Pore structure analysis of storage rocks during geological hydrogen storage: Investigation of geochemical interactions

Depleted oil and gas reservoirs have been widely studied for gas storage purposes such as carbon dioxide and methane, however, there are still some uncertainties when it comes to hydrogen operations. Understanding the geochemical interactions of hydrogen-rock-brine is a key factor for safe sub-surface hydrogen containment, due to the associated risks of altering the rock pore properties. Despite the recent interest in evaluating H₂-rock interactions, there is limited knowledge on the reactivity of sandstone and carbonate rocks to hydrogen and the potential implications on pore structure. In this work, we experimentally treated sandstone and limestone samples with hydrogen at 75 °C and 1400 psi for an extended duration of 6 months. Analytical methods including optical microscope for thin sections, Nuclear magnetic resonance (NMR), and helium gas porosity analysis were conducted before and after the hydrogen treatment. The results confirm the occurrence of geochemical reactions between rock minerals and hydrogen, however, the level of mineral dissolution and precipitation is minimal, and only minor changes were observed on the surface morphology. Consequently, insignificant changes occurred in the pore properties of the samples; the weight of the samples did not exhibit significant changes (an average increase of 0.61 %), while the average reduction in porosity and pore volume of the samples did not exceed 6 %. NMR results show no notable changes in T₂ distribution times curves, the average increase in porosity values for all the samples (sandstone and limestone) did not exceed 5 % after 6 months of H₂ treatment, suggesting that no significant alterations occurred in the pore structure or rock geometry. We conclude that the geochemical reactions of the hydrogen-rock-brine system are very limited even after 6 months of hydrogen treatment, thus the suitability of sandstone and limestone rocks for hydrogen storage is promising.