CBM drainage engineering challenges and the technology of mining protective coal seam in the Dalong Mine, Tiefa Basin, China

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As mining depths increase, new challenges, such as gas drainage, gas outbursts and rock bursts induced by mining extraction, have emerged, particularly in gassy, multiple-seam coal mines. The primary reason for this phenomenon is that increased depths lead to higher stresses, higher gas pressures and lower permeabilities in coal seams. Stress and pressure relief require temporal and spatial pre-drainage protection. However, the pursuit of increased coal production, along with economic interests, has resulted in a lack of time and space for CBM pre-drainage engineering. To solve these challenges, protective seam mining is the best way to reduce stress, increase coal permeability and increase CBM extraction efficiency. The stress relief of the rock mass below the protective seam generated five zones and three belts, providing the time and space for CBM pre-drainage engineering. Based on simulations of the outburst risk and mining economics using LaModel, the gassy No. 12 coal seam was selected as the first-mined protective seam. The floor rock roadway and cross boreholes were designed to drain mining-induced stress-relief gas from the protected seam. Therefore, they should be constructed before the protective seam mining begins. The time and space allocation pattern of CBM drainage can be summarized as follows: CBM drainage before coal seam mining using long bedding boreholes, stress-relief gas drainage during mining using floor roadway crossing boreholes and CBM extraction after mining using the roof roadway. Finally, field applications indicate that the remnant gas pressure and content of the protected seam significantly decreased after mining the protective seam. The permeability coefficient increased 1465-fold. The simultaneous extraction of CBM and coal was realized. The gas drainage rate increased from 45% to 70%, and the CBM utilization rate improved from 23% to 90%. These CBM drainage practices could provide insight and guidelines for other coal mines under similar conditions.