Permeability is the capacity of a porous material to transmit fluids. The SI unit of permeability is m² (area) but the units adopted by the petroleum industry have been named after Henry Darcy, who derived Darcy's law. A darcy is a confusing jumble of units which combines a standardized set of laboratory experiments. By definition, a material of 1 darcy permits a flow of 1 cm³/s of a fluid with viscosity 1 cP (1 mPa.s) under a pressure gradient of 1 atm/cm across an area of 1 cm².

Apart from having obscure units with an empirical origin, permeability can be an incredibly variable quantity. It can vary be as low as 10^–9 D for tight gas reservoirs and shale, to 10¹ D for unconsolidated conventional reservoirs. Just as electrical resistivity, values are plotted on a logarithmic scale. Many factors such as rock type, pore size, shape and connectedness and can effect fluid transport over volume scales from millimetres to kilometres.

Okay then, with that said, what is the upscaled permeability of the cube of rock shown here? In other words, if you only had to find one number to describe the permeability of this sample, what would it be? I'll pause for a moment while you grab your calculator... Okay, got an answer? What is it?