Method for improving annulus small-gap well cementation quality

Abstract

The invention belongs to the technical field of oil and gas field development and oil production engineering, and particularly relates to a method for improving annulus small-gap well cementation quality. According to the method, the casing pipes are tieback again in the casing pipes of the two types, and the tieback tool and the tieback process are optimized, so that the well cementation quality is gradually improved. The problems that an existing cement squeezing treatment process is low in one-time plugging success rate and short in plugging validity period, and even causes well scrapping due to severe plugging are solved, the problems that an existing conventional tieback casing treatment process is high in construction pump pressure and low in well cementation quality can be solved, and meanwhile the casing damage problem can be solved once and for all. The probability of casing damage in the later production process of an oil-water well is greatly reduced, and stable production of an oil field is facilitated.

Description

Technical Field

[0001] The invention belongs to the technical field of oil and gas field development and oil production engineering, and in particular relates to a method for improving the quality of cementing with small annular gaps. Background Art

[0002] Casing damage wells are those where casing deformation does not exceed the plastic range of the casing. As oilfields enter the middle and late stages of development, casing damage is exploding, with multi-point and multi-stage damage becoming increasingly common. This increasing severity of casing damage is severely impacting the normal operation of oilfield development and becoming a significant constraint on stable production. Currently, the primary treatment for casing damage in oilfields is through large-volume horizontal plugging techniques, supplemented by "mechanical packer + tubing liner" plugging techniques.

[0003] However, when treating long, multi-point casing damage wells, the aforementioned two methods suffer from low initial plugging success rates, short plugging durations, and even well abandonment in severe cases. To address this issue, researchers, building on existing techniques for treating casing damage with tiebacks, combined with the actual wellbore and material conditions of the oilfield, considered the risks associated with high friction, high pump pressure, and long-term pressure buildup and heat dissipation during the tieback process, which can affect cementing quality. By developing targeted countermeasures such as using heavy cementing fluid as displacement fluid, reverse circulation cementing, and initial set pressure relief, this method was successfully applied for the first time in an HDA well. Post-process logging results showed a high cementing quality pass rate of 81.62%, with an excellent cementing section of 1122.6 meters, representing 37.09%. However, a 294.9-meter, free casing section, representing 9.74% of the entire cemented section, remained. Summary of the Invention

[0004] In response to the above problems, the present invention discloses a method for improving the quality of cementing in small annular gaps during casing damage treatment, which effectively avoids the defects of low success rate and short effective period of conventional cementing plugging. At the same time, it reforms and innovates on the basis of the original casing tieback to solve the problems of high construction pump pressure and low cementing quality, providing new ideas for casing damage treatment, effectively avoiding the scrapping of old wells, and helping to stabilize oil field production.

[0005] The present invention proposes a method for improving the cementing quality of small annular gaps, which specifically includes the following steps:

[0006] Analyze the historical operation data of the casing damaged wells and determine whether the casing damaged wells need to be treated by casing tie-back according to the analysis results;

[0007] If casing tie-back is used to treat a casing-damaged well, the first type of casing and the second type of casing should be used in sequence to perform well cleaning and wall scraping on the casing-damaged well.

[0008] A double-cup packer corresponding to the second type of casing is used to temporarily seal the production layer of the casing-damaged well and perform sand filling;

[0009] Extend the simulated wellbore string to the transition point between the first type casing and the second type casing. After the wellbore is completed, remove the wellbore string and dismantle the original production spool of the damaged well. Install the neck flange, casing spool and blowout preventer assembly in the place of the original production spool.

[0010] Run the unhooked casing string into the damaged well to the transition point between the first and second type casings, adjust the casing nipple, connect the casing hanger by lowering and pressurizing, and then remove the blowout preventer group;

[0011] Installing a neck flange and a production spool on the casing spool and reinstalling the blowout preventer stack;

[0012] A cementing string is run into the unhooked casing string to a set distance above the transition point between the first type casing and the second type casing, the cementing string is adjusted and a packer is set, the well is flushed by reverse circulation, cement is squeezed and cemented, and the well is shut in and waited for setting;

[0013] Drill and cement the cement plug, flush the sand and salvage the double-cup packer, and perform engineering logging after scraping the wall of the casing damaged well.

[0014] Furthermore, the analysis of historical operation data of the casing damaged well and the determination of whether the casing damaged well needs to be treated by casing tie-back according to the analysis results specifically include the following steps:

[0015] Preliminary prediction of casing damage location based on historical operation data of casing damaged wells;

[0016] During repair, a mechanical packer capable of multiple setting is used and the binary elimination method is used to locate the casing damage point;

[0017] According to the distribution and severity of the casing damage points, it is determined whether the casing damaged well needs to be treated by casing tie-back.

[0018] Furthermore, the historical operation data includes the static temperature gradient and static pressure gradient before the logging operation. Furthermore, the first type of casing is 7″ casing, and the second type of casing is 51 / 2″

[0019] casing.

[0020] Furthermore, the simulated wellbore string includes, from the bottom of the well to the wellhead, a guide cone, a buckle, a straightening sub, a 51 / 2" unhooked casing, a straightening sub, a 51 / 2" unhooked casing, a straightening sub, a buckle, a 27 / 8" drill pipe, a buckle and a 31 / 2" drill pipe.

[0021] Furthermore, the knotless casing string includes, from the bottom of the well to the wellhead, a 51 / 2" knotless casing straightening nipple with chamfers, a 51 / 2" knotless casing 1, a 51 / 2" knotless casing nipple and a 51 / 2" knotless casing.

[0022] Furthermore, a 5.5" knotless casing straightening nipple is run into the knotless casing string structure every 200m from bottom to top.

[0023] Furthermore, the cementing string includes, from bottom to top, a round-head guide cone, a 27 / 8" tubing nipple, a 51 / 2" extended rubber sleeve RTTS packer, a variable buckle and a 27 / 8" drill pipe.

[0024] Furthermore, one straightening sub is added for every 20 27 / 8" drill pipes above the 51 / 2" extended rubber sleeve RTTS packer.

[0025] Furthermore, the set distance is 500m above the transition point between the first type casing and the second type casing.

[0026] Beneficial effects of the present invention:

[0027] The present invention first improves the casing tail pipe and adds a casing straightener, from no straightener to single mushroom head straightener, and then to the final double mushroom head straightener, to ensure the casing centering degree, while optimizing the diameter and distribution of the casing tail pipe circulation hole, to ensure the uniform return of cement slurry. Secondly, an extended rubber tube RTTS packer is inserted into the casing to change the pressure-bearing and solidification-waiting main body. During the pressure-bearing and solidification period, the pressure-bearing main body is converted from the uncoupled casing to the drill pipe pressure-bearing body, ensuring that the outer cement ring of the casing will not produce microcracks due to pressure release and shrinkage after pressure relief, thereby improving the cementing quality. Then, the original composite bridge plug and suspended plug temporary plugging process is optimized, and the double leather cup packer is inserted and filled with sand to play a temporary plugging role. The use of the double leather cup packer can reduce the plugging, drilling and other processes, greatly shortening the construction time. At the same time, there is no need for the horizontal section production layer drilling and grinding process, reducing the loss of well pressure fluid and avoiding pollution of the bottom hole production layer. The above method addresses the low casing centering and micro-gaps caused by pressure buildup and waiting for solidification, continuously optimizes tieback tools and processes, and gradually improves cementing quality. It solves the problems of the existing squeeze cement treatment process, such as low first-time plugging success rate and short plugging effectiveness, which in severe cases can even lead to well scrapping. It also solves the problems of high construction pump pressure and low cementing quality of the existing conventional tieback casing treatment process. At the same time, it can permanently solve the problem of casing damage, greatly reducing the probability of casing damage in the later production process of oil and water wells, helping to stabilize and increase oilfield production.

[0028] Other features and advantages of the present invention will be described in the following description, and in part will become apparent from the description, or will be understood by practicing the present invention. The purpose and other advantages of the present invention can be realized and obtained by the structures pointed out in the description, claims and drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following is a brief introduction to the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0030] Figure 1 A flow chart of a method for improving cementing quality with small annular gaps proposed by the present invention is shown;

[0031] Figure 2 The embodiment of the present invention is shown Schematic diagram of the righting sub;

[0032] Figure 3 It shows a schematic diagram of the casing tie-back principle in an embodiment of the present invention;

[0033] Figure 4 A schematic diagram of a circulation hole of a liner short section of a HDB well casing in an embodiment of the present invention is shown;

[0034] Figure 5 It shows a schematic diagram of a double mushroom head centralizing and circulation hole for HDD well casing in an embodiment of the present invention;

[0035] Illustrations: 1. Tieback 51 / 2" unhooked casing; 2. Casing damage section; 3. Drill pipe; 4. Centralizing nipple; 5. 51 / 2" extended rubber tube RTTS packer; 6. 51 / 2" knotless casing centralizing nipple; 7. 51 / 2" knotless casing centralizing nipple circulation hole; 8. 51 / 2" double leather cup packer; 9. Round guide cone (with 20mm water hole at the bottom) + 27 / 8" EU oil pipe nipple. DETAILED DESCRIPTION

[0036] To make the objectives, technical solutions, and advantages of the embodiments of the present invention more clear, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts shall fall within the scope of protection of the present invention.

[0037] The present invention aims to solve the problems of low one-time plugging success rate and short plugging validity period of the existing cement squeeze treatment process, which may even lead to well scrapping in severe cases, as well as the problems of high construction pump pressure and low cementing quality of the existing conventional tie-back casing treatment process. The present invention proposes a method for improving the cementing quality of small annular gaps, such as Figure 1 As shown, the method includes the following steps:

[0038] S1: Analyze the historical operation data of the casing damaged well and determine whether the casing damaged well needs to be treated by casing tie-back based on the analysis results; specifically, the following steps are included:

[0039] Preliminary prediction of the location of casing damage points based on abnormal data measured before casing damage well operation;

[0040] During a repair, a mechanical packer capable of multiple setting is used to perform a "binary" elimination method to find leaks. This allows the precise location of the casing damage to be determined, and the distribution and severity of the damage analyzed to determine whether the well requires casing tieback treatment. The abnormal data includes static temperature and pressure gradients before the operation, as well as engineering abnormality data.

[0041] When judging whether the well needs to use casing tieback to treat casing damage in actual application, it is generally selected for multi-point and long-section casing damage in 7-inch casing.

[0042] S2: If casing tieback is used to treat a damaged casing well, the first and second types of casing are used in sequence to perform well cleaning and wall scraping on the damaged casing well in advance; in an exemplary embodiment of the present invention, the first type of casing is a 7" casing to ensure that the subsequent packer is successfully set for leak detection; the second type of casing is a 51 / 2" casing, which is used for well cleaning and wall scraping. If sand is severely buried, a reasonable sand flushing (fishing) process is adopted to treat the sand to the artificial bottom of the well, while ensuring that the subsequent double-cup packer is successfully set; in the present invention, the second type of casing is below the first type of casing, and an annulus is formed between the first and second types of casing due to the size difference.

[0043] S3: using a double-cup packer of the corresponding model of the second-model casing to temporarily seal the production layer of the casing-damaged well and perform sand filling;

[0044] S4: Extend the simulated wellbore string to the place where the casing diameter changes. After the wellbore is completed, remove the simulated wellbore string, remove the original production spool of the damaged well, and then install the neck flange, casing spool and blowout preventer group in the place of the original production spool. The place where the casing diameter changes is the transition point between the first type casing and the second type casing.

[0045] The neck flange and casing spool are used to suspend the unhooked casing string used for tieback, and the blowout preventer group includes a ram blowout preventer and an annular blowout preventer to prevent oil from spraying out;

[0046] The simulated wellbore string includes, from the bottom of the well to the wellhead, a guide cone, a buckle, a straightening sub, a 5.5" unhooked casing, a straightening sub, a 5.5" unhooked casing, a straightening sub, a buckle, a 2.7 / 8" drill pipe, a buckle, and a 3.5" drill pipe. The 5.5" unhooked casing and the straightening sub can be arranged in multiple groups according to the depth of the casing-damaged well. A 5.5" unhooked casing straightening sub is run every 200m from bottom to top in the unhooked casing string structure.

[0047] S5: Lower the untied casing string to the casing reducer, adjust the casing nipple, and lower the pressurized casing hanger. Lowering the pressurized casing hanger allows the untied casing string to be tightly mounted on the casing spool by applying negative pressure, preventing the casing from floating to the wellhead due to increased buoyancy caused by increased fluid density in the well. The untied casing string is the new casing for tieback.

[0048] In one embodiment of the present invention, the knotless casing string includes, from bottom to top (from bottom to wellhead), one 5.5" knotless casing centralizing sub with chamfers, ten 5.5" knotless casings, one 5.5" knotless casing centralizing sub, and one 5.5" knotless casing; (one 5.5" knotless casing centralizing sub is run every 200m from bottom to top);

[0049] S6: Installing a neck flange and a production spool on the casing spool and reinstalling the blowout preventer stack; the neck flange and production spool are used to connect the Christmas tree required for subsequent production, and the production spool is installed on the casing spool;

[0050] S7: inserting a cementing string into the unhooked casing string to a set distance above the casing diameter change point, adjusting the cementing string and setting a packer, performing reverse circulation well washing and cementing, shutting in the well and waiting for solidification; the packer is an RTTS packer, which is used to isolate oil, gas, and water layers in downhole casing. Through the action of external force, the length of the rubber tube is shortened and the diameter is increased, thereby sealing the oil and casing annular spaces and separating the oil (gas, water) layers above and below the packer, thereby achieving isolation of the oil and gas wells;

[0051] The specific operation of squeeze cementing is to inject cement slurry into the annulus between the new and old casings through a pump truck, and circulate it out from the inside of the new casing, which is reverse circulation cementing.

[0052] In an exemplary embodiment of the present invention, the cementing string is lowered to 500m above the casing diameter change point. The cementing string includes, from bottom to top: a rounded guide cone, a 27 / 8" tubing nipple, a 51 / 2" extended rubber tube RTTS packer, a change button + 27 / 8" drill pipe to the wellhead (above the packer, every 20 27 / 8" drill pipes are added). 1 short section for straightening).

[0053] S8: Drill and cement the cement plug, flush the sand and salvage the double-cup packer, and perform engineering logging after scraping the casing damaged well.

[0054] The following is an exemplary description of the method for improving the cementing quality of small annular gaps proposed by the present invention with reference to specific embodiments, which specifically includes the following steps:

[0055] Step S10: Analyzing the operating history of the HDE well reveals that the well has undergone two casing damage treatment operations, with the casing damage sections at 2473.77-2483.43 m and 2505.85-2515.45 m, respectively. The last casing damage treatment operation occurred only two years after the current casing damage. To improve the casing damage treatment effect of the well, it was decided to use casing tieback to treat the casing damage.

[0056] Step S20: Use 7″ casing to clear the well and scrape the wall to ensure that the packer can be set smoothly after leak detection in the later stage;

[0057] Step S30: Use 51 / 2″ casing to clear the well and scrape the wall. If the sand is seriously buried, adopt a reasonable sand flushing (bailing) process to treat it to the artificial bottom of the well, and ensure the smooth setting of the double-cup packer in the later stage;

[0058] Step S40: Use a 5.5" double-cup packer to temporarily shut off the pay zone and fill it with 200 m of ceramsite sand to prevent the packer from slipping during the subsequent cementing process. The double-cup packer consists of two sets of cups with a metal skeleton structure and is used in single-tube wells with moderate pressure differentials. It relies on the interference fit between the cups and the inner wall of the casing and the pressure differential within the well to seal.

[0059] Step S50: Pass the simulated wellbore string to the place where the casing diameter changes. The simulated wellbore string structure (from bottom to top): Guide cone (bottom opening 50mm) + double ladder buckle × TP-FJ (B) with water hole buckle Centralizing nipple + 51 / 2" knotless casing 1 piece Centralizing nipple + 51 / 2" knotless casing 1 piece One straightening sub + TP-FJ(P)×210 buckle + 27 / 8" drill pipe + 211×310 buckle + 31 / 2" drill pipe to the wellhead;

[0060] Step S60: Replace the oil wellhead, install a 35-35×28-70 neck flange + a 28-70×28-70 cross, and install the blowout preventer group;

[0061] Step S70: Run 139.7mm×9.17mm unhooked casing into the well to the casing reducer, adjust the casing nipple, connect the casing hanger, and lower the pressurized casing hanger. The unhooked casing string structure from bottom to top includes: 1 51 / 2" chamfered unhooked casing straightening nipple ( × 6 holes) + 10 pieces of 5.5" knotless casing + 1 5.5" knotless casing nipple + 5.5" knotless casing to the wellhead (a 5.5" knotless casing straightening nipple is run every 200m from bottom to top);

[0062] Step S80: Replace the oil wellhead, install a 28-70×28-70 tubing head spool, and install a blowout preventer assembly;

[0063] Step S90: Cementing: Lower the cementing string to 500 m above the point where the casing diameter changes. The cementing string structure from bottom to top includes: Round guide cone (with 20mm water hole at the bottom) + 27 / 8"EU tubing nipple + 51 / 2" extended rubber tube RTTS packer + 27 / 8"EUP×210 variable buckle + 27 / 8" drill pipe to the wellhead (add 20 27 / 8" drill pipes above the packer). 1 straightening nipple). Adjust the tubing string, set the packer, flush the well with reverse circulation, then cement the well, shut in the well and wait for cementing;

[0064] The structure of the righting sub is as follows Figure 2 As shown, the main body of the righting sub is Connect both ends of the body The variable button type is 210.

[0065] Step S100, drilling cement plugs and sand flushing; displacing a certain amount of cement slurry into the casing to form a structure that meets the injection and production needs;

[0066] Step S110: salvage the double-cup packer from the well, scrape the well wall, and then perform engineering logging.

[0067] like Figure 3 The schematic diagram of the casing tieback principle in this embodiment is shown. The tied-back 51 / 2" untied casing 1 is located inside the damaged casing section 2. A drill pipe 3 is also inserted into the inner side of the 51 / 2" untied casing 1. The lower end of the drill pipe 3 is connected to Righting short section 4; Install a 51 / 2" extended rubber tube RTTS packer 5 below the straightening sub 4, install a 51 / 2" knotless casing straightening sub 6 at the lower end of the tied-back 51 / 2" knotless casing 1 and set a circulation hole 7, install a 51 / 2" extended rubber tube RTTS packer 5 below A round-head guide cone (with a 20mm water hole at the bottom) + a 27 / 8" EU tubing nipple 9 is used, and a double-cup packer 8 is installed in the pay zone of the casing-damaged well.

[0068] The logging interpretation report of the cementing quality of the HDE well in this example shows that the proportion of medium-quality cementing sections in this well exceeds 95%, and there is no free casing section, indicating that the method for improving the cementing quality of small annular gaps is becoming mature.

[0069] The method proposed in this invention for improving cementing quality in small annular gaps (i.e., the casing tie-back method) has been gradually expanded in HDB, HDC, and HDD wells, and tools and process technologies have been continuously optimized and improved. To date, a cumulative oil production increase of over 7,000 tons has been achieved. The cementing quality evaluation results in HDA, HDB, HDC, and HDD wells are shown in Table 1:

[0070] Table 1 Cementing quality evaluation

[0071]

[0072]

[0073] The structural diagram of the circulation hole of the HDB well casing tail pipe short section is as follows: Figure 4 , the casing body is The holes of HDB circulation holes are It is a through hole with 6 holes;

[0074] Schematic diagram of double mushroom head straightening and circulation hole of HDD well casing Figure 5 As shown, the sleeve body is Double mushroom head straightening block The holes of the HDD circulation hole are It is a through hole with 3 holes and the rivet hole is There are 12, non-through holes.

[0075] The strength data of the casing used for the tieback newly used in this embodiment (i.e., the 51 / 2" untied casing in the untied casing string) are shown in Table 2:

[0076] Table 2 Casing strength data

[0077]

[0078] The strength check results of the casing used in this embodiment are shown in Table 3:

[0079] Table 3 Casing strength verification

[0080]

[0081] The calculation method of the external load in Table 3 is shown in Table 4:

[0082] Table 4

[0083] Strength calculation model: uniaxial stress model Effective external extrusion force considerations: 60% hollowing of the tube Calculation of liquid column pressure outside the pipe: The whole well is calculated based on the drilling fluid density of 1.16 Bottom hole pressure calculation method: Calculate according to the well fluid density of 1.16 When calculating internal pressure, the pressure outside the pipe is calculated as 1.05 for salt water. Internal pressure calculation method: hollow out 80% of the tube

[0084] In this embodiment, when the cement slurry is injected, the maximum static pressure difference is 20.4 MPa. At this time, the static pressure difference acts on the packer cross section and the casing cross section, causing the casing buoyancy weight to decrease. The specific values ​​are shown in Table 5. At this time, the casing buoyancy weight is: 88.7 (empty casing weight) + 38 (weight of the liquid in the casing) - 85.9 (buoyancy of the casing) = 40.8 t; the casing does not float up. The upward force of 20.4 MPa acting on the 73 mm drill bit cross section is:

[0085] Table 5 Buoyancy and uplift calculation table

[0086] Drill tool outer diameter mm 73 Drill cross-sectional area <![CDATA[cm 2 ]]> 18.43 Drilling tool inner diameter mm 54.61 Upward force ton 3.8 pressure Mpa 20.4 / / /

[0087] Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein; and these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for improving the quality of cementing in small annular gaps, characterized in that: The method comprises the following steps: Analyze the historical operation data of the casing damaged wells and determine whether the casing damaged wells need to be treated by casing tie-back according to the analysis results; If casing tie-back is used to treat a casing-damaged well, the first type of casing and the second type of casing should be used in sequence to perform well cleaning and wall scraping on the casing-damaged well. A double-cup packer corresponding to the second type of casing is used to temporarily seal the production layer of the casing-damaged well and perform sand filling; Extend the simulated wellbore string to the transition point between the first type casing and the second type casing. After the wellbore is completed, remove the wellbore string and dismantle the original production spool of the damaged well. Install the neck flange, casing spool and blowout preventer assembly in the place of the original production spool. Run the unhooked casing string into the damaged well to the transition point between the first and second type casings, adjust the casing nipple, connect the casing hanger by lowering and pressurizing, and then remove the blowout preventer group; Installing a neck flange and a production spool on the casing spool and reinstalling the blowout preventer stack; A cementing string is run into the unhooked casing string to a set distance above the transition point between the first type casing and the second type casing, the cementing string is adjusted and a packer is set, the well is flushed by reverse circulation, cement is squeezed and cemented, and the well is shut in and waited for setting; Drill and cement the cement plug, flush the sand and salvage the double-cup packer, and perform engineering logging after scraping the wall of the casing damaged well.

2. The method for improving the quality of cementing in small annular gaps according to claim 1, characterized in that: The analysis of historical operation data of the casing damaged well and the determination of whether the casing damaged well needs to be treated by casing tie-back according to the analysis results specifically include the following steps: Preliminary prediction of casing damage location based on historical operation data of casing damaged wells; During repair, a mechanical packer capable of multiple setting is used and the binary elimination method is used to locate the casing damage point; According to the distribution and severity of the casing damage points, it is determined whether the casing damaged well needs to be treated by casing tie-back.

3. The method for improving the cementing quality of small annular gaps according to claim 2, characterized in that: The historical operation data includes the static temperature gradient, static pressure gradient and engineering logging number before the logging operation.

4. The method for improving the cementing quality of small annular gaps according to claim 1, characterized in that: The first type of casing is a 7" casing, and the second type of casing is a 51 / 2" casing.

5. The method for improving the cementing quality of small annular gaps according to claim 1, characterized in that: The simulated wellbore string includes, from the bottom of the well to the wellhead, a guide cone, a buckle, a straightening sub, a 5.5" unhooked casing, a straightening sub, a 5.5" unhooked casing, a straightening sub, a buckle, a 2.7 / 8" drill pipe, a buckle and a 3.5" drill pipe.

6. The method for improving the cementing quality of small annular gaps according to claim 1, characterized in that: The knotless casing string comprises, from the bottom of the well to the wellhead, a 51 / 2" knotless casing straightening nipple with chamfers, a 51 / 2" knotless casing 1, a 51 / 2" knotless casing nipple and a 51 / 2" knotless casing.

7. The method for improving the quality of cementing in small annular gaps according to claim 5, characterized in that: In the untied casing string structure, a 5.5" untied casing straightening nipple is run every 200m from bottom to top.

8. The method for improving the cementing quality of small annular gaps according to claim 1, characterized in that: The cementing string includes, from bottom to top, a round-head guide cone, a 27 / 8" tubing nipple, a 51 / 2" extended rubber sleeve RTTS packer, a variable buckle and a 27 / 8" drill pipe.

9. The method for improving the cementing quality of small annular gaps according to claim 8, characterized in that: One straightening sub is added for every 20 pieces of 27 / 8" drill pipe above the 51 / 2" extended rubber sleeve RTTS packer.

10. The method for improving cementing quality in small annular gaps according to claim 1, characterized in that: The set distance is 500m above the transition point between the first type casing and the second type casing.

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