a unit of measure of permeability. A porous medium has a permeability of 1 darcy when a pressure of 1 atmosphere on a sample 1 cm long and 1 cm' in cross-section will force a liquid of 1-cp viscosity through the sample at the rate of 1 cm' per second. The permeability of reservoir rocks is usually so low that it is measured in millidarcy units.
