Relative Permeability

Sharing the Pore Space

When oil and water share the pores, neither flows at the rock's full permeability. Relative permeability is the fraction available to each phase as a function of saturation: krw for water, kro for oil, each between zero and one.

The Mobile Band

Two endpoints bound the flow. Below the connate water saturation Swc, water clings to the smallest pores and does not move, so krw is zero. Above one minus the residual oil saturation Sor, the oil is trapped and kro is zero. Both phases flow only in the band between Swc and one minus Sor; the residual saturations are the oil a waterflood can never recover.

The Shape of the Curves

The Corey exponent sets the curvature: a higher exponent bows the curves down, so each phase is more strongly blocked by the presence of the other and less of it flows at a given saturation. Where the two curves cross, the two phases have equal relative permeability. These curves, measured on core, are what couple the two-phase flow in every simulation.