The momentum balance equation relating a pressure gradient $\begin{array}{l}\nabla p\end{array}$ in porous medium with the induced fluid flow (percolation) with velocity $\begin{array}{l}{\bf u}\end{array}$ :

(1)	$\begin{array}{l}\displaystyle {\bf u} = - \frac{k}{\mu} \ \nabla p\end{array}$

where

$\begin{array}{l}k\end{array}$	formation permeability
$\begin{array}{l}\mu\end{array}$	fluid viscosity

For a wider range of flow regimes see Forchheimer Equation.

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See also