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

2013; American Geophysical Union; Volume: 41; Issue: 1 Linguagem: Inglês

10.1002/2013gl058075

1944-8007

Ryan T. Armstrong, Apostolos Georgiadis, Holger Ott, Denis Klemin, Steffen Berg,

Groundwater flow and contamination studies

Abstract 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.