where

The Flow Zone Indicator is considered to be constant within a given lithofacies. 

This correlation was historical the first physical permeability model, based on the fluid flow in porous media with simplified structure consisted of a bunch of independent capillaries with various diameters.

Later on it's been upgraded to percolation through inter-grain porous space which specifies the Flow Zone Indicator  $//<![CDATA[ \begin{array}{l}FZI\end{array} //]]>$ as a function of grains shape, size distribution and packing.

The most popular correlation with a mean grain size  $//<![CDATA[ \begin{array}{l}D_g\end{array} //]]>$ is given as:

where coefficient  $//<![CDATA[ \begin{array}{l}a\end{array} //]]>$ is a function of grain shape, packing, inter-grain clay and, as a consequence, of inter-grain effective porosity  $//<![CDATA[ \begin{array}{l}\phi\end{array} //]]>$.

See also

Petroleum Industry / Upstream / Subsurface E&P Disciplines / Petrophysics / Absolute permeability / Absolute permeability @model