# Darcy (unit)

Physical unit
Unit name Darcy
Unit symbol ${\ displaystyle \ mathrm {D}}$
Physical quantity (s) permeability
Formula symbol ${\ displaystyle \ varkappa}$, ${\ displaystyle \ kappa}$
dimension ${\ displaystyle {\ mathsf {L ^ {2}}}}$
In SI units ${\ displaystyle \ mathrm {1 \, D = 9 {,} 869 \, 23 \ cdot 10 ^ {- 13} \; m ^ {2}}}$
Named after Henry Darcy
Derived from Poise , physical atmosphere , centimeter , second

The Darcy is a CGS unit for permeability named after the French engineer Henry Darcy . It is not a legal unit, but is still widespread , especially in petroleum and hydrogeology .

Permeability is a measure of the ability of porous media to allow liquids to flow through. Like other permeability units, the Darcy has the dimension of an area, according to the concept it corresponds approximately to a free opening of (1 µm) ².

The permeability of rock is usually given in milliDarcy (1 mD = 0.001 D), because oil and water reservoirs typically have permeabilities between 5 and 500 mD.

## definition

The Darcy is defined with the help of Darcy's Law , which can be written as follows:

${\ displaystyle v = {\ frac {\ varkappa} {\ mu}} \ cdot {\ frac {\ Delta p} {\ Delta x}} \ quad \ Leftrightarrow \ quad \ varkappa = v \ cdot \ mu \ cdot { \ frac {\ Delta x} {\ Delta p}}}$

With:

• ${\ displaystyle v}$ - filter speed in cm / s
• ${\ displaystyle \ varkappa}$ - Permeability in Darcy
• ${\ displaystyle \ mu}$- dynamic viscosity of the liquid in centipoise
• ${\ displaystyle \ Delta p}$the pressure difference between A and B in atm
• ${\ displaystyle \ Delta x}$the distance between A and B in cm

A porous medium with a permeability of 1 Darcy thus allows a liquid with a viscosity of 1 centipoise (1 mPa · s, the viscosity of water at 20 ° C.) to flow at 1 cm / s under a pressure gradient of 1 atm / cm . ${\ displaystyle {\ tfrac {\ Delta p} {\ Delta x}}}$

## Individual evidence

1. ^ HG Jerrard & DB McNeill: A Dictionary of Scientific Units. Including dimensionless numbers and scales , Chapman and Hall, London / New York, 1986, p. 33.
2. ^ The SI Metric System of Units and SPE Metric Standard. Society of Petroleum Engineers, accessed December 5, 2014 .