The ratio between the [[effective permeability]] to a given fluid at a partial [[saturation]] and the [[permeability]] at 100% saturation. The ratio of the amount of a specific fluid that will flow at a given saturation, in the presence of other fluids, to the amount that would flow at a saturation of 100%, other factors remaining the same. It ranges in value from zero at low saturation to 1.0 at 100% saturation of the specific fluid. Since different fluid phases inhibit the flow of each other, the sum of the relative permeabilities of all phases is always less than unity.

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