SPWLA Glossary

The fraction of the pore volume occupied by water in a reservoir at maximum hydrocarbon saturation. In water-wet rock. it represents the layer of adsorbed water coating solid surfaces and the pendular water around grain contacts and at pore throats. Irreducible water saturation is an equilibrium situation. It differs from “residual water saturation,” measured by core analysis because of filtrate invasion and the gas expansion that occurs when the core is removed from the bottom of the hole and brought to the surface.