Research and application of key technologies for air jet drilling pressure relief in soft coal seams

China boasts extensive distribution of soft, low-permeability coal seams, where prolonged gas control cycles severely constrain mining alternation and coal production capacity release. Hydraulic measures for pressure relief and permeability enhancement often lead to issues such as borehole blowout, collapse, and blockage, failing to efficiently resolve gas control challenges. Pneumatic permeability enhancement technology represents a feasible approach to addressing gas control difficulties in soft coal seams. However, due to factors such as cost, efficiency, and technical applicability, current pneumatic technologies have not been widely adopted. To address this, an air jet slotting pressure relief and permeability enhancement technology is proposed. To achieve efficient slotting with this technology, this study first established a calculation model for the nozzle's maximum allowable length and outer diameter. For drilling tools with a diameter of 73 mm, key parameters were determined: a maximum nozzle length of 20 mm and an outer diameter of 13 mm. Based on this, an air jet slotter was designed, and the nozzle installation space was optimized to maximize the nozzle length within the limited space of the drilling tool. Secondly, nozzle structures with different expansion ratios (n=0.5, 1.0, 1.3, and 1.4) were designed. The influence of different expansion ratios on the airflow field structure and impact pressure of the air jet was compared and analyzed through numerical simulation to determine the optimal nozzle expansion ratio and design principles. Experimental studies were conducted on the distribution characteristics of impact pressure versus target distance for nozzles with different expansion ratios, and the variation characteristics of the air jet borehole enlargement radius over time were analyzed. Integrating the above research findings, a complete air jet slotting system was developed, and field trials were ultimately conducted at Xinyi Coal Mine. The results indicate that the expansion ratio significantly influences the velocity and pressure fields of the air jet. When the expansion ratio is 1.0, the alternating development of the jet expansion and compression waves is the most gradual, heat exchange with the environment is minimized, the constant velocity core length is the longest, and the resulting jet achieves the maximum target distance and impact pressure. As the target distance increases, the impact stress of nozzles with different expansion ratios generally shows a decreasing trend. However, when the jet is under-expanded or over-expanded, the impact pressure exhibits an alternating pattern of increase and decrease with increasing target distance. In contrast, when the expansion ratio is 1.0, this alternating characteristic is not obvious, and the impact pressure consistently remains greater than that under under-expanded or over-expanded states. Under a compressed air pressure of 0.6 MPa, for a soft coal seam with a firmness coefficient (f) of 0.3, the slotting radius exceeded 0.5 m. Field trials further confirmed that the air jet slotting radius can reach over 0.56 m. Compared to hydraulic slotting technology, the gas extraction flow rate increased by 2 times, and the time required to achieve extraction standards was reduced by one-third. This effectively resolved the issue of mining imbalance and established a new model for gas control in soft coal seams.