Darcy (unit)
The darcy (or darcy unit) and millidarcy (md or mD) are
Definition
Permeability measures the ability of
where:
is the volumetric fluid flow rate through the medium is the area of the medium is the permeability of the medium is the dynamic viscosity of the fluid is the applied pressure difference 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 cm3/s of a fluid with
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.[2]
Tissue permeability, whose measurement in vivo is still in its infancy, is somewhere in the range of 0.01 to 100 darcy.[1]
Origin
The darcy is named after Henry Darcy.[1] 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.
The unit abbreviation "d" is not capitalized (contrary to industry use).[clarification needed] The American Association of Petroleum Geologists[3] uses the following unit abbreviations and grammar in their publications:
- darcy (plural darcys, not darcies): d
- millidarcy (plural millidarcys, not millidarcies): md
Conversions
Converted to
Specifically in the hydrology domain, permeability of soil or rock may also be defined as the flux of water under hydrostatic pressure (~ 0.1 bar/m) at a temperature of 20 °C. In this specific setup, 1 darcy is equivalent to 0.831 m/day. [5]
References
- ^ PMID 36340954.
- ISBN 0-939950-42-1, p. 5.
- ^ "The American Association of Petroleum Geologist Style Guides" (PDF). Archived from the original (PDF) on 2011-08-09.
- ^ The SI Metric System of Units and SPE Metric Standard (PDF) (2nd ed.). Society of Petroleum Engineers. June 1984 [First published 1982].
- ^ K. N. Duggal, J. P. Soni: Elements of Water Resources Engineering. Publisher New Age International, 1996, p. 270
- Richard Selley's "Elements of Petroleum Geology (2nd edition)," page 250.