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A darcy (or darcy unit) and millidarcy (md or mD) are units of permeability, named after Henry Darcy. They are not SI units, but they are widely used in petroleum engineering and geology. Like some other measures of permeability, a darcy has dimensional units in length². ==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: : where: : The darcy is referenced to a mixture of unit systems. A medium with a permeability of 1 darcy permits a flow of 1 cm³/s of a fluid with viscosity 1 cP (1 mPa·s) under a pressure gradient of 1 atm/cm acting across an area of 1 cm². A millidarcy (md) is equal to 0.001 darcy and a microdarcy (µd) equals 0.000001 darcy. Typical values of permeability range as high as 100,000 darcys for gravel, to less than 0.01 microdarcy for granite. Sand has a permeability of approximately 1 darcy.〔Peter C. Lichtner, Carl I. Steefel, Eric H. Oelkers, ''(Reactive Transport in Porous Media )'', Mineralogical Society of America, 1996, ISBN 0-939950-42-1, p. 5.〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Darcy (unit)」の詳細全文を読む スポンサード リンク
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