  We discuss the problem of trapping and mobilization of nonwetting fluids
during immiscible two phase displacement processes in porous media. Capillary
desaturation curves give residual saturations as a function of capillary
number. Interpreting capillary numbers as the ratio of viscous to capillary
forces the breakpoint in experimental curves contradicts the theoretically
predicted force balance. We show that replotting the data against a novel
macroscopic capillary number resolves the problem for discontinuous mode
displacement.
