Downhole coal seam pulse detonation wave directional fracturing permeability-increasing method

Abstract

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.

Claims

1. A method for improving permeability in a downhole coal seam by directional fracturing with pulsed detonation waves, the method comprising the following steps: drilling a pulsed detonation borehole from the wall of a roadway to a coal seam, and drilling four pulsed detonation guide boreholes around the pulsed detonation borehole, wherein the four pulsed detonation guide boreholes are at the same distance to the pulsed detonation borehole and are parallel to each other; connecting the input side of an explosion-proof high-voltage electrical pulse generator to an explosion-proof power cabinet via an explosion-proof switch; connecting a positive output side of the explosion-proof high-voltage electrical pulse generator to a positive electrode through a positive electrode cable, utilizing a tube in diameter to push the positive electrode to the bottom of the pulsed detonation borehole; connecting a negative output side of the explosion-proof high-voltage electrical pulse generator to a negative electrode through a negative electrode cable, utilizing a tube in diameter to push the negative electrode to the bottom of the pulsed detonation borehole; closing the explosion-proof switch to charge the explosion-proof high-voltage electrical pulse generator, and discharging from the positive electrode when the voltage of the explosion-proof high-voltage electrical pulse generator increases to a preset discharge voltage; disconnecting the explosion-proof switch when the positive electrode has discharged 20-30 times, and withdrawing the positive electrode and the negative electrode by 25 cm along the borehole; disconnecting the explosion-proof switch till the positive electrode and the negative electrode are 6 m from the wall of the roadway, and withdrawing the positive electrode and the negative electrode out of the borehole, and then connecting the pulsed detonation borehole and the pulsed detonation guide boreholes to a gas extraction pipe network for gas extraction.

2. The method according to claim 1, wherein the distance from the pulsed detonation borehole to each of the four pulsed detonation guide boreholes is 4-6 m.

3. The method according to claim 1, wherein the explosion-proof high-voltage electrical pulse generator operates at 10-50 Hz frequency and within 50-500 KV voltage range.

Description

DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic structural diagram illustrating the method for permeability improvement by directional fracturing with pulsed detonation waves according to the present invention;

(2) FIG. 2 is a top view of the arrangement of the pulsed detonation borehole and the pulsed detonation guide boreholes in a coal seam according to the present invention.

(3) In the figures: 1explosion-proof power cabinet; 2explosion-proof switch; 3explosion-proof high-voltage electrical pulse generator; 4positive output side; 5negative output side; 6positive electrode cable; 7positive electrode; 8pulsed detonation borehole; 9negative electrode cable; 10negative electrode; 11pulsed detonation guide borehole; 12coal seam.

DETAILED DESCRIPTION OF THE EMBODIMENTS

(4) Hereunder the present invention will be detailed in an embodiment with reference to the accompanying drawings.

(5) The method for permeability improvement for a downhole coal seam by directional fracturing with pulsed detonation waves provided in the present invention employs an explosion-proof high-voltage electrical pulse generator 3, and comprises the following steps: (1) drilling a pulsed detonation borehole 8 from the wall of a roadway to a coal seam 12 according to FIG. 1, and then drilling four pulsed detonation guide boreholes 11 around the pulsed detonation borehole 8, the four pulsed detonation guide boreholes 11 are at the same distance to the pulsed detonation borehole 8, and are parallel to each other; the distance from the pulsed detonation borehole 8 to each of the four pulsed detonation guide boreholes 11 is 4-6 m; (2) connecting the input side of the explosion-proof high-voltage electrical pulse generator 3 to an explosion-proof power cabinet 1 via an explosion-proof switch 2; (3) connecting a positive output side 4 of the explosion-proof high-voltage electrical pulse generator 3 to a positive electrode 7 through a positive electrode cable 6, utilizing a tube in diameter to push the positive electrode 7 to the bottom of the pulsed detonation borehole 8; (4) connecting a negative output side 5 of the explosion-proof high-voltage electrical pulse generator 3 to a negative electrode 10 through a negative electrode cable 9, utilizing a tube in diameter to push the negative electrode 10 to the bottom of the pulsed detonation borehole 11; (5) closing the explosion-proof switch 2 to charge the explosion-proof high-voltage electrical pulse generator 3, and discharging from the positive electrode 7 when the voltage increases to 260 KV discharge voltage; the explosion-proof high-voltage electrical pulse generator 3 operates at 10-50 Hz frequency and within 50-500 KV voltage range; (6) disconnecting the explosion-proof switch 2 when the positive electrode 7 has discharged for 20-30 times, and withdrawing the positive electrode 7 and the negative electrode 10 by 25 cm along the borehole; (7) repeating the steps 5 and 6 for several times, disconnecting the explosion-proof switch 2 till the positive electrode and the negative electrode are at a 6 m distance to the wall of the roadway, and withdrawing the positive electrode 7 and the negative electrode 10 out of the borehole, and then connecting the pulsed detonation borehole 8 and the pulsed detonation guide boreholes 11 to a gas extraction pipe network for gas extraction.