Millidarcies

Millidarcies (mD) are a unit of permeability used to describe the ability of porous media to transmit fluids. One millidarcy equals one thousandth of a darcy, the traditional CGS-based unit named after Henry Darcy. In SI terms, 1 darcy is approximately 9.869233×10^-13 square meters, so 1 millidarcy is about 9.869233×10^-16 m^2. There are 1000 millidarcies in 1 darcy, and 1 millidarcy equals 1000 microdarcies (μD). Conversely, 1 darcy equals 1000 millidarcies or 1,000,000 microdarcies.

The darcy is defined by Darcy’s law under standardized conditions, representing the permeability that would allow a viscous fluid to flow under a pressure gradient of 1 atmosphere per centimeter through a unit cross-sectional area and thickness at a reference temperature and viscosity. The millidarcy simply scales this quantity by 10^-3, so it is widely used in practice to express rock permeability when values fall between tens to thousands of millidarcies. The unit is not part of the International System of Units (SI), but it remains common in petroleum engineering and hydrogeology for characterizing reservoir rocks, aquifers, and other porous media.

Typical ranges vary by rock type: sandstone and carbonate rocks often measure from tens to hundreds of millidarcies, while shale and tightly fractured rocks may exhibit sub-millidarcy values. Permeability is highly dependent on pore structure, porosity, grain size, cementation, and fluid properties, and millidarcy values are frequently complemented by other measures or empirical correlations in analysis and reporting.