A novel fully coupled thermal-hydraulic-mechanical model for hydrogen storage in depleted gas reservoirs

Underground hydrogen storage (UHS) in depleted gas reservoirs provides a scalable solution for addressing energy supply-demand imbalances across short-and longterm operational horizons. Reliable prediction of hydrogen storage performance in depleted gas reservoirs requires accurate representation of coupled thermal, hydraulic, and mechanical processes. In this study, we introduce a new, fully coupled THM simulation capability for UHS by directly modifying the source code of the TOUGH2-CSM simulator. TOUGH2-CSM was originally designed for modeling CO2 sequestration in saline aquifers and lacked support for hydrogen-specific thermophysical behavior. We extend its functionality through the development and integration of a new equation-of-state (EOS) module for multicomponent hydrogen storage systems, enabling modeling of multicomponent mixtures in multiphase, non-isothermal flow conditions. The geomechanical coupling distinguishes it from hydrogen-enabled TOUGH family EOS modules, which does not account for stress-strain interactions, addressing a critical gap and enhancing its relevance for practical UHS applications. Model validation was performed using thermophysical property data from the National Institute of Standards and Technology (NIST), ensuring accurate prediction of gas behavior across a wide range of pressure-temperature conditions. The model achieves an average deviation of less than 5 % in thermophysical property predictions compared to NIST reference values. Experimental datasets further confirm its accuracy, with simulation results aligning within 5 % of measured data. Benchmark tests with existing simulator ensured consistency in pure hydrogen storage. Geomechanical consistency is validated via analytical verification against Terzaghi's one-dimensional consolidation theory, with an error of less than 1 %, confirming proper coupling of fluid flow and stress evolution. Additionally, the model's applicability is illustrated through a preliminary application case study of hydrogen storage in a depleted gas field, highlighting its potential for real-world implementation. Future work will focus on applying the developed model to real field scenarios for the design of injection and production strategies. The framework will also be extended to incorporate chemical reactions relevant to underground hydrogen storage.