Reactivity of a marly-limestone caprock in contact with an alkaline HS--rich solution: application to hydrogen storage in a saline aquifer

Underground hydrogen storage (UHS) is proposed as a carbon-free energy source derived from renewable solar and wind energies. Deep saline aquifers are proposed for UHS since can potentially contribute to large-scale renewable energy storage, providing high storage capacities required to buffer seasonal energy demands. The caprock plays an important role in the sealing capacity for safe and effective UHS. The geochemical reactions involved in H₂-saline groundwater-rock interaction are significant for the integrity of the caprock as these processes directly determine the sealing capacity during UHS.

Aqueous H₂ could react with minerals and trigger redox and dissolution/precipitation reactions, which may affect the permeability and porosity of the caprock. The resulting changes reactivate or propagate microfractures and, consequently, affect the integrity of the caprock and the long-term storage stability. UHS in formations with sulfate-rich groundwater can induce an increase in pH due to sulfate reduction (i.e., SO₄^2- + 4H₂ = HS^- + 3H₂O + OH^-). The main goal of this work is to study the effect of the increase in pH (HS^--rich water) on a marly limestone caprock.

The PHREEQC code and the phreeqc.dat database were used to simulate equilibrium of H₂ (at any pressure in a range between 1 and 100 bar) with a saline solution in equilibrium with calcite and gypsum at 60 °C. The thermodynamic calculations show that H₂ reduces sulfate and that the pH increases from 8.2 to 11.1. This high alkaline water could, therefore, affect the integrity of the caprock. We tried to prove the model results (sulfate reduction and pH increase) in a batch experiment. A saline water rich in sulfate was put in equilibrium with H₂ at 3 bars. After a week, however, the pH did not increase, suggesting that the short-term sulfate reduction does not occur in the absence of sulfate-reducing microorganisms.

A column experiment was carried out to observe potential changes in the marly limestone in contact with an alkaline (pH ≈ 12), HS^--rich solution at 60 °C. Circulation of the solution led to a release of Si and Al, i.e., dissolution of quartz and aluminosilicates. After 380 h, an increase in the flow rate (from 0.01 to 0.03 mL min^-1) resulted in a decrease in the concentrations of Si and Al, suggesting a far-from-equilibrium dissolution of the silicates (SI_quartz = -2.8; SI_albite = -5.9 and SI_illite = -10.6) although the solution was supersaturated with respect to chlorite (SI_chlorite = 5-12). The dissolution of silicates at highly alkaline (pH ≈ 12) may result in a variation of the initial properties of the UHS caprock (e.g. porosity, permeability). Numerical and experimental results of ongoing column experiments will help reveal the extent of the rock alteration.