Research on influence characteristics of water jet impact on borehole gas drainage

Water jet based borehole permeability enhancement technology is an effective way to increase coal-seam permeability and gas drainage rate. However, influence characteristics of water jet impact on borehole gas drainage are indeterminate at present. For the above problem, flow field characteristics and pressure characteristics of water jet on borehole wall are researched by use of numerical simulation method. Influence of water jet impact on coal body stress around borehole is analyzed, and gas drainage characteristics of ordinary borehole and water jet impact borehole are compared. The research results show that water jet in borehole diffusely flows with a small diffusion angle. Axial speed of water jet is the peak value. When water jet impacts the borehole wall, a circular truncated cone forms where water speed is low. There are three pressure concentration areas in process of water jet impacting borehole named exit pressure concentration area, contact surface pressure concentration area and wall pressure concentration area. Pressure on borehole wall increases with increase of water jet impact pressure. There are two stages in influence of water jet impact borehole on stress distribution of surrounding coal body named sudden stress reduction stage and stress enhancement and recovery stage separately, and stress variation degree of coal body has positive correlation with impact pressure. Whether water jet impact borehole or ordinary borehole is used for gas drainage, gas pressure of coal body around borehole decreases gradually from borehole to coal body interior with time passing. But water jet impact borehole can significantly improve drainage range and drainage rate of borehole, and the larger the water jet impact pressure is, the more obvious the influence on borehole drainage is.