A Comparative Narrative Review of Geological Risk Assessment of Hydrogen and Co2 in Depleted Reservoirs in the United States

Depleted oil and gas reservoirs are increasingly considered for carbon dioxide (CO2) sequestration and large-scale hydrogen (H2) storage, due to their established geological characterization, infrastructure, and capacity to retain hydrocarbons. However, storage strategies developed for CO2 are being applied to H2 without sufficient accounting for their physical, geochemical, and microbiological behaviors. This creates uncertainty about whether depleted U.S reservoirs can safely support both decarbonization pathways. A narrative comparative review was conducted to synthesize evidence on fluid behavior, trapping mechanisms, reservoir suitability, risk factors, and monitoring for both gases. Covering fluid behavior, trapping mechanisms, reservoir suitability, risk factors, and monitoring for both gases. The findings show that H₂ is considerably less dense and more diffusive than supercritical CO₂, making it more buoyant, prone to migration through microfractures and legacy wells. Unlike CO₂, which can utilize multiple trapping mechanisms, H₂ primarily remains in the gas phase and depends heavily on structural containment. Its storage performance may also be affected by microbial activity and mineral reactions that could form methane or hydrogen sulfide. Hence, H₂ storage requires more stringent evaluation of caprock, fault, and wellbore integrity, alongside specific hydrogen sensing systems. It is concluded that depleted reservoirs in U.S. may support both applications, but reliable H₂ storage requires tailored site selection, monitoring, and risk management frameworks, which are significant to guide safer project design, limit leakage and gas losses, inform hydrogen-specific regulation and commercial use.