A new technology for the drilling of long boreholes for gas drainage in a soft coal seam

• We improve the efficiency of draining gas in a soft coal seam by a new technology.
• We propose a drilling technology of protecting the borehole.
• We propose spraying foam concrete to protect the borehole.
• We tested this new technology in the Zhaozhuang mines.

The geostatic stress, coal mine gas and disturbances produced by external loading can undermine the stability of a borehole and cause the borehole to collapse in a soft coal seam, which results in a poor efficiency of drilling long boreholes and draining gas in a soft coal seam. To improve the efficiency of drilling boreholes and draining gas in a soft coal seam, we propose a drilling technology of protecting the borehole by spraying foam concrete slurry onto the surface of borehole wall during drilling in a soft coal seam. To realize this process, we propose spraying foam concrete onto the borehole wall during drilling with a foldable drill bit and double drill pipes. Based on this new technology, the equipment and the material required for drilling long boreholes in a soft coal seam were studied. Finally, this new technology was tested in the Zhaozhuang mines in the Shanxi Province of China. The test results indicate the following: the process of spraying foam concrete on the borehole wall can relieve borehole collapse; the plant-based foam concrete has good air permeability, and has no effect on the permeability of the borehole wall; and the novel process of drilling boreholes and spraying foam concrete is an effective drilling process, which can relieve borehole collapse, improve the drilling efficiency and improve the draining efficiency.