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:

where

• N_C = Ca = capillary number
• μ = fluid viscosity
• U = fluid velocity
• σ = surface or interfacial tension.

See: interfacial tension