  Lattice Boltzmann method (LBM) has been applied to predict flow properties of
porous media including intrinsic permeability, where it is implicitly assumed
that the LBM is equivalent to the incompressible (or near incompressible)
Navier-Stokes equation. However, in LBM simulations, high-order moments, which
are completely neglected in the Navier-Stokes equation, are still available
through particle distribution functions. To ensure that the LBM simulation is
correctly working at the Navier-Stokes hydrodynamic level, the high-order
moments have to be negligible. This requires that the Knudsen number (Kn) is
small so that rarefaction effect can be ignored. In this technical note, we
elaborate this issue in LBM modelling of porous media flows, which is
particularly important for gas flows in ultra-tight media.
