A method for enhanced extraction of coal seam gas from hydraulically driven annular fractured coal bodies

Abstract

The present invention relates to a method for enhanced extraction of coal seam gas from a hydraulically driven annular fractured coal body, and belongs to the field of coal mine safety. The present invention injects high-pressure water into a multi-directional oil cylinder through a high-pressure water injection pump or a pump group underground in the coal mine, thereby pushing an action column and a fracture arc to move in the radial direction of a gas extraction borehole, and then after contacting the inner wall of the gas extraction borehole, the water pressure in the multi-directional oil cylinder squeezes the coal body around the gas extraction borehole, thereby causing secondary fractures of varying degrees in the coal body, thereby achieving the purpose of increasing the permeability of the coal seam, and ultimately improving the fluidity of the gas in the coal body around the extraction borehole, thereby enhancing the gas extraction effect.

Description

Technical Field

[0001] The invention belongs to the field of coal mine safety and relates to a method for enhanced extraction of coal seam gas by hydraulically driving annular fractured coal bodies. Background Art

[0002] Gas is one of the main threats to mine safety. It is a combustible gas with methane as the main component, which exists in coal seams or surrounding rocks in adsorbed and free forms. At present, my country's coal industry generally adopts the method of mining protective layers and gas extraction to implement mine gas disaster management. However, due to the relatively poor gas geological conditions of coal seams in my country, most coal seams are soft and low-permeability coal seams. Therefore, in the process of implementing gas extraction, corresponding measures are often adopted to implement large-scale permeability enhancement of coal bodies in order to improve the effect of mine gas extraction and ensure that the gas extraction before the coal body of the working face is up to standard. At present, the commonly used coal seam permeability enhancement measures mainly include hydraulic fracturing, hydraulic cutting, carbon dioxide displacement, etc., and have achieved certain application effects in many engineering practices. However, due to the complex geological conditions of coal seam gas in my country, when hydraulic measures are applied to enhance the permeability of coal bodies, due to the influence of ground stress, the secondary cracks of the coal body are often closed after a certain period of time, and then its permeability characteristics are restored or approach the original values, which is actually very unfavorable for gas extraction in mines. The carbon dioxide displacement process has a limited range of action, so it is difficult to make the borehole produce high-concentration gas for a long time. In addition, hydraulic fracturing, hydraulic fracturing, and carbon dioxide displacement processes all require efficient sealing of the gas extraction boreholes after construction to ensure the safety of personnel during the permeability enhancement operation. The depth and length of the sealing put forward extremely high requirements for on-site construction personnel. Summary of the invention

[0003] In view of this, the purpose of the present invention is to provide a method for enhanced coal seam gas extraction from a hydraulically driven circumferentially fractured coal body, to solve the problem of poor gas extraction effect in low-permeability coal seams, to effectively ensure the effect of mine gas disaster management, to further ensure the continuous safe production of coal, and to effectively ensure the personal safety of underground workers.

[0004] In order to achieve the above object, the present invention provides the following technical solutions:

[0005] A method for enhanced extraction of coal seam gas by hydraulically driving annular fractured coal bodies, comprising the following steps:

[0006] S1: Use drilling equipment to construct gas extraction holes along the inclination direction of the coal seam in the working face transport chute;

[0007] S2: After the gas extraction borehole construction is completed, an end fracturing assembly is inserted into the gas extraction borehole, and then additional fracturing assemblies are added step by step through the water injection pipeline flange and the water return pipeline flange until the end fracturing assembly reaches the designed fracturing position, and then the orifice connecting pipe and the orifice water return pipeline are respectively connected to the water injection pipeline flange and the water return pipeline flange on the last additional fracturing assembly;

[0008] S3: Connect the water injection port to the high-pressure water injection pump or pump group in the coal mine, close the second valve and the fourth valve, open the first valve and the third valve, and inject high-pressure water into the multi-directional oil cylinder through the first four-way, the orifice connecting pipe, and the water injection pipeline; monitor the water pressure in the water injection pipeline by the pressure gauge set on the orifice connecting pipe, and stop injecting high-pressure water after the indication of the pressure gauge rises to a preset threshold; then open the fourth valve to allow the high-pressure water in the water injection pipeline to flow out;

[0009] S4: When the indication of the pressure gauge drops to 0, close the first valve, the third valve and the fourth valve, open the second valve, and let the high-pressure water enter the multi-directional oil cylinder through the second four-way, the orifice return pipe, and the hole return pipe, and push the action column and the fracture arc to move toward the centroid of the multi-directional oil cylinder; during this process, if the pressure gauge increases, stop injecting high-pressure water, close the second valve, open the first valve and the fourth valve, until the indication of the pressure gauge drops to 0, close the first valve and the fourth valve, open the second valve, and start the high-pressure injection. The water pump or pump group continuously injects high-pressure water into the second four-way, the orifice return water pipeline, and the inner hole return water pipeline, and enters the multi-directional oil cylinder and pushes the action column and the fracture arc to move toward the centroid direction of the multi-directional oil cylinder until the fracture arc is reset to the original position, and then stops injecting high-pressure water, closes the second valve, opens the third valve, and after the high-pressure water in the inner hole return water pipeline and the orifice return water pipeline flows out, all the fracture components are withdrawn; then the gas extraction borehole is plugged, and then it is connected to the coal mine underground gas extraction pipeline to start the gas extraction operation;

[0010] S5: Repeat S1~S4 until the distance from the working face to the cut eye is 40m, then stop constructing the gas extraction drilling holes; after the working face is cracked, the gas extraction drilling holes are connected to the underground gas extraction pipeline of the coal mine, and after a certain period of extraction, the residual gas content of the coal body of the working face is measured. If the value meets the requirements of gas extraction standards, the gas extraction drilling holes can be closed according to the on-site conditions; if the value does not meet the requirements of gas extraction standards, the connection between the gas extraction drilling holes and the underground gas extraction pipeline of the coal mine can be removed according to the on-site conditions, and S2~S4 are repeated.

[0011] Furthermore, the final hole position of the gas extraction drilling hole in step S1 is 30m away from the return air chute of the working face.

[0012] Furthermore, the fracturing assembly includes a multi-directional oil cylinder, in which a plurality of action columns are evenly distributed with the centroid of the multi-directional oil cylinder as the center, and a fracturing arc is provided at the end of the action column. A first four-way and a second four-way are provided in the multi-directional oil cylinder, and the four ends of the first four-way are respectively connected to the water injection port, the first water outlet, and the second four-way, and the rodless end of the multi-directional oil cylinder is connected through the orifice connecting pipe and the water injection pipeline; the four ends of the second four-way are respectively connected to the first four-way and the second water outlet, and the rod ends on both sides of the multi-directional oil cylinder are connected through the orifice return water pipeline and the hole return water pipeline; a first valve is provided at the connection between the orifice connecting pipe and the water injection pipeline, a second valve is provided at the connection between the first four-way and the second four-way, a third valve is provided at the connection between the second four-way and the second water outlet, and a fourth valve is provided at the connection between the first four-way and the first water outlet; a water injection pipeline flange is provided on the water injection pipeline, and a return pipeline flange is provided on the hole return pipeline; a pressure gauge is provided on the orifice connecting pipe.

[0013] Furthermore, the multi-directional oil cylinder in the end fracturing assembly is provided with a water injection pipeline and a hole return water pipeline on only one side, and the multi-directional oil cylinder of the additional fracturing assembly is provided with a water injection pipeline and a hole return water pipeline on both sides.

[0014] Further, step S3 specifically includes the following steps:

[0015] S31: Connect the water injection port to the high-pressure water injection pump or pump group in the coal mine, close the second valve and the fourth valve, and then open the first valve and the third valve to allow the high-pressure water injection pump or pump group to inject high-pressure water into the multi-directional oil cylinder through the first four-way, the orifice connecting pipe, and the water injection pipeline;

[0016] S32: During this process, a pressure gauge installed on the orifice connecting pipe monitors the water pressure in the water injection pipeline, and when the reading of the pressure gauge rises to a certain threshold, the high-pressure water injection pump or pump group is turned off; the threshold is the vertical ground stress of the coal body in the gas extraction borehole;

[0017] S33: Turn off the high-pressure water injection pump or pump group, and open the fourth valve to allow the high-pressure water in the water injection pipeline to flow out until the pressure indication reads 0 and no water flows out.

[0018] Furthermore, the vertical geostress of the coal body in the gas extraction borehole is obtained by calculation based on the lithological columnar characteristics of the coal seam.

[0019] The beneficial effects of the present invention are as follows: the present invention proposes a method for enhanced extraction of coal seam gas from a hydraulically driven annular fractured coal body, which solves the problem of poor gas extraction effect in low-permeability coal seams; that is, high-pressure water is injected into a multi-directional oil cylinder through a high-pressure water injection pump or pump group underground in a coal mine, thereby pushing the action column and the fracture arc to move in the radial direction of the gas extraction borehole, and then after contacting the inner wall of the gas extraction borehole, the water pressure in the multi-directional oil cylinder squeezes the coal body around the gas extraction borehole, thereby causing the coal body to produce secondary cracks of varying degrees, achieving the purpose of increasing the permeability of the coal seam, and ultimately improving the fluidity of the gas in the coal body around the extraction borehole, thereby enhancing the gas extraction effect. The method for enhanced extraction of coal seam gas from a hydraulically driven annular fractured coal body of the present invention has the following advantages over previous gas extraction methods:

[0020] (1) The unique hydraulically driven circumferentially fractured coal body gas enhanced extraction method of the present invention does not require high-efficiency plugging before the permeability enhancement operation of the gas extraction drilling hole, which has extremely significant economic benefits compared with the high-efficiency plugging in the traditional hydraulic permeability enhancement and carbon dioxide displacement permeability enhancement operations;

[0021] (2) The method for enhanced extraction of coal seam gas by hydraulically driven annular fracturing of coal bodies provided by the present invention can implement multiple annular fracturing according to the conditions of gas extraction by drilling, and has extremely significant technical benefits.

[0022] (3) The method for enhanced extraction of coal seam gas from a hydraulically driven annular fractured coal body provided by the present invention is simple to operate, and the core high-pressure water pressure threshold can be determined based on the vertical ground stress value of the coal body in the gas extraction borehole. When the water pressure threshold reaches the vertical ground stress value, the coal body is bound to produce secondary cracks of varying degrees, so it is easy for on-site operators to master.

[0023] Other advantages, objectives and features of the present invention will be described in the following description to some extent, and to some extent, will be obvious to those skilled in the art based on the following examination and study, or can be taught from the practice of the present invention. The objectives and other advantages of the present invention can be realized and obtained through the following description. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] In order to make the purpose, technical solutions and advantages of the present invention more clear, the present invention will be described in detail below in conjunction with the accompanying drawings, wherein:

[0025] Figure 1 It is a side view of the fracture-inducing component;

[0026] Figure 2 It is a front view of the fracture-inducing component;

[0027] Figure 3 This is a schematic diagram of the drilling and fracturing construction for gas extraction.

[0028] Figure numerals: multi-directional cylinder 1, action column 2, fracturing arc 3, first four-way valve 4, second four-way valve 5, water injection port 6, first water outlet 7, orifice connecting pipe 8, water injection pipeline 9, second water outlet 10, orifice return water pipeline 11, in-hole return water pipeline 12, first valve 13, second valve 14, third valve 15, fourth valve 16, water injection pipeline flange 17, return water pipeline flange 18, pressure gauge 19, end fracturing assembly 20, additional fracturing assembly 21. DETAILED DESCRIPTION

[0029] The following describes the embodiments of the present invention by specific examples, and those skilled in the art can easily understand other advantages and effects of the present invention from the contents disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and the details in this specification can also be modified or changed in various ways based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that the illustrations provided in the following embodiments only illustrate the basic concept of the present invention in a schematic manner, and the following embodiments and features in the embodiments can be combined with each other without conflict.

[0030] Among them, the drawings are only used for illustrative explanations, and they only represent schematic diagrams rather than actual pictures, and should not be understood as limitations on the present invention. In order to better illustrate the embodiments of the present invention, some parts of the drawings may be omitted, enlarged or reduced, and do not represent the size of actual products. For those skilled in the art, it is understandable that some well-known structures and their descriptions in the drawings may be omitted.

[0031] The same or similar numbers in the drawings of the embodiments of the present invention correspond to the same or similar parts; in the description of the present invention, it should be understood that if the terms "upper", "lower", "left", "right", "front", "rear", etc. indicate the orientation or position relationship, they are based on the orientation or position relationship shown in the drawings, which is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operate in a specific orientation. Therefore, the terms describing the position relationship in the drawings are only used for illustrative purposes and cannot be understood as limiting the present invention. For ordinary technicians in this field, the specific meanings of the above terms can be understood according to specific circumstances.

[0032] See also Figure 1 to Figure 3 , is a method for enhanced extraction of coal seam gas from a hydraulically driven annular fractured coal body, comprising the following steps:

[0033] S1: Use corresponding drilling equipment to construct gas extraction boreholes along the inclination direction of the coal seam in the working face transport drift; the final hole position of the gas extraction borehole is 30m away from the working face return air drift.

[0034] S2: After the gas extraction drilling construction is completed, an end fracturing assembly 20 is immediately inserted into the gas extraction drilling hole; the fracturing assembly includes a multi-directional oil cylinder 1, in which a plurality of action columns 2 are evenly distributed with the centroid of the multi-directional oil cylinder 1 as the center, and a fracturing arc 3 is arranged at the end of the action column 2, and a first four-way 4 and a second four-way 5 are arranged in the multi-directional oil cylinder 1, and the four ends of the first four-way 4 are respectively connected to the water injection port 6, the first water outlet 7, and the second four-way 5, and are connected to the rodless end of the multi-directional oil cylinder through the orifice connecting pipe 8 and the water injection pipeline 9; the four ends of the second four-way 5 are respectively connected to the first four-way 4, a second water outlet 10, and the rod ends on both sides of the multi-directional oil cylinder 1 are connected through the orifice return water pipeline 11 and the bore return water pipeline 12; the first valve 13 is provided at the connection between the orifice connecting pipe 8 and the water injection pipeline 9, the second valve 14 is provided at the connection between the first four-way 4 and the second four-way 5, the third valve 15 is provided at the connection between the second four-way 4 and the second water outlet 10, and the fourth valve 16 is provided at the connection between the first four-way 4 and the first water outlet 7; the water injection pipeline 9 is provided with a water injection pipeline flange 17, the bore return water pipeline is provided with a return water pipeline flange 18, and the orifice connecting pipe is provided with a pressure gauge 19. The fracturing assembly includes an end fracturing assembly 20 and an additional fracturing assembly 21. The multi-directional oil cylinder in the end fracturing assembly 20 is provided with a water injection pipeline and a bore return water pipeline only on one side, and the multi-directional oil cylinder of the additional fracturing assembly is provided with a water injection pipeline and a bore return water pipeline on both sides.

[0035] After the end fracturing assembly 20 is inserted into the gas extraction borehole, it is connected to the additional fracturing assembly 21 through the water injection pipeline flange 17 and the return water pipeline flange 18, and subsequent additional fracturing assemblies 21 are gradually added until the end fracturing assembly 20 reaches the designed fracturing position. The multi-directional cylinder 1 arranged in the end fracturing assembly 20 has only one side of the water injection pipeline 9 and the return water pipeline 12 in the hole, while the additional fracturing assembly 21 is provided with the water injection pipeline 9 and the return water pipeline 12 in the hole on both sides; then the orifice connecting pipe 8 and the orifice return water pipeline 11 are respectively connected to the water injection pipeline flange 17 and the return water pipeline flange 18 on the last additional fracturing assembly.

[0036] S3: Connect the water injection port 6 to the high-pressure water injection pump or pump group in the coal mine, close the second valve 14 and the fourth valve 16, and then open the first valve 13 and the third valve 15 to allow the high-pressure water injection pump or pump group to inject high-pressure water into the multi-directional cylinder 1 through the first four-way 4 and the orifice connecting pipe 8 and the water injection pipeline 9; in this process, the pressure gauge 19 arranged on the orifice connecting pipe 8 can monitor the water pressure in the water injection pipeline 9, and when the reading of the pressure gauge 19 rises to a certain threshold, the high-pressure water injection pump or pump group can be closed; the threshold value is the vertical geostress of the coal body in the gas extraction borehole; further, the vertical geostress of the coal body in the gas extraction borehole can be obtained by calculation through the lithological columnar characteristics of the coal seam; then close the high-pressure water injection pump or pump group, and open the fourth valve 16, so that the high-pressure water in the water injection pipeline 9 flows out until the reading of the pressure gauge 19 is 0 and no water flows out.

[0037] S4: When the indication of the pressure gauge 19 decreases to 0, the first valve 13, the third valve 15 and the fourth valve 16 are closed, the second valve 14 is opened, and the high-pressure water injection pump or pump group is turned on to allow the high-pressure water to enter the multi-directional oil cylinder 1 through the second four-way 5 and the orifice return water pipeline 11 and push the action column 2 and the fracture arc 3 to move toward the centroid direction of the multi-directional oil cylinder 1; during this process, if the indication of the pressure gauge 19 does not change, the high-pressure water injection pump or pump group is continuously turned on to inject high-pressure water into the second four-way 5, the orifice return water pipeline 11 and the hole return water pipeline 12; if the indication of the pressure gauge 19 increases, the high-pressure water injection pump or pump group can be closed at this time, and the second valve 14 can be closed, and the first valve 13 and the fourth valve 16 can be opened until the indication of the pressure gauge 19 decreases to 0, and then the first valve 13 and the fourth valve 16 are closed. 16; and open the second valve 14, start the high-pressure water injection pump or pump group to continuously inject high-pressure water into the second four-way 5, the orifice return water pipeline 11, and the inner hole return water pipeline 12 and enter the multi-directional oil cylinder 1 and push the action column 2 and the fracture arc 3 to move toward the centroid direction of the multi-directional oil cylinder 1 until the fracture arc 3 is restored to the original position, then the high-pressure water injection pump or pump group and the second valve 14 can be closed, and then the third valve 15 can be opened to make the high-pressure water in the inner hole return water pipeline 12 and the orifice return water pipeline 11 flow out, then the connection between the orifice connecting pipe 8 and the additional fracture component 21 can be removed in turn, and the additional fracture component 21 can be withdrawn until the end fracture component 20 is withdrawn; then the gas extraction borehole is plugged, and then it is immediately connected to the coal mine underground gas extraction pipeline to start the gas extraction operation;

[0038] S5: Repeat S1~S4 until the distance from the working face cut is 40m, then the construction of gas extraction drilling can be stopped, and there is no need to implement circumferential fracturing operations; after the gas extraction drilling holes after the working face fracturing are connected to the underground gas extraction pipeline of the coal mine, and after a certain period of extraction, the residual gas content of the coal body of the working face is measured. If the value meets the requirements of gas extraction standards, the corresponding gas extraction drilling holes can be closed according to the on-site conditions; if the value does not meet the requirements of gas extraction standards, the connection between the corresponding gas extraction drilling holes and the underground gas extraction pipeline of the coal mine can be removed according to the on-site conditions, and S2~S4 are repeated.

[0039] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solution of the present invention can be modified or replaced by equivalents without departing from the purpose and scope of the technical solution, which should be included in the scope of the claims of the present invention.

Claims

1. A method for enhanced extraction of coal seam gas by hydraulically driving annular fractured coal bodies, characterized in that: The following steps are involved: S1: Use drilling equipment to construct gas extraction holes along the inclination direction of the coal seam in the working face transport chute; S2: After the gas extraction borehole construction is completed, an end fracturing assembly is inserted into the gas extraction borehole, and then additional fracturing assemblies are added step by step through the water injection pipeline flange and the water return pipeline flange until the end fracturing assembly reaches the designed fracturing position, and then the orifice connecting pipe and the orifice water return pipeline are respectively connected to the water injection pipeline flange and the water return pipeline flange on the last additional fracturing assembly; The fracturing assembly comprises a multi-directional oil cylinder, in which a plurality of action columns are evenly distributed with the centroid of the multi-directional oil cylinder as the center, and the ends of the action columns are provided with fracturing arcs, and the multi-directional oil cylinder is provided with a first four-way and a second four-way, and the four ends of the first four-way are respectively connected to the water injection port, the first water outlet, and the second four-way, and the rodless end of the multi-directional oil cylinder is connected through the orifice connecting pipe and the water injection pipeline; the four ends of the second four-way are respectively connected to the first four-way and the second water outlet, and the rod ends on both sides of the multi-directional oil cylinder are connected through the orifice return water pipeline and the hole return water pipeline; a first valve is provided at the connection between the orifice connecting pipe and the water injection pipeline, a second valve is provided at the connection between the first four-way and the second four-way, a third valve is provided at the connection between the second four-way and the second water outlet, and a fourth valve is provided at the connection between the first four-way and the first water outlet; a water injection pipeline flange is provided on the water injection pipeline, and a return pipeline flange is provided on the hole return water pipeline; a pressure gauge is provided on the orifice connecting pipe; S3: Connect the water injection port to the high-pressure water injection pump or pump group in the coal mine, close the second valve and the fourth valve, open the first valve and the third valve, and inject high-pressure water into the multi-directional oil cylinder through the first four-way, the orifice connecting pipe, and the water injection pipeline; monitor the water pressure in the water injection pipeline by the pressure gauge set on the orifice connecting pipe, and stop injecting high-pressure water after the indication of the pressure gauge rises to a preset threshold; then open the fourth valve to allow the high-pressure water in the water injection pipeline to flow out; S4: When the indication of the pressure gauge drops to 0, close the first valve, the third valve and the fourth valve, open the second valve, and let the high-pressure water enter the multi-directional oil cylinder through the second four-way, the orifice return pipe, and the hole return pipe, and push the action column and the fracture arc to move toward the centroid of the multi-directional oil cylinder; during this process, if the pressure gauge increases, stop injecting high-pressure water, close the second valve, open the first valve and the fourth valve, until the indication of the pressure gauge drops to 0, close the first valve and the fourth valve, open the second valve, and start the high-pressure injection. The water pump or pump group continuously injects high-pressure water into the second four-way, the orifice return water pipeline, and the inner hole return water pipeline, and enters the multi-directional oil cylinder and pushes the action column and the fracture arc to move toward the centroid direction of the multi-directional oil cylinder until the fracture arc is reset to the original position, and then stops injecting high-pressure water, closes the second valve, opens the third valve, and after the high-pressure water in the inner hole return water pipeline and the orifice return water pipeline flows out, all the fracture components are withdrawn; then the gas extraction borehole is plugged, and then it is connected to the coal mine underground gas extraction pipeline to start the gas extraction operation; S5: Repeat S1~S4 until the distance from the working face to the cut eye is 40m, then stop constructing the gas extraction drilling holes; after the working face is cracked, the gas extraction drilling holes are connected to the underground gas extraction pipeline of the coal mine, and after a certain period of extraction, the residual gas content of the coal body of the working face is measured. If the value meets the requirements of gas extraction standards, the gas extraction drilling holes can be closed according to the on-site conditions; if the value does not meet the requirements of gas extraction standards, the connection between the gas extraction drilling holes and the underground gas extraction pipeline of the coal mine can be removed according to the on-site conditions, and S2~S4 are repeated.

2. The method for enhanced extraction of coal seam gas by hydraulically driven circumferential fracturing of coal bodies according to claim 1 is characterized in that: The final hole position of the gas extraction drilling hole in step S1 is 30m away from the return air chute of the working face.

3. The method for enhanced extraction of coal seam gas by hydraulically driven circumferential fracturing of coal bodies according to claim 1 is characterized in that: The multi-directional oil cylinder in the end fracturing assembly is provided with a water injection pipeline and a hole return water pipeline on only one side, and the multi-directional oil cylinder of the additional fracturing assembly is provided with a water injection pipeline and a hole return water pipeline on both sides.

4. The method for enhanced extraction of coal seam gas by hydraulically driven circumferential fracturing of coal bodies according to claim 1 is characterized in that: Step S3 specifically includes the following steps: S31: Connect the water injection port to the high-pressure water injection pump or pump group in the coal mine, close the second valve and the fourth valve, and then open the first valve and the third valve to allow the high-pressure water injection pump or pump group to inject high-pressure water into the multi-directional oil cylinder through the first four-way, the orifice connecting pipe, and the water injection pipeline; S32: During this process, a pressure gauge installed on the orifice connecting pipe monitors the water pressure in the water injection pipeline, and when the reading of the pressure gauge rises to a certain threshold, the high-pressure water injection pump or pump group is turned off; the threshold is the vertical ground stress of the coal body in the gas extraction borehole; S33: Turn off the high-pressure water injection pump or pump group, and open the fourth valve to allow the high-pressure water in the water injection pipeline to flow out until the pressure indication reads 0 and no water flows out.

5. The method for enhanced extraction of coal seam gas by hydraulically driven circumferential fracturing of coal bodies according to claim 4 is characterized in that: The vertical geostress of the coal body in the gas extraction borehole is obtained by calculation based on the lithological columnar characteristics of the coal seam.

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