Change Characteristics of Negative Drainage Pressure along the Drill Hole: Theoretical Analyses and Field Tests.

A proper understanding of the change characteristics of negative drainage pressure along a drilling hole is essential since gas drainage parameters are the key parameters that influence the efficiency of gas drainage. In this study, based on the coupling of gas seepage from coal seams and the gas flow along the drilling hole, a theoretical model was established to calculate the gas pressure change law along the drilling hole with different influencing factors. Subsequently, a multibranch method was applied to test the negative pressure at different drilling holes. Finally, a field test was conducted in the Jiulishan coal mine to analyze the changed characteristics of the negative drainage pressure along the drilling hole. The results show that at a constant negative drainage pressure in the borehole, the negative pressure gradually decreased with increasing depth. With an increase in negative drainage pressure at the borehole, the negative pressure loss for every 100 m substantially increased. The gas flux had the most obvious influence on the negative pressure in the drilling hole, and the pressure loss rapidly increased with increasing gas flux. When the diameter of the borehole was small, the negative pressure loss was significant; when the drilling hole was deep, the negative pressure decreased more significantly. This study has important theoretical and practical significance for improving the gas drainage effect.