It is defined as the ratio of the effective permeability of a fluid at a given value of saturation to the effective permeability of that fluid at 100% saturation. Thus, the relative permeability can be expressed symbolically as..
K rg = K g / K
K rw = K w / K
K ro = K o / K
Which are relative permeability to gas, oil, and water, respectively. It should be pointed out that when three phases are present the sum of the relative permeability (kro + krg + krw) is both variable and always less than or equal to unity. An appreciation of this observation and of its physical causes is a prerequisite to a more detailed discussion of two and three-phase relative permeability relationships. It has become a common practice to refer to the relative permeability curve for the non wetting phase as know and the relative permeability for the wetting phase as kw.
When a wetting and a non wetting phase flow together in a reservoir rock, each phase follows separate and distinct paths. The distribution of the two phases according to their wetting characteristics results in characteristic wetting and non wetting phase relative permeabilities. Since the wetting phase occupies the smaller pore openings at small saturations, and these pore openings do not contribute materially to flow, it follows that the presence of small wetting phase saturation will affect the non wetting phase permeability only to a limited extent. Since the non wetting phase occupies the central or larger pore openings which contribute materially to fluid flow through the reservoir, however, small non wetting phase saturation will drastically reduce the wetting phase permeability.