Well repair string for casing damaged well and casing damaged well cementing method

Abstract

The present invention relates to a workover string for a damaged casing well and a cementing method for a damaged casing well. The workover string for a damaged casing well comprises: an oil pipe; a small casing connected to the oil pipe through a release structure, the small casing is lowered into the damaged casing section of the original well casing through the oil pipe, so that the small casing and the original well casing form an annular space, and the length of the small casing is greater than the length of the damaged casing section; the release structure comprises an oil pipe buffer, a reducer nipple and a variable buckle nipple connected in sequence from top to bottom, the oil pipe buffer is connected to the oil pipe with a positive buckle, the reducer nipple and the variable buckle nipple are connected by a reverse buckle, the upper end of the reducer nipple is connected to the oil pipe buffer with a positive buckle, and the lower end of the variable buckle nipple is connected to the small casing with a positive buckle. Before the oil pipe is separated from the small casing, displacement cementing construction is carried out, and after the oil pipe is separated from the small casing, plugging cementing construction is carried out, so that the cement slurry can completely fill the annular space surrounded by the small casing and the original well casing, and can ensure that the cement slurry is uniform, thereby improving the cementing quality.

Description

Technical Field

[0001] The invention relates to a well repairing pipe string for a casing damaged well and a casing damaged well cementing method. Background Art

[0002] As the oilfield development enters the middle and late stages, the wellbore of the development well is affected by geological factors such as high temperature and high pressure in the formation, and casing damage accidents also occur frequently, seriously restricting the normal production of oil and gas wells. Due to the influence of factors such as corrosion and aging, sand production, large-scale measures, and changes in formation movement, the number of casing damaged wells has increased year by year. For casing damaged wells, in order to restore geological reserves, improve the injection and production well network, and improve the development level, the conventional idea is to run a four-inch casing in the original well casing, with supporting downhole tools such as hydraulic tail pipe hangers, hollow rubber plugs, ball seats, double floating collars, and guide shoes. After the pipe string is in place, mud displacement circulation cementing is configured to achieve full downhole four-inch casing or hanging four-inch casing. However, there are some shortcomings in the actual production process: first, the four-inch casing is lowered to the bottom of the artificial well, and the well section is long, resulting in a waste of four-inch casing materials; second, after the four-inch casing is cemented, the original production layer is blocked, and it is necessary to perforate again to open the production layer before production can be carried out, resulting in increased perforation costs; third, the circulation cementing passes through the production layer section, and the mud pollutes the production layer; fourth, the wells with serious casing leakage are replaced by circulation cementing, because the cementing fluid enters the casing leakage section, the circulation cannot be established, and no mud enters above the casing damage section, resulting in poor cementing quality.

[0003] In order to solve the above problems, the Chinese utility model patent with the authorization announcement number CN206111133U discloses a small casing reinforcement process string in a straight inclined well casing, including an oil pipe, a mortar injection tool and a small casing connected in sequence from top to bottom, a conical retaining ring is arranged between the outer side of the lower end of the small casing and the original well casing, and a self-sealing rubber tube is arranged between the outer side of the mortar injection tool and the original well casing, so that a closed space is formed between the original well casing and the small casing. During construction, the entire set of pipes is lowered into the damaged part of the original well casing under the guidance of the guide cone, and the mortar is injected into the annular space surrounded by the small casing and the original well casing through the mortar injection hole in the oil pipe. The mortar is sealed in the above annular space by the conical retaining ring and the upper self-sealing rubber tube, so that the small casing and the original well casing are consolidated together, so as to achieve the purpose of repairing the damaged casing section.

[0004] Since there is formation fluid flowing out of the damaged section between the original well casing and the small casing below the damaged section, and the formation fluid is incompressible, the cement slurry cannot completely fill the annular space surrounded by the small casing and the original well casing, resulting in poor cementing quality. Summary of the invention

[0005] The purpose of the present invention is to provide a workover string for a casing damaged well, so as to solve the technical problem that the small casing reinforcement process string in the prior art cannot completely fill the annular space surrounded by the small casing and the original well casing with cement slurry, resulting in poor cementing quality; the purpose of the present invention is also to provide a casing damaged well cementing method.

[0006] To achieve the above purpose, the technical solution of the workover string for casing damaged wells of the present invention is:

[0007] Well repair string for casing damaged wells, including:

[0008] Oil pipes;

[0009] The small casing is connected to the oil pipe through a release structure, and the small casing is lowered into the damaged section of the original well casing through the oil pipe, so that the small casing and the original well casing form an annular space, and the length of the small casing is greater than the length of the damaged section;

[0010] The release structure includes a tubing buffer, a reducer and a variable buckle pup joint which are connected in sequence from top to bottom. The tubing buffer is connected to the tubing with a positive buckle, the reducer and the variable buckle pup joint are connected by a reverse buckle, the upper end of the reducer pup joint is connected to the tubing buffer with a positive buckle, and the lower end of the variable buckle pup joint is connected to the small casing with a positive buckle.

[0011] The beneficial effects are as follows: a small casing is placed in the damaged section of the original well casing through the oil pipe, which reduces the amount of pipes used and saves costs; cement slurry is injected into the lower part of the annular space through the oil pipe and the small casing to perform displacement cementing operations; after the oil pipe and the small casing are separated by a tripping operation and the oil pipe is lifted to the height required for the plugging construction, cement slurry is injected into the upper part of the annular space through the oil pipe to perform plugging cementing operations, which solves the problem that the circulation cannot be established in a severely damaged well, resulting in poor cementing quality above the damaged section, and improves the cementing plugging effect. In addition, through displacement cementing operations and plugging cementing operations, the cement slurry can completely fill the annular space surrounded by the small casing and the original well casing, and can ensure that the cement slurry is uniform, thereby improving the cementing quality.

[0012] Furthermore, a check valve is provided at the lower part of the small casing to prevent cement slurry in the annular space from being spit back into the small casing.

[0013] The beneficial effect is that such a design enables cement slurry to be injected into the upper part of the annular space without solidifying the cement slurry in the lower part of the annular space, thereby improving the cementing efficiency.

[0014] Furthermore, the check valve comprises a valve body, which is positively connected to the lower end of the small sleeve, a valve seat is arranged in the valve body, a valve ball is elastically pressed on the valve seat, and a liquid flow channel is arranged below the valve seat in the valve body.

[0015] The beneficial effect is that the one-flow valve can be connected to the small sleeve as a whole by such a design, which is convenient for installing the one-flow valve.

[0016] Furthermore, a conical guide shoe with a larger upper portion and a smaller lower portion is provided below the valve body.

[0017] The beneficial effects are: not only it is convenient for the well repairing pipe string to enter the well, but also it can prevent the sand surface from blocking the bottom liquid flow channel.

[0018] Furthermore, the liquid flow channel includes a bottom liquid flow channel arranged at the bottom of the valve body and a side liquid flow channel arranged at the side of the valve body.

[0019] The beneficial effect is: the flow area is increased, thereby improving the grouting efficiency of the lower part of the annular space.

[0020] To achieve the above purpose, the technical solution of the casing damaged well cementing method of the present invention is:

[0021] The method for cementing a casing damaged well comprises the following steps:

[0022] (1) The height of the sand surface is determined based on the distance between the production layer and the casing damage section. The sand surface is below the lower interface of the casing damage section. The amount of sand filling is calculated based on the height of the sand surface and the inner diameter of the original well casing. Then, sand filling construction is carried out in the original well casing.

[0023] (2) a small casing is sent to the damaged casing section through the oil pipe of the well repair string of the damaged casing well, the small casing is above the sand surface, and the upper and lower interfaces of the damaged casing section are between the two ends of the small casing. At this time, the original well casing and the small casing form an annular space, and then a displacement circulation cementing operation is performed to make the cement slurry level in the annular space between the lower interface of the damaged casing section and the upper end of the small casing;

[0024] (3) Lower the tubing to restore the tubing buffer to a free state, then rotate the tubing forward to separate the tubing from the small casing, then lift the tubing to the height required for plugging cementing construction, and then perform plugging cementing construction to inject cement slurry into the remaining part of the annular space;

[0025] The beneficial effects are as follows: after sand filling construction is carried out in the original well casing, a small casing is placed in the casing damaged section in the original well casing through the oil pipe, which reduces the amount of pipes used and saves costs. At the same time, it effectively prevents cement slurry from entering the formation to cause pollution and reduces the cost of re-perforation; during displacement circulation cementing operations, cement slurry is injected into the lower part of the annular space through the oil pipe and the small casing; during the squeeze plugging cementing operation, the oil pipe is separated from the small casing and the oil pipe is lifted to the height required for the squeeze plugging construction, and the cement slurry is injected into the upper part of the annular space through the oil pipe, which solves the problem that the circulation cannot be established in the severely damaged well, resulting in poor cementing quality above the casing damaged section, and improves the cementing plugging effect. In addition, through displacement cementing operations and squeeze plugging cementing operations, the cement slurry can completely fill the annular space surrounded by the small casing and the original well casing, and can ensure that the cement slurry is uniform, thereby improving the cementing quality.

[0026] Furthermore, in step (3), the amount of clean water for back injection into the original well casing is calculated based on the height of the oil pipe and the inner diameter of the original well casing. After the plugging and cementing operation, the clean water for back injection is injected from the original well casing to clean the cement slurry remaining between the oil casing annulus.

[0027] The beneficial effect is: avoiding cement slurry residue between the oil casing annulus and ensuring the safety of the oil pipe in the well.

[0028] Furthermore, in step (2), the cement slurry surface after the displacement circulation cementing operation is between the upper and lower interfaces of the casing damage section.

[0029] Furthermore, in step (3), after the oil pipe is separated from the small casing, the lifting height of the oil pipe is 200-300m. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 It is a schematic diagram of the structure of displacement circulation cementing in Example 1 of the casing damaged well cementing method of the present invention;

[0031] Figure 2 for Figure 1 Schematic diagram of the structure of the middle reducer;

[0032] Figure 3 for Figure 1 Schematic diagram of the structure of the check valve;

[0033] Figure 4 It is a schematic diagram of the structure of the plugging cementing in Example 1 of the casing damaged well cementing method of the present invention;

[0034] In the figure: 11-oil pipe; 12-original well casing; 13-oil pipe buffer; 14-reducing short joint; 141-upper joint; 142-lower joint; 15-variable short joint; 16-damaged casing section; 17-small casing; 18-check valve; 181-valve seat; 182-valve ball; 183-compression spring; 184-side liquid flow channel; 185-bottom liquid flow channel; 186-conical guide shoe; 187-partition; 19-sand surface; 20-production layer. DETAILED DESCRIPTION

[0035] In order to make the purpose, technical scheme and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention, that is, the embodiments described are only part of the embodiments of the present invention, rather than all of the embodiments. The components of the embodiments of the present invention described and shown in the drawings herein can be arranged and designed in various different configurations.

[0036] Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the invention claimed for protection, but merely represents selected embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without making creative work are within the scope of protection of the present invention.

[0037] It should be noted that the relational terms such as the terms "first" and "second" are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the terms "include", "comprise" or any other variants thereof are intended to cover non-exclusive inclusion, so that the process, method, article or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device. In the absence of further restrictions, the elements defined by the sentence "including one..." do not exclude the existence of other identical elements in the process, method, article or device including the elements. In addition, the terms "upper" and "lower" are based on the orientation and positional relationship shown in the accompanying drawings, and are only for the convenience of describing the present invention, rather than indicating that the device or component referred to must have a specific orientation, and therefore cannot be understood as a limitation on the present invention.

[0038] The features and performance of the present invention are further described in detail below in conjunction with the embodiments.

[0039] Embodiment 1 of the casing damaged well cementing method of the present invention:

[0040] Before introducing the casing damaged well cementing method, the casing damaged well workover string used in this embodiment is first introduced. Figure 1 As shown, the casing well repair string includes an oil pipe 11 and a small casing 17. The lower part of the oil pipe 11 is threadedly connected with an oil pipe buffer 13 and a reducer nipple 14 in sequence; the upper end and the lower end of the small casing 17 are respectively connected with a variable thread nipple 15 and a check valve 18. The oil pipe 11 is a two-and-a-half-inch oil pipe.

[0041] In this embodiment, the small casing 17 is a four-inch casing, which is sent into the damaged section 16 of the original well casing 12 through the oil pipe 11, so that the small casing 17 and the original well casing 12 form an annular space, wherein the length of the small casing 17 is greater than the length of the damaged section 16.

[0042] In this embodiment, the tubing buffer 13 is a downhole tool in the prior art that can be extended and retracted up and down and has an anti-rotation function. The tubing buffer 13 is positively connected between the tubing 11 and the reducer nipple 14 .

[0043] like Figure 2As shown, the reducer nipple 14 is a cylinder with a central through hole, including an upper joint 141 and a lower joint 142. The diameter of the upper joint 141 is smaller than the diameter of the lower joint 142. The upper joint 141 is a positive male buckle, connected to the tubing buffer 13; the lower joint 142 is a reverse male buckle, and its buckle type is a trapezoidal buckle. The size of the lower joint 142 is consistent with the size of the variable buckle nipple 15. The lower joint 142 is connected to the variable buckle nipple 15. When disengaging, the lower joint 142 is disengaged from the variable buckle nipple 15. At the same time, during the disengagement process, the movement of the lower joint 142 relative to the variable buckle nipple 15 in the upper and lower directions is absorbed by the tubing buffer 13, so as to facilitate the disengagement of the lower joint 142 from the variable buckle nipple 15.

[0044] In this embodiment, the variable buckle short section 15 is a cylinder with a central through hole. The size of the variable buckle short section 15 is consistent with the size of the small sleeve 17. The upper end of the variable buckle short section 15 is an inverted female buckle with a trapezoidal buckle, which is connected to the lower joint 142 of the reducer short section 14; the lower end of the variable buckle short section 15 is a positive male buckle with a trapezoidal buckle, which is connected to the small sleeve 17; a chamfer is provided inside the inverted female buckle, which is conducive to the connection and separation of the reducer short section 14 and the variable buckle short section 15.

[0045] In this embodiment, the oil pipe buffer 13, the reducer nipple 14 and the variable buckle nipple 15 together constitute a release structure.

[0046] like Figure 3 As shown, the check valve 18 can prevent the cement slurry in the annular space from being spitted back into the small casing 17. In this embodiment, the check valve 18 includes a valve body, which is a cylindrical structure. The upper part of the valve body is provided with a positive female buckle, which is a trapezoidal buckle and is connected to the lower end of the small casing 17. A valve seat 181 is provided in the valve body, and a valve port is provided on the valve seat 181. A valve ball 182 is provided at the valve port, and a compression spring 183 is provided below the valve ball 182. Under the elastic force of the compression spring 183, the valve ball 182 is blocked at the valve port to prevent the cement slurry from being pushed back into the small casing 17, thereby increasing the workload of drilling.

[0047] In this embodiment, the valve body includes a partition 187, and a bottom liquid flow channel 185 is provided on the partition 187 to allow cement slurry to be injected into the lower part of the annular space through the oil pipe 11 and the small casing 17. A side liquid flow channel 184 is opened on the valve body between the partition 187 and the valve seat 181 to increase the flow area. The bottom liquid flow channel 185 and the side liquid flow channel 184 together constitute a liquid flow channel.

[0048] In this embodiment, a hollow conical guide shoe 186 is provided on the lower side of the partition 187. The conical guide shoe 186 is a structure that is larger at the top and smaller at the bottom, which not only facilitates the well repairing pipe string to enter the well, but also prevents the sand surface 19 from blocking the bottom liquid flow channel 185. In other embodiments, the conical guide shoe may not be provided.

[0049] like Figure 1 and Figure 4 As shown, the cementing method of the casing damaged well includes sand filling construction, displacement circulation cementing construction and squeeze cementing construction. Among them, displacement circulation cementing construction is to inject cement slurry from bottom to top through the oil pipe 11 and the small casing 17 into the annular space surrounded by the original well casing 12 and the small casing 17; squeeze cementing construction is to inject cement slurry from top to bottom through the oil pipe 11 into the annular space surrounded by the original well casing 12 and the small casing 17.

[0050] 1. Sand filling construction includes the following steps:

[0051] The height h of the sand surface 19 is determined according to the distance between the production layer 20 and the casing damage section 16. 1 The sand surface 19 is below the lower interface of the casing damage section 16. The sand filling volume V is calculated using formula 1. 1 , and then sand filling construction is carried out in the original well casing 12; sand filling construction can protect the production layer 20 from pollution;

[0052] V 1 =πd 1 2 h 1 / 4 (1)

[0053] Among them, d 1 It is the inner diameter of the original well casing 12.

[0054] 2. Displacement cycle cementing construction includes the following steps:

[0055] 2.1. Determine the length h of the small casing 17 according to the height of the sand surface 19 and the length of the casing damage section 16 2 and the required length h of the oil pipe 11 4 , a small casing 17 is lowered into the well above the sand surface 19 through the oil pipe 11, so that the small casing 17 and the original well casing 12 form an annular space. Among them, the upper and lower interfaces of the casing damage section 16 are between the two ends of the small casing 17;

[0056] 2.2. According to the distance between the sand surface 19 and the casing damage section 16, determine the liquid level of the cement slurry in the annular space as h 3 The cement slurry level is generally higher than the lower interface of the casing damage section 16 and lower than the upper interface of the casing damage section 16. The cement slurry consumption V during displacement circulation cementing in the annular space surrounded by the original well casing 12 and the small casing 17 is calculated using formula 2. 2 ;

[0057] V 2 =πd 1 2 h 3 / 4-πd 2 2 h 3 / 4 (2)

[0058] Among them, d 1 is the inner diameter of the original well casing 12, d 2 is the outer diameter of the small sleeve 17.

[0059] 2.3. According to the length h of the small sleeve 17 2 and the length h of the oil pipe 11 4 , use formula 3 to calculate the amount of clean water V required for displacement cycle cementing 3 ;

[0060] V 3 =πd 3 2 h 2 / 4+πd 4 2 h 4 / 4 (3)

[0061] Among them, d 3 is the inner diameter of the small sleeve 17, d 4 is the inner diameter of the oil pipe 11.

[0062] 2.4. The displacement cycle cementing slurry dosage V determined according to step 2.2 and step 2.3 2 And water consumption V 3 , displacement circulation cementing construction is carried out to inject cement slurry from bottom to top into the annular space below the upper interface of the casing damaged section 16.

[0063] 3. The plugging and cementing construction includes the following steps:

[0064] 3.1. Lower the tubing 11 to restore the tubing buffer 13 to a free state, then rotate the tubing 11 forward to separate the tubing 11 from the small casing 17, and then lift the tubing 11 to the height h required for plugging and cementing construction. 5 ,h 5 Between 200-300m, it constitutes the underground plugging cementing string;

[0065] The specific method of forming the downhole jammed pipe string is as follows: lowering the oil pipe 11, the oil pipe buffer 13 returns to a free state, the pipe string under the oil pipe buffer 13 is not subjected to tension, at this time the oil pipe 11 returns to normal hanging weight, the oil pipe 11 is rotated forward, the small casing 17 is released from the lower joint 142 of the reducer nipple 14, and the small casing 17 is separated from the oil pipe 11, and then the oil pipe 11 is lifted up to a height h 5 That's it.

[0066] 3.2. Determine the amount of cement slurry V for plugging the casing damaged section 16 based on the formation water absorption index of the casing damaged section 16. 挤 , V 挤 Generally between 10-25m 3The formation water absorption index of the casing damaged section 16 can be obtained by the positive pressure method in the prior art.

[0067] 3.3. Use Formula 4 to determine the height h of the annular space between the original well casing 12 and the small casing 17 remaining after displacement cycle cementing construction 6 , use formula 5 to determine the cement slurry volume V required for cementing the remaining annulus 4 ;

[0068] h 6 =h 2 -h 3 (4)

[0069] V 4 =πd 1 2 h 6 / 4-πd 2 2 h 6 / 4 (5)

[0070] Among them, d 1 is the inner diameter of the original well casing 12, d 2 is the outer diameter of the small sleeve 17.

[0071] 3.4. The amount of cement slurry V determined in step 3.2 挤 and the cement paste dosage V determined in step 3.3 4 , use formula 6 to determine the cement slurry dosage V for plugging and cementing 5 ;

[0072] V 5 =V 挤 +V 4 (6)

[0073] 3.5. Determine the length h of the remaining tubing 11 in the well after lifting the tubing 11 according to formula 7 7 , use formula 8 to determine the amount of clean water V injected into the plugged cementing tubing 6 ;

[0074] h 7 =h 4 -h 5 (7)

[0075] V 6 =πd 4 2 h 7 / 4 (8)

[0076] Among them, d 4 is the inner diameter of the oil pipe 11.

[0077] 3.6. According to the height h of the oil pipe 115 , use formula 9 to determine the amount of clean water V for back injection of the original well casing 12 7 ;

[0078] V 7 =πd 1 2 h 5 / 4 (9)

[0079] Among them, d 1 It is the inner diameter of the original well casing 12.

[0080] 3.7. The amount of cement slurry V determined based on steps 3.4 and 3.5 for plugging and cementing 5 、Water consumption for positive injection V 6 , the squeeze cementing construction is carried out, and the cement slurry and clean water are injected from the oil pipe 11 in accordance with the designed amount in turn on the ground, so as to ensure that all the cement slurry in the oil pipe 11 is squeezed into the annular space of the casing damaged section 16 and the original well casing 12 and the small casing 17. It should be noted that, since clean water remains in the small casing 17 after displacement circulation cementing, the cement slurry in the oil pipe 11 will not enter the small casing 17 during the squeeze cementing construction.

[0081] 3.8. The amount of clean water V for back-injection of plugging cementing determined in step 3.6 7 After step 3.7 is completed, the back injection water is injected from the original well casing 12 to avoid cement slurry residue between the oil casing annulus, ensure the safety of the oil pipe 11 in the well, and at the same time realize the plugging and cementing construction of the original well casing 12 and the upper part of the annular space of the small casing 17.

[0082] 3.9. After the cementing construction is completed, the well is shut down under pressure and waited for solidification, so that the original well casing 12 and the small casing 17 are sealed together. Under high pressure conditions, the cement slurry is more dense after solidification, and the cementing quality is better.

[0083] 3.10. After waiting for condensation for 24 hours, lower the drill plug string and drill through the check valve 18 to the bottom of the well, and then resume production.

[0084] The present invention protects the production layer by filling sand, effectively prevents cement slurry from entering the formation to cause pollution, and reduces the cost of re-perforating; a small casing is placed at the damaged section in the original well casing through the oil pipe, thereby reducing the amount of pipes used and saving costs; displacement circulation cementing is used to plug the annulus between the original well casing and the small casing below the damaged section, and squeezing cementing is used to plug the annulus between the original well casing and the small casing above the damaged section, thereby solving the problem that circulation cannot be established in severely damaged wells, resulting in poor cementing quality above the damaged section, and improving the cementing and plugging effect.

[0085] Embodiment 2 of the casing damaged well cementing method of the present invention:

[0086] The difference between this embodiment and embodiment 1 is that in embodiment 1, the cement slurry level after displacement cementing operation is between the upper and lower interfaces of the casing damage section. In this embodiment, the cement slurry level after displacement cementing operation is slightly higher than the upper interface of the casing damage section.

[0087] The embodiment of the casing damaged well workover string of the present invention has the same structure as the casing damaged well workover string described in Embodiment 1 or 2 of the casing damaged well cementing method, and will not be described in detail here.

[0088] In other embodiments, after the cement slurry after displacement circulation cementing construction is completely solidified, the plugging cementing construction is performed, and the one-way valve may not be provided at this time. In other embodiments, only the bottom liquid flow channel or the side liquid flow channel may be provided on the valve body.

[0089] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. The patent protection scope of the present invention shall be based on the claims. All equivalent structural changes made using the contents of the description and drawings of the present invention should also be included in the protection scope of the present invention.

Claims

1. A method for cementing a casing damaged well using a casing damaged well workover string, characterized in that: The following steps are involved: (1) Determine the height of the sand surface based on the distance between the production layer and the casing damage section. The sand surface is below the lower interface of the casing damage section. Calculate the amount of sand filling based on the height of the sand surface and the inner diameter of the original well casing. Then, perform sand filling construction in the original well casing. (2) The small casing is sent to the damaged casing section through the oil pipe of the workover string of the damaged casing well, the small casing is above the sand surface, and the upper and lower interfaces of the damaged casing section are between the two ends of the small casing. At this time, the original well casing and the small casing form an annular space, and then the displacement circulation cementing operation is carried out to make the cement slurry level in the annular space between the lower interface of the damaged casing section and the upper end of the small casing; (3) Lower the tubing to restore the tubing buffer to a free state, then rotate the tubing forward to separate the tubing from the small casing, then lift the tubing to the height required for plugging cementing construction, and then perform plugging cementing construction to inject cement slurry into the remaining part of the annular space; In step (3), the amount of clean water for positive injection of the tubing is calculated based on the length of the tubing remaining in the well after the tubing is lifted and the inner diameter of the tubing. During the plugging and cementing operation, cement slurry and positive injection clean water are sequentially injected from the tubing according to the designed amount, thereby ensuring that all the cement slurry in the tubing is squeezed into the casing damaged section and the annular space of the original well casing and the small casing; in step (3), the amount of clean water for reverse injection of the original well casing is calculated based on the lifting height of the tubing and the inner diameter of the original well casing. After the plugging and cementing operation, the reverse injection clean water is injected from the original well casing to clean the cement slurry remaining between the casing annulus; The well repair string for the casing damaged well comprises: an oil pipe; The small casing is connected to the oil pipe through a release structure. The small casing is lowered into the damaged casing section of the original well casing through the oil pipe so that the small casing and the original well casing form an annular space. The length of the small casing is greater than the length of the damaged casing section. The release structure includes an oil pipe buffer, which can be extended and retracted up and down and has an anti-rotation function.

2. The casing damaged well cementing method according to claim 1, characterized in that: In step (2), the cement slurry surface after the displacement circulation cementing operation is between the upper and lower interfaces of the casing damage section.

3. The casing damaged well cementing method according to claim 1, characterized in that: In step (3), after the oil pipe is separated from the small casing, the lifting height of the oil pipe is 200-300m.

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