Potential for fault reactivation during Underground Hydrogen Storage.

Underground hydrogen storage (UHS) is expected to play a key role in the emerging hydrogen economy by providing large-scale storage capacity with lower leakage risk than above-ground alternatives. While geological storage has been extensively studied for natural gas (CH₄) and carbon dioxide (CO₂), the physical behaviour of hydrogen (H₂) in the subsurface remains less understood. Owing to its lower density and viscosity and higher diffusivity, H₂ is expected to induce different coupled hydro-mechanical (HM) responses than these well-studied fluids, with potential implications for fault stability and risks of induced seismicity. This study employs a with linear elasticity and strain-dependent permeability to simulate hydrogen injection in a fractured carbonate reservoir, using the Hontomin CO₂ storage pilot site (Spain) as a geological analogue. The reservoir and caprock are represented as homogeneous continua, while major faults are modelled explicitly with varying permeability, controlling pressure diffusion and compartmentalisation. Two contrasting fault systems are considered: one critically stressed and highly sensitive to stress perturbations, and one initially more stable. The associated potential for induced seismicity is first assessed using mobilized friction to evaluate fault stability under evolving stress and pore-pressure conditions. The model is then integrated with a rate-and-state friction model framework to quantify stress-driven changes in terms of seismicity rates. The results show that pore-pressure redistribution, reservoir geometry and frictional properties are key controls on both co-injection and post-injection seismicity. Distant faults may reactivate after injection shut-in due to delayed pore-pressure diffusion and poroelastic stress. Seismicity is more likely to occur on faults with large offsets and low permeability, where pore pressure dissipation is limited. The seismic response is strongly governed by the initial proximity of faults to failure rather than injection behaviour alone. These findings highlight the necessity of detailed site-specific geological and geomechanical characterisation for assessing UHS feasibility and mitigating seismic risk. Ongoing work extends the modelling approach towards seismic waveform simulations to assess the detectability of H₂ plume evolution and migration.