Study on Air Injection to Enhance Coalbed Gas Extraction

Gas extraction is an important means to reduce coalbed gas and ensure safe coal production. Injecting N2/CO2 into a coalbed can enhance coal seam gas extraction, but problems with N2/CO2 sources underground have prevented the wide application of this technology in coal mines. The air contains a large amount of N2, but only a few studies have investigated the injection of air into coalbeds to facilitate gas extraction. In this study, a thermal–hydraulic–solid coupling model for air-enhanced coalbed gas extraction (Air-ECGE) was established. Additionally, the impact of air injection on coalbed methane extraction was simulated, and field experiments were conducted on air injection to enhance gas extraction. The results showed that injecting high-pressure air into a coalbed can effectively facilitate gas desorption and gas migration within the coalbed, greatly improving the efficiency of gas extraction in the coalbed. In addition, owing to the large pressure gradient that can lead to fast coalbed gas seepage, the gas production rate of the extraction borehole is directly proportional to the gas injection pressure. Further, the spacing of the boreholes limits the influence range of the gas injection: the larger the spacing, the larger the influence range, and the higher the gas extraction rate of the extraction borehole. After injecting air into the coalbed of the Liuzhuang coal mine, the extraction flow rate and concentration of gas from the extraction boreholes both increased significantly. A certain delay effect was also observed in the gas injection effect, and the gas extraction flow rate only decreased after a period of time after the gas injection had stopped.