A circulating gas flow liquid nitrogen fracturing device and method

Abstract

This invention discloses a circulating airflow liquid nitrogen fracturing device and method, comprising an impact section, which includes a heat insulation component disposed on the coal wall at the top of the borehole. The heat insulation component is permeated by an outer shell, the bottom of which is open. A heating component is disposed at the bottom of the inner cavity of the outer shell, and an impact component is disposed at the bottom of the heating component. A heating chamber is formed between the outer wall of the heating component and the inner wall of the outer shell. Several fourth valves are circumferentially disposed on the outer wall of the outer shell between the heating chamber and the heat insulation component. A third valve is disposed in the area between the fourth valves and the heat insulation component within the outer shell. An external connection section includes a liquid nitrogen pump station and a gas extraction station. The liquid nitrogen pump station is connected to the outer shell via a nitrogen inlet pipeline, and the gas extraction station is connected to the outer shell via an extraction pipeline. This invention can achieve efficient permeability enhancement of coal by utilizing airflow to accelerate the vaporization of liquid nitrogen and realizing the hot and cold cyclic impact of liquid nitrogen and high-temperature nitrogen gas.

Description

Technical Field

[0001] This invention relates to the field of coalbed methane extraction technology, and in particular to a circulating gas flow type liquid nitrogen fracturing device and method. Background Technology

[0002] As is well known, my country commonly has high-gas coal seams with low permeability, hindering the pre-extraction of coalbed methane. Currently, to ensure safe coal mine production and efficient coalbed methane extraction, physical and chemical methods are commonly used both domestically and internationally to enhance coal seam permeability. Existing widely used coal seam permeability enhancement technologies include hydraulic fracturing, hydrochloric acid chemical methods, hydraulic fracturing, deep-hole pre-fracturing blasting, and hydraulic perforation. These methods have significantly improved coalbed methane extraction rates, but they also have drawbacks. While hydraulic fracturing has good permeability enhancement effects, this technology uses large amounts of water mixed with chemicals, which seriously pollutes water sources, threatening the ecological environment and the health of local residents. Hydraulic fracturing requires cutting between boreholes, making it highly difficult to operate. Deep-hole blasting is complex to operate and prone to accidents, threatening safe coal mine production.

[0003] Liquid nitrogen fracturing technology, as a highly efficient and environmentally friendly anhydrous fracturing technique, can effectively improve coal seam permeability and increase coalbed methane recovery rates. Simultaneously, it can effectively conserve water resources and avoid problems such as water lock-in and water-sensitive damage. Although liquid nitrogen fracturing technology, as an emerging fracturing technique, has not yet been widely used in practice, its promising prospects have attracted increasing numbers of scholars to conduct theoretical and experimental research on it.

[0004] Existing liquid nitrogen-hot gas cooling and heating cycle liquid nitrogen fracturing devices and technologies mostly employ two methods: The first method involves heating the borehole with hot gas outside the borehole. During input, liquid nitrogen is introduced through one pipe, and hot gas is introduced through another. This method effectively meets the cooling and heating requirements of liquid nitrogen and hot gas, allowing hot gas to be quickly introduced into the borehole for rapid temperature rise and enhanced fracturing effect. However, this method requires additional gas heating equipment, increasing costs and construction difficulty and workload. The second method involves heating inside the borehole, typically using cables or similar equipment. While this method optimizes the cost of heating gas, the heating efficiency of cables is low, making it difficult to radiate heat to every part of the borehole, resulting in a significantly lower fracturing efficiency compared to the first method.

[0005] Therefore, there is an urgent need for a circulating airflow liquid nitrogen fracturing device and method to solve the above problems. Summary of the Invention

[0006] The purpose of this invention is to provide a circulating airflow liquid nitrogen fracturing device and method to solve the problems existing in the prior art. It can amplify the fracturing effect of liquid nitrogen by utilizing gas circulation and high / low temperature differences, making it applicable to boreholes with greater depths. Simultaneously, it reduces the number of equipment, lowers the operational complexity of the device, and improves its versatility. This device can heat the gas inside the borehole, optimizing the cost of heating the gas. Simultaneously, it utilizes a propeller to generate airflow, allowing the hot gas to flow rapidly within the borehole, achieving rapid temperature rise to enhance the fracturing effect. This device has low production costs and provides good fracturing effect and high fracturing efficiency.

[0007] To achieve the above objectives, the present invention provides the following solution: The present invention provides a circulating airflow liquid nitrogen fracturing device for improving the permeability of coal seams, comprising:

[0008] The impact section includes a heat insulation component disposed on the coal wall at the top of the borehole. The heat insulation component is permeated by an outer shell, the bottom surface of which is open. A heating element is disposed at the bottom of the inner cavity of the outer shell, and an impact element is disposed at the bottom of the heating element. A heating cavity is formed between the outer wall of the heating element and the inner wall of the outer shell. Several fourth valves are circumferentially disposed on the outer wall of the outer shell between the heating cavity and the heat insulation component. A third valve is disposed in the area between the fourth valves and the heat insulation component inside the outer shell.

[0009] The external connection includes a liquid nitrogen pump station and a gas extraction station. The liquid nitrogen pump station is connected to the outer shell through a nitrogen inlet pipeline, and the gas extraction station is connected to the outer shell through an extraction pipeline.

[0010] Preferably, the heat insulation component includes a plurality of heat insulation boards, which are spliced ​​together to form a heat insulation layer. Foam is injected into the joint between two adjacent heat insulation boards for sealing, and sealing tape is pasted at the joint.

[0011] Preferably, the heating element includes a heat insulation cylinder, an electric heating wire is wound around the outer wall of the heat insulation cylinder, the impact element is provided at the bottom of the inner cavity of the heat insulation cylinder, a sleeve is provided on the outer side of the heat insulation cylinder, the heating cavity is formed between the outer wall of the sleeve and the outer shell, and the sleeve and the top of the heat insulation cylinder are fixedly connected to the inner wall of the outer shell through a connector.

[0012] Preferably, the impact component includes a motor, which is fixedly connected to the bottom surface of the inner cavity of the heat insulation cylinder, and a propeller is fixedly connected to the output end of the motor through the bottom surface of the heat insulation cylinder and the sleeve in sequence.

[0013] Preferably, the nitrogen inlet pipeline includes a liquid nitrogen injection pipe, one end of which is connected to the liquid nitrogen pump station, and the other end of which is connected to the top of the outer shell. A first valve is provided on the liquid nitrogen injection pipe, and one end of the extraction pipeline is connected to the liquid nitrogen injection pipe between the first valve and the outer shell.

[0014] Preferably, the extraction pipeline includes a gas recovery pipe, one end of which is connected to the liquid nitrogen injection pipe, and the other end of which is connected to the gas extraction station. A second valve is provided on the gas recovery pipe, and safety valves are connected to both the gas recovery pipe and the liquid nitrogen injection pipe.

[0015] A circulating gas flow liquid nitrogen fracturing method includes the following steps:

[0016] S1. Drill holes at the top of the coal seam and install the impact section and external connection section;

[0017] S2. Open the first and third valves, close the second valve, safety valve and fourth valve, and start the motor to make the propeller rotate forward. After opening the liquid nitrogen pump station, inject liquid nitrogen into the borehole. After the amount of liquid nitrogen injected reaches the specified amount, close the liquid nitrogen pump station, the first valve and the third valve.

[0018] S3. Open the fourth valve and keep the propeller rotating in the forward direction.

[0019] S4. Power on the heating element and keep the motor running for 7-14 minutes, then power off the heating element and turn off the motor.

[0020] S5. Open the gas extraction station, the second valve and the third valve, close the fourth valve, and reverse the motor power supply to make the propeller reverse.

[0021] S6. If multiple liquid nitrogen fracturing operations are required, repeat S2-S5.

[0022] Preferably, the propeller radius is greater than the sleeve radius, and the propeller radius is less than the outer shell radius.

[0023] Preferably, the outer wall of the heat insulation cylinder is provided with raised threads for fixing the heating wire.

[0024] Preferably, the borehole radius is 1.5-2 times the propeller radius.

[0025] This invention discloses the following technical advantages: It optimizes the number of liquid nitrogen fracturing devices by utilizing propeller rotation to generate airflow, allowing a single device to perform multiple functions and reducing the cost associated with multiple devices. The airflow accelerates the vaporization of liquid nitrogen and achieves a hot-cold cycle impact between liquid nitrogen and high-temperature nitrogen gas, thereby efficiently enhancing the permeability of the coal seam. The device is ingeniously designed, has a simple structure, is mobile, and easy to install, enabling rapid deployment and use, which significantly improves the fracturing effect. Attached Figure Description

[0026] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0027] Figure 1 This is a schematic diagram of the structure of a circulating airflow liquid nitrogen fracturing device according to the present invention;

[0028] Figure 2 This is a schematic diagram of the liquid nitrogen injection stage in this invention;

[0029] Figure 3 This is a schematic diagram of the cyclic heating stage in this invention;

[0030] Among them, 1. Coal seam; 2. Outer shell; 3. Sleeve; 4. Heating wire; 5. Insulation cylinder; 6. Motor; 7. Propeller; 8. Safety valve; 9. First valve; 10. Second valve; 11. Third valve; 12. Fourth valve; 13. Liquid nitrogen pump station; 14. Gas extraction station; 15. Liquid nitrogen injection pipe; 16. Gas recovery pipe; 17. Insulation plate. Detailed Implementation

[0031] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0032] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0033] Reference Figures 1-3 This invention provides a circulating airflow liquid nitrogen fracturing device for enhancing the permeability of coal seam 1, comprising:

[0034] The impact section includes a heat insulation component, which is installed on the coal wall at the top of the borehole. The heat insulation component is provided through a shell 2, and the bottom surface of the shell 2 is set as an open end. A heating component is provided at the bottom of the inner cavity of the shell 2, and an impact component is provided at the bottom of the heating component. A heating cavity is formed between the outer wall of the heating component and the inner wall of the shell 2. Several fourth valves 12 are arranged circumferentially on the outer wall of the shell 2 located between the heating cavity and the heat insulation component. A third valve 11 is arranged in the area between the fourth valves 12 and the heat insulation component inside the shell 2.

[0035] The external part includes a liquid nitrogen pump station 13 and a gas extraction station 14. The liquid nitrogen pump station 13 is connected to the outer shell 2 through a nitrogen inlet pipeline, and the gas extraction station 14 is connected to the outer shell 2 through an extraction pipeline.

[0036] During liquid nitrogen injection, propeller 7 sprays the liquid nitrogen into the borehole, increasing the contact area between the liquid nitrogen and the coal face. During liquid nitrogen vaporization, propeller 7 rotates forward to blow out airflow, accelerating the vaporization. After complete vaporization, propeller 7 rotates forward while heating wire 4 heats the gas inside the borehole, generating a high-temperature airflow that rapidly raises the temperature of the vaporized liquid nitrogen, enhancing the fracturing effect. After fracturing is complete, propeller 7 reverses to draw in airflow, sending the nitrogen-gas mixture out of the borehole. This invention features good fracturing effect, simple operation, and the use of airflow circulation.

[0037] Further optimization of the solution: the heat insulation component includes several heat insulation boards 17, which are spliced ​​together to form a heat insulation layer. Foam is injected into the joint between two adjacent heat insulation boards 17 for sealing, and sealing tape is pasted at the joint.

[0038] A layer of insulation board 17 is covered on the coal wall. Different insulation boards 17 are sealed together by on-site injection foaming, and then sealing tape is evenly applied to the joint surface to ensure that the insulation board 17 is firmly bonded and has good sealing performance with the coal wall. The connection between the outer shell 2 and the insulation layer is sealed in the same way.

[0039] The design is further optimized. The heating element includes a heat insulation cylinder 5. The outer wall of the heat insulation cylinder 5 is wrapped with an electric heating wire 4. An impact element is provided at the bottom of the inner cavity of the heat insulation cylinder 5. A sleeve 3 is provided on the outer sleeve of the heat insulation cylinder 5. A heating cavity is formed between the outer wall of the sleeve 3 and the outer shell 2. The top of the sleeve 3 and the heat insulation cylinder 5 are fixedly connected to the inner wall of the outer shell 2 through a connector.

[0040] The heating wire 4 is wound around the outside of the heat insulation cylinder 5. The heat insulation cylinder 5 isolates the heat generated by the heating wire 4 after it is energized, and is used to protect the motor 6 inside. The motor 6 is located at the bottom of the heat insulation cylinder 5. The heat insulation cylinder 5 is wrapped inside the sleeve 3 and its length is slightly less than that of the sleeve 3. The shaft at the bottom of the motor 6 passes through the heat insulation cylinder 5 and the sleeve 3 and is connected to the propeller 7 at the bottom of the sleeve 3 to control the rotation of the propeller 7.

[0041] The surface of the heat insulation cylinder 5 is engraved with threads and has protrusions to fix the heating wire 4. The heating wire 4 itself should be made into a wavy shape to increase its heating area. The heating wire 4 is densely and evenly wound on the heat insulation cylinder 5 to increase its heating area.

[0042] The design is further optimized so that the impact component includes a motor 6, which is fixed to the bottom surface of the inner cavity of the heat insulation cylinder 5. The output end of the motor 6 passes through the bottom surface of the heat insulation cylinder 5 and the sleeve 3 and is fixed to a propeller 7.

[0043] The propeller 7, the outer casing 2, and the heating wire 4 all need to be made of low-temperature resistant materials, and the radius of the propeller 7 should be larger than the radius of the sleeve 3 and close to the radius of the outer casing 2, so as to ensure that it can withstand the impact of liquid nitrogen and generate a sufficient flow of air.

[0044] The scheme is further optimized. The nitrogen inlet pipeline includes a liquid nitrogen injection pipe 15. One end of the liquid nitrogen injection pipe 15 is connected to the liquid nitrogen pump station 13, and the other end of the liquid nitrogen injection pipe 15 is connected to the top of the outer shell 2. A first valve 9 is installed on the liquid nitrogen injection pipe 15. One end of the extraction pipeline is connected to the liquid nitrogen injection pipe 15 between the first valve 9 and the outer shell 2.

[0045] The scheme is further optimized. The extraction pipeline includes a gas recovery pipe 16. One end of the gas recovery pipe 16 is connected to the liquid nitrogen injection pipe 15, and the other end of the gas recovery pipe 16 is connected to the gas extraction station 14. A second valve 10 is installed on the gas recovery pipe 16, and safety valves 8 are connected to both the gas recovery pipe 16 and the liquid nitrogen injection pipe 15.

[0046] A circulating gas flow liquid nitrogen fracturing method includes the following steps:

[0047] S1. Drill holes at the top of coal seam 1 and install the impact section and external connection section;

[0048] In S1, the drilling depth and layout are designed according to the actual geological conditions on site. A layer of heat insulation board 17 is covered on the coal wall. Different heat insulation boards 17 are sealed together by on-site injection foaming. Then, sealing tape is used to evenly apply the joint surface to ensure that the heat insulation board 17 is firmly bonded and has good sealing performance with the coal wall.

[0049] When installing the impact unit, first place the motor 6 into the bottom of the heat insulation cylinder 5 and fix it. The shaft of the motor 6 passes through the small hole at the bottom of the heat insulation cylinder 5. Then, the heating wire 4 is evenly wound around the heat insulation cylinder 5. Then, the heat insulation cylinder 5 with the heating wire 4 wound around it is placed into the sleeve 3. The shaft of the motor 6, which extends from the small hole at the bottom of the heat insulation cylinder 5, passes through the small hole at the center of the bottom of the sleeve 3. Then, the propeller 7 is connected to the shaft. Then, the assembled device is placed into the outer shell 2. The upper end of the sleeve 3 is fixed near the lower part of the fourth valve 12. The third valve 11 is located above the fourth valve 12. Then, drill a hole in the heat insulation plate 17 with the same radius as the outer shell 2. The assembled motor 6, heat insulation cylinder 5, heating wire 4, sleeve 3, propeller 7, outer shell 2, fourth valve 12, and third valve 11 are all fed into the drilled hole through the hole in the heat insulation plate 17 to ensure a good seal between the outer shell 2 and the heat insulation plate 17.

[0050] When installing the external parts, take the liquid nitrogen injection pipe 15, connect one end to the liquid nitrogen pump station 13, and the other end to the upper inlet of the outer casing 2. Take the gas recovery pipe 16, connect one end to the gas extraction station 14, and the other end to the liquid nitrogen injection pipe 15. The control of the liquid nitrogen injection pipe 15 is completed by the safety valve 8 and the first valve 9, and the control of the gas recovery pipe 16 is completed by the safety valve 8 and the second valve 10.

[0051] S2. Open the first valve 9 and the third valve 11, close the second valve 10, the safety valve 8 and the fourth valve 12, and at the same time start the motor 6 to make the propeller 7 rotate forward. After opening the liquid nitrogen pump station 13, inject liquid nitrogen into the borehole. After the amount of liquid nitrogen injected reaches the specified amount, close the liquid nitrogen pump station 13, the first valve 9 and the third valve 11.

[0052] At this time, the heating wire 4 is not energized, the liquid nitrogen temperature is -210℃ to -180℃, the liquid nitrogen flows through the rotating propeller 7 and sprays onto the borehole. After filling with enough liquid nitrogen, the liquid nitrogen pump station 13 is shut down, and the first valve 9 and the third valve 11 are closed.

[0053] S3. Open the fourth valve 12 and keep propeller 7 rotating forward.

[0054] At this time, the heating wire 4 is not powered, and the propeller 7 drives the gas flow in the borehole, accelerating the vaporization speed of liquid nitrogen in the borehole. The propeller 7 runs for 3 to 6 minutes to ensure that the liquid nitrogen in the borehole is completely vaporized into nitrogen gas.

[0055] S4. The heating wire 4 is powered on and the motor 6 is kept running for 7-14 minutes. The heating wire 4 is powered off and the motor 6 is turned off.

[0056] When the heating wire 4 is energized, it begins to heat up. The propeller 7 drives the gas flow inside the borehole. The gas outside the outer casing 2 flows into the device through the fourth valve 12. The gas is heated as it flows through the pipe next to the heating wire 4. After flowing out of the heating chamber, it flows into the depth of the borehole.

[0057] S5. Open the gas extraction station 14, the second valve 10 and the third valve 11, close the fourth valve 12, and reverse the power supply of the motor 6 to make the propeller 7 reverse.

[0058] When propeller 7 reverses, the gas flow direction changes to enter the device from propeller 7, flow through gas recovery pipe 16, and gas extraction station 14 collects the mixed gas of coal seam gas and nitrogen.

[0059] S6. If multiple liquid nitrogen fracturing operations are required, repeat S2-S5 until the desired fracturing effect is achieved.

[0060] Further optimization of the design: the radius of propeller 7 is greater than the radius of sleeve 3, and the radius of propeller 7 is less than the radius of outer shell 2.

[0061] Sleeve 3 and heat insulation cylinder 5 need to be made of materials that are resistant to both low and high temperatures. Sleeve 3 and heat insulation cylinder 5 need to be in contact with low-temperature liquid nitrogen and close to high-temperature heating wire 4, so they need to meet both low-temperature resistance and high-temperature resistance conditions at the same time.

[0062] To further optimize the design, the outer wall of the heat insulation cylinder 5 is provided with raised threads for fixing the heating wire 4.

[0063] The design was further optimized so that the drilling radius was 1.5-2 times the radius of propeller 7.

[0064] The radius of the borehole should be 1.5-2 times the radius of the propeller 7 to ensure that the distance between the inner shell 2 and the sleeve 3 is equivalent to the distance between the inner shell 2 and the coal wall, thereby ensuring that the gas can form a continuous airflow.

[0065] The minimum drilling depth should be 1.5 times the height of the outer casing 2 to ensure sufficient space for airflow circulation; the maximum drilling depth depends on the rotational speed of the propeller 7. For a higher power motor 6, the propeller 7 can generate a larger airflow that radiates to deeper places, thus allowing for deeper drilling.

[0066] Motor 6 needs to be a three-phase asynchronous motor because the propeller 7 it controls needs to frequently switch between forward and reverse rotation. Therefore, a forward / reverse switch is installed on motor 6 to control the rotation direction of propeller 7, thereby controlling the flow direction of airflow in the borehole.

[0067] In the description of this invention, it should be understood that the terms "longitudinal", "lateral", "up", "down", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, and are only for the convenience of describing this invention, and are not intended to indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this invention.

[0068] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A circulating airflow liquid nitrogen fracturing device for enhancing the permeability of coal seams (1), characterized in that, include: The impact section includes a heat insulation component, which is disposed on the coal wall at the top of the borehole. The heat insulation component is provided with a shell (2) through it. The bottom surface of the shell (2) is set as an open end. A heating component is provided at the bottom of the inner cavity of the shell (2). An impact component is provided at the bottom of the heating component. A heating cavity is formed between the outer wall of the heating component and the inner wall of the shell (2). Several fourth valves (12) are provided circumferentially on the outer wall of the shell (2) between the heating cavity and the heat insulation component. A third valve (11) is provided in the area between the fourth valves (12) and the heat insulation component inside the shell (2). The external connection includes a liquid nitrogen pump station (13) and a gas extraction station (14). The liquid nitrogen pump station (13) is connected to the outer shell (2) through a nitrogen inlet pipeline, and the gas extraction station (14) is connected to the outer shell (2) through an extraction pipeline. The heating element includes a heat insulation cylinder (5), the outer wall of which is wound with an electric heating wire (4), the bottom of the inner cavity of the heat insulation cylinder (5) is provided with the impact element, the heat insulation cylinder (5) is covered with a sleeve (3), the outer wall of the sleeve (3) and the outer shell (2) form the heating cavity, and the top of the sleeve (3) and the heat insulation cylinder (5) are fixedly connected to the inner wall of the outer shell (2) through a connector; The impact component includes a motor (6), which is fixed to the bottom surface of the inner cavity of the heat insulation cylinder (5). The output end of the motor (6) passes through the bottom surface of the heat insulation cylinder (5) and the sleeve (3) and is fixed to a propeller (7). The nitrogen inlet pipeline includes a liquid nitrogen injection pipe (15), one end of which is connected to the liquid nitrogen pump station (13), and the other end of which is connected to the top of the outer shell (2). A first valve (9) is provided on the liquid nitrogen injection pipe (15), and one end of the extraction pipeline is connected to the liquid nitrogen injection pipe (15) between the first valve (9) and the outer shell (2). The extraction pipeline includes a gas recovery pipe (16), one end of which is connected to the liquid nitrogen injection pipe (15), and the other end of which is connected to the gas extraction station (14). A second valve (10) is provided on the gas recovery pipe (16), and safety valves (8) are provided on both the gas recovery pipe (16) and the liquid nitrogen injection pipe (15).

2. The circulating gas flow type liquid nitrogen fracturing device according to claim 1, characterized in that: The heat insulation component includes several heat insulation boards (17), which are spliced ​​together to form a heat insulation layer. Foam is injected into the joint between two adjacent heat insulation boards (17) for sealing, and sealing tape is pasted at the joint.

3. A circulating gas flow liquid nitrogen fracturing method, wherein the circulating gas flow liquid nitrogen fracturing device according to claim 1 is characterized in that, Includes the following steps: S1. Drill holes at the top of the coal seam (1) and install the impact part and the external connection part; S2. Open the first valve (9) and the third valve (11), close the second valve (10), the safety valve (8) and the fourth valve (12), and start the motor (6) to make the propeller (7) rotate forward. After opening the liquid nitrogen pump station (13), inject liquid nitrogen into the borehole. After completing the liquid nitrogen injection, close the liquid nitrogen pump station (13), the first valve (9) and the third valve (11). S3. Open the fourth valve (12) and keep the propeller (7) rotating forward. S4. The heating wire (4) is energized and the motor (6) continues to run. The heating wire (4) is de-energized and the motor (6) is turned off. S5. Open the gas extraction station (14), the second valve (10) and the third valve (11), close the fourth valve (12), and reverse the power supply of the motor (6) to make the propeller (7) reverse. S6. If multiple liquid nitrogen fracturing operations are required, repeat S2-S5.

4. The circulating gas flow liquid nitrogen fracturing method according to claim 3, characterized in that: The radius of the propeller (7) is greater than the radius of the sleeve (3), and the radius of the propeller (7) is smaller than the radius of the outer shell (2).

5. The circulating gas flow liquid nitrogen fracturing method according to claim 4, characterized in that: The outer wall of the heat insulation cylinder (5) is provided with raised threads for fixing the heating wire (4).

6. The circulating gas flow liquid nitrogen fracturing method according to claim 5, characterized in that: The radius of the borehole is 1.5-2 times the radius of the propeller (7).

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