• Relative-permeability hysteresis is identified as the dominant mechanism controlling hydrogen recovery losses. • Hydrogen solubility and molecular diffusion have only a minor impact on recovery and plume evolution. • Multicycle injection–withdrawal conditions the reservoir, increasing recovery from ∼62% to ∼86%, while perforation depth has a negligible effect on recovery and purity. • Inert cushion gases (N 2 and CH 4 ) provide the highest hydrogen recovery and purity. • CO 2 significantly degrades performance when methanation is active under assumed reaction kinetics, and the behavior of binary and ternary mixtures is governed by CO 2 fraction. Understanding how reservoir conditioning and cushion-gas composition influence multicycle performance remains a key requirement for reliable underground hydrogen storage (UHS) in saline aquifers. This study investigates the coupled effects of relative-permeability hysteresis, hydrogen solubility, molecular diffusion, and cushion-gas selection on hydrogen recovery and produced-gas purity using fully compositional simulations. Five storage cycles revealed a progressive improvement in system performance, as trapped gas accumulated near the structural crest and around the wellbore, forming a stabilizing internal buffer that suppressed water encroachment and increased peak hydrogen recovery from approximately 62% in Cycle 1 to about 86% in Cycle 5. Cushion-gas selection exerted a strong control on both recovery and produced-gas purity. Inert gases (N 2 and CH 4 ) provided the most stable multicycle behavior, achieving high final-cycle recovery and hydrogen purity in the range of ∼0.87–0.89. In contrast, Under the assumed generic methanation kinetics adopted in this study, chemical conversion of H 2 to CH 4 and H 2 O is the dominant cause of the degraded CO 2 -cushion-gas performance, although it behaved comparably to inert gases when the reaction pathway was disabled. Mixed-gas scenarios displayed a clear performance hierarchy governed by CO 2 fraction, with both recovery and purity declining as CO 2 content increased. Sensitivity analysis showed that perforation depth had negligible influence on recovery efficiency and hydrogen purity. Overall, the results demonstrate that UHS in saline aquifers becomes more efficient with repeated cycling, but performance remains strongly controlled by hysteresis effects and cushion-gas reactivity. Inert cushion gases, either pure or in mixtures, therefore, represent the most effective strategy for maximizing hydrogen recovery and maintaining high-purity hydrogen production during cyclic storage operations.
Aboushanab et al. (Wed,) studied this question.