Failure mechanism of gas extraction boreholes in small coal pillar gob-side entry and full cycle tight hole sealing technology

To address the problems of intense mine pressure behavior and poor gas extraction performance in small coal pillar gob-side entry, the stress distribution characteristics on the solid coal side of 10 m and 30 m coal pillars were comparatively analyzed. The failure mechanism of extraction boreholes in small coal pillar gob-side entry was clarified, and the full cycle tight hole sealing technology for extraction boreholes was developed using a time staged and zonal control method. The key parameters of the full cycle tight hole sealing technology were determined and subsequently applied in the field at Gaohe Energy. The results indicate that the solid coal side of small coal pillar gob-side entry bears most of the roof pressure. The peak abutment pressure on the solid coal side reaches 32.66 MPa, which is much higher than the 16.45 MPa observed in the large coal pillar. Under the superimposed influence of multiple mining induced stresses, fractures in the coal mass become more developed, thereby causing air leakage channels to form readily within the borehole sealing section. In addition, borehole collapse and blockage tend to occur at the bottom of the sealing pipe, while the sealing section is susceptible to shear failure, ultimately leading to borehole failure. During the initial stage, an integrated shallow pressure relief and deep sealing technology was adopted, in which pressure relief was conducted in the shallow section and tight sealing was implemented in the deep section, thereby enabling the coordinated control of mine pressure and gas. During the intermediate stage, accurate plugging of secondary grouting was used to seal air leakage channels and restore the gas extraction capacity of the boreholes. During the later stage, after the deformation of the roadway surrounding rock became relatively stable, the three sealing and two grouting technology was applied to grout the pressure relief section, thereby improving the sealing quality and gas extraction performance. In the initial sealing stage, the average gas extraction concentration reached 44.28%, which was 3.29 times that of the two sealing and one grouting sealing process. During the intermediate stage, after accurate plugging of secondary grouting was applied, the average gas extraction concentration increased from 24.65% to 67.61%. During the later stage, after the three sealing and two grouting technology was used to grout the pressure relief section, the average gas extraction concentration increased from 27.57% to 39.27%. These results demonstrate that the full cycle tight hole sealing technology can effectively enhance gas extraction performance and shorten the time required to meet the gas extraction standard.