Experimental study on hole collapse characterization of gas drainage holes in this coal seam based on fiber bragg grating

Drill hole gas extraction is an important method to solve the problem of gas overrun. However, the mine gas extraction drilling hole is prone to collapse and deformation due to the influence of ground stress and drilling disturbance, which seriously affects the effect of gas treatment. Aiming at the problem that it is difficult to monitor the location of collapsed hole in the gas extraction drillhole of coal seam, a monitoring technology for collapsed hole based on fiber grating intelligent sensing technology was proposed. Firstly, the principle of monitoring drilling hole collapse by fiber grating was explained from the theory of stress monitoring by fiber grating sensors. Based on this, an experimental platform for the hole collapse characterization based on fiber grating was built. The hole collapse monitoring experiments of three types of grating arrangements (0°, 90° and 180°) under different amount of collapsed coal were carried out, the changes of grating measurement wavelength offset and extraction flow rate with the amount of collapsed coal were studied, and the collapse level of drillhole was classified. The experimental results shown that, the accuracy of collapsed hole monitoring was the highest when the grating measurement point was located below the substrate material, the relationship between the central wavelength offset Δλ and the amount of collapsed coal m was m = 3.017\Delta \lambda , and the relationship between the extraction flow rate Q and the wavelength offset Δλ was Q = 8.72E - 7x^3 + 2.30E - 4x^2 - 0.18x + 30 . The decay rate of extraction flow was used as the criterion of collapsing hole to divide the three levels of collapsed hole: Class I collapsed hole with the extraction flow decayed to 90%, Class II collapsed hole with the extraction flow decayed from 90% to 50%, and Class III collapsed hole with the extraction flow decayed to less than 50%. Finally, the effectiveness of fiber grating collapsed hole monitoring technology was verified through a field test in 2802 working face of Zhangcun coal mine of Lu'an Group, and it was found that the distribution of collapsed-hole locations after 30 days of drilling was similar to that of the initial period after the drilling was completed. In the field test, five drill holes were successfully located and repaired based on the proposed collapsed hole monitoring technique, and the pure volume of gas extraction from the repaired drill holes was improved by 37.79% compared to that before the repair.