Evolutionary Laws and Preventative Measures for the Coal and Rock Dynamic Disasters Around the Boreholes During Coal Bed Methane Extraction in Low-Temperature Oxidation Conditions: A Case Study of the No. \(8_{2}\) Reservoir of the Yangliu Coal Mine

The purpose of this research is to solve the problem of unpredictable coal and rock dynamic disasters that frequently occur near extraction boreholes during the extraction of underground coal bed methane (CBM). The development laws of pores with different diameters inside the coal masses and the internal mechanisms during the low-temperature oxidation were studied using methods such as nuclear magnetic resonance, P wave tests, and gas chromatographs. Using the experimental result as a theoretical basis, a hole-sealing technique for the sealing–isolation integration was proposed to prevent the oxidation of the coal around the extraction borehole by combining the distribution of stresses around the borehole and fissure zones. The result showed that the pores with different diameters inside of the coal masses synchronously developed during the low-temperature oxidation; the micropores developed first, followed by the mesopores, and finally, the macropores. Furthermore, the development of the pores damaged the coal integrity and structural strength, which resulted in a higher risk of coal and rock dynamic disasters. Compared with the ordinary hole-sealing technique using cement mortar, the extraction concentration increased by 3.28 times when using the proposed hole-sealing technique for the sealing–isolation integration within 90 days of the test period for the extraction. Moreover, the borehole temperature decreased by \(121\,{^{\circ }}\hbox {C}\). The technique not only improves the extraction efficiency of the CBM but also inhibits the spontaneous combustion of the coal around the boreholes, which guarantees the safe and efficient extraction of the No. \(8_{2}\) CBM reservoir of the coal mine.