A double-string wellbore cleaning structure and method suitable for low-pressure oil and gas wells

Abstract

The present application relates to a double string wellbore cleaning structure and method suitable for low pressure oil and gas wells. The double string wellbore cleaning structure comprises a tubing, a coiled tubing, a coiled tubing operation well control device, and a drainage cross, a control valve, a gas injection cross and a wellhead device connected in sequence from top to bottom and sealingly connected to the lower end outlet of the coiled tubing operation well control device. The wellhead device is arranged at the wellhead. The gas injection cross is provided with a tubing hanger. The tubing is vertically arranged in the well, with its upper end connected to the tubing hanger and its lower end connected to a tubing tool. The coiled tubing sequentially passes through the coiled tubing operation well control device, the drainage cross, the control valve, the gas injection cross, the wellhead device and the tubing, and extends to the horizontal section in the well, with its lower end connected to a coiled tubing tool. The advantage is that the circulation channel is constructed in sections, the entire wellbore is divided into a vertical section and a horizontal section, the vertical section is lifted by a fixed pump, and the horizontal section is cleaned by a coiled tubing drag jet tool, so that the cleaning operation can be performed at very low wellbore pressure.

Description

Technical Field

[0001] This invention relates to the field of wellbore cleaning technology for oil and gas wells, and particularly to a dual-string wellbore cleaning structure and construction method suitable for low-pressure oil and gas wells. Background Technology

[0002] During oil and gas development, near-wellbore blockage can easily occur due to formation particle migration, sand production, and wellbore scaling, significantly impacting production. Currently, the main conventional wellbore cleaning method is to establish a full wellbore circulation system, relying on gas foam to reduce the density of the fluid medium, thereby reducing wellbore pressure and reservoir contamination. However, this method is less applicable and effective for low-pressure wells. The concentric tube jet local negative pressure sand-driving technology is limited by the tubing string size, resulting in small displacement, high friction, and low efficiency. In actual production operations, how to clean the wellbore at low cost and high efficiency under conditions where the bottom hole pressure is lower than the reservoir pressure is a major technical challenge in the later stages of oil and gas well development, especially for low-pressure and ultra-low-pressure wells. Conventional wellbore cleaning methods involve running tubing / coiled tubing into the target formation, pumping flushing fluid (or annular pumping flushing fluid) into the tubing to create a forward (reverse) circulation, carrying debris from the wellbore to the surface. For scale and other contaminants, a vortex flushing tool can be used for cleaning, followed by removal to the surface. Patent application number 202021787939.4 discloses a wellbore descaling device for oil extraction. This patent utilizes the combined action of high-pressure water flow and scrapers to effectively remove crude oil scale from the wellbore wall, achieving the purpose of cleaning the wellbore. Patent authorization number CN202210159276 discloses a pollution-free downhole electric-driven circulation cleaning device and a horizontal wellbore cleaning method. This method is based on a downhole electric-driven circulation device, which is lowered to the target formation via a continuous tubing cable to establish a fixed-point circulation, collecting scale, drill cuttings, and other contaminants to the bottom of the device, achieving pressurized wellbore cleaning without establishing circulation. These three wellbore cleaning methods have the following two problems:

[0003] (1) For oil and gas wells with low formation pressure coefficient and serious leakage, conventional circulation well washing may cause serious leakage and cannot effectively clean the formation;

[0004] (2) While it is possible to protect the formation during low-pressure well operations by using local circulation cleaning tools, the tool power and screen capacity are limited, and the processing capacity for gas wells with severe sand production and scaling cannot meet the actual needs. In addition, it requires cable operation, which is costly.

[0005] In response to the problems with wellbore cleaning processes for the aforementioned low-pressure oil and gas reservoir characteristics, there is currently a lack of applicable technologies for large-volume horizontal well operations with low formation pollution.

[0006] Therefore, it is necessary to develop a dual-string wellbore cleaning structure and construction method to overcome the above-mentioned technical problems. Summary of the Invention

[0007] The technical problem to be solved by the present invention is to provide a dual-string wellbore cleaning structure and construction method suitable for low-pressure oil and gas wells, which effectively overcomes the defects of the prior art.

[0008] The technical solution of the present invention to solve the above-mentioned technical problems is as follows:

[0009] A dual-string wellbore cleaning structure suitable for low-pressure oil and gas wells includes tubing, coiled tubing, coiled tubing well control equipment, and a drain four-way, control valve, gas injection four-way, and wellhead device that are sequentially and sealed in series from top to bottom at the lower outlet of the coiled tubing well control equipment. The wellhead device is located at the wellhead. A tubing suspension is installed in the gas injection four-way. The tubing is vertically installed in the well, with its upper end connected to the tubing suspension and its lower end connected to a tubing tool. The coiled tubing sequentially passes through the coiled tubing well control equipment, drain four-way, control valve, gas injection four-way, wellhead device, and tubing, and extends through the downhole directional section to the horizontal section downhole. The lower end of the coiled tubing is connected to the coiled tubing tool.

[0010] Based on the above technical solution, the present invention can be further improved as follows.

[0011] Furthermore, the aforementioned control valve is a flat plate valve.

[0012] Furthermore, the aforementioned coiled tubing well control equipment includes an injection head, a blowout preventer box, a blowout preventer, and a blowout preventer pipe connected in series from top to bottom, with the lower end of the blowout preventer pipe connected to the upper end of the aforementioned drainage four-way valve.

[0013] Furthermore, the upper and lower interfaces of the aforementioned drain four-way valve are respectively connected to the lower outlet of the aforementioned coiled tubing well control equipment and the control valve. The drain four-way valve has two interfaces on both sides, and each is connected to a drain pipe with a valve.

[0014] Furthermore, the upper and lower ports of the aforementioned gas injection four-way valve are respectively connected to the aforementioned control valve and wellhead device, and the aforementioned gas injection four-way valve has two ports on both sides, which are respectively connected to gas injection pipes with valves.

[0015] Furthermore, the aforementioned tubing tool includes a lifting pump, a packer, and a centralizer connected in series from top to bottom at the lower end of the tubing, with the lower end of the continuous tubing passing through the internal air cavities of the lifting pump, packer, and centralizer in sequence.

[0016] Furthermore, the aforementioned lifting pump includes a cylindrical pump body with a channel in the middle for a continuous oil pipe to pass through. The lower part of the side wall of the channel is provided with an ejector port, and the upper part of the inner side wall of the lifting pump is provided with an injection port. The ejector port extends upward and communicates with the injection port, and a first check valve is provided at the communication point. The lower part of the outer side wall of the pump body is provided with an air injection port, which extends upward and communicates with the ejector port, and a second check valve is provided at the communication point between the two.

[0017] Furthermore, the aforementioned coiled tubing tool includes a connector, a check valve, a release mechanism, and a flushing head. The lower end of the coiled tubing is connected to the check valve via the connector, and the check valve is connected to the flushing head via the release mechanism.

[0018] The beneficial effects of this invention are: by running conventional tubing and coiled tubing to form a downhole tubing string combination, constructing circulation channels in sections, the entire wellbore is divided into vertical and horizontal sections. The vertical section is lifted by a fixed pump, and the horizontal section is cleaned by a coiled tubing-driven jet tool. This allows for cleaning operations to be carried out under extremely low wellbore pressure, thereby achieving the goals of protecting the reservoir and restoring and increasing production.

[0019] A construction method for a dual-string wellbore cleaning structure suitable for low-pressure oil and gas wells is also provided, including the following steps:

[0020] Step 1: Assemble the entire twin-string wellbore cleaning structure at the wellhead, excluding the coiled tubing.

[0021] Step 2: Install coiled tubing at the well control equipment for coiled tubing operation, open the control valve, and lower the coiled tubing downhole to the working depth. Then, begin injecting fluid into the coiled tubing at a low flow rate.

[0022] Step 3: Inject working gas into the annulus between the tubing and the well through the four-way injection valve, and continuously inject fluid through the coiled tubing to establish circulation;

[0023] Step 4: Lower the coiled tubing for sand flushing, using a segmented sand flushing method, dragging the coiled tubing back and forth during the lowering process;

[0024] Step 5: Remove the coiled tubing and end the operation.

[0025] The beneficial effects are: the construction method is fast and safe, and can effectively achieve the purpose of wellbore cleaning operations for low-pressure oil and gas wells.

[0026] Based on the above technical solution, the present invention can be further improved as follows.

[0027] Furthermore, the working gas mentioned above is either nitrogen or carbon dioxide. Attached Figure Description

[0028] Figure 1This is a schematic diagram of the dual-string wellbore cleaning structure applicable to low-pressure oil and gas wells according to the present invention;

[0029] Figure 2 This is a schematic diagram of the tubing tool in the dual-string wellbore cleaning structure applicable to low-pressure oil and gas wells of the present invention;

[0030] Figure 3 This is a schematic diagram of the lifting pump in the dual-string wellbore cleaning structure applicable to low-pressure oil and gas wells of the present invention;

[0031] Figure 4 This is a schematic diagram of the coiled tubing tool in the dual-string wellbore cleaning structure applicable to low-pressure oil and gas wells of the present invention;

[0032] Figure 5 This is a flowchart illustrating the construction method of the dual-string wellbore cleaning structure applicable to low-pressure oil and gas wells according to the present invention.

[0033] The attached diagram lists the components represented by each number as follows:

[0034] 1. Coiled tubing well control equipment; 2. Fluid drain four-way; 3. Control valve; 4. Gas injection four-way; 5. Wellhead equipment; 6. Tubing suspension; 7. Tubing tools; 8. Coiled tubing tools; 10. Tubing; 11. Injection head; 12. Blowout preventer box; 13. Blowout preventer; 14. Blowout preventer pipe; 20. Coiled tubing; 71. Lift pump; 72. Packer; 73. Centralizer; 81. Connector; 82. Check valve; 83. Release valve; 84. Flushing head; 711. Injector port; 712. Injector port; 713. First check valve; 714. Gas injection port; 715. Second check valve. Detailed Implementation

[0035] The principles and features of the present invention are described below with reference to the accompanying drawings. The examples given are only for explaining the present invention and are not intended to limit the scope of the present invention.

[0036] Example 1

[0037] like Figure 1As shown, the dual-string wellbore cleaning structure for low-pressure oil and gas wells in this embodiment includes tubing 10, coiled tubing 20, coiled tubing well control equipment 1, and a drainage four-way 2, a control valve 3, an injection four-way 4, and a wellhead device 5, which are sequentially and sealed in series from top to bottom at the lower outlet of the coiled tubing well control equipment 1. The wellhead device 5 is located at the wellhead. The injection four-way 4 contains a tubing suspension 6. The tubing 10 is vertically installed in the well, with its upper end connected to the tubing suspension 6 and its lower end connected to a tubing tool 7. The coiled tubing 20 sequentially passes through the coiled tubing well control equipment 1, the drainage four-way 2, the control valve 3, the injection four-way 4, the wellhead device 5, and the tubing 10, and extends through the downhole directional section to the horizontal section downhole. The lower end of the coiled tubing 20 is connected to the coiled tubing tool 8.

[0038] This embodiment describes a dual-string wellbore cleaning structure for low-pressure oil and gas wells. The tubing 10, coiled tubing 20, coiled tubing well control equipment 1, drainage four-way 2, control valve 3, gas injection four-way 4, and wellhead device 5 are all existing tools related to oil and gas wells; their specific structures are not detailed here. During assembly, the upper interface of the drainage four-way 2 is bolted to the lower end of the coiled tubing well control equipment 1, the lower interface of the drainage four-way 2 is bolted to the upper end of the control valve 3, the lower end of the control valve 3 is bolted to the upper interface of the gas injection four-way 4, and the lower interface of the gas injection four-way 4 is bolted to the upper end of the wellhead device 5. The tubing suspension 6 is rigidly connected to the outer wall of the tubing 10, and the tubing suspension 6 is seated on a matching suspension base inside the gas injection four-way 4 (meaning the upper end of the tubing 10 is not connected to the gas injection four-way 4; liquid discharged through the tubing 10 can only be discharged through the drainage four-way 2). In this embodiment, nitrogen is pumped into the annulus during operation using the drainage four-way 2; the nitrogen and well-washing fluid injected into the well are returned using the gas injection four-way 4; the lower part of the tubing suspension 6 is connected to the tubing 10, and the tubing 10 is suspended by cooperation with the drainage four-way 2.

[0039] In this embodiment, the control valve 3 can be a flat plate valve of an oil and gas well-related type, and its specific structure will not be described in detail here.

[0040] In a preferred embodiment, the above-mentioned coiled tubing well control equipment 1 includes an injection head 11, a blowout preventer box 12, a blowout preventer 13 and a blowout preventer pipe 14 connected in series from top to bottom, with the lower end of the blowout preventer pipe 14 connected to the upper end of the drainage four-way 2.

[0041] In the above implementation scheme, the injection head 11, blowout preventer box 12, blowout preventer 13, and blowout preventer tubing 14 are all oil and gas well-related work equipment. Their specific structures will not be detailed here. During construction, the coiled tubing 20 enters from the top of the injection head 11, passing through the internal channels of the blowout preventer box 12, blowout preventer 13, blowout preventer tubing 14, drainage four-way 2, gas injection four-way 4, tubing 10, and tubing tool 7. Its lower end connects to the coiled tubing tool 8. The injection head 11 is used to deliver the coiled tubing 20.

[0042] The upper and lower ports of the aforementioned drain four-way 2 are respectively connected to the lower outlet of the aforementioned continuous tubing well control equipment 1 and the control valve 3. The aforementioned drain four-way 2 has two ports on both sides, and each is connected to a drain pipe with a valve.

[0043] The upper and lower ports of the aforementioned gas injection four-way valve 4 are respectively connected to the aforementioned control valve 3 and wellhead device 5. The aforementioned gas injection four-way valve 4 has two ports on both sides, and each is connected to a gas injection pipe with a valve.

[0044] In this embodiment, as Figure 2 As shown, the tubing tool 7 includes a lift pump 71, a packer 72, and a centralizer 73 connected sequentially from top to bottom at the lower end of the tubing 10. The lower end of the coiled tubing 20 passes through the internal air cavities of the lift pump 71, packer 72, and centralizer 73. The upper part of the lift pump 71 is connected to the tubing 10 via a threaded connection, and the lower part is conventionally connected to the packer 72. The lower part of the packer 72 is conventionally connected to the centralizer 73. The tubing tool 7 mainly uses the packer 72 to seal the upper annulus of the casing and the wellbore. Simultaneously, under the action of working gas injected from the surface, the lift pump 71 is used to lift the fluid in the well. The coiled tubing 20 can be lowered from the tubing suspension 6. Through the internal channels of the tubing 10 and the tubing tool 7, the coiled tubing tool 8 is transported to clean the wellbore and pump fluid, while simultaneously constructing a tubing-coiled tubing annulus to provide a return flow channel for the wellbore fluid.

[0045] In this embodiment, as Figure 3As shown, the lifting pump 71 includes a cylindrical pump body with a channel in the middle through which the continuous oil pipe 20 passes. The lower part of the side wall of the channel is provided with an ejector port 711. The upper part of the inner side wall of the lifting pump 71 is provided with an injection port 712 that connects to the channel. The ejector port 711 extends upward and communicates with the injection port 712. A first check valve 713 is provided at the connection point. The lower part of the outer side wall of the pump body is provided with an air injection port 714. The air injection port 714 extends upward and communicates with the ejector port 711. A second check valve 715 is provided at the connection point between the two. During the operation, working gas is injected into the annulus between the tubing 10 and the wellbore via the gas injection four-way 4. The working gas enters through the gas injection port 714 of the lift pump 71 and flows into the injector port 711 through the second one-way valve 715. Simultaneously, the liquid generated from flushing the coiled tubing 20 enters through the annulus between the coiled tubing 20 and the tubing 10 and enters the injector port 711. Mixed with the injected gas, it is discharged upwards through the first one-way valve 713 and the injection port 712. Due to the design of the blowout preventer box 12 in the coiled tubing well control equipment 1, the backflow fluid can only be discharged from the interface of the drain four-way 2. The well-constructed annulus between the tubing and the coiled tubing provides a backflow channel for the wellbore fluid. The effective use of the gas injection via the lift pump 71 overcomes the hydrostatic pressure in the vertical well section and controls the wellbore pressure in the horizontal section to be lower than the reservoir pressure, effectively preventing well flushing fluid loss.

[0046] In this embodiment, as Figure 4 As shown, the coiled tubing tool 8 includes a connector 81, a check valve 82, a release valve 83, and a flushing head 84. The lower end of the coiled tubing 20 is connected to the check valve 82 via the connector 81, and the check valve 82 is connected to the flushing head 84 via the release valve 83. Liquid injected into the coiled tubing 20 passes through the inner cavity of the connector 81, the valve cavity of the check valve 82, and the inner cavity of the release valve 83 into the flushing head 84, and is discharged from the flushing holes on the surface of the flushing head 84.

[0047] Example 2

[0048] like Figure 5 As shown, the construction method for the dual-string wellbore cleaning structure applicable to low-pressure oil and gas wells in this embodiment includes the following steps:

[0049] Step 1: Assemble the entire twin-string wellbore cleaning structure at the wellhead, excluding the coiled tubing 20.

[0050] Step 2: Install coiled tubing 20 into the well control equipment 1 and open control valve 3 to lower coiled tubing 20 downhole to the working depth, and start injecting fluid into coiled tubing 20 at a low flow rate.

[0051] More specifically, in step two, control valve 3 is opened, and fluid is injected into the well through coiled tubing 20 at a low flow rate. The running speed is strictly controlled as the tubing passes the wellhead, the build-up section, and the tail end of tubing 10. The coiled tubing 20 is temporarily suspended below the tail end of tubing 10. Preferably, the running speed is ≤5 m / min in the well depth 0-50 m and within 100 m of the tail end of tubing 10; ≤20 m / min in the vertical section (A in the diagram); ≤15 m / min from the build-up point to the target point; and ≤10 m / min in the horizontal section (B in the diagram). The target depth of the coiled tubing 20 is 100 m below the tail end of tubing 10.

[0052] Step 3: Inject working gas into the annulus between the tubing 10 and the well via the gas injection four-way valve 4, and continuously inject fluid through the coiled tubing 20 to establish circulation;

[0053] In step three, more specifically, a gas pump is pre-connected to the interface of the gas injection four-way 4. Then, the gas pump is turned on, and working gas is injected into the four-way 4. During the injection process, well-washing fluid is injected into the coiled tubing 20 according to the pressure of the controlled gas pump, establishing circulation. The displacement of the working gas is 55Sm. 3 / min, the well washing fluid discharge rate is 400-500L / min;

[0054] Step 4: Lower the coiled tubing 20 for sand flushing. Use a segmented sand flushing method, dragging the coiled tubing 20 back and forth during the lowering process.

[0055] More specifically, in step four, a 20mm continuous tubing is run in, and the speed is controlled during the running process. A segmented sand flushing method is adopted, with a 50m pullback after every 50m of advance, and repeated flushing is performed until the maximum depth is reached and the well is circulated at a fixed point. The running and pullback speeds are controlled to not exceed 5m / min, the change in the suspended weight of the workover rig does not exceed 2t, and the well is flushed at a fixed point for more than 1.5 cycles.

[0056] Step 5: Remove the coiled tubing 20mm to complete the operation;

[0057] More specifically, in step five, the retrieval speed is strictly controlled during the retrieval process. After cyclically retrieval the coiled tubing 20 to 100m from the bottom of tubing 10, fixed-point circulation begins. Once no sand is returned, the coiled tubing 20 is retrieved twice within 100m of the bottom of tubing 10. After no sand is returned, the pump flow rate of the injected fluid is reduced, and the retrieval speed is controlled. The depth and weight display are monitored. After the coiled tubing tool 8 is pulled out of the drain valve 2, the main valve of the wellhead device 5 is closed, the wellhead is restored, and the operation is completed. The retrieval speed should not exceed 5m / min, and in the vertical well section, the retrieval speed should not exceed 20m / min. After no sand is returned, the pump flow rate of the coiled tubing 20 is reduced to 200L / min.

[0058] In this embodiment, the working gas in step three is nitrogen or carbon dioxide.

[0059] This invention relates to a dual-string wellbore cleaning method suitable for low-pressure oil and gas wells. Through wellhead equipment, downhole tools, and construction methods, it progressively achieves the installation of the working tubing string and the implementation of wellbore cleaning operations, ultimately achieving the goal of cleaning the wellbore of low-pressure oil and gas wells. This method is fast and safe. Conventional tubing with a packer is run to the bottom of the vertical well section, dividing the wellbore into vertical and horizontal sections. Coiled tubing is then run through the tubing to the horizontal section for drag-and-flush sand removal and scale removal, achieving wellbore cleaning and improving operational efficiency. A lift pump installed at the bottom of the tubing overcomes the hydrostatic pressure in the vertical section, controlling the wellbore pressure in the horizontal section to be lower than the reservoir pressure, preventing well-washing fluid loss. The flushing tools are carried through the tubing via the coiled tubing, effectively cleaning sand plugs, sand beds, and scale in the horizontal section. Simultaneously, a large-volume pump can be injected into the tubing to enhance the cleaning effect. This wellbore cleaning method completely avoids reservoir contamination during cleaning operations, effectively solving the problem of reduced production in low-pressure oil and gas wells caused by severe sand production and scaling.

[0060] In the description of this invention, it should be understood that the terms "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "clockwise," "counterclockwise," "axial," "radial," and "circumferential" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing this invention and simplifying the description, 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. Therefore, they should not be construed as limitations on this invention.

[0061] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this invention, "a plurality of" means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0062] In this invention, unless otherwise explicitly specified and limited, the terms "installation," "connection," "linking," and "fixing," etc., should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components, unless otherwise explicitly limited. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0063] In this invention, unless otherwise explicitly specified and limited, "above" or "below" the second feature can mean that the first feature is in direct contact with the second feature, or that the first feature is in indirect contact with the second feature through an intermediate medium. Furthermore, "above," "over," and "on top" of the second feature can mean that the first feature is directly above or diagonally above the second feature, or simply that the first feature is at a higher horizontal level than the second feature. "Below," "below," and "under" the second feature can mean that the first feature is directly below or diagonally below the second feature, or simply that the first feature is at a lower horizontal level than the second feature.

[0064] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the present invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0065] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention.

Claims

1. A dual-string wellbore cleaning structure suitable for low-pressure oil and gas wells, characterized in that: The well includes tubing (10), coiled tubing (20), coiled tubing well control equipment (1), and a drain four-way valve (2), a control valve (3), an injection four-way valve (4), and a wellhead device (5) that are sequentially sealed and connected in series from top to bottom at the lower outlet of the coiled tubing well control equipment (1). The wellhead device (5) is located at the wellhead. The injection four-way valve (4) contains a tubing suspension (6). The tubing (10) is vertically installed in the well, with its upper end connected to the tubing suspension (6) and its lower end connected to a tubing tool (7). The coiled tubing (20) sequentially passes through the coiled tubing well control equipment (1), the drain four-way valve (2), the control valve (3), the injection four-way valve (4), the wellhead device (5), and the tubing (10), and extends through the downhole directional section to the horizontal section downhole. The lower end of the coiled tubing (20) is connected to the coiled tubing tool (8).

2. The dual-string wellbore cleaning structure suitable for low-pressure oil and gas wells according to claim 1, characterized in that: The control valve (3) is a flat plate valve.

3. The dual-string wellbore cleaning structure suitable for low-pressure oil and gas wells according to claim 1, characterized in that: The continuous tubing well control equipment (1) includes an injection head (11), a blowout preventer box (12), a blowout preventer (13), and a blowout preventer pipe (14) connected in series from top to bottom. The lower end of the blowout preventer pipe (14) is connected to the upper end of the drainage four-way valve (2).

4. The dual-string wellbore cleaning structure suitable for low-pressure oil and gas wells according to claim 1, characterized in that: The upper and lower ports of the drain four-way (2) are respectively connected to the lower outlet of the continuous tubing well control equipment (1) and the control valve (3). The drain four-way (2) has two ports on both sides, and each is connected to a drain pipe with a valve.

5. A dual-string wellbore cleaning structure suitable for low-pressure oil and gas wells according to claim 1, characterized in that: The upper and lower ports of the gas injection four-way (4) are respectively connected to the control valve (3) and the wellhead device (5). The gas injection four-way (4) has two ports on both sides, and each is connected to a gas injection pipe with a valve.

6. The dual-string wellbore cleaning structure for low-pressure oil and gas wells according to claim 1, characterized in that: The tubing tool (7) includes a lifting pump (71), a packer (72) and a centralizer (73) connected in series from top to bottom at the lower end of the tubing (10). The lower end of the continuous tubing (20) passes through the internal air cavities of the lifting pump (71), the packer (72) and the centralizer (73) in sequence.

7. A dual-string wellbore cleaning structure suitable for low-pressure oil and gas wells according to claim 6, characterized in that: The lifting pump (71) includes a cylindrical pump body with a channel in the middle through which a continuous oil pipe (20) passes. The lower part of the side wall of the channel is provided with an ejector port (711). The upper part of the inner side wall of the lifting pump (71) is provided with an injection port (712). The ejector port (711) extends upward and communicates with the injection port (712), and a first check valve (713) is provided at the communication point. The lower part of the outer side wall of the pump body is provided with an air injection port (714). The air injection port (714) extends upward and communicates with the ejector port (711), and a second check valve (715) is provided at the communication point between the two.

8. A dual-string wellbore cleaning structure suitable for low-pressure oil and gas wells according to any one of claims 1 to 7, characterized in that: The coiled tubing tool (8) includes a connector (81), a check valve (82), a release handle (83), and a flushing head (84). The lower end of the coiled tubing (20) is connected to the check valve (82) via the connector (81), and the check valve (82) is connected to the flushing head (84) via the release handle (83).

9. A construction method for a dual-string wellbore cleaning structure applicable to low-pressure oil and gas wells as described in any one of claims 1 to 8, characterized in that, Includes the following steps: Step 1: Assemble the entire twin-string wellbore cleaning structure at the wellhead, excluding the coiled tubing (20); Step 2: Install coiled tubing (20) at the coiled tubing operation well control equipment (1), open the control valve (3), lower the coiled tubing (20) downhole to the operating depth, and start injecting fluid into the coiled tubing (20) at a low flow rate; Step 3: Inject working gas into the annulus between the tubing (10) and the well through the gas injection four-way valve (4), and continuously inject fluid into the coiled tubing (20) to build a circulation; Step 4: Lower the coiled tubing (20) for sand flushing. Use a segmented sand flushing method and drag the coiled tubing (20) back and forth during the lowering process. Step 5: Remove the coiled tubing (20) and end the operation.

10. A construction method for a dual-string wellbore cleaning structure suitable for low-pressure oil and gas wells according to claim 9, characterized in that: The working gas is nitrogen or carbon dioxide.

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