Controls of Geological Factors on H₂ Adsorption in the Subsurface

Underground hydrogen storage has been regarded as a sustainable and reliable way to maintain a stable national-scale energy supply in the development of the hydrogen economy. Various risks influencing subsurface H₂ recovery due to interactions between H₂, water, and rocks remain to be evaluated thoroughly. In this study, the effects of geological factors on H₂ sorption are discussed among the organic-matter-rich and lean samples. The measurements of H₂ and CH₄ adsorption on coals with different maturities and smectite, which has large pore surface areas but no organic oxygen-functional groups, were made at temperatures from 35 to 65 °C and pressures up to 15 MPa. The adsorbed amounts of H₂ and CH₄ in coal samples were much higher than those in smectite at different temperatures, as verified by Langmuir monolayer modeling. Our study indicates that the BET surface area of the sample material is an important factor influencing H₂ sorption, providing the sorption sites on the rock surface, but this will greatly be influenced by the chemical structures due to the interaction of H₂ with organic oxygen-containing functional groups. This is verified by a comparison of H₂ sorption between coals and smectite. For the hydrogen geological storage, we find that at least 30% more H₂ can be stored in the coal seam due to mainly physical sorption, compared with the H₂ amounts in the free phase at the same geological condition. Our study deepens our understanding of the controlling effect of geological factors on H₂ storage in the subsurface.