The development of large-scale, flexible, and safe hydrogen storage is critical for enabling a low-carbon energy system. Deep saline aquifers (DSAs) offer substantial theoretical capacity and broad geographic distribution, making them attractive options for underground hydrogen storage. However, hydrogen storage in DSAs presents complex technical, geochemical, microbial, geomechanical, and economic challenges that must be addressed to ensure efficiency, safety, and recoverability. This study synthesizes current knowledge on hydrogen behavior in DSAs, focusing on multiphase flow dynamics, capillary trapping, fingering phenomena, geochemical reactions, microbial consumption, cushion gas requirements, and operational constraints. Advanced numerical simulations and experimental observations highlight the role of reservoir heterogeneity, relative permeability hysteresis, buoyancy-driven migration, and redox-driven hydrogen loss in shaping storage performance. Economic analysis emphasizes the significant influence of cushion gas volumes and hydrogen recovery efficiency on the levelized cost of storage, while pilot studies reveal strategies for mitigating operational and geochemical risks. The findings underscore the importance of integrated, coupled-process modeling and comprehensive site characterization to optimize hydrogen storage design and operation. This work provides a roadmap for developing scalable, safe, and economically viable hydrogen storage in DSAs, bridging the gap between laboratory research, pilot demonstration, and commercial deployment.

The growing mismatch between renewable energy supply and demand necessitates large-scale seasonal underground hydrogen storage (UHS) in deep saline aquifers. This study presents a field-scale compositional numerical simulation assessment of operational strategies for UHS in homogeneous and heterogeneous anticline deep saline aquifers using CMG simulator on an optimized 91 x 44 x 10 corner-point grid model. Cushion gas type and fraction, well configuration, reservoir petrophysical properties, and cyclic injection-production schedules were systematically evaluated against cumulative hydrogen mass stored and recovery efficiency. Results shows that nitrogen consistently outperformed CO2 and CH4 across all cushion gas fractions, yielding 268,706.5 kg at 20% fraction, attributable to its chemical inertness, low aqueous solubility, and favorable gravitational segregation from hydrogen. Reservoir heterogeneity enhanced stored hydrogen mass by approximately 12 to 15% relative to the homogeneous formation by enhancing plume distribution and improved volumetric sweep efficiency. Recovery efficiency remained insensitive to injector-to-producer spacing across 0 to 60 m, peaking at 91.48% at 15 m spacing. Mole fraction analysis identified nitrogen as providing the best operational producer placement, offering field operators a flexibility in well positioning without losing hydrogen purity. This study provides practical screening and operational design criteria for large-scale UHS in saline aquifer systems.