Abstract:
In response to the frequent problems of hole collapse, pipe sticking, and low drilling efficiency during gas drainage drilling with traditional high-pressure water drilling in the No. 3 and No. 15 broken and soft coal seams of Wenzhuang Mine, Lu’an Chemical Group, and based on the characteristics of the mine’s coal seams and existing equipment conditions, innovatively proposed a nitrogen pneumatic drilling technology scheme of “nitrogen generation on the surface-pressurization underground” and systematically carried out on-site tests of underground nitrogen pneumatic rotary (ordinary drilling) and directional drilling. Combined with engineering practice and based on the principle of energy conservation of gas-solid two-phase flow, the study deeply analyzed the energy loss mechanism during the drilling process, constructed mathematical models for the minimum air pressure and air volume requirements of nitrogen pneumatic drilling applicable to ordinary drilling and directional drilling respectively, and quantitatively revealed the internal correlations among the drilling speed, the rotation speed of drilling tools, and the nitrogen air volume and air pressure. In the excavation working face of the return air crossheading in Wenzhuang Coal Mine, 16 drill holes were designed (8 for ordinary drilling and 8 for directional drilling), and key parameters under working conditions with nitrogen pressures ranging from 0.6 to 1.4 MPa and flow rates from 600 to 800 m
3/h were systematically tested. The results show that compared with the traditional high-pressure water drilling process, the nitrogen pneumatic drilling technology has significant advantages in broken and soft coal seams. The average drilling efficiency of ordinary drilling has nearly doubled, reaching up to 51 m/h, and the efficiency of directional drilling has also increased. The incidence of hole collapse and pipe sticking has been greatly reduced, with the hole formation rate of ordinary drilling reaching 94% and that of directional drilling reaching 87%. Meanwhile, the calculated values of the theoretical model are in high agreement with the on-site measured data, with the average error controlled within 12%, which can effectively guide the rational setting and optimization of core process parameters in drilling. This technology makes full use of existing infrastructure, with mature and reliable core equipment, and the mode of “nitrogen generation on the surface - pressurization underground” is easy to promote. It not only significantly reduces the use of drilling fluid and its potential impact on the groundwater aquifer but also effectively prevents the risk of bit combustion with high-purity nitrogen, significantly improving the construction efficiency and safety of gas drainage drilling. It provides important technical reference and practical guidance for safe and efficient drilling in broken and soft coal seams and gas control in the coal industry.