Geochemical Investigation of the Hydrogen Gas – Quartz – Porewater System for Understanding Underground Hydrogen Storage in Siliciclastic Reservoirs

Underground hydrogen storage (UHS) is a key opportunity in the transition to sustainable energy economy as it addresses the challenge of intermittent renewable energy production. However, a better understanding of the pore scale processes in the rock-porewater-hydrogen system is crucial for secure UHS. To address these geochemistry-related questions, the interaction between quartz and hydrogen gas was investigated in this study. Quartz is a major constituent of siliciclastic rocks, therefore for the batch experiment two types of quartz grains were used: the grains of a natural, inclusion free quartz crystal and quartz grains separated by hand picking from a typical reservoir sandstone of the Carpathian-Pannonian region. Batch experiments combined with geochemical modeling (PHREEQC) were carried out to match the experimental results.

For the batch experiments, 2 g of quartz and 70 ml of deionized water were mixed in a reactor vessel. The experiments were conducted under varying pressures (50–100 bar) and temperatures (80–100 °C), corresponding to the expected conditions for underground hydrogen storage. Throughout the 72-hour experiments, the chemical composition of the solution was monitored through sampling and analyzed by inductively coupled plasma optical emission spectrometry (ICP-OES). Quartz grains were examined before and after the experiments via scanning electron microscope (SEM-BSE) and Fourier-transform infrared spectroscopy (FTIR) to observe any effect of dissolution or surface alteration on the quartz grains. Reference experiments were conducted with helium gas under the same p-T conditions.

Results show that quartz reactivity with hydrogen remained quite low in all experimental runs. The pH displayed considerable increase during some of the experimental runs, which was unforeseen in the geochemical models. Quartz solubility was found to be primarily pH-dependent, as reflected by Si concentrations in solution samples from experiments. Lower solubility (~2 mg/l) was observed in acidic and neutral pH ranges, whereas somewhat higher solubility (~6 mg/l) was observed under alkaline conditions. Silanol groups on the surface of the powdered quartz grains, confirmed by FTIR, may have contributed to the observed increase in pH and enhanced quartz solubility, and should be accounted for the geochemical models.

In the experiment, involving quartz grains from the sandstone reservoir, significantly higher dissolved Si concentrations were measured compared to the experiments with pure quartz under the same conditions. This difference was likely due to the dissolution of other rock forming minerals (e.g., kaolinite) remaining in trace amounts on the surface of the grains despite careful preparation.

In conclusion, quartz is a less reactive mineral under the typical pressure and temperature conditions of subsurface hydrogen storage, therefore quartz dominant rocks seem to be favorable for future hydrogen storage. Further study of silanol behavior and its integration into geochemical modeling may enhance the accuracy of future predictions.