Darcy
1507685
217792519
2008-06-07T18:25:30Z
Mwtoews
711150
[[WP:UNDO|Undid]] revision 217679361 by [[Special:Contributions/68.151.193.193|68.151.193.193]] ([[User talk:68.151.193.193|talk]]) - it is typically κ (a kappa) and not k
{{lowercase|title=darcy}}
{{for|the 19th-century French scientist|Henry Darcy}}
{{otheruses1|the unit of fluid permeability}}
A '''darcy''' is a [[Units of measurement|unit]] of [[Permeability (fluid)|permeability]], named after [[Henry Darcy]]. It is not an [[SI|SI unit]], but it is widely used in [[petroleum engineering]] and [[geology]]. Like other measures of permeability, a darcy has the same units as [[area]].
==Definition==
Permeability measures the ability of fluids to flow through rock (or other porous media). The darcy is defined using [[Darcy's law]] which can be written as:
:<math>v = \frac{\kappa \Delta P}{\mu \Delta x}</math>
where:
:<math>v</math> is the superficial (or bulk) fluid flow rate through the medium
:<math>\kappa</math> is the permeability of a medium
:<math>\mu</math> is the dynamic [[viscosity]] of the fluid
:<math>\Delta P</math> is the applied [[pressure]] difference
:<math>\Delta x</math> is the thickness of the medium
The darcy is referenced to a mixture of unit systems. A medium with a permeability of 1 darcy permits a flow of 1 cm/s of a fluid with viscosity 1 [[centipoise|cP]] (1 [[pascal second|mPa·s]]) under a pressure gradient of 1 atm/cm .
==Origin==
The darcy is named after [[Henry Darcy]]. Rock permeability is usually expressed in '''[[milli]]darcies (mD)'''—1 mD is equal to 0.001 darcy—because rocks found in oil and water reservoirs typically have permeabilities in the range of 5 to 500 mD.
The odd combination of units comes from Darcy's original studies of water flow through columns of sand. [[Water]] has a viscosity of 1.0019 cP at about room temperature.
==Conversions==
Converted to [[SI]] units, 1 darcy is equivalent to 9.86923×10<sup>−13</sup> [[Square metre|m²]] or 0.986923 [[micrometre|µm]]². This conversion is usually approximated as 1 µm². Note that this is the reciprocal of 1.01325—the conversion factor from atmospheres to [[bar (unit)|bars]].
==References==
*[http://www.spe.org/spe-site/spe/spe/papers/authors/Metric_Standard.pdf Society of Petroleum Engineers (SPE) metric standard]
*Richard Selley's "Elements of Petroleum Geology (2nd edition)," page 250
[[Category:Units of measure]]
[[Category:Geology]]
[[Category:Hydraulics]]
[[Category:Hydraulic engineering]]
[[Category:Hydrology]]
[[Category:Hydrogeology]]
[[Category:Soil mechanics]]
[[Category:Soil physics]]
[[de:Darcy (Einheit)]]
[[ru:Дарси]]
[[uk:Дарсі (одиниця)]]