Biogeochemical Reactions in a Diffusion-Limited Zone Around Wellbore Cement Relevant to Underground Hydrogen Storage

Hydrogen (H2) is a promising energy carrier to balance the production-consumption mismatch of renewable energy sources. To store H2, depleted reservoirs are a prospective option for large scale underground hydrogen storage. However, H2 could boost biogeochemical reactions that consume hydrogen, generate undesired gases, and alter pore structures of wellbores. Here, we present observations of biogeochemical alteration of wellbore cement after interactions with H2. Experiments were conducted with a wellbore cement core and shale submerged in synthetic formation brine. The reactor vessel was pressurized with 95 bar H2 and 5 bar carbon dioxide (CO2) gas and maintained at 60 ^oC for 16 weeks. A sulfide odor, indicative of microbial activity, was noted during degassing of the reactor. The cement sample was cross-sectioned and characterized using multiple techniques, with particular focus around aragonite crystals formed during the reaction in a diffusion limited zone. Utilizing synchrotron X-ray fluorescence mapping, a sulfide- and iron(II)-enriched zone measuring 0 – 80 μm into the surface of the cement was observed along the rim of the cement cross section as a result of microbial sulfate and iron reduction. The effects of the reaction were minimal adjacent to the aragonite while more pronounced at a distance of 300 – 500 μm away. In addition to sulfur and iron reduction, cement carbonation was observed using SEM. These findings demonstrate wellbore cement alteration by H2-triggered biogeochemical reactions and that the biotic reactions are largely impacted by mass transport in a diffusion limited zone. These observations provide a scientific basis for estimating the extents of biogeochemical reactions and their impacts on wellbores during geologic hydrogen storage.