Onset and dynamics of density-driven convection in underground hydrogen storage sites

Achieving net-zero carbon emissions in a hydrogen economy will require large-scale storage sites, in which geological porous formations can play a crucial role. However, the efficiency of hydrogen storage sites strongly depends on their ability to minimize hydrogen losses posed by the diffusion and dissolution phenomena. While density-driven convection is a well-known phenomenon in carbon dioxide storage sites, its role in hydrogen storage has not been thoroughly understood. The main objective of this study is to develop a series of mathematical formulations to model onset and dynamics of density-driven convection mixing during underground hydrogen storage in the presence of different cushion gases. A separate set of simulation workflow was also developed, and the solubility of different cushion gases was determined and calibrated against available experimental data. The Stuttgart formation from the Ketzin storage site was considered as a case study to show the application of the methodology developed. The results obtained indicated that a hydrogen loss of about 30% can occur after 20,2 years, when the injected hydrogen occupies 50% of the conceptual aquifer volume. It was also observed that N 2 is the most effective cushion gas for minimizing density-driven convection and hydrogen loss, followed closely by CH 4 . These findings highlight the importance of careful selection of storage sites and cushion gas to limit hydrogen loss due to density-driven flow.