Background Gas drainage via ultra-long directional boreholes plays a significant role in gas control in coal mines. To ensure safe and efficient gas drainage using these boreholes, it is critical to determine the mechanisms behind positive- negative pressure coupling, as well as the dominant mechanisms governing gas migration. However, the evolutionary patterns of positive-negative pressure zoning within the boreholes and the primary factors influencing gas drainage efficiency have yet to be accurately characterized.

Methods Under the engineering background of the Shendong Baode Coal Mine, this study developed a multi-field coupling model integrating diffusion, seepage, and in-borehole flow fields based on dual media involving pores and fractures. Using this model, gas evolution in the ultra-long directional borehole during gas drainage was simulated, followed by validation using field monitoring data. Finally, the gas migration patterns and parameter optimization paths were discovered, and the primary factors influencing gas drainage efficiency were determined.

Results and Conclusions  During gas drainage, a gas flow pattern characterized by axial traction coupled with radial gas supply was formed within the ultra-long directional borehole. The borehole section near the borehole head represented a zone of high-intensity gas drainage predominantly driven by negative pressure, while the deep part of the borehole showed a stable gas supply achieved by positive-pressure-controlled gas diffusion. Accordingly, the borehole was generally characterized by zoning of gas drainage driven by negative pressure locally and positive pressure supply widely. Over time, the positive-negative pressure zoning and equilibrium point moved exponentially from the borehole head inwards. The zone with effective negative pressure was concentrated within approximately 100 m from the borehole head, while positive-pressure supply played a dominant role in long-term gas drainage. A resistance evolution model considering borehole collapse and blockage was introduced for simulations under varying resistance coefficients. A comparison between the simulation results and underground monitoring data revealed that an equivalent resistance coefficient of a₁ = 400 was more consistent with the in situ conditions and enabled accurate simulation of the evolution of both positive-negative pressure zoning and daily gas drainage volume. In terms of the matching between the lengths of the borehole and coal seams, short and long boreholes highlighted single-borehole production capacity and regional gas control effectiveness, respectively. In contrast, the moderate ratios of the coal seam length to the borehole length enabled balanced efficiency and adaptability. In engineering applications, the length matching should be optimized based on specific targets. The radius of the zone with qualified gas drainage reduced gently first and then dramatically along the axial direction of the borehole, corresponding to the equilibrium diffusion section and the attenuation-controlled section, respectively. The analysis of the axial and radial contributions indicates that the radial gas diffusion driven by positive pressure contributed 81.3% of the gas supply, while the axial gas drainage governed by negative-pressure traction was concentrated within approximately 100 m from the borehole head. This result reflects a spatial gas drainage pattern where radial desorption and seepage predominated, followed by axial short-distance traction. To address the contraction of the gas drainage radius and limited gas flow at the end of the borehole, this study proposed local reinforcement by taking auxiliary measures at the borehole end. The results of this study offer a theoretical basis and engineering guidance for the layout of ultra-long boreholes, the design of gas drainage systems, and the compliance control of gas drainage.