Quality inspection and quantitative evaluation method for borehole sealing in gas drainage

Sealing quality is one of the key factors to determine the effect of borehole extraction in gas drainage. For a long time, sealing quality is indirectly described by parameters such as extraction concentration and extraction purity. There is no quantitative detection method and quantitative evaluation method for sealing quality. In order to accurately locate the air leakage points in gas drainage boreholes, the air leakage quantity of each air leakage points in boreholes is calculated quantitatively, and the classification and evaluation method of the sealing quality of gas drainage boreholes is established. Based on the analysis of the influencing factors of the sealing quality of boreholes, the variation law of gas concentration and negative pressure of gas drainage in boreholes under the condition of air leakage is studied. It is found that the gas concentration and negative pressure at the upstream and downstream of the air leakage point will change abruptly, while the gas concentration in the non-air leakage area will remain basically stable. Based on this, the calculation methods of air leakage quantity and air leakage rate in boreholes are established. The sampling principle of negative pressure ejection is adopted here, the parameters of gas concentration and negative pressure in the different depths of boreholes can be determined by extending a probe rod into the different depths of boreholes, so as to accurately determine the location of air leakage points. Because the gas in the borehole comes from the deep part of the borehole, the total amount of gas in the sealing section remains unchanged. According to the change of extraction parameters, the air leakage quantity and the air leakage rate of each air leakage point can be calculated quantitatively. Taking Wangjialing Coal Mine as a test site, two different sealing methods are used to detect and quantitatively evaluate the sealing quality of extraction boreholes. The results show that the number of air leakage boreholes in extraction boreholes with the pressure sealing method of "two blockages and one injection" only accounts for 5.0% of the total number of boreholes, the average air leakage rate is 3.28%, and the maximum air leakage rate of a single borehole is 3.28%.When the polyurethane-cement mortar sealing method at atmospheric pressure is adopted, the number of air leakage boreholes in the extraction boreholes accounted for 60% of the total boreholes, the average air leakage rate was 46.03%, and the maximum air leakage rate was 83.84%.Finally, according to the influence of air leakage in boreholes on extraction effect, the classification index of sealing quality grade and corresponding treatment measures are put forward. Field application proves that the method of sealing quality detection is simple, stable and reliable. It is feasible and effective to quantitatively evaluate the sealing quality of extraction boreholes by using air leakage rate and other indicators. © 2019, Editorial Office of Journal of China Coal Society. All right reserved.