To see a rock in a grain of sand

Permeability is perhaps one of the most important yet elusive reservoir properties. Unlike porosity, it correlates poorly with the elastic properties of rocks and, as a result, cannot be mapped remotely. Rather, it has to be directly measured in the lab on core plugs extracted from a well. In such lab experiments, a pressure gradient is applied to a fluid-filled rock sample, the fluid's volume flux is recorded, and the absolute permeability is calculated from Darcy's law by relating the flux to the pressure gradient.