Formation permeability to a given fluid $\begin{array}{l}\alpha\end{array}$ -phase normalised to the reference value, usually to air permeability $\begin{array}{l}k_{air}\end{array}$ :

(1)	$\begin{array}{l}\displaystyle k_{r\alpha} = \frac{k_\alpha}{k_{air}}\end{array}$

Units = dimensionless

The concept of relative permeability is closely related to Wettability.

Mathematical models of relative permeability are called RPM and summarized in the following table:

2-phase RPM

3-phase RPM

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See also