Investigation of the suitability of underground gas storage facilities for hydrogen storage : an experimental study on mixing processes in porous media

The global transition to renewable energy sources necessitates efficient and large-scale energy storage solutions to balance energy demand and supply. Hydrogen emerges as a promising energy carrier and storage medium for the future. Among various options for storing hydrogen produced from surplus energy, underground hydrogen storage (UHS) in porous reservoirs stands out as particularly promising. Understanding gas mixing processes, which are strongly influenced by molecular diffusion and mechanical dispersion, is critical for optimising these systems to ensure both efficiency and safety. Specialised experimental setups were designed and built to investigate hydrogen diffusion and dispersion properties within geological formations relevant to UHS. Two modified versions of the Wicke-Kallenbach method were employed to assess diffusion in barrier samples (caprock and cement) and reservoir samples (sandstones), while mechanical dispersion was evaluated using a slim tube coil setup. The experiments revealed diffusion coefficients for barrier samples ranging from 1 · 10−11 to 3.1 · 10−10m2/s, confirming their effectiveness as barriers to hydrogen migration. Reservoir rocks exhibited coefficients between 5.00 · 10−9 and 3.71 · 10−7m2/s, indicating dependence on rock porosity and water saturation. Dispersivity values ranged from 0.0042 to 0.06 meters, reflecting variability based on flow conditions. These findings validate the robustness of the measurement approaches and provide essential data for enhancing predictive models and simulation efforts. Using analytical solutions, laboratory measurements were scaled to assess the potential impacts of hydrogen mixing in larger scenarios. These analyses of the scaled laboratory measurements indicated that caprock leakage is negligible, with an annual diffusion loss of only 0.005 % of the total storage volume, highlighting the caprock’s effectiveness as a barrier for hydrogen. Within the reservoir, the mixing zone for molecular diffusion is limited to a few meters, while mechanical dispersion expands it to approximately 30 meters, providing valuable insights for storage strategy optimisation. The monitoring of UHS requires specialised techniques distinct from natural gas storage due to hydrogen’s unique characteristics and the potential for microbial activity. Key methods include continuous pressure and temperature monitoring using advanced sensors placed strategically at wellheads and reservoirs, ensuring real-time data collection for early anomaly detection. Gas chromatography and mass spectrometry are employed to assess gas composition continuously, safeguarding purity. Unlike natural gas, UHS demands additional microbial monitoring to prevent potential storage integrity issues due to biochemical interactions. The UHS monitoring program integrates continuous and periodic assessments aligned with international guidelines, focusing on maintaining both engineering and geological integrity. Preventive measures are strategically implemented to detect and address potential issues early, minimising risks and ensuring the facility’s safety and efficiency.