Critical capillary number: Desaturation studied with fast X‐ray computed microtomography

For subsurface flow, the correct definition for the balance of viscous and capillary forces, the so‐called capillary number ( Ca ), which predicts the mobilization of nonwetting phase, has been a long‐standing controversy. The most common microscopic definition results in nonwetting phase mobilization at Ca ~10 −5 , which is counterintuitive. Rather, mobilization should occur at Ca ≥ 1. As demonstrated herein, by using fast synchrotron‐based X‐ray computed microtomography and averaging of thereby accessible pore‐scale parameters to macroscale values, a macroscale Ca definition is validated and shown to correctly describe mobilization at Ca ~1. The presented methodology provides a connection between desaturation and pore‐scale fluid topology and gives insight into when and how Ca changes with system size. The broader implication implies that it makes a difference whether desaturation is driven by an increase in flow rate or viscosity or decrease in interfacial tension since Ca incorporates nonwetting phase cluster length, which is process‐dependent.