3.2. Field Application of the “Concentric Ring” Strengthening Sealing Method
An on-site test was conducted in the N2106 transportation roadway of a mining area in Shanxi Province, China. The coal body of the working face has a firmness coefficient
fc < 0.5, which makes it a soft coal seam. The N2106 transportation roadway has no special geological structure, and the on-site measured coal seam gas content is 7.82 m
3/t, with a total gas emission of 2.16 m
3/min. On-site drilling is gas extraction drilling along the coal seam, which is drilled from the N2106 transport roadway towards the coal body of the working face. The hole is opened from the upper side of the tunnel, perpendicular to the tunnel, and continuously extends into the coal body. Each drilling hole should be sealed and integrated into the gas extraction network in a timely manner after construction. The coal body in the experimental area has not been disturbed by mining, and its strength is relatively weak, belonging to a soft coal seam. The layout of the experimental boreholes is shown in
Figure 16.
This on-site test was divided into three groups, each consisting of ten boreholes. The first group used ordinary cloth-bag-type “two plugs and one injection” sealing technology, and the sealing material used was an ordinary expanded cement sealing material. The second group adopted a new reinforcement sealing technology, and its sealing material was an ordinary expansion cement sealing material. On the basis of adopting new reinforcement sealing technology, the third group used new sealing materials for grout sealing. The three sets of test boreholes were recorded as A1–A10, B1–B10, and C1–C10, respectively.
In this on-site test, a total of 13 boreholes were sealed using the ordinary bag-type “two plugs and one injection” sealing technology, and 10 boreholes were effectively drilled. Among those, 7 boreholes experienced collapse at the hole mouth after drilling back and were later cleared through secondary drilling. Using new reinforcement and sealing technology to seal 16 boreholes, we effectively drilled 10 boreholes without any occurrence of hole collapse at the borehole opening position. There were 12 boreholes sealed with new reinforcement sealing technology and materials, and 10 effective boreholes were drilled without any collapse at the borehole opening.
In order to investigate the on-site application effect of new reinforcement sealing technology and new sealing materials, more than 30 coal seam extraction boreholes were constructed in the N2106 working face with an inclination angle of −1–2° and an angle of 90 ° with the centerline of the tunnel. The designed depth of the boreholes was 130 m, and the average depth of the completed boreholes was 127 m. According to the research results of the new sealing technology, drill bits with a diameter of 140 mm were selected for drilling the new reinforced sealing holes in Groups B and C, with a pre-reinforcement depth of 6 m, and the hole opening was sealed with expansion capsules for reinforcement grouting. The sealing depth of drilling holes in Group A was 16 m, and the sealing depth area of bag sealing in Groups B and C was 6–16 m. Under the same conditions, three sets of test boreholes were monitored on-site for a period of three months.
According to the three-month gas concentration detection results of three sets of testing boreholes at the underground site, as shown in
Figure 17,
Figure 18 and
Figure 19, the interval between testing boreholes was one day, and the final average value was taken for each shift on the same day of testing.
By comparing and analyzing
Figure 17,
Figure 18 and
Figure 19, it can be found that after 30 days of extraction, the concentration of test boreholes in Group A decreased to within 30%. The average concentration of boreholes in Group A at 30 days of extraction was 21.2%, and the gas concentration in boreholes decreased significantly from 30 to 60 days of extraction. The average concentration of extraction at 60 and 90 days of extraction was only 10.1% and 2.0%, respectively. The gas concentration in the test boreholes of Group B was basically above 30% within 30 days before extraction, but after 30 days of extraction, most boreholes began to decline and decrease to within 30%. The average gas concentration on the 60th day of extraction was 18.4%. After 60 days of extraction, the gas concentration in Group B boreholes was relatively low, with an average concentration of only 10.9% at 90 days. The test boreholes in Group C ensured a gas extraction concentration of over 30% at 60 days before extraction, with high stability and sustainability of gas concentration. The gas concentration at 30 days of extraction was 53.9%, and the average gas concentration at 60 days of extraction was still greater than 30%.
In order to improve and compare the sealing effect of three test boreholes, four representative boreholes were selected from each group after 30 days of extraction. The negative pressure at the orifice and the pure gas flow rate that characterized the sealing quality were selected for comparative analysis. The comparison results are shown in
Figure 20. It was found that under the same extraction time conditions, the gas concentration extracted from Group C was significantly higher than the other two groups, fully demonstrating that the use of new reinforcement sealing technology and new sealing materials greatly improved the gas extraction effect.
According to comprehensive comparison of the sealing effect parameters of the three test boreholes according to
Table 6, it can be found that the negative pressure at the orifice of the Group C boreholes was significantly higher than that of the other two test boreholes. Based on the concentration and mixed flow rate, the final gas pure flow rate was calculated, and the gas pure flow rate of Group C was also significantly higher than that of the other two groups. In addition, the suction negative pressure of the N2106 working face was 15 kPa, and the negative pressure at the orifice of the Group C test boreholes still maintained a high level after 30 days of extraction. This indicates that the sealing quality of the Group C test boreholes was stable for a long time and there would be no collapse or leakage.
Based on the above analysis, it can be concluded that the testing drilling hole of Group C using new reinforcement sealing technology and materials ensured a good gas extraction effect for a long time. Compared to Group A, which used a conventional bag-type with two plugs and one injection to test the sealing effect of the drilling hole, Group B used a new reinforcement sealing technology to avoid collapse before sealing the drilling hole, and the drilling hole also had a better sealing effect compared to the testing drilling hole of Group A due to the double grouting and pre-reinforcement of the broken coal body. However, due to the inherent defects of the sealing material, it could not resist the instability of the drilling sealing section, so the test drilling hole of Group B could not guarantee a high extraction level for a long time.