Numerical Modelling of Biomethanation in UHS/UMR: The Role of Gas Solubility in Long-Term Dynamics

The European Green Deal aims for a gradual reduction in CO2 emissions while simultaneously increasing the share of renewable energy sources (RES). A key challenge is balancing the variable production of green energy with the seasonal demand for the energy. One way to balance the supply of energy with the demand for it is to store its surpluses, e.g., through underground hydrogen storage (UHS). Another gas that requires storage (CCUS) is carbon dioxide (CO2). Under certain reservoir conditions, H2 in contact with CO2 in the presence of methanogenic Archaea may undergo biomethanation, a process that under abiotic conditions would normally require very high temperatures. This process may become an alternative method of utilizing surplus CO2 production and converting H2 into CH4, which is easier to store, transport, and integrate into existing gas systems. This study presents a numerical workflow for modelling the biomethanation phenomenon in the commercial Eclipse reservoir simulator. The methodology was validated through both quantitative and qualitative analyses of results from modelling the operation of an underground biomethanation reactor (UMR). Particular attention was given to the role of gas solubility (H2, CO2, CH4) in long-term reservoir dynamics. The simulations assessed not only the total CH4 production but also the fraction of injected H2 converted into CH4, as well as the effects of varying compositions of injected gas. To the best of the authors‘ knowledge, this is the first study to explicitly link gas solubility with H2 utilization efficiency in UMR simulations, providing new insights into long-term underground biomethanation.