Molecular Simulation on H₂ Huff-n-Puff in a Depleted Shale Gas Reservoir and Its Implications on Underground Hydrogen Storage

Depleted shale gas reservoirs are recognized as promising geological sites for underground hydrogen storage due to their strong adsorption and sealing properties. The injection, storage, and recovery of hydrogen from underground can be described as a typical huff-n-puff process. In this work, molecular dynamics simulations are conducted to study the entire huff-n-puff process of H2 in shale nanopores at reservoir conditions. The effects of pore type, pore size, and gas mole fraction on H2 storage capacity and recovery efficiency are explicitly examined. The results indicate that organic nanopores show a greater adsorption capacity for hydrogen than inorganic nanopores. The injected hydrogen forms two adsorption layers in the two nanopores. The first adsorption peak of H2 is lower than the second adsorption peak in organic pores, while the distribution of H2 exhibits an inverse trend in inorganic pores. This phenomenon is attributed to the fact that higher amounts of CH4 adsorb on the organic surface due to its stronger affinity. Compared with organic pores, the adsorption of CH4 on the inorganic surface is not saturated at the same pressure, resulting in more adsorption sites for H2. The puff process allows for the recovery of 49.9% and 40.0% of H2 molecules from organic mesopores and micropores, respectively. For comparison, a greater proportion of H2 molecules can be recovered from inorganic pores (70.0% and 63.3% in mesopores and micropores, respectively). As the amount of injected H2 increases, the H2 recovery efficiency from shale pores also increases. This is particularly evident when the H2 mole fraction increases from 0.3 to 0.5. The preceding results demonstrate that the H2 huff-n-puff process exhibits superior performance in inorganic pores, particularly in mesopores. This work presents a framework for performing the huff-n-puff process of H2 in depleted shale gas reservoirs, thereby providing insights into the underground hydrogen storage under reservoir conditions.