The network capacity is based on the flow capacity of the simulated void network for the trickle flow of non-compressible fluids. Each arc of the network has flow capacity Farccalculated from parametrized Navier-Stokes equations (Matthews, Canonville and Moss, 2006):
where h is the length of a throat of radius r connecting two cubic pores with sides L1 and L2, respectively, and λ is the mean free path between collisions in the fluid.
Except for very straight forward pore networks, such as those within clean, highly porous outcrop sandstone and track etch membranes, the network capacity is less than the actual absolute permeability. The extent of the discrepancy is shown in our property simulation validation, and arises because of the simplifying flow assumptions, and the fact that PoreXpert cannot replicate the true complexity and intricacy of void networks in most natural materials. Larger unit cells provide more realistic values of permeability because there are fewer unit cell replications to represent the experimental sample, but take longer as detailed below.
The network capacity is calculated in milliDarcy units.
The calculated value includes the slip/Knudsen flow of molecules of gases, caused by molecules bouncing their way through throats within the network. The effect is most noticeable, relative to laminar / Poiseuillian flow, for small molecules in low density gases passing through narrow throats. The difference between gas and liquid permeability corresponds to the Klinkenberg effect.