Research progress of secondary biogenic coalbed methane in China

To summarize the development status of theoretical research and engineering practice of secondary biogenic coalbed methane in China, the geochemical data of coalbed water and gas production in areas with rich secondary biogas are comprehensively collected, the previous research results are summarized, and discussed from the perspectives of formation mechanism, formation environment, gas production characteristics and coalbed methane bioengineering. The results show that secondary biogas exists in coal-bearing reservoirs and coal mining areas with low coal rank, medium coal rank and high coal rank in our country. Generally, the groundwater dynamic conditions in areas rich in secondary biogas are not strong, and they are generally in a hydrodynamic environment with weak runoff, and the water types are mostly Na-HCO₃ and Na-Cl·HCO₃. In addition, secondary biogas and thermogenic gas are often mixed in coal seams, and some of the thermogenic gas shows signs of biological retransformation, and most of the gas generation pathways are CO₂ reduction, with relatively dry gas components. At present, the amount of artificial secondary biogas has not yet reached the industrial scale, and CBM bioengineering is still in the early stage of testing, which is mainly limited by the low coal biotransformation efficiency and the immature large-scale biogasification technology. Therefore, the discovery and cultivation of efficient methanogens, the combination of CBM bioengineering and coal and gas production integration technology, and the realization of large-scale biodegradation gas production, are necessary for the sustainable development of engineering on microbial stimulation coalbed methane.