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

where

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

average grain size

One of the proposed qualification of reservoir quality based on FZI is given by the Table 1.

Table1. One of the grade systems on reservoir quality based on FZI.

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See also