Geochemical influences of hydrogen storage in depleted gas reservoirs with N₂ cushion gas

Geochemical investigation of hydrogen (H₂), brine, rock formations, and residual gases (nitrogen – N₂, methane – CH₄, and carbon dioxide – CO₂) is crucial for understanding H₂ storage processes in depleted gas reservoirs. This study examines the effects of injecting a gas mixture (60% H₂ + 30% N₂ + 5% CH₄ + 5% CO₂) into Bandera Grey sandstone with notable clay mineral content. The experiment lasted 50 days at 42 °C, 1350 psi, and 5 wt% NaCl brine, with detailed analytical characterization before and after aging. Results show minimal sample reactivity, causing slight alterations in petrophysical characteristics. Dissolution primarily affected dolomite and barite minerals, while illite and kaolinite caused pore blockage. Gas chromatography data indicate only 0.45% of injected H₂ is lost (via dissolution in brine and reaction with CO₂), CH₄ increased by 6.22%, N₂ remained stable, and CO₂ decreased by 1%. These results provide valuable insights into the feasibility and safety of utilizing depleted gas reservoirs for H₂ storage.