Definition - What does Capillary Pressure mean?
Capillary Pressure is the difference in pressure between two immiscible phases of fluids occupying similar pores due to interfacial tension between the phases. This interfacial tension has to be overcome so as to initiate the flow. The fluid is responsible for wetting the surface of the formation rock known as wetting phase in the presence of non-wetting phase. Water is generally the wetting phase in oil-water systems and oil in gas-oil systems. Gas is always the non-wetting phase in both the systems.
Petropedia explains Capillary Pressure
Capillary Pressure interaction forces act between and within fluids and bounding solids of those fluids. The forces include both the adhesive and cohesive forces. Liquid is considered as wetting phase when the adhesive force is greater than the cohesive force. On the other hand, when the cohesive force is greater than the adhesive force, the liquid is considered as non-wetting phase. The Capillary Pressure is essential in the reservoir studies as the reservoir rocks are approximated by the bundle of capillaries in which hydrocarbons are the non-wetting phase and water the wetting phase.