Characteristics of gas bursting and application of layered cutting drilling in outburst coal seam

Objective The overlimit gas bursting in the drilling process of outburst coal seam is a safety hazard that threatens the efficient production of the mine. The layered cutting drilling method helps to weaken the burst gas pressure in the borehole, but the characteristics of gas release around the trapezoidal borehole formed by layered cutting are unknown, which limits the structural design and application effect of the layered cutting drilling tool.

Methods Herein, study was conducted based on the layered cutting boreholes with different layer spacing and layer thickness difference. Specifically, a mathematical model was established to analyze the gas flow law around the borehole, and the relationship between the gas flow rate of the layered cutting borehole and the layer spacing and the layer thickness difference was quantitatively described. On this basis, the gas release control mechanism of the layered cutting drilling tool was analyzed by numerical simulation, the structure of the layered cutting drilling tool was optimized, and the industrial test of field drilling was carried out.

Results and Conclusions The results show that: (1) Using the multi-cutting hole-forming method, the exposed area of the borehole gradually increases, which effectively controls the gas release area, and then regulates the gas release flow. By reasonably designing the layer thickness difference H_j and the layer spacing L_i of the borehole, the gas release flow could be reduced by more than 18.3%. (2) In a certain range of layer spacing, the control effect of the double-layer cutting drilling tool will be better than that of the three-layer cutting drilling tool. Under the condition with the thrust of 140 kN and the torque of 4 600 N·m, the grooved damping drill pipe with diameter of 63.5 mm has the minimum safety factor of 3.009, which meets the strength design requirements. (3) The optimized double-layer cutting drilling tool has an exposing diameter of 73 mm, a final hole diameter of 113 mm, and a layer spacing of 0.8 m. As shown in the industrial tests, the gas volume fraction in the borehole is reduced by 37.7% during the drilling into high-pressure gas-rich areas, and the gas volume fraction in the borehole does not exceed 0.5%. This indicates that the layered cutting drilling tool has a significant effect on regulating the gas release amount of the coal body around the borehole and weakening the gas release intensity, which provides a new solution for preventing the overlimit gas bursting in the borehole.