# Capillary Number

## Definition - What does Capillary Number mean?

Capillary Number is the dimensionless group which shows how the viscous forces and the surface tensions acting across the liquid and gas interface or between two immiscible liquids affect each other. It is related to fluid dynamics and can be expressed as:

Where,

Nc = Ca = Capillary Number

*ยต* = Liquid dynamic viscosity

U = Liquid velocity

= Surface tension or interfacial tension between liquid and gas or two immiscible liquids.

## Petropedia explains Capillary Number

Capillary Number in oilfield is represented by "Nc," whereas in chemical engineering field it is represented by "Ca." By equation, we can say that viscous forces are inversely proportional to the surface tensions. When the fluid is flowing and Nc >> 1, then the viscous forces are stronger than the interfacial forces or tensions and when Nc << 1, then the viscous forces are very weak and are negligible to the interfacial tension. For high velocity flows, Capillary Numbers are generally high and for low velocity flows, they are low.

Capillary Number for flow through reservoir pores is 10^{-6}.

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Capillary Number for flow through a producing well is approximately close to 1.

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