Impact of H₂-CH₄ mixture on pore structure of sandstone and limestone formations relevant to subsurface hydrogen storage

Subsurface Hydrogen (H₂) storage is a promising technology to achieve net-zero carbon emissions. Depleted gas reservoirs can provide safe storage of hydrogen in the long term, and effective recovery during the cycling process. The pre-existing Methane gas (CH₄) or its traces in these formations can mix with the injected hydrogen and might cause in-situ geochemical reactions with the storage rock. Despite the recent interest in evaluating H₂-rock interactions, the available knowledge on the impact of H₂–CH₄ mixture on the petrophysical properties of storage rocks is scarce. Motivated by the lack of experimental work that exists on this topic, we provide an experimental investigation on the reactivity of sandstone and limestone formations to H₂–CH₄ mixture. Samples from Bandera Gray Sandstone and Indiana Limestone samples were aged in H₂–CH₄ mixture at 75 °C and 1400 psi for 90 days, and Nuclear Magnetic Resonance (NMR) and gas porosity analysis were performed. The results showed limited reactivity of the samples to H₂–CH₄ mixture, and subsequently, insignificant changes occurred in the petrophysical properties. The average porosity of BG and IL samples decreased by 3.32 % and 1.00 % respectively. NMR analysis indicates no significant changes in T₂ distribution times curves, indicating no major alterations occurred in the pore structure or rock geometry. Moreover, the average NMR porosity of BG was reduced by 3 %, while the reduction of IL porosity did not exceed 6 %. We conclude that the potential geochemical reactions (dissolution/precipitation) between the H₂–CH₄ mixture and rock minerals are very weak within 90 days of H₂–CH₄ mixture treatment, thus the suitability of sandstone and limestone rocks for hydrogen storage is promising.