Darcy

A darcy is a unit of permeability, named after Henry Darcy. It is not an SI unit, but it is widely used in petroleum engineering and geology. Like other measures of permeability, a darcy has the same units as area.

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:

<math>v = \frac{\kappa \Delta P}{\mu \Delta x}</math>

where:

<math>v</math> is the superficial (or bulk) fluid flow rate through the medium

<math>\kappa</math> is the permeability of a medium

<math>\mu</math> is the dynamic viscosity of the fluid

<math>\Delta P</math> is the applied pressure difference

<math>\Delta x</math> 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 cm/s of a fluid with viscosity 1 cP (1 mPa·s) under a pressure gradient of 1 atm/cm .

Origin

The darcy is named after Henry Darcy. Rock permeability is usually expressed in millidarcies (mD)—1 mD is equal to 0.001 darcy—because rocks found in oil and water reservoirs typically have permeabilities in the range of 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.

Conversions

Converted to SI units, 1 darcy is equivalent to 9.86923×10⁻¹³ m² or 0.986923 µm². This conversion is usually approximated as 1 µm². Note that this is the reciprocal of 1.01325—the conversion factor from atmospheres to bars.

References

• Society of Petroleum Engineers (SPE) metric standard
• Richard Selley's "Elements of Petroleum Geology (2nd edition)," page 250