Modelling Underground Hydrogen Storage: A State-of-the-art Review of Fundamental Approaches and Findings

This review presents a state-of-the-art of geochemical, geomechanical, and hydrodynamic modelling studies in the Underground Hydrogen Storage (UHS) domain. Geochemical modelling assessed the reactivity of hydrogen and res pective fluctuations in hydrogen losses using kinetic reaction rates, rock mineralogy, brine salinity, and the integration of hydrogen redox reactions. Existing geomechanics studies offer an array of coupled hydromechanical models, suggesting a decline in rock failure during the withdrawal phase in aquifers compared to injection phase. Hydrodynamic modelling evaluations indicate the critical importance of relative permeability hysteresis in determining the UHS performance. Solubility and diffusion of hydrogen gas appear to have minimal impact on UHS. Injection and production rates, cushion gas deployment, and reservoir heterogeneity however significantly affect the UHS performance, stressing the need for thorough modelling and experimental studies. Most of the current UHS modelling efforts focus on assessing the hydrodynamic aspects which are crucial for understanding the viability and safety of UHS. In contrast, the lesser-explored geochemical and geomechanical considerations point to potential research gaps. A variety of modelling software tools such as CMG, Eclipse, COMSOL, and PHREEQC evaluated those UHS underlying effects, along with a few recent applications of datadriven-based Machine Learning (ML) techniques for enhanced accuracy. This review identified several unresolved challenges in UHS modelling: pronounced lack of expansive datasets, leading to a gap between model predictions and their practical reliability; need robust methodologies capable of capturing natural subsurface heterogeneity while upscaling from precise laboratory data to field-scale conditions; demanding intensive computational resources and novel strategies to enhance simulation efficiency; and a gap in addressing geological uncertainties in subsurface environments, suggesting that methodologies from oil reservoir simulations could be adapted for UHS. This comprehensive review offers a critical synthesis of the prevailing approaches, challenges, and research gaps in the domain of UHS, thus providing a valuable reference document for further modelling efforts, facilitating the informed advancements in this critical domain towards the realization of sustainable energy solutions.