One of the most important properties for understanding multiphase flow in porous media is relative permeability. In two-phase flow, the relative permeability to a given phase is generally assumed only to be a function of its saturation, independent of the properties of fluids involved and/or flow conditions and ranging from zero to unity. Considering the physical principles of multiphase flow through porous media, the momentum transfer or viscous coupling appears as a hidden driving mechanism that might lead to higher than expected oil-flow rates. In an effort to provide a better understanding of oil mobility in heavy-oil reservoirs, a capillary model is used to assess the importance of lubrication in two-phase flow and to determine the effect of the viscous coupling on relative permeabilities. Different cross-sectional geometries are analyzed. The problem is addressed analytically even for the unequal-viscosity case by making use of the Galerkin method. The concept of contact angle is used in the determination of the fluid distribution inside a noncircular channel. The model results show that the viscosity ratio theoretically affects relative permeabilities, especially in systems involving heavy oil.