Dictionary:Irreducible water saturation
The fraction of the pore volume occupied by water in a reservoir at maximum hydrocarbon saturation. It represents water that has not been displaced by hydrocarbons because it is trapped by adhering to rock surfaces, trapped in small pore spaces and narrow interstices, etc. Irreducible water saturation is an equilibrium situation. It differs from residual water saturation, the value measured by core analysis, because of filtrate invasion and the gas expansion that occurs when a core is removed from the bottom of the hole to the surface. Also called immovable water.