Development of a Novel Directional Hydraulic Flushing Technology for Enhanced Gas Drainage in Soft and Low-Permeability Coal Seams

Abstract Although directional drilling technology is extensively employed in underground coal seam gas drainage, its efficiency remains limited in soft and low-permeability (SLP) coal seams due to their poor gas flow properties. To address this, a novel directional hydraulic flushing technology is introduced in this work. Based on the specific geological conditions in the Erjing Mine, Yangquan coalfield, China, this study first analyzes the challenges associated with gas drainage. Subsequently, the gas flow enhancement mechanisms of the proposed technology and its key influencing factors are systematically revealed through numerical simulation. Finally, a detailed field investigation was conducted to gauge the efficacy of this new technology. The results demonstrated that hydraulic flushing significantly enlarges the zones of matrix fragmentation and permeability improvement around the borehole. This notably enhances the coal’s gas diffusion and seepage properties, thereby improving overall gas flow behavior. Additionally, increasing the borehole radius or reducing the borehole spacing can amplify this enhancement effect. Furthermore, field application in the Erjing Mine showed that the average gas concentration from directional boreholes increased by approximately 20%, and the average pure drainage volume yielded a substantial 1.29-fold increase, conclusively confirming the technology’s effectiveness.