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Formation permeability to a given fluid -phase normalised to the reference value, usually to air permeability :

k_{r\alpha} = \frac{k_\alpha}{k_{air}}

Units = dimensionless

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 relative permeability depends on the current rock saturation:

k_{r\alpha} = k_{r\alpha}(s = \{s_\alpha\})

For 3-phase Oil + Gas + Water fluid model this is going to be:

k_{rw} = k_{rw}(s_w, s_g)

k_{ro} = k_{ro}(s_w, s_g)

k_{rg} = k_{rg}(s_w, s_g)

Mathematical models of relative permeability are often abbreviated RPM.

See also

Physics /  Fluid Dynamics / Percolation

Petroleum Industry / Upstream / Subsurface E&P Disciplines / Field Study & Modelling 

[ Petrophysics ] [ Basic reservoir properties ] [ Wettability ] [ Permeability ] [ Absolute permeability ]  [Relative permeability] 

Reference

Fekete – Relative Permeability Correlations