A combined casing string for preventing casing deformation and its running method and application

Abstract

The application discloses a combined casing string for preventing casing deformation and a running method and application thereof, and the combined casing string comprises at least one combined casing and a casing nipple; the combined casing comprises a casing and a rubber sleeve sleeved outside the casing; the casing nipple is connected with the casing through a connecting part, the casing nipple is used for mounting a centralizer, so that the combined casing running into a wellbore is positioned at a center position of the wellbore; and the centralizer is axially limited through the connecting part. The combined casing string has a simple structure and can solve the problem that a rubber structure is damaged in the running process of the combined casing, that is, solve the problem of installation of the centralizer in the case of the rubber sleeve mounted outside the casing, and the centralizer does not contact the rubber sleeve when being installed, so that the rubber sleeve can be fully water-absorbed and expanded, and then the problem of large deformation of the casing caused by fault sliding can be prevented.

Description

Technical Field

[0001] This invention relates to the field of oil and gas extraction technology, and in particular to a rubber composite casing string for preventing casing deformation, its installation method, and its application. Background Technology

[0002] In the prior art, methods for preventing or reducing casing deformation during fracturing operations include: using "low-modulus" cement paste for fracturing operations, i.e., the cement paste has the lowest possible modulus of elasticity while meeting the compressive strength requirements; and adding an elastic rubber ring to the outside of the casing, which can absorb the sliding deformation of the crack / fault if a high-risk crack / fault slips during fracturing, thereby protecting the inner casing from being affected and reducing casing deformation during fracturing operations. Summary of the Invention

[0003] The inventors have discovered that the method of installing a rubber sleeve outside the casing to absorb sliding deformation of fractures or faults has several drawbacks in practical applications. Because a centralizer needs to be installed outside the casing to ensure its proper position during casing string lowering, and this centralizer is placed outside the rubber sleeve, the centralizer and rubber sleeve come into contact and experience relative displacement during casing lifting and lowering. This relative displacement can damage the rigid structure of the centralizer, thereby compromising the rubber sleeve's protective function against casing deformation. Furthermore, the resulting rubber debris can clog the wellbore, affecting normal fracturing operations. Therefore, determining the correct installation position of the rubber sleeve and centralizer when using a rubber sleeve for casing deformation protection is a problem that requires further investigation.

[0004] In view of the above problems, the present invention is proposed to provide a combined sleeve string for preventing sleeve deformation, a method for installing the same, and an application thereof to overcome or at least partially solve the above problems.

[0005] This invention provides a combined bushing string for preventing bushing deformation, comprising: at least one combined bushing section and a bushing stub; The combined sleeve includes: a sleeve and a rubber sleeve fitted over the outside of the sleeve; The casing short section is connected to the casing via a connecting part. The casing short section is used to install a centralizer so that the combined casing, when lowered into the wellbore, is positioned at the center of the wellbore. The centralizer is axially limited by the connecting part.

[0006] In some alternative embodiments, the main body length of the sleeve sub is greater than the sum of the depth of the jaws of the pliers and the height of the centralizer.

[0007] In some alternative embodiments, the body length of the sleeve sub is less than the sum of the depth of the pliers jaws and twice the height of the centralizer.

[0008] In some optional embodiments, the casing strength of the casing sub is not lower than that of Q125 grade casing.

[0009] In some alternative embodiments, the outer diameter of the main body of the sleeve sub is equal to the outer diameter of the main body of the sleeve.

[0010] In some optional embodiments, the connecting part is a coupling, the sleeve short section and the sleeve are connected to the coupling by threads, and the outer diameter of the coupling is larger than the inner diameter of the centralizer.

[0011] In some optional embodiments, when the combined sleeve and the sleeve short section are configured as multiple segments, the sleeve short section and the combined sleeve are alternately connected.

[0012] In some optional embodiments, the combined sleeve string further includes a centralizer, which is sleeved on the sleeve stub and the centralizer and the sleeve stub are in a clearance fit.

[0013] In some alternative embodiments, the centralizer is a rigid centralizer.

[0014] Secondly, embodiments of the present invention also provide a method for installing the above-mentioned combined bushing string, wherein when the combined bushing string includes multiple combined bushing segments and bushing stubs, the method includes: Lower one end of the combined casing into the well; Lower the casing short section to a first predetermined distance from the other end of the first combined casing, attach the centralizer to the casing short section, and keep the centralizer stationary. Continue lowering the casing short section to a second predetermined distance from the other end of the first combined casing, fasten the casing short section and the combined casing together, drive the hydraulic tongs to fasten, and lower the casing short section and the combined casing so that one end of the casing short section enters the well. Lower a combined casing to a third predetermined distance from the other end of the casing short section, fasten the combined casing and the casing short section together, drive the hydraulic tongs to fasten, and lower the combined casing and the casing short section so that one end of the combined casing enters the well. Repeat the steps of lowering the bushing section until the assembled bushing string is completely lowered.

[0015] Thirdly, embodiments of the present invention also provide an application of the above-mentioned combined casing string for preventing large deformation of the casing in the field of natural gas extraction technology.

[0016] The beneficial effects of the above-described technical solutions provided in the embodiments of the present invention include at least the following: The combined casing string for preventing casing deformation provided in this embodiment of the invention has a rubber sleeve installed outside the casing. This sleeve absorbs the sliding deformation of high-risk fractures / faults during fracturing, thus protecting the inner casing from damage. Simultaneously, a casing sub is connected to one end of the casing for installing a centralizer. Installing the centralizer on the casing sub ensures that the running casing string remains centered in the wellbore during casing installation. Furthermore, compared to structures where the centralizer is installed outside the casing, since the centralizer is confined to the casing sub via a connecting part, it prevents contact between the centralizer and the rubber sleeve during the sliding process of lowering and raising the combined casing, thereby avoiding contact between the centralizer and the rubber sleeve. When the rubber sleeves come into contact and relative displacement occurs, damage to the rubber sleeves is caused, thus ensuring the integrity of the combined casing. Furthermore, this casing assembly installs the rubber sleeves and centralizers on the casing and casing subsection respectively, eliminating the need to consider the influence of the centralizer's inner diameter on the rubber sleeves during expansion. This means that the rubber sleeves can fully expand during the application of the combined casing. As the rubber thickness increases with water absorption and expansion, the amount of fault slip that the rubber can absorb increases, thereby reducing the casing deformation. This effectively addresses the problem of large casing deformation caused by fault slip, providing sufficient protection for the inner casing and significantly contributing to cost reduction and efficiency improvement in oil and gas well development. Moreover, the combined casing string structure of this embodiment is simple, solving the problem of centralizer installation when rubber sleeves are installed outside the casing.

[0017] Other features and advantages of the invention will be set forth in the description which follows, and will be apparent in part from the description, or may be learned by practicing the invention. The objects and other advantages of the invention may be realized and obtained by means of the structures particularly pointed out in the written description, claims, and drawings.

[0018] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0019] The accompanying drawings are provided to further illustrate the invention and form part of the specification. They are used in conjunction with embodiments of the invention to explain the invention and do not constitute a limitation thereof. In the drawings: Figure 1 This is a schematic diagram of the combined bushing string structure according to an embodiment of the present invention; Figure 2 This is a schematic diagram of the combined sleeve structure according to an embodiment of the present invention; Figure 3 This is a structural example diagram of the sleeve short section according to an embodiment of the present invention; Figure 4 This is a front view of the rigid roller straightener according to an embodiment of the present invention; Figure 5 This is a top view of the rigid roller straightener according to an embodiment of the present invention.

[0020] Explanation of reference numerals in the attached figures: 1-Rubber cylinder, 2-Connecting part, 3-Center, 4-Sleeve short section. Detailed Implementation

[0021] Exemplary embodiments of the present disclosure will now be described in more detail with reference to the accompanying drawings. While exemplary embodiments of the present disclosure are shown in the drawings, it should be understood that the present disclosure may be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the disclosure to those skilled in the art.

[0022] In the description of this invention, it should be noted that the terms "center," "upper," "lower," "left," "right," "vertical," "horizontal," "inner," and "outer," etc., 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 the invention and for simplifying the description, and do not 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 the invention. Furthermore, the terms "first," "second," and "third" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0023] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; 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; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0024] To prevent casing deformation during fracturing, the inventors proposed the concept of "using flexibility to overcome rigidity." Based on this concept, they proposed a method to prevent casing deformation using "low-modulus" cement stone, meaning that the compressive strength of the cement stone meets requirements while its elastic modulus is as low as possible. This method has achieved certain results in field applications. The inventors found that when using "low-modulus" cement stone for fracturing operations, the addition of elastic materials to the cement reduces the elastic modulus of the cement stone, thus decreasing its compressive strength. The more elastic material added, the greater the strength reduction. Since cementing quality has minimum strength requirements, the amount of material added is limited, and therefore, the reduction in casing deformation is also limited. This method is more suitable for cases with small deformation amounts and is not applicable to cases with large deformation amounts.

[0025] To address the problem of large deformation in casing, the inventors proposed a structure that adds an elastic rubber sleeve to the outside of the casing, namely a rubber composite casing. At high-risk crack / fault slip locations, the rubber composite casing is lowered, with the rubber sleeve added to the outside. This allows the rubber sleeve to absorb the slip deformation of the crack / fault, thereby reducing damage to the inner casing. However, the inventors discovered during their research that adding an elastic rubber sleeve to the outside of the casing causes rubber debris to accumulate inside the wellbore, leading to blockage. This problem cannot be solved by changing the material of the rubber sleeve. Further observation revealed that a centralizer is required during casing installation. A typical casing centralizer is mounted on the casing and can move freely between the couplings at both ends during casing lowering and raising. However, when a rubber sleeve is fitted over the outer casing, the centralizer comes into contact with the rubber sleeve during lowering and raising of the combined casing, causing relative displacement. The rigid structure of the centralizer scratches the rubber sleeve and generates significant friction, damaging the rubber structure. The resulting rubber debris can block the wellbore and even cause major construction accidents. Conversely, if a centralizer is not installed outside the combined casing, the rubber sleeve is easily scratched and generates significant friction during casing lowering, damaging the rubber structure. Furthermore, there is currently no established method for installing the centralizer and the combined rubber casing during construction using rubber combined casing.

[0026] To address the problems existing in the prior art, embodiments of the present invention provide a combined sleeve string for preventing sleeve deformation, its installation method, and its application.

[0027] Reference Figures 1-3 As shown, the combined sleeve string for preventing sleeve deformation provided in this embodiment of the invention includes: at least one combined sleeve and sleeve short section 4; The combined sleeve includes: a sleeve (not shown in the figure) and a rubber sleeve 1 that is fitted over the sleeve; The casing short section 4 is connected to the casing via the connecting part 2. The casing short section 4 is used to install the centralizer 3 so that the combined casing that is lowered into the wellbore is in the center position of the wellbore; and the centralizer 3 is axially limited by the connecting part 2.

[0028] The combined casing string for preventing casing deformation provided in this embodiment of the invention has a rubber sleeve 1 installed outside the casing. This rubber sleeve 1 absorbs the sliding deformation of the fracture / fault during high-risk fracture / fault sliding conditions during fracturing, thereby protecting the inner casing from damage. Simultaneously, a casing sub 4 is connected to one end of the casing for installing a centralizer 3. Installing the centralizer 3 on the casing sub 4 ensures that the running casing string remains centered in the wellbore during casing installation. Furthermore, compared to structures where the centralizer 3 is installed outside the casing, since the centralizer 3 is confined to the casing sub 4 by the connecting part 2, it prevents contact between the centralizer 3 and the rubber sleeve 1 installed outside the casing during the lowering and raising of the combined casing, thus avoiding contact between the centralizer 3 and the rubber sleeve 1. When the rubber sleeve 1 comes into contact with the centralizer 3 and relative displacement occurs, the rubber sleeve 1 is damaged, thus ensuring the integrity of the combined casing. Furthermore, this casing assembly installs the rubber sleeve 1 and the centralizer 3 on the casing and casing section 4 respectively, eliminating the need to consider the influence of the centralizer 3's inner diameter on the rubber sleeve 1 during expansion. This means that the rubber sleeve 1 can fully expand during the application of the combined casing. As the thickness of the rubber increases with water absorption and expansion, the amount of fault slip that the rubber can absorb increases, thereby reducing the casing deformation. This effectively addresses the problem of large casing deformation caused by fault slip, providing sufficient protection for the inner casing and significantly contributing to cost reduction and efficiency improvement in oil and gas well development. Moreover, the combined casing string of this embodiment has a simple structure, solving the installation problem of the centralizer 3 when the rubber sleeve 1 is installed outside the casing.

[0029] Specifically, the main body length of the sleeve section 4 is greater than the sum of the depth of the pliers jaws and the height of the centralizer 3. The depth of the pliers jaws refers to the depth required to clamp the sleeve section 4 during installation. Generally, the pliers consist of an upper clamp and a lower clamp. The lower clamp clamps the lower pipe body or connector, and the upper clamp clamps the upper pipe body or connector before rotation installation. The jaw depth of the pliers referred to here is the sum of the jaw depths of the upper and lower clamps. The main body length of the sleeve section 4 is the total length of the sleeve section minus the length of the threads at both ends. Setting the main body length of the sleeve section 4 to be greater than the sum of the pliers jaw depth and the height of the centralizer 3 provides sufficient space for clamping the pliers during sleeve section installation, facilitating on-site sleeve installation and allowing space for the movement of the centralizer 4.

[0030] Furthermore, the main body length of casing sub 4 is less than the depth of the large clamp jaws and twice the height of the centralizer. In the combined casing string, since there is no rubber sleeve at casing sub 4, it cannot absorb the stratum slip caused by faults / natural fractures. Therefore, the length of casing sub 4 must be appropriate to allow sufficient space for on-site construction and the centralizer, so as to avoid casing deformation at casing sub 4 due to excessive length.

[0031] For example, for a combined casing string with an outer diameter of 139.7mm, the main body length of the casing stub 4 can be 0.7m, which is greater than the sum of the height of the centralizer 3 (250mm) and the jaw depth of the large clamp (420mm), but less than twice the sum of the height of the centralizer (500mm) and the jaw depth of the large clamp (420mm).

[0032] In an optional embodiment, the casing sub has a casing strength not lower than that of Q125 grade casing to better suit the downhole working environment.

[0033] Optionally, the outer diameter of the selected sleeve sub 4 can be equal to the outer diameter of the sleeve; furthermore, when selecting the sleeve sub 4, except that its length is different from the length of the sleeve, the other specifications can be the same as the parameters of the sleeve, so as to better achieve the limiting effect of the centralizer on the sleeve. The other specifications of the sleeve sub may include parameters such as the outer diameter and wall thickness of the sleeve sub.

[0034] In an optional embodiment, the connecting part 2 can be a coupling, with the sleeve stub 4 and the sleeve connected to the coupling by threads. The outer diameter of the coupling is larger than the inner diameter of the centralizer 3 to prevent the centralizer 3 from sliding to the outside of the sleeve assembly and contacting the rubber sleeve 1 during sliding. It should be noted that the connecting part 2 connecting the sleeve stub 4 and the sleeve can be a coupling or other connecting structures, as long as it can achieve the connection between the sleeve stub 4 and the sleeve and play a role in axial positioning of the centralizer 3. This embodiment of the invention does not specifically limit this.

[0035] Optionally, when the combined sleeve and sleeve section 4 are set to multiple segments, the sleeve section 4 and the combined sleeve are connected alternately. That is, during the process of lowering the combined sleeve string, the combined sleeve is lowered first, then the sleeve section 4 is lowered, and the stabilizer 3 is put on the sleeve section 4. This cycle is repeated until the entire rubber combined sleeve string is lowered.

[0036] In an optional embodiment, the combined casing string further includes a centralizer 3, which is sleeved on the casing sub 4, with a clearance fit between the centralizer and the casing sub, so as to use the centralizer to position the combined casing string running into the wellbore at the center position; wherein the type of centralizer 3 can be selected by those skilled in the art according to the site requirements, and the embodiments of the present invention do not specifically limit it, such as a rigid centralizer, specifically a rigid roller centralizer 3, the structure of which is as follows. Figure 4 and Figure 5 As shown, this ensures that the lowered rubber casing assembly is centered in the wellbore. The size of the centralizer 3 can be selected according to the casing size and relevant regulations. For example, for a casing assembly with an outer diameter of 139.7 mm, the maximum outer diameter of the rigid roller centralizer 3 can be 205 mm, the main body outer diameter can be 200 mm, the inner diameter can be 143 mm, and the height can be 250 mm.

[0037] The casing assembly provided in this embodiment of the invention includes two parts: a rubber cylinder 1 and a casing. By utilizing the water absorption and expansion properties of rubber, it absorbs the sliding deformation caused by the sliding of high-risk cracks / faults during the fracturing process, thereby protecting the inner casing from being affected.

[0038] The rubber sleeve 1 attached to the outer casing increases the resistance when the casing is lowered. In order to reduce the resistance, a special material water-swellable rubber can be used for the rubber sleeve 1, that is, the rubber sleeve 1 only absorbs water and expands during the thickening process of cementing slurry.

[0039] When selecting the rubber sleeve 1, the thickness of the rubber sleeve 1 can be selected from 5mm to 12.7mm, and the length of the rubber sleeve 1 can be the same as the length of the main body of the sleeve. The length of the main body of the sleeve is the total length of the sleeve minus the length of the threads at both ends. Designing the length of the rubber sleeve 1 to be the same as the length of the main body of the sleeve can better protect the sleeve. The inner diameter of the rubber sleeve 1 can be selected to be no less than the outer diameter of the sleeve, so as to facilitate the fitting of the rubber sleeve 1 onto the sleeve. For example, for a combined sleeve with an outer diameter of 139.7mm, a main body length of 10m, and a wall thickness of 12.7mm, the length of the rubber sleeve 1 can be 10m, which is the same as the length of the main body of the sleeve, the inner diameter can be 139.7mm, which is equal to the outer diameter of the sleeve used, and the wall thickness can be 12.7mm.

[0040] This invention also provides a method for installing the above-mentioned combined bushing string, wherein when the combined bushing string includes multiple combined bushing segments and bushing stubs, the method includes: Lower one end of the combined casing into the well; Lower the casing short section to a first predetermined distance from the other end of the first combined casing, attach the centralizer to the casing short section and keep the centralizer stationary, continue lowering the casing short section to a second predetermined distance from the other end of the combined casing, fasten the casing short section and the combined casing together, drive the hydraulic tongs to fasten, and lower the casing short section and the first combined casing so that one end of the casing short section enters the well; Lower a combined casing to a third predetermined distance from the other end of the casing short section, fasten the rubber combined casing and the casing short section together, drive the hydraulic tongs to fasten, and lower the combined casing and the casing short section so that one end of the combined casing enters the well; the specific lengths of the first predetermined distance, the second predetermined distance and the third predetermined distance can be selected according to the actual needs of the construction site, and the embodiments of the present invention do not make specific limitations on this.

[0041] Repeat the steps of lowering the casing section until the combined casing string is fully installed. Regarding the installation method of the combined casing string, this embodiment of the invention uses the installation of a combined casing string with an outer diameter of 139.7 mm in a deep shale gas well as an example. The combined casing string has a casing wall thickness of 12.7 mm and a length of 11.350 m; the rubber sleeve has an outer diameter of 165.1 mm, a wall thickness of 12.7 mm, and a length of 10.968 m. The installation steps are illustrated below: (1) Lower one end of the combined casing into the well through the rotary table hole and blowout preventer; (2) Lower the casing short section. When one end of the casing short section (male thread end) is 0.5 meters away from the other end (coupling end) of the wellhead assembled casing, brake. The casing short section parameters are: casing outer diameter 139.7 mm, casing wall thickness 12.7 mm, and casing length 0.882 m. (3) Fit the rigid roller centralizer onto the casing short section and keep the centralizer stationary. Continue to lower the casing short section. When the male thread end of the casing short section is 0.2 meters away from the rubber combination casing coupling at the wellhead, brake. The parameters of the rigid roller centralizer are: maximum outer diameter 205mm, main body outer diameter 200mm, inner diameter 143mm, and length 250mm. (4) Connect the short sleeve section to the rubber combination sleeve and drive the hydraulic pliers to fasten it.

[0042] (5) The combined casing and one end of the casing short section equipped with a rigid roller centralizer are lowered into the well through the rotary table hole and the blowout preventer.

[0043] (6) Lower a combined casing. When one end of the casing combination (male end) is 0.2 meters higher than the other end (coupling) of the casing short section at the wellhead, brake.

[0044] (7) Connect the combined sleeve to the sleeve short section and drive the hydraulic pliers to engage.

[0045] (8) The casing sub and one end of the combined casing are lowered into the well through the rotary table hole and blowout preventer.

[0046] (9) Repeat steps (2)-(8) until all the combined bushings are installed.

[0047] The following table shows the sequence of the above casing assemblies and casing subsections.

[0048] Table 1. Partial List of Casing Strings with an Outer Diameter of 139.7 mm Run into the Wellbore

[0049] It should be noted that the wall thickness in the table refers to the thickness of the casing stub, casing, and rubber sleeve; it should also be noted that the table only provides an illustrative example of the partially installed casing assembly, and the installation of the remaining casing stubs and casing assemblies is consistent with the above process.

[0050] The present invention also provides an application of the above-mentioned combined casing string for preventing large deformation of casing in the field of natural gas extraction technology.

[0051] The combined casing string for preventing large casing deformation provided in this invention is applicable to the field of natural gas extraction technology, especially to hydraulic fracturing operations. While ensuring the integrity of the rubber sleeve, it solves the problem of installing the centralizer in the rubber combined casing, allowing the rubber sleeve to fully expand. The centralizer can move freely between the couplings at both ends of the casing short section without contacting the rubber sleeve, allowing the combined casing to function fully. This prevents casing deformation caused by fault / fracture slippage, and is of great significance for cost reduction and efficiency improvement in oil and gas well development.

[0052] It should be understood that the specific order or hierarchy of steps in the disclosed process is an example of an exemplary method. Based on design preferences, it should be understood that the specific order or hierarchy of steps in the process may be rearranged without departing from the scope of this disclosure. The appended method claims provide elements of various steps in an exemplary order and are not intended to limit the scope to the specific order or hierarchy described.

[0053] In the detailed description above, various features are combined together in a single embodiment to simplify this disclosure. This approach to disclosure should not be construed as reflecting an intention that embodiments of the claimed subject matter require more features than are explicitly stated in each claim. Rather, as reflected in the appended claims, the invention is presented with fewer features than all of the features in a single disclosed embodiment. Therefore, the appended claims are hereby explicitly incorporated into the detailed description, with each claim representing a separate preferred embodiment of the invention.

[0054] The foregoing description includes examples of one or more embodiments. It is certainly impossible to describe all possible combinations of components or methods in order to describe the above embodiments, but those skilled in the art will recognize that further combinations and arrangements of the various embodiments are possible. Therefore, the embodiments described herein are intended to cover all such changes, modifications, and variations that fall within the scope of the appended claims. Furthermore, the term "comprising" as used in the specification or claims is interpreted in a manner similar to the term "including," as interpreted when used as a conjunction in the claims. Additionally, the use of any term "or" in the specification of the claims is intended to mean "non-exclusive or."

Claims

1. A combined bushing string for preventing bushing deformation, characterized in that, include: At least one section of combined casing and casing sub; The combined sleeve includes: a sleeve and a rubber sleeve fitted over the outside of the sleeve; The casing short section is connected to the casing via a connecting part. The casing short section is used to install a centralizer so that the combined casing, when lowered into the wellbore, is positioned at the center of the wellbore. The centralizer is axially limited by the connecting part, so that the centralizer is confined at the casing short section. During the lowering and raising of the combined casing, the centralizer is prevented from contacting the rubber sleeve outside the casing during sliding. The main body length of the sleeve section is greater than the sum of the depth of the pliers jaws and the height of the centralizer, but less than the sum of the depth of the pliers jaws and twice the height of the centralizer.

2. The combined bushing string as described in claim 1, characterized in that, The casing strength of the casing short section shall not be lower than that of Q125 steel grade casing.

3. The combined bushing string as described in claim 1, characterized in that, The outer diameter of the main body of the sleeve sub is equal to the outer diameter of the main body of the sleeve.

4. The combined bushing string as described in claim 1, characterized in that, The connecting part is a coupling, and the sleeve short section and the sleeve are connected to the coupling by threads, and the outer diameter of the coupling is larger than the inner diameter of the centralizer.

5. The combined bushing string as described in claim 1, characterized in that, When the combined sleeve and the sleeve short section are configured as multiple segments, the sleeve short section and the combined sleeve are connected alternately.

6. The combined bushing string as described in any one of claims 1-5, characterized in that, Also includes: A centralizer is fitted onto the sleeve short section, and the centralizer and the sleeve short section are clearance-fitted.

7. The combined bushing string as described in claim 6, characterized in that, The centralizer is a rigid centralizer.

8. A method for installing a combined sleeve string as described in any one of claims 1-7, characterized in that, When the combined bushing string includes multiple combined bushing segments and bushing stubs, it includes: Lower one end of the combined casing into the well; Lower the casing short section to a first predetermined distance from the other end of the first combined casing, attach the centralizer to the casing short section, and keep the centralizer stationary. Continue lowering the casing short section to a second predetermined distance from the other end of the first combined casing, fasten the casing short section and the combined casing together, drive the hydraulic tongs to fasten, and lower the casing short section and the combined casing so that one end of the casing short section enters the well. Lower a combined casing to a third predetermined distance from the other end of the casing short section, fasten the combined casing and the casing short section together, drive the hydraulic tongs to fasten, and lower the combined casing and the casing short section so that one end of the combined casing enters the well. Repeat the steps of lowering the bushing section until the assembled bushing string is completely lowered.

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