Solvent-Induced Organic Adsorption Variations in Calcite/H₂/Brine Systems: Implications for Underground Hydrogen Storage

Hydrogen (H₂) geo-storage is being increasingly identified as a vital, long-term solution for powering a cleaner, more sustainable hydrogen economy. There is an increased interest in utilizing saline aquifers and depleted oil and gas reservoirs for large-scale hydrogen storage and to fulfill the increasing energy demands of the future. The wettability properties of gas and rock-fluid interaction play a key role in determining the containment security, fluid flow, and storage capacity. Despite extensive research, a comprehensive understanding of the influence of rock surface roughness,and the choice of solvents used in organic acid solution preparation on wettability remains elusive. This study aims to evaluate the solvent-induced organic adsorption variations in calcite/H₂/brine systems under various thermophysical conditions. In this study, experiments are conducted at 0.1, 5, 10, 15, and 20 MPa pressures and 298, 323, and 343 K temperatures using a Kruss Drop Shape Analyzer (DSA-100). Methanol and n-decane solvents mixed with 10^-2 mol/L concentration of stearic acid solutions are used to identify the variation in wettability due to organic adsorption. Firstly, calcite chips are prepared and cleaned with DI water, followed by 15 minutes of air plasma treatment for contaminant evacuation. Afterwards these chips are immersed in a 2 wt% solution of NaCl, and pH is maintained at 4 using aqueous hydrochloric acid (HCl) for surface ionization. The chips are then dried and aged in prepared solvents for 10 days. The residual fluids are then removed from the surface using a process of vacuum drying, and the contact angle measurements are taken using ImageJ software. The results indicated a pronounced shift towards hydrophobicity in the calcite/H₂/brine system as pressure increased, while a transition from hydrophobic to hydrophilic is observed as the temperature is increased. For methanol/stearic-acid aged substrates, higher contact angles are obtained in comparison to n-decane/stearic acid treatments, which indicates the role of solvent type in wettability alteration. The higher values for methanol than n-decane can be further explained by surface energy, entropy effects, and molecular interactions. The increase in hydrophobicity with the increase in pressure can be justified by the increase in H₂ gas density and intermolecular interactions between H₂ molecules and the calcite surface. At higher temperatures, intermolecular interactions weaken the effect of H₂ on the wettability, and lower contact angles are obtained with the increase in temperature. The results highlight the impact of solvent-induced organic adsorption on wettability alteration in calcite/H2/brine systems. This understanding becomes essential for assessing the performance of UHS, i.e. underground hydrogen storage. The impact of pressure, temperature, organic solvent, and surface roughness on wettability have high importance for further reservoir modeling and evaluating the feasibility of large-scale H₂ storage.