Aspects of hydrogen flow in water-saturated limestone

The feasibility of underground hydrogen storage (UHS) in carbonate formations relies on a comprehensive understanding of multiphase flow and geomechanical effects under in-situ conditions. This study examines the two-phase flow (water and hydrogen) in Indiana limestone as a representative carbonate reservoir under steady-state conditions and 21 days of hydrogen treatment. Experimental results indicate a good agreement with Brooks-Corey model: the exponent values during primary drainage and imbibition are 5.0 for water and 4.9-5.3 for hydrogen, with minimal hysteresis effect, which is aligned with previously reported results on porous rock. Additionally, the endpoint water saturation after imbibition is recorded at 0.92, corresponding to an irreducible hydrogen saturation of 0.08 and indicating that residual gas trapping plays an important role in hydrogen retention within the pore structure. After the treatment, calcite expansion-driven porosity reduction is observed, suggesting mineral alterations induced by prolonged hydrogen exposure. Potential calcite dissolution raises the necessity for further long-term observations to assess its implications on reservoir integrity and storage performance over extended operational timescales.