The darcy (or darcy unit) and millidarcy (md or mD) are
units of
permeability, named after
Henry Darcy. They are not
SI units, but they are widely used in
petroleum engineering and
geology
Geology (). is a branch of natural science concerned with the Earth and other astronomical objects, the rocks of which they are composed, and the processes by which they change over time. Modern geology significantly overlaps all other Earth ...
. 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
dimensions of
length
Length is a measure of distance. In the International System of Quantities, length is a quantity with Dimension (physical quantity), dimension distance. In most systems of measurement a Base unit (measurement), base unit for length is chosen, ...
2.
Definition
Permeability measures the ability of
fluids to
flow through rock (or other porous media). The darcy is defined using
Darcy's law, 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
Viscosity is a measure of a fluid's rate-dependent drag (physics), resistance to a change in shape or to movement of its neighboring portions relative to one another. For liquids, it corresponds to the informal concept of ''thickness''; for e ...
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 is still in its infancy, is somewhere in the range of 0.01 to 100 darcy.
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
Water is an inorganic compound with the chemical formula . It is a transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance. It is the main constituent of Earth's hydrosphere and the fluids of all known liv ...
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