Volume 57

Insights into the Biochemical and Geochemical Interplay in CO₂ /H₂ Underground Bio-methanation Lin Wu, Zhengmeng Hou, Tao Zhu

Abstract

COâ‚‚/Hâ‚‚ underground bio-methanation (UBM) technology offers considerable promise because of its versatile functions. However, alongside biocatalysts methanogens, sulfate-reducing bacteria (SRB) and acetogens also utilize CO₂ / H₂ as substrates, initiating a range of complex geochemical reactions that may impact both the efficiency and safety of the process. To elucidate the complex biochemical and geochemical interplay in UBM, this study utilized an innovative microbial kinetic model that incorporates environmental factors and subsurface space limitations, implemented within the geochemical simulation platform PHREEQC. This framework facilitated the analysis of scenarios where methanogens, acetogens, and SRB exist individually within different mineral systems, enhancing understanding of each microbe’s influence. Building on this, the study further examined complex interactions when all three microbes are present together, as well as the effects of reservoir temperature and gas pressure. The results indicate that both methanogen and SRB metabolism tend to raise the pH of formation water, while acetogen metabolism has the opposite effect. When all three microbes are present, the buffering effect of carbonate minerals enhances methanogen activity, causing a rise in pH during CO₂ /H₂ conversion, which leads to the re-dissolution of hydrogen sulfide gas produced by SRB into the formation water, thereby reducing safety risks. Ultimately, CH4 makes up as much as 96% of the gas phase. Additionally, when the temperature is near the optimal growth range for methanogens, the conversion rate is rapid; however, the competitive consumption of CO₂ and H₂ by acetogens also intensifies. Moreover, an increase in the total pressure of CO₂ and H₂ results in a lower pH in the formation water. This shift suppresses methanogen function, lowering the conversion efficiency and causing a greater proportion of CO₂ and H₂ to be transformed into acetate. The findings of this study contribute to understanding the complex biochemical and geochemical interplay in UBM and provide guidance for selecting sites and optimizing design in UBM projects.

Keywords Biogeochemical interaction, Carbon circular utilization, Large-scale energy storage, Renewable natural gas, Underground bio-methanation