Numerical simulation of borehole wall remodel during gas drainage

Abstract

Gas drainage bearing coal and rock mass can be simplified as isotropic homogeneous body with gas saturation. Numerical method was used to study the influence of borehole wall remodel-size, remodel-length and negative pressure on gas drainage effect which based on gas diffusion and seepage equation of pore fracture system and governing equation of coal and rock mass degeneration. The results showed that borehole wall remodel can maintain the support of coal skeleton in borehole and prevent blocking the gas flow channel, gas pressure drop in remodelled section is more obvious than that in the non-remodelled section, the longer the remodelled length of borehole wall the more smooth the gas flow channel between borehole and coal body around borehole, with the increase of negative pressure, the decrease range of gas pressure at the same depth increases, but the difference is small under different negative pressure. On the premise of preventing borehole creep instability, borehole wall remodel can provide high-quality gas flow channel for coal seam gas drainage, maintain the continuous influence of negative pressure on coal body along borehole depth to extend the life cycle of gas drainage and is not sensitive to borehole wall remodel size and negative pressure.