Reservoir modeling of subsurface methanation for synthetic natural gas generation from co-injected hydrogen and carbon dioxide

The underground co-storage of carbon dioxide (CO₂) and hydrogen (H₂) in deep saline aquifers offers a promising strategy for coupling climate mitigation with renewable energy storage. A particularly attractive outcome of such co-injection is in situ methanation, whereby CO₂ and H₂ react to generate methane (CH₄), a synthetic natural gas that can be stored underground and distributed through existing natural gas infrastructure. This study employs compositional reservoir simulation to evaluate the co-injection behavior, methane yield, and storage efficiency of such a process over a 100-year time frame, including 20 years of injection followed by 80 years of monitoring. This work presents a conceptual, simulation-based assessment of subsurface methanation under idealized reservoir conditions. The methanation process is represented using an idealized conceptual assumption to evaluate the upper-limit potential of methane formation. The model represents a homogeneous aquifer with three injection wells, into which a gas mixture of 20% CO₂ and 80% H₂ (molar basis) was introduced. The simulation results indicate that approximately 2.05 × 10⁹ gmol of CO₂ and 8.20 × 10⁹ gmol of H₂ can be stored, with methane conversion rates reaching up to 95% in the most reactive zones. Residual and solubility trapping mechanisms were found to dominate long-term containment of CO₂. A sensitivity analysis was conducted to quantify the influence of key reservoir and operational parameters, including porosity, permeability, pore compressibility, pressure-dependent fluid properties, and temperature. Porosity emerged as the most influential factor, with higher porosity enabling greater injectivity and enhanced methane generation. Moreover, the interaction between porosity and permeability was shown to strongly affect overall storage potential. Although the model assumes idealized microbial conversion, the findings demonstrate the dual benefits of subsurface methanation as a CO₂ utilization pathway and synthetic natural gas production method . Future work should integrate kinetic modeling and heterogeneous geological settings to improve predictive reliability and field applicability.