Permeability (porous media)

The concept of permeability is of importance in determining the flow characteristics of hydrocarbons in oil and gas reservoirs,[4] and of groundwater in aquifers.

[5] For a rock to be considered as an exploitable hydrocarbon reservoir without stimulation, its permeability must be greater than approximately 100 md (depending on the nature of the hydrocarbon – gas reservoirs with lower permeabilities are still exploitable because of the lower viscosity of gas in comparison with oil).

Rocks with permeabilities significantly lower than 100 md can form efficient seals (see petroleum geology).

Permeability is part of the proportionality constant in Darcy's law which relates discharge (flow rate) and fluid physical properties (e.g. dynamic viscosity), to a pressure gradient applied to the porous media:[6] Therefore: where: In naturally occurring materials, the permeability values range over many orders of magnitude (see table below for an example of this range).

The global proportionality constant for the flow of water through a porous medium is called the hydraulic conductivity (K, unit: m/s).

Given the value of hydraulic conductivity for a studied system, the permeability can be calculated as follows: Tissue such as brain, liver, muscle, etc can be treated as a heterogeneous porous medium.

[8] Permeability needs to be measured, either directly (using Darcy's law), or through estimation using empirically derived formulas.

However, for some simple models of porous media, permeability can be calculated (e.g., random close packing of identical spheres).

These terms refer to the quality that the permeability value in question is an intensive property of the medium, not a spatial average of a heterogeneous block of material equation 2.28[clarification needed][further explanation needed]; and that it is a function of the material structure only (and not of the fluid).