Interfacial viscous coupling: a myth or reality?

This paper is a review of interfacial viscous coupling in multiphase porous media flow which has been a matter of debate for the last two decades. Several researchers have viewed the occurrence of interfacial viscous coupling phenomena as a reality and strongly recommended its incorporation into the existing Darcy formulation for multiphase flow through porous media. Other investigators, however, have argued that the effect(s) of mutual transfer of momentum between flowing fluids is extremely small; hence, it can be ignored and there is no real need to modify the conventional Darcy formulation. Others, however, disagree, even with the existence of such phenomena. This paper is an attempt to resolve this controversy. To accomplish this task, and to present a fair judgment about the controversy, three theoretical approaches are reviewed. These are (1) the volume averaging methods, (2) the irreversible thermodynamic methods and (3) the use of analogous models, which were used to develop a theoretical understanding of viscous coupling phenomena. On the basis of this review, it was found that the phenomena of interfacial viscous coupling is closer to a reality rather than a myth. Therefore, it is strongly recommended that the conventional two-phase flow formulation of Darcyʹs law should be modified. Some of the important problems, related to validation of interfacial viscous coupling theory, and possible means for resolving them, are identified.