Darcy (unit)

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:

${\displaystyle Q={\frac {Ak\,\Delta P}{\mu \,\Delta x}}}$

where:

${\displaystyle Q\,}$	is the volumetric fluid flow rate through the medium
${\displaystyle A\,}$	is the area of the medium
${\displaystyle k\,}$	is the permeability of the medium
${\displaystyle \mu \,}$	is the dynamic viscosity of the fluid
${\displaystyle \Delta P\,}$	is the applied pressure difference
${\displaystyle \Delta x\,}$	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 viscosity 1 cP (1 mPa·s) under a pressure gradient of 1 atm/cm acting across an area of 1 cm².

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]

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). 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

Converted to SI units, 1 darcy is equivalent to 9.869233×10⁻¹³ m² or 0.9869233 µm².[4] This conversion is usually approximated as 1 µm². This is the reciprocal of 1.013250—the conversion factor from atmospheres to bars.[1]

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]

• Richard Selley's "Elements of Petroleum Geology (2nd edition)," page 250.