(1)	$\begin{array}{l}\displaystyle j = \frac{J}{h_e} =\frac{q}{h_e \cdot \|p_e - p_{wf}\|}\end{array}$

where

$\begin{array}{l}J\end{array}$	productivity index
$\begin{array}{l}q\end{array}$	flowrate (the specification of which may differ, see below)
$\begin{array}{l}p_e\end{array}$	formation pressure
$\begin{array}{l}p_{wf}\end{array}$	bottom hole pressure at sandface
$\begin{array}{l}h_e\end{array}$	effective formation thickness

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