Permeability in fluid mechanics and the earth sciences (commonly symbolized as κ, or k) is a measure of the ability of a porous material (often, a rock or an unconsolidated material) to allow fluids to pass through it.
Contents

Permeability 1

Units 2

Applications 3

Description 4

Relation to hydraulic conductivity 4.1

Determination 5

Death row 6

Facilities 7

Permeability model based on conduit flow 7.1

Facilities 8

Fallen officers 9

Employee Organizations 10

See also 11

References 12

External links 13
Permeability
Permeability is the property of rocks that is an indication of the ability for fluids (gas or liquid) to flow through rocks. High permeability will allow fluids to move rapidly through rocks. Permeability is affected by the pressure in a rock. The unit of measure is called the darcy, named after Henry Darcy (18031858). Sandstones may vary in permeability from less than one to over 50,000 millidarcys (md). Permeabilities are more commonly in the range of tens to hundreds of millidarcies. A rock with 25% porosity and a permeability of 1 md will not yield a significant flow of water. Such “tight” rocks are usually artificially stimulated (fractured or acidized) to create permeability and yield a flow.
Units
The SI unit for permeability is m^{2}. A practical unit for permeability is the darcy (d), or more commonly the millidarcy (md) (1 darcy \approx10^{−12}m^{2}). The name is in honor to the French Engineer Henry Darcy who first described the flow of water through sand filters for potable water supply. Permeability values for sandstones range typically from a fraction of a darcy to several darcys. The unit of cm^{2} is also sometimes used (1 cm^{2} = 10^{−4} m^{2} \approx 10^{8} d).
Applications
The concept of permeability is of importance in determining the flow characteristics of hydrocarbons in oil and gas reservoirs, and of groundwater in aquifers.
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 with respect to oil). Rocks with permeabilities significantly lower than 100 md can form efficient seals (see petroleum geology). Unconsolidated sands may have permeabilities of over 5000 md.
The concept has also many practical applications outside of geology, for example in chemical engineering (e.g., filtration).
Description
Permeability is part of the proportionality constant in Darcy's law which relates discharge (flow rate) and fluid physical properties (e.g. viscosity), to a pressure gradient applied to the porous media:

v = \frac {\kappa}{\mu} \frac{\Delta P}{\Delta x}
Therefore:

\kappa = v \frac{\mu \Delta x}{\Delta P}
where:

v is the superficial fluid flow velocity through the medium (i.e., the average velocity calculated as if the fluid were the only phase present in the porous medium) (m/s)

\kappa is the permeability of a medium (m^{2})

\mu is the dynamic viscosity of the fluid (Pa·s)

\Delta P is the applied pressure difference (Pa)

\Delta x is the thickness of the bed of the porous medium (m)
In naturally occurring materials, permeability values range over many orders of magnitude (see table below for an example of this range).
Relation to hydraulic conductivity
The proportionality constant specifically for the flow of water through a porous media is called the hydraulic conductivity; permeability is a portion of this, and is a property of the porous media only, not the fluid. Given the value of hydraulic conductivity for a subsurface system, the permeability can be calculated as follows:
K = \kappa \frac {\rho g} {\mu}

where

\kappa is the permeability, m^{2}

K is the hydraulic conductivity, m/s

\mu is the dynamic viscosity of the fluid, kg/(m·s)

\rho is the density of the fluid, kg/m^{3}

g is the acceleration due to gravity, m/s^{2}.
Determination
Permeability is typically determined in the lab by application of Darcy's law under steady state conditions or, more generally, by application of various solutions to the diffusion equation for unsteady flow conditions.^{[1]}
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).
Permeability model based on conduit flow
Based on the Hagen–Poiseuille equation for viscous flow in a pipe, permeability can be expressed as:

{\kappa}_{I}=C \cdot d^2
where:

{\kappa}_{I} is the intrinsic permeability [length^{2}]

C is a dimensionless constant that is related to the configuration of the flowpaths

d is the average, or effective pore diameter [length].
Estimation of permeability distribution in subsurface reservoirs
Permeability distribution in subsurface reservoirs is typically estimated using inverse problem theory.^{[2]}
Intrinsic and absolute permeability
The terms intrinsic permeability and absolute permeability states that the permeability value in question is an intensive property (not a spatial average of a heterogeneous block of material), that it is a function of the material structure only (and not of the fluid), and explicitly distinguishes the value from that of relative permeability.
Permeability to gases
Sometimes permeability to gases can be somewhat different that those for liquids in the same media. One difference is attributable to "slippage" of gas at the interface with the solid^{[3]} when the gas mean free path is comparable to the pore size (about 0.01 to 0.1 μm at standard temperature and pressure). See also Knudsen diffusion and constrictivity. For example, measurement of permeability through sandstones and shales yielded values from 9.0x10^{−19} m^{2} to 2.4x10^{−12} m^{2} for water and between 1.7x10^{−17} m^{2} to 2.6x10^{−12} m^{2} for nitrogen gas.^{[4]} Gas permeability of reservoir rock and source rock is important in petroleum engineering, when considering the optimal extraction of shale gas, tight gas, or coalbed methane.
Tensor permeability
To model permeability in anisotropic media, a permeability tensor is needed. Pressure can be applied in three directions, and for each direction, permeability can be measured (via Darcy's law in 3D) in three directions, thus leading to a 3 by 3 tensor. The tensor is realised using a 3 by 3 matrix being both symmetric and positive definite (SPD matrix):

The tensor is symmetric by the Onsager reciprocal relations.

The tensor is positive definite as the component of the flow parallel to the pressure drop is always in the same direction as the pressure drop.
The permeability tensor is always diagonalizable (being both symmetric and positive definite). The eigenvectors will yield the principal directions of flow, meaning the directions where flow is parallel to the pressure drop, and the eigenvalues representing the principal permeabilities.
Ranges of common intrinsic permeabilities
These values do not depend on the fluid properties; see the table derived from the same source for values of hydraulic conductivity, which are specific to the material through which the fluid is flowing.
Permeability

Pervious

SemiPervious

Impervious

Unconsolidated Sand & Gravel

Well Sorted Gravel

Well Sorted Sand or Sand & Gravel

Very Fine Sand, Silt, Loess, Loam


Unconsolidated Clay & Organic


Peat

Layered Clay

Unweathered Clay

Consolidated Rocks

Highly Fractured Rocks

Oil Reservoir Rocks

Fresh Sandstone

Fresh Limestone, Dolomite

Fresh Granite

κ (cm^{2})

0.001

0.0001

10^{−5}

10^{−6}

10^{−7}

10^{−8}

10^{−9}

10^{−10}

10^{−11}

10^{−12}

10^{−13}

10^{−14}

10^{−15}

κ (millidarcy)

10^{+8}

10^{+7}

10^{+6}

10^{+5}

10,000

1,000

100

10

1

0.1

0.01

0.001

0.0001

Source: modified from Bear, 1972
See also

^

^ Arizona Law

^ Corrections at a glance

^ COTA

^ "ADC Contact Information." Arizona Department of Corrections. Retrieved on December 7, 2009.

^ 2008 Budget

^ 2009 Budget

^ 2013 Budget

^

^ "Death Row Information and Frequently Asked Questions." Arizona Department of Corrections. Retrieved on August 16, 2010.

^ "Death Row Information and Frequently Asked Questions." Arizona Department of Corrections. Retrieved on August 16, 2010.

^ http://www.nytimes.com/2007/07/31/us/31prisons.html?pagewanted=all&_r=0

^ http://www.allgov.com/news/controversies/housingprisonersfromotherstateshasbecomea320milliondollarayearindustry?news=851761

^ The Officer Down Memorial Page
References

Bear, Jacob, 1972. Dynamics of Fluids in Porous Media, Dover. — ISBN 0486656756

Wang, H. F., 2000. Theory of Linear Poroelasticity with Applications to Geomechanics and Hydrogeology, Princeton University Press. ISBN 0691037469
External links

Defining Permeability

Permeability of Porous Media

Graphical depiction of different flow rates through materials of differing permeability

Webbased porosity and permeability calculator given flow characteristics

Florida Method of Test For Concrete Resistivity as an Electrical Indicator of its Permeability
This article was sourced from Creative Commons AttributionShareAlike License; additional terms may apply. World Heritage Encyclopedia content is assembled from numerous content providers, Open Access Publishing, and in compliance with The Fair Access to Science and Technology Research Act (FASTR), Wikimedia Foundation, Inc., Public Library of Science, The Encyclopedia of Life, Open Book Publishers (OBP), PubMed, U.S. National Library of Medicine, National Center for Biotechnology Information, U.S. National Library of Medicine, National Institutes of Health (NIH), U.S. Department of Health & Human Services, and USA.gov, which sources content from all federal, state, local, tribal, and territorial government publication portals (.gov, .mil, .edu). Funding for USA.gov and content contributors is made possible from the U.S. Congress, EGovernment Act of 2002.
Crowd sourced content that is contributed to World Heritage Encyclopedia is peer reviewed and edited by our editorial staff to ensure quality scholarly research articles.
By using this site, you agree to the Terms of Use and Privacy Policy. World Heritage Encyclopedia™ is a registered trademark of the World Public Library Association, a nonprofit organization.