One of the measures of rock quality in terms of availability for flow:

$\begin{array}{l}\displaystyle { \rm FZI } = \frac{\rm RQI}{\phi_z}\end{array}$

where

$\begin{array}{l}\rm RQI\end{array}$	Rock Quality Index
$\begin{array}{l}\phi_z\end{array}$	normalised porosity

Unit = μm

It has much weaker dependence on effective porosity and shaliness and can be often considered constant within a given lithofacies which makes it a very efficient tool for lithofacies analysis

The constant FZI means that permeability strictly follows Cozeny-Karman permeability @model, although it's rarely met in practise.

The FZI depends on grains size distribution, shape and packing and can be modelled as follows:

(1)	$\begin{array}{l}\displaystyle {\rm FZI} = \frac{1}{\sqrt{F_S} \, S_{gV} \, \tau }\end{array}$

where

$\begin{array}{l}S_{gV} = \Sigma_e/V_\phi\end{array}$	surface pore area per unit pore volume	$\begin{array}{l}\Sigma_e\end{array}$	pore surface area
$\begin{array}{l}F_S\end{array}$	pore shape factor	$\begin{array}{l}V_\phi\end{array}$	pore volume
$\begin{array}{l}\tau\end{array}$	pore channel tortuosity

In some practical cases this can be further simplified to:

(2)	$\begin{array}{l}\displaystyle {\rm FZI} = 0.0037 \cdot \frac{d}{\tau }\end{array}$

where

$\begin{array}{l}d\end{array}$

average grain size

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See also