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.

Abstract

CO2 geological sequestration has been widely adopted as a key strategy for mitigating greenhouse-gas emissions and addressing climate change. Previous simulation techniques typically coupled thermal, hydraulic, mechanical and chemical (THMC) processes, yet they often omit the fracture initiation and evolution mechanisms induced by high injection pressures and temperature gradients. In this study, a novel numerical framework is introduced to simulate fracture initiation and extension during CO2 storage. This method integrates the thermal, hydraulic, mechanical and chemical reaction (THMC) processes with dynamic fracture generation and propagation to enable accurate prediction of caprock integrity and long-term storage safety. The proposed numerical framework extended the Embedded Discrete Fracture Modeling (EDFM) to fracture initiation and extension based on tension and shear failure criteria, and dynamically updates the hydraulic properties of fractures throughout the simulation. Meanwhile, heat transfer between the injected CO2 and the reservoir, stress redistribution, and relevant geochemical reactions are fully coupled to assess the impacts of fracture evolution on multi-field dynamics. The proposed framework was demonstrated using a two-dimensional In Salah CO2 storage model. The results indicated that fractures first initiated around the CO2 injection well after approximately 5 years of injection, and then extended along the caprock-saline aquifer interface. In addition, it is observed that fracture evolution exerted a pronounced influence on surface deformation. The proposed framework provides a rigorous, quantitative tool for fracture evolution forecast and containment risk management in CO2 storage projects.