Industrial-scale underground hydrogen storage (UHS) in carbonate formations can facilitate the realization of carbon dioxide-free economy, as well as global energy security through replacement of fossil fuel with clean renewable energy. Although carbonate minerals-hydrogen-brine interactions plays significant role in residual trapping of H2 in calcite-dominated host-rock, however, there is limited information on impacts of hydrogen-brine-limestone reactions on storage potentials and sustainability of carbonate formation for long term underground hydrogen storage. To address this concern, two sisters outcrop limestone samples were saturated with 3 wt NaCl brine (pH = 7) and pressurized with H2 for 125 days at 75 °C and 1400 psi to evaluate the hydrogen-brine-limestone reactions. The cores were scanned through X-ray computed micro-tomography (μCT) and visually inspected for modifications of pore structures through calcite dissolution and pore-filling. μCT and nuclear magnetic resonance (NMR) porosities before and after hydrogen injection were computed and the gas chromatography (GC) analysis of the effluent fluids was conducted to determine if any methane is produced from the reactions. Results showed that the extent of calcite dissolution was trivial in first limestone sample and almost insignificant in second sample. The small fraction of dissolved calcite in sample 1 resulted in slight change in NMR porosity from 18.55 to 20.96 and micro-CT porosity from 5 to 6.6 whereas the NMR and micro-CT porosities of sample 2 were almost the sample before and after hydrogen injection. No methane production and significant H2 loss was detected from GC analysis, suggesting that just teeny portion of hydrogen penetrated the limestone. The results largely suggest that UHS in carbonate reservoirs may not be significantly affected by H2/brine/limestone reactions within 125 days of hydrogen storage cycle. The study provides significant insight into the stability of carbonate rock in the presence of brine and H2 at short storage cycle during UHS. © 2023 Elsevier Ltd