Investigating the effects of flow regime on hydrogen transport in salt rock

Underground hydrogen storage (UHS) in salt caverns is emerging as a promising solution for the transition to a sustainable energy future. However, a thorough understanding of hydrogen flow mechanisms through salt rock is essential to ensure safe and efficient storage operations. In this study, we conducted hydrogen flow experiments in salt rocks using the pressure pulse decay (PPD) method, covering a range of hydrogen pore pressures from 0.4 MPa to 7.5 MPa within the slip and transitional flow regimes (Knudsen numbers between 0.04 and 1.5). The Knudsen numbers were determined by measuring the pore size distribution (PSD) of the salt rock samples and assigning an average pore size to each sample based on the measured PSD. Our results indicate that the intrinsic permeability of the tested salt rock samples ranges from 5 × 10⁻²¹ m² to 1.0 × 10⁻²⁰ m². However, a significant enhancement in apparent permeability, up to 10 times the intrinsic permeability was observed, particularly at lower pressures. This permeability enhancement is attributed to the nanoscale pore structure of salt rocks, where the mean free path of hydrogen becomes comparable to the pore sizes, leading to a shift from slip flow to the transitional flow regime. The results further reveal that the first-order slip model underestimates the apparent permeability in the transitional flow regime, despite its satisfactory accuracy in the slip region. Moreover, the higher-order slip model demonstrates acceptable accuracy across both the slip and transitional flow regimes.