Challenges in the simulation of underground hydrogen storage: A review of relative permeability and hysteresis in hydrogen-water system

Underground hydrogen storage (UHS) and production in depleted gas reservoirs, aquifers, and salt caverns is a promising solution to balance supply and demand on a large scale. Because of the cyclical nature of the hydrogen storage and production process and the presence of cushion gas, compared to other underground gas storage operations like carbon capture and storage (CCS), accurate application of hydrogen-water dynamic properties in the UHS simulation process is critical. In this paper, we present a state-of-the-art literature review on relative permeability of the hydrogen-water system, and the determination methods. To further highlight the existing data deficiency and simulation challenges, we performed reservoir-scale flow simulations using a conceptual geological model to study the impact of hysteresis and reservoir mixing between hydrogen and cushion gas on flow functions, and the efficiency of the operation. The results reveal that hysteresis and gas mixing significantly affect the total volume of hydrogen produced, the water-gas ratio, the purity of the produced gas, and the reservoir pressure. On average, the ultimate recovery factor in the presence of hysteresis decreased by 16 % to 25 %, while increasing water production from the field. The simulation results for the effect of in-situ gas mixing show a decrease in the hydrogen recovery factor by about 5 %. This review emphasizes the importance of considering hysteresis and gas mixing in UHS simulation studies. To improve the accuracy and reliability of the simulation results of UHS process for field deployment, it is necessary to conduct further research and acquire field-specific saturation functions for pure and mixed gases.