Optimized Cuttings Removal for Mechanical Cavity-Making in Downward Boreholes: A Strategy for Coal Seam Permeability Enhancement

Mechanical cavity-making has been applied in many coal mines in China because it can improve pressure relief and increase the permeability of coal around boreholes. However, for downward cross-seam boreholes, the application of this technology is still restricted, mainly because cuttings are difficult to remove during cavity-making. To address this problem, this study combined numerical simulation with field tests to examine the cuttings output required for effective cavity-making and to compare the gas extraction performance under different cuttings-removal methods. The simulation results indicated that the required cuttings output was about 0.30 t / m . Field tests then were carried out using three cuttings-removal methods. Among them, the combined high-flow mud pump and air compressor method performed best, and the cuttings output reached 0.45 t / m . Boreholes treated with this method also had better gas extraction performance than those under the other two methods. The average gas extraction concentration reached 67%, and the average pure gas extraction rate reached 0.39 m 3 / min . These results show that improving cuttings removal is important for the field application of downward borehole mechanical cavity-making, and can provide a practical reference for enhancing coal seam permeability and improving coalbed gas extraction in low-permeability coal seams.