Study on secondary grouting technology for coal seam gas extraction failure in borehole pipe bottom

In order to solve the failure results in the decrease of gas concentration extraction hole leakage problems, according to the mechanism of internal and external leakage, borehole outside air leakage can be gained by analyzing the borehole axial wall broken flat belt and radial borehole surrounding rock of roadway coal gas leakage circle of influence, regional distribution of hole leakage hole sealing material leakage resistance coefficient, hole sealing length, the influence of multiple factors such as support strength, for failure of drilling was targeted secondary grouting. The technology of secondary grouting at the bottom of the failed drilling pipe is described. According to the variation trend of detected gas concentration, the specific leakage area of the borehole is obtained, and the specific location and length of the borehole that needs secondary grouting are determined. The special grouting equipment is used to inject the grout into the leakage passage near the sealing section of the leakage borehole. With the flow, filling and solidification of the grout in the penetrating crack, the borehole passage is blocked, and the resistance of the air outside the hole to enter the hole increases or it cannot enter the hole, so as to realize the targeted treatment of the leakage passage. The equipment for detecting the location of the hole leakage and the secondary grouting and thickening at the bottom of the pipe has been developed, which is mainly composed of sealing air bag, double-channel connecting pipe, orifice tee and other components. Combined with the actual size of the existing drilling holes in the S5101 mining face of a mine in Shanxi, the overall size of the device is designed. KFGP-III micro-expansion cement-based special material for gas hole sealing is selected, and its material properties, admixtures and grout ratio were tested. The effectiveness of the secondary grouting at the bottom of the pipe is verified by simulation. The simulation results show that the leakage channels formed by the cracks around the borehole and the shrinkage failure of the primary sealing section are effectively blocked under the condition of the secondary grouting at the bottom of the pipe. Industrial tests were carried out in Yuwu Coal Industry, and the concentration of the test borehole was generally increased to 35%～80% after grouting. The test data showed that the grouting not only significantly enhanced the concentration of the test borehole, but also improved the gas concentration of the drainage borehole within the adjacent range of the grouting borehole to a certain extent.