A dimensionless group used in analysis of fluid flow that characterizes the ratio of viscous forces to surface or interfacial tension forces. It is usually denoted NC in the oil field and Ca in chemical engineering. For a flowing liquid, if NC >>1, then viscous forces dominate over interfacial forces; however if NC <<1, then viscous forces are negligible compared with interfacial forces. Capillary numbers are usually large for high-speed flows and low for low-speed flows; thus, typically for flow through pores in the reservoir NC is ~10−6, and for flow in production tubulars NC is ~1.
Capillary number equation:
NC = Ca = capillary number
μ = fluid viscosity
U = fluid velocity
σ = surface or interfacial tension.