A method for permeability improvement for a downhole
coal seam by directional fracturing with pulsed
detonation waves, which is applicable to gas control in
coal seam areas with high
gas concentration and low air permeability. The permeability improvement method is as follows: first, drilling a pulsed
detonation borehole and pulsed
detonation guide boreholes from a
coal roadway to a coal seam respectively; then, pushing a positive
electrode connected to a positive output side of an explosion-proof high-
voltage electrical
pulse generator to the bottom of the pulsed detonation borehole and pushing a negative
electrode connected to a negative output side of the explosion-proof high-
voltage electrical
pulse generator to the bottom of the pulsed detonation guide borehole; connecting the pulsed detonation borehole and the pulsed detonation guide boreholes to an extraction pipeline for gas extraction, after electrical pulsed detonation fracturing for the coal seam is carried out. The method disclosed in the present invention utilizes the high
instantaneous energy provided by electrical pulsed detonation
waves to fracture a coal
mass, so as to form a
fissure network in the coal
mass between the pulsed detonation borehole and the pulsed detonation guide boreholes; thus, the air
permeability coefficient of the coal
mass can be increased by 200-400 times, the effective influence scope of gas extraction of a single borehole for gas extraction can be enlarged by 3-4 times, the extracted gas volume from the borehole can be increased by 3-8 times, and the coal seam gas pre-extraction time can be shortened effectively.