Molecular Dynamics Study of the H2-CO2-H2O Mixture Flow Through Silica Nanopores

Summary Underground hydrogen storage (UHS) can be a viable solution for large-scale, long-term seasonal hydrogen (H2) storage. It could potentially address the energy peak-shaving issues due to the seasonal power supply and demand fluctuations. Recent studies have reported the promising sealing efficacy and storage capacity of shale as potential UHS hosts. However, the hydrogen transportation mechanisms in shale nanopores with various component mixtures remain unclear. In this work, the H2 flow processes in nonprotonated and protonated silica nanopores with varying carbon dioxide (CO2) and water (H2O) contents are investigated using a dynamical molecular simulation protocol. The results show that a water film forms along the pore walls and plays a more important role in reducing H2 diffusion loss than CO2. At 30 mol% H2O content, some H2O clusters become enveloped by the gases in the middle of the nonprotonated silica pore, while the water bridge structure transitions into a water film in the protonated silica pore as the driving force increases. Increasing the driving force further enhances the separation of H2 from fluid mixtures. The water film is disrupted under high driving force at 10 mol% H2O content, potentially leading to bypass flow patterns. These findings highlight the importance of controlling moisture levels and avoiding excessive pressure gradients during field operations. This study could provide a reference for experimental studies and large-scale modeling studies using CO2 as a cushion gas and offer insights into the design of injection-production strategies for UHS in nanoscale porous media.