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

[1] 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.