Permeability Converter

The darcy (symbol D) is the customary unit of intrinsic permeability used in petroleum engineering and hydrogeology. One darcy is defined as the permeability of a porous medium through which a fluid of one centipoise viscosity flows at one centimetre per second under a pressure gradient of one atmosphere per centimetre. In SI units, 1 darcy equals approximately 9.869233e-13 square metres. Because most reservoir rocks are far less permeable, the millidarcy (one thousandth of a darcy) is the everyday working unit.

Intrinsic permeability has SI units of square metres because it describes a geometric property of the pore structure alone, independent of the fluid. It appears in Darcy's law, where flow rate is proportional to permeability times the pressure gradient divided by fluid viscosity. The fluid's viscosity is accounted for separately, so the rock property reduces to an area. The square metre is large for this purpose, so values are often quoted in darcy or millidarcy, or in square micrometres in some scientific literature.

1 darcy = 9.869233e-13 m² (approximately). This factor follows from the darcy's definition using one atmosphere (101,325 Pa) per centimetre and one centipoise (0.001 Pa·s). One millidarcy equals 9.869233e-16 m². This converter uses the standard factor 9.869233e-13 m² per darcy.

Gravel and clean sand can exceed several darcies. Good oil-reservoir sandstone is often tens to hundreds of millidarcies. Tight gas sands and shales fall below one millidarcy, sometimes into the microdarcy or nanodarcy range. Unfractured granite and intact clay are nearly impermeable, below a microdarcy. Always confirm values against site-specific core measurements rather than generic ranges.