Numerical analysis of bio-methanation process during underground hydrogen storage

Geological formations can provide cost-effective storage capacity at scale and therefore are increasingly considered for hydrogen (H2) storage. However, H2 may trigger the bio-methanation process when carbon dioxide (CO2) is used as cushion gas. This process may lead to H2 loss and the contamination of the back produced gas. The impact of the methanation process on the H2 recovery performance is analysed using a series of fine-scale numerical flow simulations. A gravity-dominated operational strategy is designed to mimic seasonal H2 supply-demand patterns. Although gravity can drive the segregation between H2 and CO2, permeability heterogeneities lead to flow dispersions and gas mixing. In turn, they provide the base condition (local mixing of H2 and CO2) for the subsequent methanogenesis to occur. In one scenario, approximately 30% of H2 has been converted to CH4 using a methanation rate from the literature. Compared with the case without methanogenesis, H2 recovery is reduced by 17% and 26% with reference to H2 purity levels of 98% and 90% in the case with methanogenesis considered. Water-breakthrough also occurs when the methanogenesis is activated. This is because of the decrease in the total gas volume and the newly formed water, as a result of the methanation reaction.