1.
n. [Enhanced Oil Recovery]
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 N_{C} in the oil field and Ca in chemical engineering. For a flowing liquid, if N_{C} >>1, then viscous forces dominate over interfacial forces; however if N_{C} <<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 N_{C} is ~10^{−6}, and for flow in production tubulars N_{C} is ~1.
Capillary number equation:
N_{C} =
Ca = (μ
U) / σ
where
N_{C} = Ca = capillary number
μ = fluid viscosity
U = fluid velocity
σ = surface or interfacial tension.