Method for integrated drilling, flushing, slotting and thermal injection for coalbed gas extraction

Abstract

A method for combining integrated drilling, flushing and slotting with thermal injection to enhance coalbed gas extraction, applicable to managing gas extraction from microporous, low-permeability, high-adsorption coalbed areas. A gas extraction borehole is drilled within a certain distance of a predetermined drilling, flushing and slotting borehole, and, once sealed, is used for gas extraction. An integrated drilling, flushing and slotting drill bit is used to sink the borehole, which is then sealed. Concentration variation in the gas extraction borehole is monitored in real time, and when concentration is below 30%, borehole is opened and high-temperature steam is injected by means of a steam generator, after which the borehole is again closed. Drilling a drilling, flushing and slotting borehole increases pressure relief space and the surface of exposed coal, relieves stress on the coal body, and increases gas permeability of the coalbed, while the injection of high-temperature steam promotes gas desorption in the coal body, promotes crack propagation around the borehole, and increases channels for gas flow, thus achieving highly efficient extraction of gas from the coalbed.

Claims

1. A method for coal seam gas extraction enhanced by integrated drilling, flushing, slotting and heat injection in combination, comprising drilling, hydraulic flushing, hydraulic slotting, sealing, and gas extraction, through the following steps: a. selecting a site of a heat injection borehole in an extraction lane of a coal seam floor, drilling 3 to 6 gas extraction boreholes around the heat injection borehole in the extraction lane of the coal seam floor through the coal seam floor toward a coal seam to a roof of the coal seam, and then withdrawing a drill stem, sealing the gas extraction boreholes in a conventional way, and carrying out gas extraction from the gas extraction boreholes; b. drilling the heat injection borehole from the extraction lane of the coal seam floor through the coal seam floor and the coal seam to the roof of the coal seam with an integrated drilling and slotting drill bit; c. injecting pressurized water at a pressure of 5 MPa to 10 MPa while withdrawing the drill stem, pulling and rotating the drill stem repeatedly to carry out hydraulic flushing on a section of the coal seam so that the heat injection borehole forms a drilling and flushing borehole having a bore diameter of approximately 0.4 m to 0.8 m; d. adjusting the water pressure from 15 MPa to 25 MPa, pulling the drill stem repeatedly to cut a slot parallel to the axial direction of the drilling and flushing borehole; turning the drill stem for 45 to 180 and pulling the drill stem repeatedly to cut several slots parallel to the axial direction of the drilling and flushing borehole so as to form a drilling, flushing, and slotting borehole; then, sealing the drilling, flushing, and slotting borehole; and e. monitoring gas concentration change in the gas extraction borehole in real time; opening an orifice of the drilling, flushing, and slotting borehole when the gas concentration in the drilling, flushing, and slotting borehole is lower than 30%, and injecting high-temperature steam into the drilling, flushing, and slotting borehole via a high-temperature heat-insulating heat supply pipeline from a steam generator for 1 to 3 hours, in order to increase the gas concentration in the gas extraction borehole.

2. The method according to claim 1, wherein the slots have a width of approximately 0.5 m to 1 m and a height of approximately 0.02 m to 0.05 m.

3. The method according to claim 2, wherein the number of the slots parallel to the axial direction of the drilling and flushing borehole is from two (2) to eight (8).

4. The method according to claim 1, wherein the number of the slots parallel to the axial direction of the drilling and flushing borehole is 2 to 8.

5. The method according to claim 1, wherein the high-temperature steam has a temperature of 150 C. to 450 C.

6. A method of coal seam gas extraction, the method comprising: drilling from three to six gas extraction boreholes around a heat injection borehole in an extraction lane of a coal seam floor through the coal seam floor toward a coal seam to a roof of the coal seam, withdrawing a drill stem, sealing the gas extraction boreholes, and extracting gas from the gas extraction boreholes; drilling the heat injection borehole from the extraction lane of the coal seam floor through the coal seam floor and the coal seam proximate the coal seam roof with an integrated drilling and slotting drill bit; injecting water into the heat injection borehole at a water pressure of from 5 MPa to 10 MPa while withdrawing the drill stem, pulling and rotating the drill stem repeatedly to carry out hydraulic flushing on a section of the coal seam so that the heat injection borehole forms a drilling and flushing borehole having a bore diameter of approximately 0.4 m to 0.8 m; adjusting the water pressure from 15 MPa to 25 MPa, pulling the drill stem repeatedly to cut a slot parallel to the axial direction of the drilling and flushing borehole; turning the drill stem for 45 to 180 and pulling the drill stem repeatedly to cut several slots parallel to the axial direction of the drilling and flushing borehole so as to form a drilling, flushing, and slotting borehole; then, sealing the drilling, flushing, and slotting borehole; monitoring the gas concentration change in the gas extraction borehole; and opening an orifice of the drilling, flushing, and slotting borehole when the gas concentration in the drilling, flushing, and slotting borehole is lower than 30%, and injecting high-temperature steam therein via a high-temperature heat-insulating heat supply pipeline for from one (1) to three (3) hours so as to increase the gas concentration in the gas extraction borehole.

7. The method according to claim 6, wherein the slots have a width of approximately 0.5 m to 1 m and a height of approximately 0.02 m to 0.05 m.

8. The method according to claim 6, wherein the number of the slots parallel to the axial direction of the drilling and flushing borehole is from two (2) to eight (8).

9. The method according to claim 6, wherein the high-temperature steam has a temperature of from 150 C. to 450 C.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a cross-sectional view according to this disclosure; and

(2) FIG. 2 is a schematic diagram of borehole position according to this disclosure.

(3) Among the figures, the following reference numerals and definitions are used: 1heat injection borehole; 2gas extraction borehole; 3extraction lane on coal seam floor; 4coal seam floor; 5roof of coal seam; 6coal seam; 7high-temperature heat-insulating heat supply pipeline; 8steam generator; 9drilling, flushing, and slotting borehole; 10drilling and flushing borehole; and 11slot.

DETAILED DESCRIPTION

(4) This disclosure will be hereinafter detailed in an embodiment with reference to the accompanying drawings.

(5) The method for coal seam gas extraction enhanced by integrated drilling, flushing, slotting and heat injection in combination provided in this disclosure comprises sequentially drilling, hydraulic flushing, hydraulic slotting, sealing, injecting hot steam, and gas extraction in a coal seam, through the following steps: a. selecting a site of heat injection borehole 1 in the extraction lane 3 on the coal seam floor, drilling approximately 3 to 6 gas extraction boreholes 2 around the heat injection borehole 1 in the extraction lane 3 on the coal seam floor through the coal seam floor 4 toward the coal seam 6 to the roof 5 of the coal seam, and then withdrawing the drill stem, sealing the boreholes 2 in a conventional way, and carrying out gas extraction from the gas extraction boreholes 2; b. drilling a heat injection borehole 1 from the extraction lane 3 on the coal seam floor through the coal seam floor 4 and the coal seam 6 to the roof 5 of the coal seam with an integrated drilling and slotting drill bit; c. injecting pressurized water at a pressure of 5 MPa to 10 MPa into the borehole while withdrawing the drill stem, pulling and rotating the drill stem repeatedly to carry out hydraulic flushing on the section of the coal seam 6 to enlarge the bore radius, so that the heat injection borehole 1 forms a drilling and flushing borehole 10 having a bore diameter of approximately 0.4 m to 0.8 m; d. adjusting the water pressure to 15 MPa to 25 MPa, repeatedly pulling the drill stem to cut a symmetric slot 11, which is parallel to the axial direction of the borehole in the drilling and flushing borehole 10; turning the drill stem for 45 to 180 and pulling the drill stem repeatedly to cut several slots 11 parallel to the axial direction of the borehole, so as to form a drilling, flushing, and slotting borehole 9; then, sealing the drilling, flushing, and slotting borehole 9; the slots 11 parallel to the axial direction of the borehole having a width of 0.5 m to 1 m and a height of approximately 0.02 m to 0.05 m, and having a quantity of 2 to 8. e. monitoring the gas concentration change in the gas extraction borehole 2 in real time; opening the orifice of the drilling, flushing, and slotting borehole 9 when the gas concentration in the borehole 9 is lower than 30%, and injecting high-temperature steam at 150 C. to 450 C. into the drilling, flushing, and slotting borehole 9 via a high-temperature heat insulating heat supply pipeline 7 from a steam generator 8 for approximately 1 to 3 hours, in order to increase the gas concentration in the gas extraction borehole 2 by means of the high-temperature steam.