In the majority of practical cases the low permeability values have very weak dependence on low porosity values.

This can be fairly discounted from potential production and considered as no-flow rocks with zero permeability thus leading to the following permeability modification:

k = 
\left\{
\begin{aligned}
k(\phi) , \, \phi> \phi_0 \\ 
0 , \, \phi \le \phi_0 \\
\end{aligned}
\right.

where is minimal porosity contributing to flow.

See also