@wikipedia

Formation permeability to a given fluid  $//<![CDATA[ \begin{array}{l}\alpha\end{array} //]]>$-phase normalised to the reference value, usually to air permeability  $//<![CDATA[ \begin{array}{l}k_{air}\end{array} //]]>$:

The concept of relative permeability implies that different phases sweep through the same rock and under the same pressure gradient at different pace even if their viscosities are the same.

It is closely related to Wettability.

The re

Mathematical models of relative permeability are called RPM.

The usual practice is to build 

See also

Petroleum Industry / Upstream / Subsurface E&P Disciplines / Petrophysics

[ Permeability ] [ Absolute permeability ] [ Wettability ]

Reference

[1] Fekete – Relative Permeability Correlations