foam state. We show that the diffusion, whether introduced artificially by the finite-difference scheme or representing physical dispersion, damps this slight instability. We obtain good agreement with finite-element simulations with and without diffusion.