Underground Bio-methanation of Carbon Dioxide and Hydrogen in Depleted Gas Reservoirs: Site Selection and Biogeochemical Modelling (English shop)

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The thesis focuses on Underground Bio-Methanation (UBM) technology, which utilizes methanogens to convert CO2 and H2 into methane (CH4) within porous rocks. This technology offers benefits such as carbon circular utilization, large-scale underground energy storage, and renewable CH4 production, making it increasingly relevant in the context of global warming and the transition to renewable energy. However, methanogens are sensitive to environmental factors, and the injection of external gases can lead to complex interactions.

To address these challenges, the thesis introduces a novel evaluation criteria system for UBM site selection, consisting of four main criteria (technology, safety, society, and economy) and 20 sub-criteria. An integrated multi-criteria decision-making (MCDM) method is proposed, which balances subjective and objective factors. A case study in the Sichuan Basin identified site A1 as the most suitable for UBM, with stable ranking results prompting further biogeochemical modeling.

The thesis also develops a microbial growth kinetics model using PHREEQC software to analyze competition among methanogens, acetogens, and sulfate-reducing bacteria (SRB) during UBM. Findings indicate that methanogens thrive in the presence of carbonate minerals, with significant CO2 and H2 utilization for CH4 synthesis. However, factors like pH changes and SRB metabolism can hinder methanogen function.

Additionally, the research assesses the effects of cyclic UBM on the gas-water-rock system, revealing that alternating acidity and alkalinity during cycles lead to minimal changes in porosity. Increased cycles result in decreased salinity and gas storage capacity, while temperature changes due to methanogen metabolism are influenced by heat loss.

In summary, this thesis provides a comprehensive study of UBM site selection and offers valuable insights for underground storage of gases. The findings enhance understanding of microbial interactions in UBM and the impact of cyclic processes, serving as a theoretical guide for future site selection and optimization in this field.