Capillary pressure

In fluid statics, capillary pressure is the difference in pressure across the interface between two immiscible fluids, and thus defined as
In oil-water systems, water is typically the wetting phase, while for gas-oil systems, oil is typically the wetting phase.
The Young–Laplace equation states that this pressure difference is proportional to the interfacial tension, , and inversely proportional to the effective radius, , of the interface, it also depends on the wetting angle, , of the liquid on the surface of the capillary.
The equation for capillary pressure is only valid under capillary equilibrium, which means that there can not be any flowing phases.
In porous media
In porous media, capillary pressure is the force necessary to squeeze a hydrocarbon droplet through a pore throat (works against the interfacial tension between oil and water phases) and is higher for smaller pore diameter. The expression...