Research on negative pressure distribution laws of drainage borehole with different deformation and instability

A solid-flow coupling model of gas drainage was established which synthetically considered dynamic variation of negative pressure of drainage borehole according to variation mass flow characteristic of gas flowing in the borehole and its deformation situation. Taking Xi'an Coal Mine as an example, distribution of negative pressure of drainage borehole with different deformation and instability were numerically simulated by use of multi physical fields coupling software Comsol Multiphysics. The simulation results show that: ① Coal-seam gas distribution near hole collar and hole bottom of complete borehole is almost the same. Negative pressure loss is little. ② When borehole is collapsed, coal-seam gas distribution around collapsed section is only slightly different from that in the complete borehole. The total negative pressure loss is larger than that in the complete borehole, but still lower than that at hole collar. ③ When borehole is blocked, the difference of coal-seam gas pressure between hole collar and hole bottom is significant, effective drainage length of borehole reduces, and blank tape will appear in coal-seam near the blocked section. Drainage pressure in the complete section varies little and is still negative pressure, while that in the blocked section becomes positive. What's more, the drainage pressure in the blocked section is close to coal-seam gas pressure gradually with approach to hole bottom. The field test results show that the negative pressure loss of complete borehole is small and decreases with prolongation of drainage time, which is consistent with numerical simulation results.