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 H₂/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 H₂/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 H₂ fraction increases in the dry pores but is nearly not affected by the pore size in the range of 5-200 nm. For the H₂ fraction below 20%, the percentage of the H₂ stored in the pores from the injected gas mixture is higher than that of CH₄, and the larger pores are more preferential for H₂ storage, but the H₂ 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° or 70° toward the surface, while the H₂ molecules are perpendicular to the surface. But both the molecular structures of H₂ and CH₄ are not affected by the gibbsite/siloxane structure or water content. However, the siloxane pores have a better selective storage capacity for H₂ from the H₂/CH₄ mixture. The existence of water in the formation weakens both the H₂ and CH₄ storage densities. For gibbsite pores, the water molecules form a film on the surface, driving the H₂ 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 H₂ and CH₄ storage, which causes a weaker H₂ storage capacity than that of the gibbsite pores.