Experimental Characterization of H2-Brine-Cement Interactions on Wellbore Integrity During Underground Hydrogen Storage in Depleted Gas Reservoirs

Abstract The transition to renewable energy sources has increased the need for efficient and sustainable energy storage solutions, with Underground Hydrogen Storage (UHS) in depleted gas reservoirs emerging as a promising option. Maintaining wellbore integrity in such environments is critical, yet the interactions between hydrogen, brine, and cement, particularly with chemical additives, remain insufficiently understood. This study addresses this gap by investigating the effects of hydrogen and brine on the mineralogy and microstructure of cement with and without chemical additives. We conducted a series of experiments, including X-ray diffract (XRD), scanning electron microscopy (SEM), micro-computed tomography (micro-CT), and water chemistry measurements, to evaluate changes in cement composition and integrity over time. Key findings reveal that while the primary mineral compositions of cement, such as Portlandite and Calcium-Silicate-Hydrate (C-S-H) compound, transform under hydrogen and brine exposure, chemical additives such as fluid loss, dispersants and retarder show minimal impact on these changes. The average size of the interconnected pore networks increased significantly, but the increase was much lower in cement with additives (38.3%) compared to without (324.6%), indicating that additives help inhibit pore network expansion. Despite this, the overall porosity remained stable. Micro-fractures were more uniformly distributed in cement with additives, whereas in cement without additives, cracks connected existing pores, forming larger void networks. However, the generation and propagation of micro-fractures are more likely due to the changes of temperature and pressure during the samples loading rather than the impact of H2. Ion concentrations, including Na+, K+, Ca2+, and Cl-, remained largely stable, with only a slight increase in SO42- due to gypsum dissolution. These results suggest that the impact of hydrogen on Class G cement is generally negligible, with minimal changes observed in overall mineral composition. While chemical additives offer some resistance to structural changes induced by hydrogen and brine, their overall effectiveness is limited. The study underscores the need for developing more robust solutions to enhance the durability and strength of cement in UHS applications, which is crucial for the long-term stability and safety of subsurface hydrogen storage systems.