@wikipedia

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

k_f = \mu_f \frac{| {\bf v}|}{ | \nabla p |}

where

	fluid viscosity
	fluid velocity
	pressure gradient

Units = 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:

k_f = k_a \cdot k_{r\alpha}

where

	absolute permeability to air which is defined by the reservoir pore structure only, also denoted as or
	relative permeability to a given fluid which is defined by the interaction between fluid and reservoir matrix

See also