Study on in-situ microbial methanogenesis of CO₂ and in-situ inhibition of H₂S and increase of coalbed methane production in low-quality coalbed methane of the Southern Junggar Basin

The development and utilization of coalbed methane (CBM) not only ensures national energy security, but also provides a boost to carbon neutrality. The southern edge of the Junggar Basin is a major CBM resource area and a key development hotspot in China. However, the region has few high-yield wells, and maintaining stable production is challenging. There is an urgent need for a technology that can increase the production of CBM resources within the control range of a single well. Additionally, high volume fractions of CO₂ and H₂S have become common in the CBM of this region, creating an urgent need for in-situ disposal technology for these gases. Coalbed gas bioengineering offers a promising technology for enhancing both the quality and production of CBM in this region. On-site monitoring and laboratory simulation experiments indicate that the CO₂ volume fraction is closely linked to reservoir temperature. Acid-producing fermentative bacteria and Hydrogen-producing acetic acid bacteria remain active and continue to produce CO₂ across a broad range of reservoir temperatures. At lower reservoir temperatures, the metabolism of hydrogenotrophic methanogens is weak and CO₂ is difficult to be reduced, which is the main reason for the high CO₂ volume fraction in this area. It was also found that the microbial community in the groundwater interacts with organic matter and \mathrmSO_4^2- during migration. H₂S is generated when the groundwater recharge and drainage rates are consistent with the metabolic cycle of methanogens, which is called epigenetic H₂S. The presence of these two acidic gases not only compromises production safety but also significantly reduces the quality of CBM. This study introduces a key technology for in-situ microbial-mediated enhancement of CBM quality and production, addressing the issue of low-quality CBM in southern Junggar Basin. The necessity and feasibility of this technology are discussed, highlighting its potential to enhance CBM production, facilitate in-situ microbial conversion of CO₂, and inhibit H₂S generation. The fundamental concept of this technology is to utilize the coal reservoir as an anaerobic fermentation site, with the coal and CO₂ present in the reservoir serving as fermentation substrates. This approach aims to achieve in-situ suppression of H₂S while enabling the biomethanation of CO₂. The key challenges of this technology include cultivating efficient microbial communities, especially hydrogenotrophic methanogens that can thrive across a wide temperature range, developing bio-fracturing fluids for in-situ H₂S suppression, and establishing effective evaluation methods for enhancing CBM quality. Physical simulations of CO₂ microbial methanogenesis showed that cumulative methane production by hydrogenotrophic methanogens increased with rising reservoir temperatures, reaching a peak of 8.5 m³/t at 55℃. At this temperature, the abundance of key enzymes involved in glycolysis, pyruvate metabolism, and the TCA cycle was significantly higher compared to other in-situ anaerobic fermentation systems, enhancing both CO₂ conversion efficiency. Furthermore, in an anaerobic fermentation system without CO₂ and with added biological inhibitors, biomethane production reached 4.5 m³/t, slightly higher than that of the control group (4.38 m³/t). Notably, the gaseous H₂S volume fraction was reduced by 88.8% compared to the control group. And the H₂S volume fraction was always zero from the 9^th day to the end of gas production during the anaerobic fermentation, achieving in-situ inhibition of H₂S.