Measurement of pressure drop in drainage boreholes and its effects on the performance of coal seam gas extraction: a case study in the Jiulishan Mine with strong coal and gas outburst dangers

Although it is a well-accepted belief in the petroleum industry that horizontal well productivity can be limited by the pressure drop within the wellbore, little has been reported regarding how this pressure drop affects gas extraction from a coal seam and its further effects on mitigating coal and gas outburst dangers in coal. One of the major reasons for this scarcity is that the pressure-drop distribution in horizontal drainage boreholes is difficult to obtain. In this study, measurements of pressure drops in 54 drainage boreholes were performed in the No. 2₁ coal seam, which is the primary mining layer of Jiulishan Mine and poses a strong danger of coal and gas outbursts. Next, a coupled governing finite-element model, which includes the pressure drop in the borehole, Darcy flow in fractures, gas diffusion in the matrix blocks, and the dynamic evolution of the permeability of coal, was developed and implemented using a finite-element method to quantify the pressure-drop effects. Field tests of the pressure drops indicate that the pressure increases in a parabolic form with the increasing depth of the borehole, and lower outer end pressure is associated with larger pressure increments. The numerical results indicate that the pressure drop does affect the coal seam gas extraction, the pressure around the borehole increases with increasing borehole depth, and the increment of the pressure becomes larger when the borehole’s drainage effect is enhanced. However, the impact is small and can be ignored in engineering.