Fracturing tubular column

Abstract

The utility model provides a fracturing tubular column, and belongs to the technical field of methods for promoting yield by forming fractures or cracks in oil well exploitation. The fracturing string comprises an upper oil pipe, a double-pipe section and a lower oil pipe, the double-pipe section comprises an inner pipe and an outer pipe, a double-pipe annulus is formed between the inner pipe and the outer pipe, the lower oil pipe is connected with a fracturing packer set above a fracturing layer, the lower end of the lower oil pipe is open, and the lower oil pipe is communicated with the inner pipe and the upper oil pipe. The outer pipe is connected with an upper packer and a lower packer which are arranged on the upper side and the lower side of a leakage layer in a setting mode, a liquid inlet structure used for enabling equilibrium liquid in an oil sleeve annulus to enter a double-pipe annulus is arranged on the outer pipe and located above the upper packer, and a liquid outlet structure used for enabling the equilibrium liquid to be discharged out of the double-pipe annulus is arranged below the lower packer. According to the utility model, a leakage layer is isolated, so that equilibrium liquid pumped into the oil sleeve annulus can reach the upper part of the fracturing packer, the pressure of the lower part of the fracturing packer is balanced, the upper and lower pressure difference borne by the fracturing packer is reduced, and the fracturing packer is protected.

Description

Technical Field

[0001] The utility model relates to a fracturing string, belonging to the technical field of methods for promoting production by forming fractures or cracks in oil well exploitation. Background Art

[0002] During the oilfield development process, when an oil well is put into production or after production reaches a certain stage, in order to alleviate the contradiction between oil layers, improve the utilization rate of oil wells, narrow the difference in interlayer permeability, improve the production capacity of oil wells, and increase the ultimate recovery rate, low-permeability formations are often fractured. As Figure 1 shown, the packer 100 in the fracturing string is set on the upper part of the fracturing layer 200, and fracturing fluid is injected into the fracturing layer 200. Since the formation fracture pressure increases with the increase of depth, the construction pressure during fracturing is relatively high, and the pressure difference borne by the packer 100 is large. In order to prevent the failure of the packer 100 and cause fracturing failure, balancing fluid is usually injected from the annulus between the tubing and the casing to keep a certain pressure in the annulus between the tubing and the casing, so as to reduce the upper and lower pressure differences borne by the packer 100. However, when there is a leakage layer 300 above the fracturing layer 200, the balancing fluid injected from the annulus between the tubing and the casing quickly leaks into the leakage layer 300, resulting in the inability to maintain a certain balance pressure in the annulus between the tubing and the casing, and the upper and lower pressure differences borne by the packer 100 are still large, and the stable operation of the packer 100 cannot be guaranteed. Summary of the Utility Model

[0003] The purpose of the utility model is to provide a fracturing string to solve the problem in the prior art that when there is a leakage layer above the fracturing layer, the balancing fluid injected from the annulus between the tubing and the casing quickly leaks into the leakage layer, resulting in the inability to maintain a certain balance pressure in the annulus between the tubing and the casing, the upper and lower pressure differences borne by the packer are still large, and the stable operation of the packer cannot be guaranteed.

[0004] To achieve the above purpose, the following technical solutions are adopted in the utility model:

[0005] A fracturing string includes an upper tubing, a double-tube section, and a lower tubing arranged from top to bottom. The double-tube section includes an inner tube and an outer tube, and a double-tube annulus is formed between the two. A fracturing packer for setting on the upper part of the fracturing layer is connected to the lower tubing. The lower end of the lower tubing is open, and the lower tubing is communicated with the inner tube and the upper tubing to allow the fracturing fluid to be discharged to the fracturing layer through the upper tubing, the inner tube, and the lower tubing. An upper packer and a lower packer for respectively setting on the upper and lower sides of the leakage layer are connected to the outer tube. A liquid inlet structure for allowing the balancing fluid in the annulus between the tubing and the casing to enter the double-tube annulus is arranged on the outer tube above the upper packer. A liquid outlet structure for allowing the balancing fluid to be discharged from the double-tube annulus to the annulus between the outer tube and the casing is arranged on the outer tube below the lower packer.

[0006] The beneficial effects of the above technical solution are as follows: The present utility model belongs to a pioneering invention. When in use, the fracturing packer is set on the upper part of the fracturing layer to block the upper part of the fracturing layer; the upper packer and the lower packer are respectively set on the upper and lower sides of the leakage layer to isolate the leakage layer; during the fracturing construction, the fracturing fluid is discharged to the fracturing layer through the lower open ends of the upper tubing, the inner tubing and the lower tubing. At the same time, the balancing fluid injected from the annulus between the tubing and the casing enters the double-tubing annulus through the liquid inlet structure on the outer tubing, and then is discharged to the outer annulus between the outer tubing and the casing through the liquid outlet structure, reaching above the fracturing packer, so as to balance the pressure below the fracturing packer, reduce the differential pressure between the upper and lower parts borne by the fracturing packer, protect the fracturing packer and enable it to operate stably.

[0007] Further, the lower tubing includes a tubing body and a bell tube connected to the lower end of the tubing body, and the orifice of the bell tube is an open end.

[0008] Further, an insertion pipe assembly is connected below the inner tubing. The insertion pipe assembly includes a seal. An adapter is connected below the outer tubing. The lower tubing is connected below the adapter. The adapter includes a sealing cylinder. The insertion pipe assembly is inserted into the sealing cylinder and the seal is in sealing cooperation with the inner wall of the sealing cylinder.

[0009] Further, the insertion pipe assembly includes an upper connection pipe, an intermediate pipe and a lower connection pipe. The upper connection pipe and the intermediate pipe, and the intermediate pipe and the lower connection pipe are both connected by threaded fit. The seal includes a first seal assembly clamped between the upper connection pipe and the intermediate pipe and a second seal assembly clamped between the intermediate pipe and the lower connection pipe.

[0010] Further, both the first seal assembly and the second seal assembly include V-shaped sealing rings with opposite openings.

[0011] Further, a guide shoe structure for guiding the insertion pipe assembly to be inserted into the sealing cylinder is provided at the lower end of the insertion pipe assembly.

[0012] Further, a reaming section for guiding the insertion pipe assembly to be inserted into the sealing cylinder is provided at the upper end opening of the sealing cylinder.

[0013] Further, the adapter includes an adapter body threadedly connected to the sealing cylinder. The upper part of the adapter body is provided with a first inner thread of the body for connecting the sealing cylinder and a second inner thread of the body for connecting the outer tubing. The inner diameter of the first inner thread of the body is smaller than the inner diameter of the second inner thread of the body. The lower part of the adapter body is provided with an outer thread of the body for connecting the lower tubing.

[0014] Furthermore, the upper tubing, the inner pipe, and the outer pipe are connected by a three-pipe joint. The upper part of the three-pipe joint is provided with a first inner thread of the joint for connecting the upper tubing and a second inner thread of the joint for connecting the inner pipe. The first inner thread of the joint and the second inner thread of the joint are symmetrically arranged up and down. The lower part of the three-pipe joint is provided with an external thread of the joint for connecting the outer pipe.

[0015] Furthermore, an upper screen pipe and a lower screen pipe are connected to the outer pipe. The screen holes on the upper screen pipe form a liquid inlet structure, and the screen holes on the lower screen pipe form a liquid outlet structure. Brief Description of the Drawings

[0016] Figure 1 is a schematic structural diagram of a fracturing string in the prior art;

[0017] Figure 2 is a schematic structural diagram of an embodiment of the fracturing string of the present invention;

[0018] Figure 3 is a structural diagram of the three-pipe joint in the embodiment of the fracturing string of the present invention;

[0019] Figure 4 is a structural diagram of the insertion pipe assembly in the embodiment of the fracturing string of the present invention;

[0020] Figure 5 is a structural diagram of the socket in the embodiment of the fracturing string of the present invention.

[0021] In the figure: 100, packer; 200, fracturing layer; 300, lost circulation zone; 1, upper tubing; 2, three-pipe joint; 21, first inner thread of the joint; 22, second inner thread of the joint; 23, external thread of the joint; 3, upper screen pipe; 4, inner pipe; 5, outer pipe; 6, lower packer; 7, lower screen pipe; 8, insertion pipe assembly; 81, upper connection pipe; 82, intermediate pipe; 83, lower connection pipe; 831, guide shoe structure; 84, upper V-shaped sealing ring; 85, upper spacer ring; 86, lower V-shaped sealing ring; 87, lower spacer ring; 88, upper O-shaped sealing ring; 89, lower O-shaped sealing ring; 9, socket; 91, socket body; 911, first inner thread of the body; 912, second inner thread of the body; 913, external thread of the body; 92, sealing cylinder; 921, reaming section; 10, lower tubing; 101, bell mouth; 11, fracturing packer; 12, casing; 13, annulus between tubing and casing; 14, annulus between double pipes; 15, upper packer; 16, outer annulus. Detailed Description of the Embodiment

[0022] The features and performance of the present invention will be further described in detail below in conjunction with the embodiments.

[0023] In view of the technical problems existing in the prior art, the basic technical concept of the present utility model is to design a fracturing string including an upper tubing, a double-tube section, and a lower tubing. The double-tube section includes an inner tube and an outer tube, and a double-tube annulus is formed between the two. The lower tubing is communicated with the inner tube and the upper tubing to supply fracturing fluid to be discharged to the fracturing layer through the upper tubing, the inner tube, and the lower tubing. An upper packer and a lower packer for respectively setting and sealing on the upper and lower sides of the lost circulation zone are connected to the outer tube. An inlet structure for allowing the balance fluid in the oil-casing annulus to enter the double-tube annulus is arranged above the upper packer on the outer tube, and an outlet structure for allowing the balance fluid to be discharged from the double-tube annulus to the outer annulus between the outer tube and the casing is arranged below the lower packer on the outer tube. The present utility model isolates the lost circulation zone, enabling the balance fluid to reach above the fracturing packer, reducing the differential pressure between the upper and lower sides borne by the fracturing packer, and protecting the fracturing packer.

[0024] An embodiment of the fracturing string in the present utility model:

[0025] As Figure 2 shown, it includes an upper tubing 1, a double-tube section, and a lower tubing 10 arranged from top to bottom. An oil-casing annulus 13 is formed between the upper tubing 1 and the casing 12. The double-tube section includes an inner tube 4 and an outer tube 5, and a double-tube annulus 14 is formed between the two. An outer annulus 16 is formed between the outer tube 5 and the casing 12.

[0026] The upper tubing 1, the inner tube 4, and the outer tube 5 are connected by a three-tube joint 2. As Figure 3 shown, the three-tube joint 2 has a through structure. Its upper part is provided with a first inner thread 21 of the joint for connecting the upper tubing 1 and a second inner thread 22 of the joint for connecting the inner tube 4. The first inner thread 21 of the joint and the second inner thread 22 of the joint are symmetrically arranged up and down, and both have the same specifications and are taper threads. The lower part of the three-tube joint 2 is provided with an external thread 23 of the joint for connecting the outer tube 5, and the external thread 23 of the joint is also a taper thread. Using taper threads can achieve a better sealing effect.

[0027] As Figure 2 shown, an upper packer 15 and a lower packer 6 for respectively setting and sealing on the upper and lower sides of the lost circulation zone 300 are connected to the outer tube 5. The two packers can isolate the lost circulation zone 300, and both packers are Y111 packers. An inlet structure for allowing the balance fluid in the oil-casing annulus 13 to enter the double-tube annulus 14 is arranged above the upper packer 15 on the outer tube 5, and an outlet structure for allowing the balance fluid to be discharged from the double-tube annulus 14 to the outer annulus 16 is arranged below the lower packer 6 on the outer tube 5. Specifically, in this embodiment, an upper screen pipe 3 and a lower screen pipe 7 are connected to the outer tube 5. The screen holes on the upper screen pipe 3 form the inlet structure, and the screen holes on the lower screen pipe 7 form the outlet structure.

[0028] The upper end of the inner pipe 4 is connected to the second inner thread 22 of the three-way joint 2, and an insertion pipe assembly 8 is connected below the inner pipe 4. The insertion pipe assembly 8 includes a seal. The upper end of the outer pipe 5 is connected to the outer thread 23 of the three-way joint 2, and a socket 9 is connected below the outer pipe 5. The lower oil pipe 10 is connected below the socket 9. As Figure 5 shown, the socket 9 includes a sealing cylinder 92 and a socket body 91 threadedly connected to the sealing cylinder 92. The sealing cylinder 92 is a cylindrical tube with a smooth inner wall. The insertion pipe assembly 8 is inserted into the sealing cylinder 92 and the seal is in sealing cooperation with the inner wall of the sealing cylinder 92. An expanded hole section 921 for guiding the insertion pipe assembly 8 into the sealing cylinder 92 is provided at the upper end opening of the sealing cylinder 92. The upper part of the socket body 91 is provided with a first inner thread 911 of the body for connecting the sealing cylinder 92 and a second inner thread 912 of the body for connecting the outer pipe 5. The first inner thread 911 of the body is located below the second inner thread 912 of the body, and the inner diameter of the first inner thread 911 of the body is smaller than the inner diameter of the second inner thread 912 of the body to avoid interference between the outer pipe 5 and the sealing cylinder 92. The lower part of the socket body 91 is provided with an outer thread 913 of the body for connecting the lower oil pipe 10. The outer thread 913 of the body, the first inner thread 911 of the body and the second inner thread 912 of the body are all tapered threads to ensure a good sealing effect.

[0029] As Figure 4 shown, the insertion pipe assembly 8 has a through structure and includes an upper connection pipe 81, an intermediate pipe 82 and a lower connection pipe 83. The upper connection pipe 81 and the intermediate pipe 82, and the intermediate pipe 82 and the lower connection pipe 83 are all threadedly connected. Specifically, the upper end of the upper connection pipe 81 is provided with a female thread for connecting to the lower end of the inner pipe 4. The lower end of the upper connection pipe 81 is provided with a male thread for threaded connection to the upper end of the intermediate pipe 82. The outer diameter of the upper connection pipe 81 includes a large diameter section, a medium diameter section and a small diameter section from top to bottom. The female thread for connecting the inner pipe 4 is provided in the large diameter section. A step surface is formed between the medium diameter section and the small diameter section. The small diameter section includes a smooth rod section and a screw rod section. The male thread for threaded connection to the intermediate pipe 82 is provided on the screw rod section. The above-mentioned seal includes a first seal assembly sleeved outside the smooth rod section of the upper connection pipe 81. The first seal assembly includes a plurality of upper V-shaped sealing rings 84 with opposite openings. The first seal assembly further includes upper spacer rings 85 located on the upper and lower sides of each upper V-shaped sealing ring 84. The upper end face of the intermediate pipe 82 is used to press the first seal assembly so that it is clamped between the step surface of the upper connection pipe 81 and the upper end face of the intermediate pipe 82. At the same time, an upper O-shaped sealing ring 88 is also provided between the upper connection pipe 81 and the intermediate pipe 82.

[0030] As Figure 4As shown, the upper end of the intermediate pipe 82 is provided with a female thread for connection with the male thread of the upper connection pipe 81. The lower end of the intermediate pipe 82 is provided with a male thread for threaded connection with the lower connection pipe 83. The intermediate pipe 82 includes a large-diameter section and a small-diameter section. A stepped surface is formed between the large-diameter section and the small-diameter section. The female thread for threaded connection with the upper connection pipe 81 is arranged inside the large-diameter section. The small-diameter section includes a smooth rod section and a screw rod section. The male thread for threaded connection with the lower connection pipe 83 is arranged on the screw rod section. The above-mentioned seal also includes a second seal assembly sleeved outside the smooth rod section of the intermediate pipe 82. The second seal assembly includes a plurality of lower V-shaped sealing rings 86 with opposite openings. The second seal assembly also includes lower spacer rings 87 located on the upper and lower sides of each lower V-shaped sealing ring 86. The upper end surface of the lower connection pipe 83 is used to press against the second seal assembly, sandwiching it between the stepped surface of the intermediate pipe 82 and the upper end surface of the lower connection pipe 83. At the same time, a lower O-shaped sealing ring 89 is also arranged between the lower connection pipe 83 and the intermediate pipe 82.

[0031] By using V-shaped sealing rings with opposite openings, no matter whether the intubation assembly 8 moves upward or downward relative to the sealing cylinder 92, the sealing effect between the intubation assembly 8 and the sealing cylinder 92 can be ensured. In addition, O-shaped sealing rings are used for sealing between the upper connection pipe 81 and the intermediate pipe 82, and between the intermediate pipe 82 and the lower connection pipe 83, which can ensure the connection sealing performance of the intubation assembly 8 itself, making the channels of the fracturing fluid and the balance fluid completely independent and not affecting each other.

[0032] In addition, as Figure 4 shown, a guide shoe structure 831 for guiding the intubation assembly 8 to insert into the sealing cylinder 92 is arranged on the lower connection pipe 83. The guide shoe structure 831 is a horseshoe-shaped inclined section, which is convenient for processing and manufacturing.

[0033] As Figure 2 shown, a fracturing packer 11 for setting on the upper part of the fracturing layer 200 is connected to the lower oil pipe 10. The fracturing packer 11 adopts an RTTS fracturing packer, which can separately isolate the fracturing layer 200. And the lower end of the lower oil pipe 10 is open. The lower oil pipe 10 is connected to the inner pipe 4 and the upper oil pipe 1, so that the fracturing fluid can be discharged to the fracturing layer 200 through the upper oil pipe 10, the inner pipe 4 and the lower oil pipe 10, facilitating the fracturing construction. In this embodiment, the lower oil pipe 10 includes an oil pipe body and a bell mouth pipe 101 connected to the lower end of the oil pipe body. The pipe orifice of the bell mouth pipe 101 is open. The bell mouth pipe 101 mainly provides a bell mouth, thereby having a certain guiding effect on the discharged fracturing fluid and preventing the sand grains in the fracturing fluid from swirling upward to damage the setting effect of the fracturing packer 11.

[0034] The working principle of the fracturing string in the present utility model is as follows:

[0035] During use, when the fracturing string is lowered to an appropriate position in the wellbore and each packer is set, the sand-carrying fracturing fluid injected into the upper tubing 1 flows through the three-way joint 2, the inner tube 4, the insertion assembly 8, the socket 9, the central passage of the fracturing packer 11, and the bell mouth of the lower tubing 10 and is injected into the fracturing formation 200; the balancing fluid is injected from the annulus 13 between the tubing and the casing, passes through the upper screen pipe 3, the double-tubing annulus 14, and the lower screen pipe 7 to reach the outer annulus 16 between the lower packer 6 and the fracturing packer 11, balancing the pressure at the lower part of the fracturing packer 11, reducing the differential pressure borne by the fracturing packer 11, and protecting the fracturing packer 11. The fracturing string of the present utility model is applicable to fracturing operations where there is a leakage formation 300 above the fracturing formation 200 and it is impossible to directly inject the balancing fluid from the annulus 13 between the tubing and the casing.

[0036] In other embodiments of the fracturing string: instead of connecting the upper screen pipe and the lower screen pipe to the outer pipe, holes can be directly drilled in the outer pipe to form a liquid inlet hole and a liquid outlet hole, respectively constituting a liquid inlet structure and a liquid outlet structure.

[0037] In other embodiments of the fracturing string: the external thread of the joint connecting the three-way joint to the outer pipe can also be replaced with an internal thread. In this case, to avoid interference between the inner pipe and the outer pipe, the diameter of the internal thread connecting the outer pipe needs to be set larger than the diameter of the internal thread connecting the inner pipe; in other embodiments, the first internal thread and the second internal thread of the joint connecting the upper tubing and the inner pipe can also be replaced with external threads. In this case, the external thread connecting the inner pipe is located below the external thread connecting the outer pipe, and the diameter of the external thread connecting the outer pipe is larger to avoid interference; in other embodiments, the three-way joint and the upper tubing can be integrally connected, which is equivalent to having no first internal thread of the joint, that is, there is no independent joint component. In this case, the internal thread connecting the inner pipe and the external thread connecting the outer pipe are equivalent to being provided at the lower end of the upper tubing.

[0038] In other embodiments of the fracturing string: the socket body and the sealing cylinder can also be integrally connected. In this case, it is equivalent to having no first internal thread of the body, and the internal thread connecting the outer pipe and the external thread connecting the lower tubing are equivalent to being provided at the lower end of the sealing cylinder. Of course, the internal thread connecting the outer pipe can also be replaced with an external thread, and similarly, the external thread connecting the lower tubing can also be replaced with an internal thread.

[0039] In other embodiments of the fracturing string: the reaming section may not be provided at the upper end opening of the sealing cylinder, and the inner diameter of the part of the sealing cylinder for inserting the insertion assembly is of equal diameter.

[0040] In other embodiments of the fracturing string: the lower end of the insertion assembly may not be provided with a guide shoe structure, and its lower end is a complete round pipe structure.

[0041] In other embodiments of the fracturing string: both the first sealing assembly and the second sealing assembly can also be all O-ring seals.

[0042] In other embodiments of the fracturing string: The intubation assembly may also include an upper joint and a lower joint connected by a direct threaded fit. In this case, the intermediate pipe is no longer provided, and a seal is clamped between the upper joint and the lower joint. Of course, in other embodiments, the intubation assembly may include a single pipe, and a seal mounting groove is provided on the outer peripheral surface of the pipe for mounting a seal that seals with the inner wall of the seal cylinder.

[0043] In other embodiments of the fracturing string: The intubation assembly and the socket can be omitted, and instead a joint is provided. The upper part of the joint is directly connected to the lower ends of the inner pipe and the outer pipe, and the lower part of the joint is directly connected to the lower tubing. The specific connection method can be a threaded connection.

[0044] In other embodiments of the fracturing string: The lower tubing may only include a tubing body, which is a cylindrical pipe with an open lower end for directly discharging the fracturing fluid. In this case, there is no flared structure.

[0045] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. The patent protection scope of the present invention is subject to the claims. All equivalent structural changes made by using the description and drawings of the present invention should be equally included in the protection scope of the present invention.

Claims

1. A fracturing string, characterized in that: The utility model comprises an upper oil pipe, a double pipe section and a lower oil pipe arranged from top to bottom, the double pipe section comprises an inner pipe and an outer pipe and a double pipe annulus is formed between the two, the lower oil pipe is connected with a fracturing packer for sealing above the fracturing layer, the lower end of the lower oil pipe is open, and the lower oil pipe is connected with the inner pipe and the upper oil pipe, so that the fracturing fluid can be discharged to the fracturing layer through the upper oil pipe, the inner pipe and the lower oil pipe, the outer pipe is connected with an upper packer and a lower packer for sealing at the upper and lower sides of the leakage layer respectively, an inlet structure for supplying balance fluid in the oil casing annulus to enter the double pipe annulus is arranged above the upper packer on the outer pipe, and a outlet structure for supplying balance fluid from the double pipe annulus to the outer sleeve annulus between the outer pipe and the casing is arranged below the lower packer on the outer pipe.

2. The fracturing string according to claim 1, characterized in that: The lower oil pipe comprises an oil pipe body and a bell pipe connected to the lower end of the oil pipe body, and the pipe opening of the bell pipe is open.

3. The fracturing string according to claim 1 or 2, characterized in that: A plug assembly is connected below the inner tube, the plug assembly includes a seal, a socket is connected below the outer tube, the lower oil pipe is connected below the socket, the socket includes a sealing cylinder, the plug assembly is inserted into the sealing cylinder, and the seal is sealed with the inner wall of the sealing cylinder.

4. The fracturing string according to claim 3, characterized in that: The insert tube assembly includes an upper connecting tube, an intermediate tube and a lower connecting tube. The upper connecting tube and the intermediate tube, as well as the intermediate tube and the lower connecting tube, are all threadedly connected. The seal includes a first sealing component clamped between the upper connecting tube and the intermediate tube, and a second sealing component clamped between the intermediate tube and the lower connecting tube.

5. The fracturing string according to claim 4, characterized in that: The first sealing assembly and the second sealing assembly both include V-shaped sealing rings with openings arranged opposite to each other.

6. The fracturing string according to claim 3, characterized in that: The lower end of the insert tube assembly is provided with a guide shoe structure for guiding the insert tube assembly to be inserted into the sealing cylinder.

7. The fracturing string according to claim 3, characterized in that: The upper end of the sealing cylinder is provided with an expansion section for guiding the insertion of the inserting tube assembly into the sealing cylinder.

8. The fracturing string according to claim 3, characterized in that: The socket includes a socket body which is threadedly connected to the sealing sleeve. The upper part of the socket body is provided with a first internal thread of the body for connecting to the sealing sleeve and a second internal thread of the body for connecting to the outer pipe. The inner diameter of the first internal thread of the body is smaller than the inner diameter of the second internal thread of the body. The lower part of the socket body is provided with an external thread of the body for connecting to the lower oil pipe.

9. The fracturing string according to claim 1 or 2, characterized in that: The upper oil pipe, the inner pipe and the outer pipe are connected by a three-pipe joint. The upper part of the three-pipe joint is provided with a first internal thread of the joint for connecting the upper oil pipe and a second internal thread of the joint for connecting the inner pipe. The first internal thread of the joint and the second internal thread of the joint are symmetrically arranged up and down. The lower part of the three-pipe joint is provided with an external thread of the joint for connecting the outer pipe.

10. The fracturing string according to claim 1 or 2, characterized in that: An upper sieve tube and a lower sieve tube are connected to the outer tube. The sieve holes on the upper sieve tube constitute a liquid inlet structure, and the sieve holes on the lower sieve tube constitute a liquid outlet structure.

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