1. n. [Formation Evaluation]

The extra resistance of a porous medium to fluid flow, beyond that predicted by Darcy's law, caused by local accelerations within the tortuous pore volume. The inertial resistance is proportional to the fluid density times the flow rate. The inertial resistance is significant with gas because flow rates can be high. In laboratory measurements of permeability, inertial resistance can be important for high-permeability samples where high flow rates are needed to have significant pressure gradients. The effect is corrected by using the Forchheimer equation and making measurements at several flow rates (often through an unsteady state test).

See: core plug,  permeameter,  pressure gradient,  routine core analysis,  steady state,  unsteady state