Altering carbonate wettability for hydrogen storage: The role of surfactant and CO₂ floods

Underground hydrogen storage (UHS) in depleted oil and gas fields is pivotal for balancing large-scale renewable-energy systems, yet the wettability of reservoir rocks in contact with hydrogen after decades of Enhanced Oil Recovery (EOR) operations remains poorly quantified. This work experimentally investigates how two common EOR legacies, cationic surfactant (city-trimethyl-ammonium bromide, CTAB) and supercritical carbon dioxide (SC–CO₂) flooding, alter rock–water–Hydrogen (H₂) wettability in carbonate formations. Contact angles were measured on dolomite and limestone rock slabs at 30–75 °C and 3.4–17.2 MPa using a high-pressure captive-bubble cell. Crude-oil aging shifted clean dolomite from strongly water-wet (θ ~ 28–29°) to intermediate-wet (θ ≈ 84°). Subsequent immersion in dilute CTAB solutions (0.5–2 wt %) fully reversed this effect, restoring or surpassing the original water-wetness (θ ≈ 21–28°). Limestone samples exposed to SC-CO₂ at 60–80 °C became more hydrophilic (θ ≈ 18–30°) relative to untreated controls; moderate carbonate dissolution (≤6 × 10³ ppm Ca²⁺) produced the most significant improvement in water-wetness, whereas severe dissolution yielded diminishing returns. These findings show that many mature reservoirs are already water-wet (post-CO2) or can be easily re-wetted (via residual CTAB). Across all scenarios, sample wettability showed little sensitivity to pressure, but higher temperature consistently promoted stronger water-wetness. Future work should include dynamic core-flooding experiments with realistic reservoir.