Mechanism of Coal Permeability Enhancement by a Series of Mechanically Over-excavated Cavities Along Drainage Boreholes

To solve the difficult problems of gas drainage in coal seams with a high gas content and low permeability, a novel technology based on a series of mechanically over-excavated cavities along drainage boreholes in coal seams to increase permeability was proposed. In this paper, the fluid–solid coupled model of the mechanically over-excavated borehole and the gas drainage in soft coal seams was analyzed, and COMSOL Multiphysics numerical simulation software was used for validation. The influence of the parameters for the mechanically over-excavated borehole (i.e., cavity diameter, cavity length, and interspace between the cavities and cavity number) and the reservoir properties of coal seams (i.e., initial permeability, initial porosity and elastic modulus) on gas drainage were investigated. The results show that the gas drainage volume increases with cavity volume and has little effect when the interspace between the cavities exceeds 0.5 m. This indicates that mechanically over-excavated boreholes can significantly increase the coal seam permeability and gas drainage volume. Based on numerical analyses, a single-wing embedded mechanically over-excavated borehole drill bit was developed. The effectiveness of the drill bit was tested in situ at the Licun Coal Mine of the Shanxi Lu’an Group. The results indicate that the effective gas drainage volume of a mechanically over-excavated borehole is 147% higher than that of an ordinary borehole and is not much different from that of a hydraulic flushing borehole. However, mechanically over-excavated borehole technology has great advantages with respect to operational efficiency and cost.