Experimental assessment of methanogenic activity in reservoir analogues for underground hydrogen storage: impact of pore volume, surface area, and gas-liquid interfacial area

Hydrogen storage within porous geological formations initiates the methanogenesis process, leading to the conversion of hydrogen into methane. Understanding and quantifying the impact of pore characteristics, including porosity, surface area, and interfacial area between liquid and gas phases on hydrogen conversion rates is crucial for evaluating both the risks of hydrogen loss during underground hydrogen storage and the potential benefits for efficient bio-methanation. This study explores the impact of surface area of reservoir rocks on methanogenic activity by employing various techniques including nitrogen physisorption, mercury injection capillary pressure (MICP), nuclear magnetic resonance (NMR), scanning electron microscopy (SEM), and X-ray micro-computed tomography (µCT). The research examines reservoir analogues for hydrogen storage, from the Cretaceous (Bentheimer Sandstone and Oberkirchner Sandstone) and Triassic periods (Red and Grey Weser Sandstone), varying from tight to permeable. The cell size of Methanothermococcus thermolithotrophicus ranges from 1 to 2 µm, suggesting that these archaea can access pores larger than this threshold. Microbial activity within the pore space of the reservoir rocks was assessed by monitoring pressure changes and gas compositions. Upon normalization of microbial activities on pore volume and interfacial area, the findings correlate with the specific surface area of accessible pores obtained from MICP, NMR, and SEM methods. These correlations emphasize the stimulating effect of surface area on microbial activity. The normalized activities demonstrate increments ranging from 0.19 to 0.44 mM/(h∙cm³∙cm²) as the specific surface area increases, varying depending on the method. Furthermore, a predictive model integrating pore volume, SSA, and interfacial area has been established to estimate reliable hydrogen conversion rates in porous media, crucial for assessing the economic viability of UHS and bio-methanation projects.