Multi-stage self-triggering type liquid carbon dioxide grading fracturing method

Abstract

The invention discloses a multi-stage self-triggering type liquid carbon dioxide grading fracturing method. The method comprises the following steps that early-stage preparation work is conducted on a multi-stage self-triggering type fracturing pipe; liquid carbon dioxide is filled into the multi-stage self-triggering type fracturing pipe; the multi-stage self-triggering type fracturing pipe is placed in a drill hole in the target fracturing position and then connected with an exploder; the exploder is started, and the multi-stage self-triggering type fracturing pipes conduct multi-stage fracturing in sequence; and after fracturing is completed, the multi-stage self-triggering type fracturing pipe is recycled for reuse. The grading self-triggering fracturing device is constructed on the basis of the gas transfer rule and the memory alloy material, the liquid carbon dioxide single-point excitation and multi-stage self-triggering grading fracturing method is provided, and the problems that power of a liquid carbon dioxide fracturing single pipe is insufficient, and wiring of multiple pipes is complex are solved.

Description

Technical Field

[0001] This invention belongs to the technical fields of mining, geotechnical engineering and oil and gas well production enhancement, specifically involving a multi-stage self-triggered liquid carbon dioxide graded fracturing method. Background Technology

[0002] Liquid carbon dioxide fracturing technology, as a novel rock-breaking technique, is widely used in high-gas mines, sensitive urban areas, and oil and gas well production enhancement applications where safety requirements are stringent. This is because it does not produce open flames, high temperatures, or toxic gases during the fracturing process, and its vibration effect is relatively small. The basic principle of this technology is to fill the fracturing tube with liquid carbon dioxide, heat it through an excitation tube, causing it to vaporize and expand rapidly. The pressure inside the storage tube also increases accordingly. When the pressure exceeds the strength of the pressure relief plate, the plate ruptures, and high-pressure carbon dioxide gas is ejected from the relief port. The coal and rock mass surrounding the borehole is fractured by the impact of the carbon dioxide. From the perspective of fracturing mechanism, the fracturing process of liquid carbon dioxide is similar to that of explosive blasting, and can be divided into two stages: The first stage is the dynamic loading of the blasting stress wave, that is, the stress wave generated by the instantaneous release of high-pressure gas impacts the rock mass around the blast hole. When its peak pressure exceeds the dynamic compressive strength of the coal and rock mass, a fracture zone is formed and initial cracks are generated; The second stage is the quasi-static loading of the blasting gas, that is, the gas continues to act on the already generated cracks, causing the cracks to further expand and extend, forming a fracture zone.

[0003] However, compared with explosive blasting, liquid carbon dioxide fracturing technology is less effective in practical applications, especially in fracturing hard rock masses, mainly in the following two aspects: (1) Insufficient fracturing power of single-tube: Explosive blasting can generate extremely high peak pressure within microseconds, thus forming a large initial fracture zone. However, the fracturing process of liquid carbon dioxide is relatively mild, and the peak pressure generated is much lower than that of explosive blasting. That is, the dynamic energy provided by the dynamic loading stage of blasting stress wave is smaller, and the impact force on the rock mass is insufficient. Therefore, the area of ​​the fracture zone formed around the blast hole is much smaller than that of explosive blasting. Due to the small fracture zone and initial fracture range formed by liquid carbon dioxide fracturing, the subsequent gas propagation effect will be greatly reduced, ultimately resulting in poor overall fracturing effect.

[0004] (2) Complex wiring for multi-tube coordinated operation: In order to make up for the insufficient power of a single liquid carbon dioxide fracturing tube, multiple fracturing tubes were connected in series or in parallel in the project. When multiple tubes are used in coordinated operation, each fracturing tube needs to be filled and wired separately, which makes the construction process complicated, the fracturing efficiency low, and the fracturing tubes often interfere with each other, which seriously restricts the large-scale application of liquid carbon dioxide fracturing technology.

[0005] Therefore, those skilled in the art urgently need a liquid carbon dioxide fracturing technology solution that has good fracturing effect and is easy to operate, to overcome the core defects of the existing technology, fundamentally improve the fracturing effect of liquid carbon dioxide, and promote the large-scale industrial application of liquid carbon dioxide fracturing technology. Summary of the Invention

[0006] This invention overcomes the shortcomings of the prior art and provides a multi-stage self-triggered liquid carbon dioxide graded fracturing method, which realizes single-point excitation and multi-stage self-triggering of liquid carbon dioxide. The equipment has a reasonable structure, is easy to operate, and has adjustable power, which can solve the problems of insufficient power of single tube fracturing and complex wiring of multiple tubes.

[0007] To solve the above-mentioned technical problems, the present invention adopts the following technical solution: a multi-stage self-triggered liquid carbon dioxide graded fracturing method, comprising the following steps: S1. Preliminary preparations for multi-stage self-triggered fracturing tubes; S2. Fill the multi-stage self-triggered fracturing tube with liquid carbon dioxide; S3. Place the multi-stage self-triggered fracturing tube into the borehole at the target fracturing location, and then connect the detonator. S4. Start the detonator, and the multi-stage self-triggered fracturing tube will sequentially perform multi-stage fracturing. S5. After fracturing is completed, the multi-stage self-triggered fracturing tubes are recycled for reuse.

[0008] The multi-stage self-triggered fracturing tube includes a vertically arranged reservoir tube with an upper cap and a lower cap at its upper and lower ends, respectively. The upper cap has a handle, an injection valve, an injection head, an activation tube, and a wiring hole. The lower end of the injection head communicates with the inside of the reservoir tube, and the upper end of the injection head is connected to the lower end of the injection valve. The wiring hole is located in the center of the upper cap, and the wiring end of the activation tube is located in the wiring hole. The lower charging end of the activation tube extends into the reservoir tube. The reservoir tube has multiple compartments arranged sequentially from top to bottom, and the reservoir tube wall has multiple energy release structures that communicate with each compartment. A sealing ring is provided between the top surface of the lower cap and the lower end of the reservoir tube.

[0009] The liquid storage tube has three chambers from top to bottom. The wall of the liquid storage tube has two arc-shaped slot holes spaced at intervals. An upper isolation valve and a lower isolation valve are respectively inserted into the upper and lower arc-shaped slot holes and inserted into the liquid storage tube. The upper isolation valve and the lower isolation valve divide the internal space of the liquid storage tube into the three chambers, namely, chamber I, chamber II and chamber III from top to bottom.

[0010] Both the upper and lower isolation valves include a plug-in sealing frame that mates with an arc-shaped slot hole. The plug-in sealing frame has a circular shape memory alloy isolation plate with a diameter equal to the inner diameter of the liquid storage tube. The shape memory alloy isolation plate is made of nickel-titanium alloy material with shape memory effect. The phase change triggering temperature of the shape memory alloy isolation plate is higher than the liquid carbon dioxide storage temperature (approximately -20°C to -30°C) and lower than the high temperature generated when the excitation tube is activated (typically >500°C). The triggering temperature of the lower shape memory alloy isolation plate is lower than that of the upper shape memory alloy isolation plate to improve the triggering sensitivity of the lower shape memory alloy isolation plate.

[0011] Each energy release structure includes a vortex acceleration energy release hole opened on the wall of the liquid storage pipe, and a constant pressure energy release plate is provided inside the vortex acceleration energy release hole to close the vortex acceleration energy release hole. The centerline of the vortex acceleration energy release hole is set along the radial direction of the liquid storage pipe. The vortex acceleration energy release hole is a conical structure with a smaller inner diameter and a larger outer diameter. The conical surface inside the vortex acceleration energy release hole is provided with a guide groove based on the Archimedean spiral structure. The guide groove is a variable pitch design, with the pitch gradually increasing from the inside to the outside.

[0012] The specific process of step S1 is as follows: open the liquid injection valve on the upper plug, connect the external liquid carbon dioxide filling device to the injection head, pull out the upper isolation valve and the lower isolation valve, and use a sealing film to stick to the outer wall of the liquid storage tube to seal the arc-shaped slot hole, so that the liquid storage tube is in a sealed state.

[0013] The specific process of step S2 is as follows: Start the external liquid carbon dioxide filling device, and fill the storage tube with liquid carbon dioxide through the injection head. When the liquid carbon dioxide level rises to the lower edge of the lower arc-shaped slot hole, puncture the lower sealing membrane of the lower isolation valve and insert it into the lower arc-shaped slot hole. At this time, chamber III is filled with liquid carbon dioxide. Continue to fill with liquid carbon dioxide. When the liquid carbon dioxide level rises to the lower edge of the upper arc-shaped slot hole, puncture the upper sealing membrane of the upper isolation valve and insert it into the upper arc-shaped slot hole. At this time, chamber II is filled with liquid carbon dioxide. Continue to fill chamber I with liquid carbon dioxide. After chamber I is filled, close the external liquid carbon dioxide filling device and the injection valve.

[0014] The specific process of step S3 is as follows: Hold the handle and put the filled multi-stage self-triggered fracturing tube into the borehole at the target fracturing position. Then, lead out the detonation wire of the trigger tube through the wiring hole and connect it to the external detonator.

[0015] Step S4 specifically includes the following sub-steps: 1) Primary fracturing: The detonator is activated, and the excitation tube is detonated in chamber I. The heat generated causes the liquid carbon dioxide in chamber I to rapidly vaporize, and the internal pressure of chamber I rises sharply. When the internal pressure of chamber I reaches the rupture pressure of the constant pressure relief plate, the constant pressure relief plate ruptures, and the high-pressure carbon dioxide gas is ejected outward through the vortex acceleration relief hole connected to chamber I. The high-speed swirling shock wave formed by the vortex acceleration relief hole acts on the coal and rock mass to achieve primary fracturing. 2) Secondary self-triggered fracturing: The heat generated by the liquid carbon dioxide inside chamber I acts on the upper isolation valve, causing it to reach the phase change temperature. After the valve reaches the phase change temperature, the shape memory alloy isolation plate opens, and high-pressure gas rushes into chamber II, causing the pressure inside chamber II to rise sharply. This causes the constant pressure venting plate in chamber II to rupture. After the liquid carbon dioxide in chamber II loses its confining pressure, it rapidly undergoes a phase change and expands. The high-pressure carbon dioxide gas is ejected outward through the vortex acceleration venting hole connected to chamber II. The high-speed swirling shock wave formed by the vortex acceleration venting hole acts on the coal and rock mass to achieve secondary fracturing. 3) Three-stage self-triggered fracturing: The heat generated by the liquid carbon dioxide inside chamber I acts on the lower isolation valve, causing it to reach the phase change temperature. After the valve reaches the phase change temperature, the shape memory alloy isolation plate opens, and high-pressure gas rushes into chamber III, causing the internal pressure of chamber III to rise sharply. This causes the constant pressure venting plate in chamber III to rupture. After the liquid carbon dioxide in chamber III loses its constrained pressure, it rapidly undergoes a phase change and expands. The high-pressure carbon dioxide gas is ejected outward through the vortex acceleration venting hole connected to chamber III. The high-speed swirling shock wave formed by the vortex acceleration venting hole acts on the coal and rock mass to achieve three-stage fracturing.

[0016] The specific process of step S5 is as follows: the multi-stage self-triggered fracturing tube is recycled and reused. After replacing the constant pressure relief plate, isolation valve and excitation tube, the next fracturing can be carried out.

[0017] Using the above technical solution, the upper plug integrates a handle (for easy insertion of the entire liquid storage tube into the borehole), a liquid injection valve, a liquid injection head, an activation tube, and a wiring hole. By opening the liquid injection valve and connecting to an external filling device, liquid carbon dioxide can be filled into the liquid storage tube. The liquid storage tube is divided into three chambers in series: chamber I, chamber II, and chamber III (the number of chambers can be adjusted according to engineering requirements) by two isolation valves. The activation tube acts on chamber I to achieve the first fracturing. The high temperature generated triggers the opening of the upper isolation valve, allowing high-pressure gas to rush into chamber II, causing its pressure to rise sharply to achieve the second fracturing. Subsequent chambers are triggered in sequence according to this principle, ultimately forming multiple superimposed shock waves. Each chamber of the liquid storage tube has multiple vortex acceleration energy release holes arranged circumferentially on its sidewall. The inner side of the energy release hole has a guide groove designed based on the Archimedes spiral principle, which can convert the flowing gas from a straight flow to a high-speed vortex, significantly enhancing the shearing and crushing effect on the coal and rock mass. The rear plug is sealed to the end of the liquid storage tube through a sealing ring.

[0018] This invention has outstanding substantive features and significant progress compared to the prior art, specifically: 1) Significantly enhanced single-tube fracturing effect: Multi-stage continuous fracturing is achieved through a single tube, allowing energy to be released and superimposed in stages. At the same time, through the unique vortex acceleration energy release hole design, the straight gas impact is transformed into a high-speed rotating jet, which significantly enhances the crushing efficiency of coal and rock mass. The fracturing effect is far greater than that of traditional liquid carbon dioxide fracturing tubes.

[0019] 2) Simple and reliable operation: When filling with liquid carbon dioxide, the two isolation valves are punctured and inserted into the storage tube one after another, so that the cavity inside the storage tube forms a three-stage fracturing chamber with successive explosions. Only one activation tube needs to be wired. Subsequent fracturing is completed by high-pressure gas transmission and self-triggering by shape memory alloy isolation plates, avoiding the complicated wiring problems of multi-tube layout.

[0020] 3) Economical, environmentally friendly and reusable: The main body of the multi-stage self-triggered liquid carbon dioxide fracturing tube is recyclable and can be reused after replacing worn parts, which reduces long-term operating costs and is in line with the concept of green mining. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the three-dimensional structure of a multi-stage self-triggered fracturing tube.

[0022] Figure 2 This is a front view of a multi-stage self-triggered fracturing tube.

[0023] Figure 3 This is a right-side plan view of a multi-stage self-triggered fracturing tube.

[0024] Figure 4 for Figure 3 Top view.

[0025] Figure 5 This is a schematic diagram illustrating the gas release principle of the vortex acceleration energy release port.

[0026] Figure 6 This is a plan view of an isolation valve.

[0027] Reference numerals: 1-Handle; 2-Upper cap; 3-Injection valve; 4-Injection head; 5-Excitation tube; 6-Wiring hole; 7-Storage tube; 8-Upper isolation valve; 9-Lower isolation valve; 10-Vortex acceleration energy release hole; 11-Constant pressure energy release plate; 12-Sealing ring; 13-Lower cap; 14-Guide groove; 15-High-speed vortex shock wave; 16-Sealing membrane; 17-Plug-in sealing frame; 18-Shape memory alloy isolation plate. Detailed Implementation

[0028] like Figures 1-6 As shown, the multi-stage self-triggered liquid carbon dioxide staged fracturing method of the present invention includes the following steps: S1. Preliminary preparations for multi-stage self-triggered fracturing tubes; S2. Fill the multi-stage self-triggered fracturing tube with liquid carbon dioxide; S3. Place the multi-stage self-triggered fracturing tube into the borehole at the target fracturing location, and then connect the detonator. S4. Start the detonator, and the multi-stage self-triggered fracturing tube will sequentially perform multi-stage fracturing. S5. After fracturing is completed, the multi-stage self-triggered fracturing tubes are recycled for reuse.

[0029] The multi-stage self-triggered fracturing tube includes a vertically arranged liquid storage tube 7. The upper and lower ends of the liquid storage tube 7 are respectively provided with an upper cap 2 and a lower cap 13. The upper cap 2 is provided with a handle 1, an injection valve 3, an injection head 4, an activation tube 5, and a wiring hole 6. The lower end of the injection head 4 is connected to the inside of the liquid storage tube 7, and the upper end of the injection head 4 is connected to the lower end of the injection valve. The wiring hole 6 is opened in the center of the upper cap 2. The wiring end of the activation tube 5 is located in the wiring hole 6. The lower charging end of the activation tube 5 extends into the liquid storage tube 7. The liquid storage tube 7 is provided with multiple compartments arranged sequentially from top to bottom. The wall of the liquid storage tube 7 is provided with multiple energy release structures that are connected to each compartment. A sealing ring 12 is provided between the top surface of the lower cap 13 and the lower end of the liquid storage tube 7.

[0030] The liquid storage tube 7 has three chambers from top to bottom. The wall of the liquid storage tube 7 has two arc-shaped slot holes spaced at intervals. An upper isolation valve 8 and a lower isolation valve 9 are respectively inserted into the upper and lower arc-shaped slot holes and inserted into the liquid storage tube 7. The upper isolation valve 8 and the lower isolation valve 9 divide the internal space of the liquid storage tube 7 into the three chambers, namely, chamber I, chamber II and chamber III from top to bottom.

[0031] Both the upper isolation valve 8 and the lower isolation valve 9 include a plug-in sealing frame 17 that mates with the arc-shaped slot hole. The plug-in sealing frame 17 is provided with a circular shape memory alloy isolation plate 18. The diameter of the shape memory alloy isolation plate 18 is equal to the inner diameter of the liquid storage tube 7. The shape memory alloy isolation plate 18 is made of nickel-titanium alloy material with shape memory effect. The phase change triggering temperature of the shape memory alloy isolation plate 18 is higher than the liquid carbon dioxide storage temperature (about -20°C to -30°C) and lower than the high temperature generated when the excitation tube 5 is in operation (usually >500°C). The triggering temperature of the lower shape memory alloy isolation plate 18 is lower than that of the upper shape memory alloy isolation plate 18 to improve the triggering sensitivity of the lower shape memory alloy isolation plate 18.

[0032] Each energy release structure includes a vortex acceleration energy release hole 10 opened on the wall of the liquid storage pipe 7, and a constant pressure energy release plate 11 is provided inside the vortex acceleration energy release hole 10 to close the vortex acceleration energy release hole 10. The centerline of the vortex acceleration energy release hole 10 is set along the radial direction of the liquid storage pipe 7. The vortex acceleration energy release hole 10 is a conical structure with a smaller inner diameter and a larger outer diameter. The conical surface inside the vortex acceleration energy release hole 10 is provided with a guide groove 14 based on the Archimedes spiral structure. The guide groove 14 is a variable pitch design, and the pitch size gradually increases from the inside to the outside.

[0033] The specific process of step S1 is as follows: open the liquid injection valve 3 on the upper plug 2, connect the liquid carbon dioxide external filling device to the liquid injection head 4, pull out the upper isolation valve 8 and the lower isolation valve 9, and use a sealing film to stick to the outer wall of the liquid storage tube 7 to seal the arc-shaped slot hole, so that the liquid storage tube 7 is in a sealed state.

[0034] The specific process of step S2 is as follows: Start the external liquid carbon dioxide filling equipment, and fill the storage tube 7 with liquid carbon dioxide through the injection head 4. When the liquid carbon dioxide level rises to the lower edge of the lower arc-shaped slot hole, pierce the lower sealing membrane of the lower isolation valve 9 and insert it into the lower arc-shaped slot hole. At this time, chamber III is filled with liquid carbon dioxide. Continue to fill with liquid carbon dioxide. When the liquid carbon dioxide level rises to the lower edge of the upper arc-shaped slot hole, pierce the upper sealing membrane of the upper isolation valve 8 and insert it into the upper arc-shaped slot hole. At this time, chamber II is filled with liquid carbon dioxide. Continue to fill chamber I with liquid carbon dioxide. After chamber I is filled, close the external liquid carbon dioxide filling equipment and the injection valve 3.

[0035] The specific process of step S3 is as follows: Hold the handle 1 and put the filled multi-stage self-trigger fracturing tube into the borehole at the target fracturing position. Then, lead out the detonation wire of the trigger tube 5 through the wiring hole 6 and connect it to the external detonator.

[0036] Step S4 specifically includes the following sub-steps: 1) Primary fracturing: The detonator is activated, and the excitation tube 5 is detonated in chamber I. The heat generated causes the liquid carbon dioxide in chamber I to rapidly vaporize, and the internal pressure of chamber I rises sharply. When the internal pressure of chamber I reaches the rupture pressure of the constant pressure relief plate 11, the constant pressure relief plate 11 ruptures, and the high-pressure carbon dioxide gas is ejected outward through the vortex acceleration relief hole 10 connected to chamber I. The high-speed swirling shock wave 15 formed by the vortex acceleration relief hole 10 acts on the coal and rock mass to achieve primary fracturing. 2) Secondary self-triggered fracturing: The heat generated by the liquid carbon dioxide inside chamber I acts on the upper isolation valve 8, causing it to reach the phase change temperature. After the valve reaches the phase change temperature, the shape memory alloy isolation plate 18 opens, and high-pressure gas rushes into chamber II, causing the pressure inside chamber II to rise sharply. This causes the constant pressure venting plate 11 in chamber II to rupture. After the liquid carbon dioxide in chamber II loses its constrained pressure, it rapidly undergoes a phase change and expands. The high-pressure carbon dioxide gas is ejected outward through the vortex acceleration venting hole 10 connected to chamber II. The high-speed swirling shock wave 15 formed by the vortex acceleration venting hole 10 acts on the coal and rock mass to achieve secondary fracturing. 3) Three-stage self-triggered fracturing: The heat generated by the liquid carbon dioxide inside chamber I acts on the lower isolation valve 9, causing it to reach the phase change temperature. After the valve reaches the phase change temperature, the shape memory alloy isolation plate 18 opens, and high-pressure gas rushes into chamber III, causing the internal pressure of chamber III to rise sharply. This causes the constant pressure venting plate 11 in chamber III to rupture. After the liquid carbon dioxide in chamber III loses its constrained pressure, it rapidly undergoes a phase change and expands. The high-pressure carbon dioxide gas is ejected outward through the vortex acceleration venting hole 10 connected to chamber III. The high-speed swirling shock wave 15 formed by the vortex acceleration venting hole 10 acts on the coal and rock mass to achieve three-stage fracturing.

[0037] The specific process of step S5 is as follows: the multi-stage self-triggered fracturing tube is recycled and reused. After replacing the constant pressure relief plate 11, the isolation valve and the excitation tube 5, the next fracturing can be carried out.

[0038] The above embodiments illustrate the basic principles and features of the present invention, but are merely preferred embodiments and are not limited to these embodiments. Those skilled in the art, inspired by this patent, can make many modifications and improvements without departing from the spirit and scope of the claims, all of which fall within the scope of protection of the present invention. Therefore, the scope of this patent and its protection should be determined by the appended claims.

Claims

1. A multi-stage self-triggered liquid carbon dioxide staged fracturing method, characterized in that: Includes the following steps: S1. Preliminary preparations for multi-stage self-triggered fracturing tubes; S2. Fill the multi-stage self-triggered fracturing tube with liquid carbon dioxide; S3. Place the multi-stage self-triggered fracturing tube into the borehole at the target fracturing location, and then connect the detonator. S4. Start the detonator, and the multi-stage self-triggered fracturing tube will sequentially perform multi-stage fracturing. S5. After fracturing is completed, the multi-stage self-triggered fracturing tubes are recycled for reuse.

2. The multi-stage self-triggered liquid carbon dioxide staged fracturing method according to claim 1, characterized in that: The multi-stage self-triggered fracturing tube includes a vertically arranged reservoir tube with an upper cap and a lower cap at its upper and lower ends, respectively. The upper cap has a handle, an injection valve, an injection head, an activation tube, and a wiring hole. The lower end of the injection head communicates with the inside of the reservoir tube, and the upper end of the injection head is connected to the lower end of the injection valve. The wiring hole is located in the center of the upper cap, and the wiring end of the activation tube is located in the wiring hole. The lower charging end of the activation tube extends into the reservoir tube. The reservoir tube has multiple compartments arranged sequentially from top to bottom, and the reservoir tube wall has multiple energy release structures that communicate with each compartment. A sealing ring is provided between the top surface of the lower cap and the lower end of the reservoir tube.

3. The multi-stage self-triggered liquid carbon dioxide staged fracturing method according to claim 2, characterized in that: The liquid storage tube has three chambers from top to bottom. The wall of the liquid storage tube has two arc-shaped slot holes spaced at intervals. An upper isolation valve and a lower isolation valve are respectively inserted into the upper and lower arc-shaped slot holes and inserted into the liquid storage tube. The upper isolation valve and the lower isolation valve divide the internal space of the liquid storage tube into the three chambers, namely, chamber I, chamber II and chamber III from top to bottom.

4. The multi-stage self-triggered liquid carbon dioxide staged fracturing method according to claim 3, characterized in that: Both the upper and lower isolation valves include a plug-in sealing frame that mates with an arc-shaped slot hole. The plug-in sealing frame is equipped with a circular shape memory alloy isolation plate. The diameter of the shape memory alloy isolation plate is equal to the inner diameter of the liquid storage tube. The shape memory alloy isolation plate is made of nickel-titanium alloy material with shape memory effect. The phase change triggering temperature of the shape memory alloy isolation plate is higher than the liquid carbon dioxide storage temperature but lower than the high temperature generated when the excitation tube is activated. The triggering temperature of the lower shape memory alloy isolation plate is lower than that of the upper shape memory alloy isolation plate to improve the triggering sensitivity of the lower shape memory alloy isolation plate.

5. The multi-stage self-triggered liquid carbon dioxide staged fracturing method according to claim 4, characterized in that: Each energy release structure includes a vortex acceleration energy release hole opened on the wall of the liquid storage pipe, and a constant pressure energy release plate is provided inside the vortex acceleration energy release hole to close the vortex acceleration energy release hole. The centerline of the vortex acceleration energy release hole is set along the radial direction of the liquid storage pipe. The vortex acceleration energy release hole is a conical structure with a smaller inner diameter and a larger outer diameter. The conical surface inside the vortex acceleration energy release hole is provided with a guide groove based on the Archimedean spiral structure. The guide groove is a variable pitch design, with the pitch gradually increasing from the inside to the outside.

6. The multi-stage self-triggered liquid carbon dioxide staged fracturing method according to claim 5, characterized in that: The specific process of step S1 is as follows: open the liquid injection valve on the upper plug, connect the external liquid carbon dioxide filling device to the injection head, pull out the upper isolation valve and the lower isolation valve, and use a sealing film to stick to the outer wall of the liquid storage tube to seal the arc-shaped slot hole, so that the liquid storage tube is in a sealed state.

7. The multi-stage self-triggered liquid carbon dioxide staged fracturing method according to claim 6, characterized in that: The specific process of step S2 is as follows: Start the external liquid carbon dioxide filling device, and fill the storage tube with liquid carbon dioxide through the injection head. When the liquid carbon dioxide level rises to the lower edge of the lower arc-shaped slot hole, puncture the lower sealing membrane of the lower isolation valve and insert it into the lower arc-shaped slot hole. At this time, chamber III is filled with liquid carbon dioxide. Continue to fill with liquid carbon dioxide. When the liquid carbon dioxide level rises to the lower edge of the upper arc-shaped slot hole, puncture the upper sealing membrane of the upper isolation valve and insert it into the upper arc-shaped slot hole. At this time, chamber II is filled with liquid carbon dioxide. Continue to fill chamber I with liquid carbon dioxide. After chamber I is filled, close the external liquid carbon dioxide filling device and the injection valve.

8. The multi-stage self-triggered liquid carbon dioxide staged fracturing method according to claim 7, characterized in that: The specific process of step S3 is as follows: Hold the handle and put the filled multi-stage self-triggered fracturing tube into the borehole at the target fracturing position. Then, lead out the detonation wire of the trigger tube through the wiring hole and connect it to the external detonator.

9. The multi-stage self-triggered liquid carbon dioxide staged fracturing method according to claim 8, characterized in that: Step S4 specifically includes the following sub-steps: 1) Primary fracturing: The detonator is activated, and the excitation tube is detonated in chamber I. The heat generated causes the liquid carbon dioxide in chamber I to rapidly vaporize, and the internal pressure of chamber I rises sharply. When the internal pressure of chamber I reaches the rupture pressure of the constant pressure relief plate, the constant pressure relief plate ruptures, and the high-pressure carbon dioxide gas is ejected outward through the vortex acceleration relief hole connected to chamber I. The high-speed swirling shock wave formed by the vortex acceleration relief hole acts on the coal and rock mass to achieve primary fracturing. 2) Secondary self-triggered fracturing: The heat generated by the liquid carbon dioxide inside chamber I acts on the upper isolation valve, causing it to reach the phase change temperature. After the valve reaches the phase change temperature, the shape memory alloy isolation plate opens, and high-pressure gas rushes into chamber II, causing the pressure inside chamber II to rise sharply. This causes the constant pressure venting plate in chamber II to rupture. After the liquid carbon dioxide in chamber II loses its confining pressure, it rapidly undergoes a phase change and expands. The high-pressure carbon dioxide gas is ejected outward through the vortex acceleration venting hole connected to chamber II. The high-speed swirling shock wave formed by the vortex acceleration venting hole acts on the coal and rock mass to achieve secondary fracturing. 3) Three-stage self-triggered fracturing: The heat generated by the liquid carbon dioxide inside chamber I acts on the lower isolation valve, causing it to reach the phase change temperature. After the valve reaches the phase change temperature, the shape memory alloy isolation plate opens, and high-pressure gas rushes into chamber III, causing the internal pressure of chamber III to rise sharply. This causes the constant pressure venting plate in chamber III to rupture. After the liquid carbon dioxide in chamber III loses its constrained pressure, it rapidly undergoes a phase change and expands. The high-pressure carbon dioxide gas is ejected outward through the vortex acceleration venting hole connected to chamber III. The high-speed swirling shock wave formed by the vortex acceleration venting hole acts on the coal and rock mass to achieve three-stage fracturing.

10. The multi-stage self-triggered liquid carbon dioxide staged fracturing method according to claim 9, characterized in that: The specific process of step S5 is as follows: the multi-stage self-triggered fracturing tube is recycled and reused. After replacing the constant pressure relief plate, isolation valve and excitation tube, the next fracturing can be carried out.

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