The evolution characteristics of gas pressure around coal seam directional long extraction boreholes with nonlinear extraction negative pressure

Abstract

Nonlinear pressure of coal seam gas in the directional borehole and surrounding areas affects extraction efficiency. A multi-field coupling numerical model, including gas desorption, diffusion, fracture seepage, and borehole extraction, is developed for the No. 3 coal seam of the GJH mine. Using the numerical simulation method of control variables, extraction was developed to investigate the nonlinear evolution of axial pressure gradients in directional long borehole and the spatiotemporal response characteristics of gas pressure. Results show that coal seam seepage pressure exhibits a nonlinear radial gradient attenuation around the borehole, evolving through rapid pressure change, stabilization, and slow adjustment. The pressure difference between the initial gas extraction pressure at any position in the borehole and the pressure at the end of extraction gradually decreases as the distance from the orifice, with significant pressure response delays observed beyond 400 m depth. The long-hole system demonstrates superior pressure relaxation (5.8% rate) compared to short borehole. Near-borehole monitoring points (≤ 5 m) show pronounced pressure drops, while far-field regions (> 5 m) exhibit gradual declines. Once far-field pressure reaches a critical threshold, decay rates sharply decrease. These findings provide theoretical guidance for optimizing borehole layouts and regulating gas extraction systems.