Potential analysis of in-situ hydrogen generation in narrow reservoir with alternating steam-oxygen injection

The growing demand for carbon-neutral energy has driven advances in clean energy utilization and hydrocarbon conversion. This study introduces a novel in-situ hydrogen generation method using steam–oxygen alternating injection and high-temperature thermal reactions within a heterogeneous reservoir model. The workflow integrates WinProp/CMG thermal–reaction simulation with three sub-models that capture porosity–permeability contrasts, saturation variations, and fault barriers. Before gas breakthrough, the system maintains 600–800 ℃ with a hydrogen yield of 1781.3 m 3 per m 3 of fossil fuel and 34.94 % oil recovery. Sub-model 1 shows that fault-induced breakthrough delay extends the hydrogen-generation period, reaching a peak of 2221.7 m 3 at approximately 700 ℃. A categorical operating window emerges: below 600 ℃ favours combustion-dominated reactions, while above 800 ℃ accelerates coke oxidation and suppresses H 2 yield. After a breakthrough, continued O 2 injection lowers temperature and sharply reduces productivity, emphasizing the importance of thermal management and cycle timing. Compared with conventional underground coal gasification, alternating injection improves hydrogen recovery by 8.8 % and reduces greenhouse gas emissions by 8.2–15.6 % by suppressing H 2 and CO combustion. 3D simulations reveal conical cavity growth driven by pressure gradients and fault-moderated continuity, supporting the feasibility of screening for unconventional settings. • Steam–oxygen alternating injection enables in-situ hydrogen production. • Stable reaction temperature (600–800 ℃) and high H₂ yield (1781.3 m 3 /m 3 ). • H₂ recovery improves by 8.8 % over conventional gasification. • CO and H 2 oxidation suppressed, cutting GHG emissions by 8.2–15.6 %. • Faults delay breakthrough and enhance hydrogen recovery.