Molecular simulation on H2 adsorption in nanopores and effects of cushion gas: Implications for underground hydrogen storage in shale reservoirs

Hydrogen (H2), a renewable and clean energy source, plays an important role in the implementation of a low-carbon economy; however, hydrogen storage is challenging. Underground hydrogen storage (UHS) in shale reservoirs is a potential option for large-scale and long-term storage of H2. In this work, we used Grand Canonical Monte Carlo (GCMC) simulations to investigate the adsorption of pure H2 and H2 mixtures with CH4 or CO2 in kerogen and montmorillonite (MMT) nanopores under storage conditions. We found that pure H2 is more strongly adsorbed on kerogen, resulting in higher excess adsorption and consequently larger storage capacities. In the presence of CH4, H2 adsorption on kerogen is gradually weakened, indicating a stronger affinity between CH4 and kerogen surfaces. In contrast, co-adsorption of H2 and CH4 occurred on MMT surfaces. Moreover, H2 was almost completely desorbed by CO2, resulting in higher CO2 adsorption and selectivity (compared to CH4). Consequently, CH4 or CO2 can be used as the cushion gas as it greatly reduces the surface adsorption of H2, which is favorable for the retrieval of H2. Due to the stronger competitive adsorption ability, CO2 might perform better than CH4 in producing higher-purity H2. However, the presence of cushion gas significantly reduces H2 storage capacity, especially in smaller pores (<2 nm). This study provides important insights into H2 adsorption mechanisms and cushion gas effects in depleted shale reservoirs, thus helping to implement an industrial-scale hydrogen economy.