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. 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.

Tissue permeability, whose measurement is still in its infancy, is somewhere in the range of 0.01 to 100 darcy. 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&nbsp;darcy is equivalent to or 0.9869233&nbsp;μm<sup>2</sup>. This conversion is usually approximated as 1&nbsp;μm<sup>2</sup>. This is the reciprocal of 1.013250&mdash;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 (≈&nbsp;0.1&nbsp;bar/m) at a temperature of 20&nbsp;°C. In this specific setup, 1 darcy is equivalent to 0.831&nbsp;m/day.

References

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