Molecular Simulation of H₂/CH₄ Mixture Storage and Adsorption in Kaolinite Nanopores for Underground Hydrogen Storage

Hydrogen has been regarded as an important type of renewable energy; however, the usage of pure hydrogen remains a challenge due to the difficulty of transportation and storage. To facilitate the hydrogen usage, it has been proposed to add hydrogen into the existing natural gas pipeline and storage systems, and therefore, the blended hydrogen is injected into the underground gas reservoirs, which is often used for the seasonal storage of natural gas. However, the mechanism of the H-2/CH4 mixture storage in the porous formations of underground reservoirs still remains unclear. In this work, we have used molecular simulations to study the storage of the H-2/CH4 mixture in dry and wet kaolinite pores of gibbsite and siloxane structures aiming at the clays in the underground porous reservoirs. The results showed that the hydrogen storage density increases as the injected H-2 fraction increases in the dry pores but is nearly not affected by the pore size in the range of 5-200 nm. For the H-2 fraction below 20%, the percentage of the H-2 stored in the pores from the injected gas mixture is higher than that of CH4, and the larger pores are more preferential for H-2 storage, but the H-2 molecule distribution is more diffused than that of CH4 across the pore. The CH4 molecules adopt the tripod and inclined configurations with an angle of 110 degrees or 70 degrees toward the surface, while the H-2 molecules are perpendicular to the surface. But both the molecular structures of H-2 and CH4 are not affected by the gibbsite/siloxane structure or water content. However, the siloxane pores have a better selective storage capacity for H-2 from the H-2/CH4 mixture. The existence of water in the formation weakens both the H-2 and CH4 storage densities. For gibbsite pores, the water molecules form a film on the surface, driving the H-2 and CH4 molecules to the middle of the pore. But for siloxane pores, the water molecules form water cluster across the pore, leaving less pore volumes for H-2 and CH4 storage, which causes a weaker H-2 storage capacity than that of the gibbsite pores.