Capillary Pressure

When two immiscible fluid phases, such as oil and water, are present in a porous
medium a pressure differential is observed between the two fluid phases that can be
expressed as capillary pressure.

A generalized expression for capillary pressure, as it relates to fluid phases in porous media, is the difference between the pressure exerted by the nonwetting phase and the pressure exerted by the wetting phase pressure. The magnitude of the capillary pressure in a porous medium is influenced by fluid saturations, interfacial tension between the two fluid phases, and the radius of the pore
and pore throat, among other factors.

Where:

Pc = Capillary Pressure

Pnw= Nonwetting Phase Pressure

Pw = Wetting Phase Pressure

In an oil–water system, water is generally the wetting phase, and oil is the nonwetting phase.

References:

• Satter, A., and M. Iqbal, G. Reservoir Engineering The Fundamentals, Simulation, and Management of Conventional and Unconventional Recoveries. Elsevier, 2016.