A laboratory observation for gases transport in shale nanochannels: Helium, nitrogen, methane, and helium-methane mixture

Abstract

Mudstones are widely distributed in sedimentary basins with abundant nanopores and has very low fluid mobility. It is widely accepted that gas migration in micro-nanochannels no longer follows Darcy's law and the Hagen-Poiseuille equation and that molecular kinetic features profoundly influence gas migration in micro-nanochannels. Here, we characterize the nanochannel sizes of Longmaxi Shale cores under different overburden pressure and pore pressure conditions. And the apparent permeabilities of individual component gases (helium, nitrogen, and methane) in different size nanochannels are compared. Then the variations for components and methane carbon isotope values of the overflowing helium-methane mixture are analyzed. The results show that the molecular slippage effect contributes significantly to the transport of the three gases, especially at low pore pressures; helium has the largest apparent permeability because of its largest molecular mean free path (or Knudsen number). Methane has the smallest molecular mean free path, and the adsorption effect reduces channel size and increases surface roughness, which inhibits the molecular slip effect and therefore results in methane having the smallest apparent permeability. Helium has a transport advantage in the helium-methane mixture due to its Knudsen number advantage, and the larger Knudsen number of 12CH4 likewise leads to a transport advantage over 13CH4. This study suggests that the molecular slippage effect may be an important molecular dynamics mechanism for the efficient development of shale gas. Pressure gradient-driven transport of helium-bearing gas in nanochannels is an essential mechanism for helium enrichment, and high-grade helium reservoirs should occur in secondary low-pressure reservoirs in the basin.