Gas storage and transport in porous media: From shale gas to helium-3

Helium-3, a potential fusion fuel which is widely deposited in the lunar regolith, may act as a leading role in the future energy supply. Despite abundant investigations of the lunar regolith properties, the research of the transport behavior of helium-3 in porous media is still lacking, which is essential for the prediction of its distribution characteristics. In this work, the storage mechanisms of helium-3 in the lunar regolith are firstly reviewed, and it is found that the primary driving force for the diffusion of helium-3 in porous media arises from its local supersaturation caused by the solar wind flux. After an estimation of the Knudsen number (Kn), it is concluded that Knudsen diffusion is the only transfer regime for the transport of helium-3 in the lunar subsurface because of a rarefaction effect. Moreover, the diffusion coefficient of helium-3 in porous media is predicted and the factors affecting this coefficient are analyzed. Furthermore, a sensitivity analysis is performed due to the uncertainty of the geometric factors of the lunar regolith. Finally, the storage and transport behavior for shale gas and helium-3 are compared, and the prospect for the mining of helium-3 is discussed.