Given the fact that the nature of fluid flow varies from Darcy flow to slip flow for a given mean pore diameter (say, 100 nm) – when the reservoir pressure drops down by an order from its initial value (say, from 10,000 psi to 1000 psi) – assuming the reservoir temperature hangs around at the lower end of the gas window.
(May be - the hypothetical - " Darcy-Knudsen Boundary Pressure " needs to be deduced as a function of reservoir pressure, reservoir temperature and mean pore diameter in order to clearly delineate the nature of different fluid regimes of a shale gas reservoir with time)