Origin
The darcy is named after Henry Darcy. Rock permeability is usually expressed in millidarcys (md) because rocks hosting hydrocarbon or water accumulations typically exhibit permeability ranging from 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.
Although the unit is named after Henry Darcy, it should be noted that the unit abbreviation is not capitalized (contrary to industry use). The American Association of Petroleum Geologists use the following unit abbreviations and grammar in their publications:
- darcy (plural darcys, not darcies): d
- millidarcy (plural millidarcys, not millidarcies): md
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