Low-temperature hydrogen generation from fractured ultramafic rock under dynamic flow: implications for engineered geological hydrogen systems

Geologic hydrogen is emerging as a potential low-carbon energy resource. However, the viability of extracting hydrogen from shallow, low-temperature ultramafic reservoirs depends on sustaining reaction rates under dynamic subsurface conditions. This study investigates low‐temperature hydrogen (H 2 ) generation from fractured ultramafic rock under continuous flow-through conditions representative of subsurface environments. Artificially fractured dunite was subjected to coreflood experiments at 100°C and an effective pressure of 5.5 MPa during brine circulation and subsequent acid stimulation. Baseline H 2 production was modest (< 5 µg) but increased nearly thirtyfold following acid injection, reaching 175 µg, accompanied by enhanced Fe release and fracture widening confirmed by micro-CT imaging. H 2 generation declined rapidly before acid stimulation, and only about 23% of the fracture surface contributed to active reactions, emphasizing the critical role of the reactive surface area in sustaining hydrogen productivity. The results demonstrate a coupled relationship between fracture evolution, mineral dissolution, and hydrogen generation under flow-through conditions. These findings provide new insight into the geochemical and transport processes governing subsurface reactivity and have implications for the design of engineered geological hydrogen systems in ultramafic formations.