In this paper, we investigate theoretically and experimentally the combined effects of gravity and viscous forces on residual nonwetting-phase saturation in immiscible displacements. We develop simplified models of oil entrapments to account for the combined effects of gravity and viscous forces on snap-off and bypass processes. Both models demonstrate theoretically that the combined effects of gravity and viscous forces can be represented by the Brownell-Katz number (N-BK), the sum of a capillary number (ratio of viscous force to capillary force, del Pk/sigma) and a Bond number (ratio of gravity force to capillary force, Delta rho k/sigma). The magnitude of the Bond number in comparison to capillary number depends on the permeability of the medium and the wetting-phase relative permeability. Immiscible displacements were conducted in a vertical sandpack (119 cm long and 1.95 cm in diameter) to test the proposed theory. We obtained a wide range of Bond numbers and capillary numbers by varying interfacial tension (IFT), phase density difference, and injection rates for mixtures of brine, isopropanol, and iso-octane. Results from displacements with gravity forces assisting and with gravity forces impeding recovery support the linear combination of capillary and Bond numbers (N-BK). The experimental results presented along with previously available experimental data verify that the scaling results hold for variation of any of the parameters that appear in the Bond and capillary numbers.