1 millidarcy is one-thousandth of a darcy (1 mD = 0.001 darcy).
Darcy is the unit of permeability commonly used in oil and water reservoir to express the ability of fluid to flow through a porous media (let’s say a sandstone reservoir for gas).
1 darcy will require that the
1. fluid has a viscosity of 1 cp (centipoise),
2. the fluid is flowing at a rate of 1 cubic cm per second,
3. and passing a length of 1 cm of the porous media which has a cross-sectional area of 1 square cm.
4. while all these happens, atmospheric pressure drops 1 unit (means 1 atmosphere pressure drop).
One example of very low-permeability reservoir is the South Korea’s offshore natural gas reservoir in the East Sea, where in the deformed sandstone reservoir the maximum permeability reaches to about 0.365 mD (at a depth of about 3000 m). In undeformed sandstone reservoir it reaches up to 81.6 mD (at a depth of about 3000 m). Source: low-permeability reservoir, southeast Korea.