Permeability of the composite coal in the grouting section around the CBM drainage borehole in the coal mine.

The leakage of borehole during CBM drainage leads to a significant decrease in concentration, severely restricting the effectiveness of gas drainage in the coal seam. After grouting in the gas pre-drainage boreholes, the slurry around the borehole forms a grout-graded composite structure with the broken coal, and its permeability is a key factor affecting the sealing. Triaxial seepage tests of the composite specimen were conducted with different degrees of fragmentation and moisture content to investigate its permeability. The relationship between structure of the specimen, stress and permeability was studied. The results showed that the polarization effect and the water plug effect were observed in the graded and composite specimens respectively. The water plug effect in the composite specimens inhibited the permeation, as evidenced by a decrease in permeability with increasing equivalent Talbot index or Talbot index of the lower part of composite coal (LPC), which was further aggravated by higher moisture content of the LPC. The intensity of the water plug effect was related to these factors, and their combined increase further enhanced the effect. This study provides insights into the gas seepage mechanism around boreholes and offers a theoretical foundation for the CBM drainage in coal mine.