Hydrogen has emerged as a key energy carrier in the global transition toward low-carbon energy systems, driving increasing interest in Underground Hydrogen Storage (UHS) as a large-scale energy balancing solution. Repurposing existing Underground Gas Storage (UGS) facilities provides a practical pathway for early UHS deployment by leveraging established subsurface infrastructure. This study investigates the technical feasibility of converting a depleted gas reservoir from UGS to UHS using an energy-based framework, in which storage and withdrawal performance are evaluated in terms of energy units (TJ/day) rather than gas volume or mass. A three-dimensional compositional reservoir model of a dome-shaped depleted gas reservoir was developed using CMG-GEM to simulate cyclic natural gas and hydrogen storage operations. The results indicate a pronounced reduction in deliverable energy following conversion, with hydrogen storage providing approximately 12 TJ/day compared to about 48 TJ/day during natural gas storage under equivalent volumetric operating conditions. Increasing the hydrogen injection rate to four times that of natural gas restores the energy injection capacity to levels comparable to UGS. Several operational strategies were evaluated to enhance UHS performance, including cushion gas reduction, extension of the hydrogen injection period, and the use of horizontal wells. Among these, reducing cushion gas volume proved most effective in improving early-cycle hydrogen injectivity. Caprock integrity was assessed under cyclic loading using geo-mechanical scenarios representing stress-independent, elastic, and elasto-plastic behavior. The results demonstrate that stress-induced deformation and hysteresis can significantly increase hydrogen leakage, highlighting the critical role of pressure management in UHS operations. In addition, a screening-level assessment of hydrogenotrophic methanogenesis was incorporated through a simplified reaction model, indicating that microbial hydrogen consumption remains limited under the temperature and salinity conditions of the studied reservoir.
