A multiscale approach was employed to shed light on the critical challenge of hydrogen leakage during underground hydrogen storage (UHS). This study investigated both the effectiveness of cap rocks and aquifers as upper and lower bounds in preventing hydrogen leakage. For cap rock integrity, the research identified crucial factors influencing leakage rates, which are pore size, tortuosity, and reservoir pressure. Smaller pores (less than 1 nm), higher tortuosity (more intricate pore network), and higher pressure were found to significantly enhance the cap rock's ability to contain hydrogen. Shifting focus to hydrogen aquifer interactions, the study explored the impact of the temperature on hydrogen diffusion within the water-saturated rock formations. Simulations demonstrated a strong correlation with higher temperatures leading to increased hydrogen mobility and potential leakage. This finding underscores the importance of considering realistic reservoir conditions, particularly temperature, during UHS assessments. The combined effects of leakage through both cap rocks and aquifers necessitate a comprehensive geological and environmental assessment of the entire UHS system. Such an assessment should meticulously evaluate the cap rock integrity, aquifer properties, and anticipated reservoir temperatures. By incorporation of these factors, researchers and engineers can develop UHS with minimized leakage risks. A holistic understanding of these factors, coupled with the findings presented here, will lead to the development of safer and more efficient UHS solutions, paving the way for a clean, sustainable hydrogen economy.
