A centimeter-gram-second unit of permeability to fluid flow, used, for example, by geologists to describe rocks in an oil field. A porous substance has a permeability of 1 darcy if, in 1 second, 1 cubic centimeter of a gas or liquid with a viscosity of 1 centipoise will flow through a section 1-centimeter thick with a cross section of 1 square centimeter, when the difference between the pressures on the two sides of the section is 1 atmosphere. Sandstone typically has a permeability of a few darcys. Converted to SI, one darcy becomes about 9.869233 × 10⁻¹³ square meters, but it is not a unit of area.
The darcy is not an SI unit. The SI unit of permeability to fluid flow is defined as the amount of permeability that permits 1 cubic meter of fluid of a viscosity of 1 pascal per second to flow through a section 1 meter thick with a cross section of 1 square meter in 1 second at a pressure difference of 1 pascal. That unit has no special name. The SI unit of permeability = 1.013 25 × 10¹² darcy.
The darcy is named for the French scientist H. Darcy (1803-1858), an early investigator of fluid flow in porous materials. It was first proposed in 1933.¹
R.D. Wycoff, H.G. Botset, M. Muskat and D.W. Reed.
Review of Scientific Instruments (GB) volume 4, page 395 (1933).
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Last revised: 4 February 2002.