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 \begin{document}${\mathrm{SO}}_4^{2-} $\end{document} 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 9th day to the end of gas production during the anaerobic fermentation, achieving in-situ inhibition of H2S.