Underground Hydrogen Storage: Insights into hydrogen reactivity and porosity dynamics for optimizing clean energy storage in depleted reservoirs

Underground Hydrogen Storage (UHS) is an emerging clean energy solution, particularly in depleted oil and gas reservoirs. These formations often contain varying amounts of carbonates, yet existing studies report contradictory findings on their reactivity with hydrogen. This study addresses these inconsistencies through long-term experimental investigations of hydrogen–rock interactions under both low (2.8 MPa, 28 °C) and high (12 MPa, 70 °C) pressure-temperature conditions, across varying water saturation levels, for up to 180 days. Using XRD, ICP-OES, SEM, and CT imaging, we observed limited mineral reactivity in carbonate-rich samples, with a maximum of ∼1 % calcite dissolution and ∼1 % gupeiite precipitation. Despite minimal elemental variation, up to 25 % hydrogen loss occurred within 10 days, suggesting physical and chemical interactions at play. Notably, CT scans revealed up to 63 % porosity increase, with SEM confirming significant pore alteration. Comparative N₂ control experiments confirmed that hydrogen-specific reactivity, especially under higher water-to-rock ratios, plays a dominant role in mineral alteration and gas loss. These findings provide new evidence that, while carbonate minerals are largely inert to hydrogen under standard conditions, high W/R environments can significantly influence hydrogen loss and storage performance. This work delivers novel insights into long-term hydrogen retention mechanisms in carbonate-containing formations, helping inform the design of safer and more efficient UHS systems.