Ability of reservoir to translate fluid volumes throughout the pore volume:

(1)	$\begin{array}{l}\displaystyle \sigma = \bigg< \frac{k}{\mu} \bigg> \ h\end{array}$

where

$\begin{array}{l}\bigg< \frac{k}{\mu} \bigg>\end{array}$	multiphase fluid mobility
$\begin{array}{l}h\end{array}$	effective formation thickness

For a single-phase fluid it simplifies to:

(2)	$\begin{array}{l}\displaystyle \sigma = \frac{kh}{\mu}\end{array}$

where

$\begin{array}{l}k\end{array}$	formation permeability to a single-phase fluid
$\begin{array}{l}\mu\end{array}$	fluid viscosity
$\begin{array}{l}h\end{array}$	effective formation thickness

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See also