Permeability enhancements of borehole outburst cavitation in outburst-prone coal seams

Borehole outbursts eject coal power, create cavities and relieve in-situ stress while drilling cross-measure boreholes into outburst coal seams for pre-drainage. This paper highlights the borehole outburst cavitation to enhance the permeability of coal for the first time. Borehole outburst is a mechanical behavior, and the outburst trigger and development depend on the status of gas pressure gradient near the opening coal wall. It was testified that the high-pressure water, with 10-15 MPa in pressure and 20-30 m3/h in flow, was feasible to remove the failure coal near the opening wall and stimulate a new set of outbursts, thus creating bigger cavities and better permeability enhancement. Furthermore, an outburst control system was designed for borehole outburst cavitation. The system contains an outburst stimulation subsystem, a gas drainage subsystem and a separation subsystem, thus ensuring the flow and the separation of outburst ejections within a relatively closed environment. The outburst cavity reconstitutes the in-situ stress and permeability of the surrounding coal mass, leading to a failure zone, a plastic zone and a visco-elastic zone. The failure zone and the plastic zone due to borehole outburst cavitation are both effective in permeability enhancement. Finally, a field test in the No. 10 coal seam at the Luling mine proved that the sufficient permeability enhancement scope is approximately twelve times of the cavity radius. The permeability enhancement promotes the increase of borehole distance from 5 m to 12 m, thus reducing 78.7% of boreholes in number for the pre-drainage of the No. 10 coal seam.