The darcy (or darcy unit) and millidarcy (md or mD) are
units
Unit may refer to:
Arts and entertainment
* UNIT, a fictional military organization in the science fiction television series ''Doctor Who''
* Unit of action, a discrete piece of action (or beat) in a theatrical presentation
Music
* ''Unit'' (al ...
of
permeability, named after
Henry Darcy
Henry Philibert Gaspard Darcy (, 10 June 1803 – 3 January 1858) was a French engineer who made several important contributions to hydraulics, including Darcy’s law for flow in porous media.
Early life
Darcy was born in Dijon, France, on J ...
. They are not
SI units
The International System of Units, known by the international abbreviation SI in all languages and sometimes Pleonasm#Acronyms and initialisms, pleonastically as the SI system, is the modern form of the metric system and the world's most wid ...
, but they are widely used in
petroleum engineering and
geology. The unit has also been used in biophysics and biomechanics, where the flow of fluids such as blood through capillary beds and cerebrospinal fluid through the brain interstitial space is being examined.
A darcy has
dimensional units of
length2.
Definition
Permeability measures the ability of
fluids to
flow
Flow may refer to:
Science and technology
* Fluid flow, the motion of a gas or liquid
* Flow (geomorphology), a type of mass wasting or slope movement in geomorphology
* Flow (mathematics), a group action of the real numbers on a set
* Flow (psyc ...
through rock (or other porous media). The darcy is defined using
Darcy's law
Darcy's law is an equation that describes the flow of a fluid through a porous medium. The law was formulated by Henry Darcy based on results of experiments on the flow of water through beds of sand, forming the basis of hydrogeology, a branch o ...
, 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 cm
3/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 cm
2.
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.
Tissue permeability, whose measurement ''in vivo'' is still in its infancy, is somewhere in the range of 0.01 to 100 darcy.
Origin
The darcy is named after
Henry Darcy
Henry Philibert Gaspard Darcy (, 10 June 1803 – 3 January 1858) was a French engineer who made several important contributions to hydraulics, including Darcy’s law for flow in porous media.
Early life
Darcy was born in Dijon, France, on J ...
.
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). The American Association of Petroleum Geologists 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
SI units, 1 darcy is equivalent to or 0.9869233
µm2.
This conversion is usually approximated as 1 µm
2. This is the reciprocal of 1.013250—the conversion factor from atmospheres to
bars.
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.
[K. N. Duggal, J. P. Soni: Elements of Water Resources Engineering.
Publisher New Age International, 1996, p. 270]
References
{{reflist
* Richard Selley's "Elements of Petroleum Geology (2nd edition)," page 250.
Units of measurement
Hydraulics
Hydraulic engineering
Hydrology
Hydrogeology
Soil mechanics
Soil physics