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A measure of ability of a porous formation to allow a certain fluid to pass through it:

(1)	$\begin{array}{l}\displaystyle k = \mu \cdot \frac{\| {\bf v}\|}{ \| \nabla p \|}\end{array}$

where

$\begin{array}{l}\mu\end{array}$	fluid viscosity
$\begin{array}{l}\| {\bf v}\|\end{array}$	fluid velocity
$\begin{array}{l}\nabla p\end{array}$	pressure gradient

Symbol	Dimension	SI units	Oil metric units	Oil field units
$\begin{array}{l}k\end{array}$	L²	m²	md	md

Permeability depends on fluid type, filling the porous media and the fluid type which is sweeping through it which leads to splitting its value into a product of two components:

(2)	$\begin{array}{l}\displaystyle k = k_a \cdot k_r\end{array}$

where

$\begin{array}{l}k_a\end{array}$	absolute permeability to air which is defined by the reservoir pore structure only, also denoted as $\begin{array}{l}k_{abs}\end{array}$ or $\begin{array}{l}k_{air}\end{array}$
$\begin{array}{l}k_r\end{array}$	relative permeability to a given fluid which is defined by the interaction between fluid and reservoir matrix

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Permeability (formation)

See also