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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