Fault-controlled natural hydrogen reservoirs as underground hydrogen storage sites.

Underground hydrogen storage requires geological formations with demonstrated hydrogen containment, yet no conventional storage candidate (salt caverns, aquifers, or depleted hydrocarbon reservoirs) offers direct, prior evidence of H 2 -specific sealing. We propose that subcommercial natural hydrogen reservoirs, where H 2 has accumulated over geological timescales, can serve as precertified storage sites. This proposition is evaluated using the Bulqizë fault-controlled natural H 2 reservoir in Albania. We develop a coupled geofluid-geomechanical model from exploration data incorporating depth-dependent temperature and permeability gradients across the 5-km vertical extent of the fault zone, hydrogen dissolution under two end-member hydrogeological configurations, and fault stability assessed through slip tendency analysis. Our results show that hydrogen losses from dissolution and abiotic consumption remain below 2% of total inventory under conservative upper-bound assumptions-independently consistent with the sustained natural H 2 accumulation observed in the reservoir. We identify two distinct, permeability-dependent fault reactivation modes: localized instability near the injection interval at low permeabilities, and instability at the fault-caprock interface at high permeabilities. Parametric analysis across 72 combinations of injection rate, location, and permeability defines safe operating envelopes and yields optimized storage capacities of 3.5 to 4.8 × 10 8 m 3 . A 10-y cyclic injection-production simulation demonstrates stable throughput of 7.5 × 10 5 m 3 ·d -1 with progressively decreasing fault reactivation risk-a counterintuitive finding driven by pressure homogenization through gas redistribution. These results establish natural hydrogen reservoirs as a viable class of underground storage sites.