Method for evaluating shaft sealing integrity under ultra-high-temperature complex working condition

Abstract

The invention belongs to the technical field of oil and gas production, and particularly provides a shaft sealing integrity evaluation method under an ultra-high-temperature complex working condition, which comprises the following steps of: establishing a shaft pipe column-cement sheath-stratum thermosetting coupling shaft temperature and stress field finite element model under the ultra-high-temperature complex working condition according to a well body structure and production parameters; determining shaft formation temperature field distribution conditions and shaft tubular column stress distribution conditions under different use conditions; establishing a three-dimensional finite element model of the sealing performance of the threads of the shaft tubular column according to the threads, and obtaining the stress conditions and the sealing performance of the casing tubular column in different operation stages and under different load conditions. The problems that in the prior art, the ultra-high temperature service condition cannot be met, a completed oil and gas well is tripped into a leakage detection tool for detection, detection belongs to postmortem detection, and well completion design and risk identification cannot be guided are solved. The method is suitable for an ultra-high-temperature shaft environment, and for a completed oil and gas well, a leakage detection tool does not need to be put down for detection, and well completion design and risk identification can be guided.

Description

Technical Field

[0001] The present invention belongs to the technical field of oil and gas production, and particularly relates to a method for evaluating the wellbore seal integrity under ultra-high temperature and complex working conditions. Background Art

[0002] In oil and gas field development, in order to improve the recovery rate, heating processes such as formation heating and steam injection are often adopted to change the reservoir seepage channels and the physical properties of crude oil, etc., so as to increase the production of a single well.

[0003] In a certain special enhanced oil recovery process, the operating temperature of the heat source well is 550 - 650°C, the bottom hole pressure can reach about 20 - 40 MPa, the wellbore temperature of the oil and gas production well is 350 - 400°C, and the bottom hole flowing pressure is 2 - 5 MPa. Both the heat source well and the production well are in ultra-high temperature and high temperature environments.

[0004] For such a wellbore under ultra-high temperature and complex working conditions, the wellbore seal integrity is very important. The threaded connection is the weak link of the wellbore seal integrity. Under the complex load environment such as high temperature, radial and axial stresses, it is easy to cause the failure of the threaded seal, and the leakage of formation fluids such as oil, gas, and water will affect the normal production of oil and gas wells and pose potential hazards to safety, environmental protection, etc.

[0005] CN104963813B "Oil and gas well pipe string threaded seal performance detection equipment and method" discloses an oil and gas well pipe string threaded seal performance detection equipment and method, and the invented oil and gas well pipe string threaded seal performance detection equipment. The invented oil and gas well pipe string threaded seal performance detection equipment and method are especially for the detection of the seal performance of small-sized oil casing pipe strings, and can effectively avoid the large reaction force generated by the detection gas on the detection equipment after the detection, and prevent the detection equipment from being flushed out of the oil casing by high-pressure gas.

[0006] CN203275027U "Detection tool for detecting the seal performance of pipe threads" discloses a detection tool for detecting the seal performance of pipe threads. The compressed nitrogen in the tool pushes the sliding sleeve to reset to achieve automatic unsealing. It achieves the purpose of fully automatic detection of the seal performance of pipe threads, shortening the detection time, improving the success rate, and saving costs.

[0007] CN105241612B "A dynamic test device and method for the threaded seal performance of a tubing joint" relates to a dynamic test device and method for the threaded seal performance of a tubing joint in the technical field of oil pipes. Its characteristics are that a mandrel is placed inside the tubing specimen to form an annular cavity, and then both ends are sealed. The annular cavity is pressurized by a booster, and then a vibration load, a bending moment load, and an axial tensile load are respectively applied to the tubing specimen through a vibrator, a bending moment loading module, and a static load tensile module.

[0008] CN 114544081A, "Method and Device for Determining Safety Load of Threaded Pipe", discloses a method and device for determining the safety load of a threaded pipe. The technical solution provided in this document determines multiple groups of test loads that can ensure the safety and effectiveness of the threaded pipe by loading the test load spectrum of the target threaded pipe on the geometric model of the target threaded pipe and judging according to the thread sealing ability and the corresponding required seal of each group of test loads in the test load spectrum. It provides a method for quantitatively analyzing the thread sealing performance and improves the accuracy of the analysis of the thread sealing performance.

[0009] CN 103994863A, "A Gas Sealing Detection Method for Oil Casing Thread Connections", discloses a gas sealing detection method for oil casing thread connections. By using a detection tool, a gas collection sleeve and a leak detector, the gas tightness of the oil casing threads that have been made up on the well site can be detected one by one. The detection medium is a tracer gas, which can give a timely warning for unsealed threads and effectively eliminate leaking threads, providing safety guarantee for on-site detection of the overall gas tightness of oil pipes and casings.

[0010] CN 207280702U, "An Experimental Device for Continuously Testing the Gas Leakage Rate of Special Thread Sealing Surfaces", discloses an experimental device for continuously testing the gas leakage rate of special thread sealing surfaces. By an automatic make-up device, it continuously controls the contact length, interference amount and average contact stress of the sealing surface, and tests the gas leakage rate of the special thread sealing surface under high temperature and high pressure environment, providing a theoretical basis for quantitatively evaluating the gas sealing performance of special threads and optimizing the sealing parameters.

[0011] At present, there are mainly two types of evaluations of thread sealing performance at home and abroad. One is through experimental methods and theoretical methods. The evaluation methods specified in standards such as ISO13679 and API RP 5C5 establish empirical formulas based on test data and are mainly applied to environments with a temperature ≤ 180°C. The above-mentioned documents, existing devices and evaluation methods cannot meet the service conditions of ultra-high temperature (350 - 650°C). The other is for oil and gas wells that have been completed, and leakage detection tools are lowered for detection, but this belongs to post-detection and cannot guide well completion design and risk identification. Summary of the Invention

[0012] One objective of the method for evaluating wellbore seal integrity under ultra-high temperature and complex working conditions provided by the present invention is to overcome the problem that existing devices and evaluation methods cannot meet the ultra-high temperature service conditions in the prior art; the second objective is to overcome the problem that for oil and gas wells that have been completed, lowering leakage detection tools for detection belongs to post-detection and cannot guide well completion design and risk identification in the prior art.

[0013] To this end, the present invention provides a method for evaluating wellbore seal integrity under ultra-high temperature and complex working conditions, including the following steps:

[0014] S1. Establish a finite element model of the thermal-solid coupling wellbore temperature and stress field of the wellbore string-cement sheath-formation under ultra-high temperature and complex working conditions according to the actual wellbore structure and production parameters, and obtain the temperature field and stress field results at the thread position; the production parameters include the wellbore string size and the cement sheath return depth.

[0015] S2. Establish a three-dimensional solid model of the thread according to the structural parameters of the thread; the thread includes an internal thread and an external thread.

[0016] S3. Import the three-dimensional solid model of the thread into the finite element analysis software, and establish a three-dimensional finite element model of the thread sealing performance of the wellbore string by combining the temperature field and stress field results at the thread position in step 1 during the analysis.

[0017] S4. After the analysis is completed, obtain the stress, strain and contact pressure of the three-dimensional finite element model of the thread sealing performance of the wellbore string.

[0018] S5. Calculate the critical sealing performance index of the thread and the thread sealing performance index under the effective sealing contact length, and complete the evaluation and analysis of the thread sealing performance according to the comparison relationship between the critical sealing performance index of the thread and the thread sealing performance index under the effective sealing contact length.

[0019] Preferably, step S1 of establishing a finite element model of the thermal-solid coupling wellbore temperature and stress field of the wellbore string-cement sheath-formation under ultra-high temperature and complex working conditions according to the actual wellbore structure and production parameters includes the following steps:

[0020] S1.1. According to the actual wellbore structure, divide the coupled finite element model of the casing-cement sheath-formation under ultra-high temperature and complex loads according to different wellbore string sizes and cement sheath return depths respectively, and assign material properties to each component after division; the material properties include elastic modulus, Poisson's ratio and heat transfer coefficient.

[0021] S1.2. Use adaptive meshing, and combine the characteristics of pipe string lowering, cementing and production operations to control the sealing effect generated between the wellbore cement sheath and the casing at different stages in the contact setting.

[0022] S1.3. Set the load: Apply the self-weight to each stage of the wellbore string during the lowering stage, and apply friction at the lower part of the wellbore string at the same time.

[0023] S1.4. Set the constraints and boundary conditions: Apply axial constraints to the top of each stage of the wellbore string; constrain the lower part of the pipe string to the current position; during the cementing stage, apply constraints in the U2 direction to the upper and lower parts of the formation, and apply temperature to the whole formation according to the geothermal gradient; during the production stage, apply the wellbore temperature to the wellbore fluid.

[0024] S1.5, Mesh Generation: The mesh is generated with a gradually increasing density from the far-end formation to the wellbore. The mesh uses a coupled temperature-displacement mesh to establish a finite element model of the thermal-solid coupling wellbore temperature and stress field for the wellbore string-cement sheath-formation under ultra-high temperature and complex working conditions.

[0025] Preferably, in the coupled finite element model of the casing-cement sheath-formation under ultra-high temperature and complex loads in step S1.1, all components adopt a hexahedron mesh form.

[0026] Preferably, in step S2, a three-dimensional solid model of the thread is established using a parameter optimization design method based on a sketch.

[0027] Preferably, after importing the three-dimensional solid model of the thread into the finite element analysis software in step S3, material properties are assigned to the internal thread and the external thread respectively.

[0028] Preferably, in step S3, 3 analysis steps are set during the analysis. The 3 analysis steps are a static analysis step, a first temperature-displacement coupled analysis step, and a second temperature-displacement coupled analysis step; the static analysis step is a simulation of the pre-tightening analysis step of the make-up torque, the first temperature-displacement coupled analysis step is an analysis step where the internal thread and the external thread bear internal and external pressures and temperature loads, and the second temperature-displacement coupled analysis step is an analysis step where the internal thread bears axial external loads and temperature loads.

[0029] Preferably, after setting the 3 analysis steps in step S3, boundary conditions are set, specifically:

[0030] The internal thread remains fully constrained in all 3 analysis steps;

[0031] For the external thread, the displacement in the axial direction of the three-dimensional solid model of the thread is constrained in the static analysis step to simulate the make-up process of the thread, and it is not constrained in the subsequent analysis steps; the temperature boundary condition is applied to the entire three-dimensional solid model of the thread; no external load needs to be set in the analysis step of simulating the make-up process.

[0032] In the first temperature-displacement coupled analysis step, a pressure load is applied to the inner surfaces of the internal thread component and the external thread component.

[0033] In the second temperature-displacement coupled analysis step, an axial tensile or compressive load is applied to the reference point of the external thread.

[0034] Preferably, for the critical sealing performance index W ac of the thread, the calculation formula is where: p gas is the atmospheric pressure, MPa; p atm is the internal pressure in the pipe to be sealed by the thread, MPa.

[0035] Preferably, the calculation formula of the thread sealing performance index under the effective sealing contact length is: Where: L es is the effective sealing contact length, mm; P is the contact pressure.

[0036] Preferably, the effective sealing contact length includes three effective contact lengths, namely: the contact length between the shoulder and the sealing surface, the contact length between the shoulder and the half of the threaded meshing area, and the entire contact length between the shoulder and the threaded meshing area.

[0037] Beneficial effects of the present invention:

[0038] 1. The method for evaluating the sealing integrity of wellbore under ultra-high temperature and complex working conditions provided by the present invention establishes a finite element model of the wellbore temperature and stress field of the wellbore tubing-cement ring-formation thermal-solid coupling under ultra-high temperature and complex working conditions according to the wellbore structure and service environment, and determines the distribution of the wellbore formation temperature field and the stress distribution of the wellbore tubing under different use conditions (casing, cementing, and oil and gas well production); for the threads, a three-dimensional finite element model of the sealing performance of the wellbore tubing threads is established to obtain the stress conditions and sealing performance of the casing tubing under different operation stages and different load conditions. This method is particularly suitable for ultra-high temperature wellbore environments, and can also be widely promoted in the fields of thermal recovery wells, geothermal wells, etc. For completed oil and gas wells, the present invention can guide the problems of completion design and risk identification without the need to insert leakage detection tools for detection.

[0039] 2. The method for evaluating the wellbore sealing integrity under ultra-high temperature and complex working conditions provided by the present invention can simultaneously consider complex working conditions such as internal pressure, temperature, axial load, bending load, etc., and obtain the sealing performance and connection strength of the thread under different working conditions by extracting the stress, strain and contact pressure of the three-dimensional finite element model of the wellbore tubing thread sealing performance under different working conditions. Finally, the change in the sealing performance of the target thread during the in-situ conversion of shale oil is obtained through multiple groups of finite element simulation results.

[0040] 3. The method for evaluating the sealing integrity of wellbore under ultra-high temperature and complex working conditions provided by the present invention uses interference fit to simulate the make-up torque of the target thread. It is simple and efficient while ensuring the make-up torque, and the calculation results are easy to converge. During the meshing process, the nonlinear geometric contour area (shoulder, sealing surface, thread engagement area) of the contact between the internal and external threads is meshed, and a sensitivity analysis is performed on the number of meshes, ultimately obtaining a suitable mesh density that can save computer resources while ensuring accurate calculation results. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] The present invention will be further described in detail below with reference to the accompanying drawings.

[0042] Figure 1It is a schematic flow diagram of the wellbore seal integrity evaluation method under ultra-high temperature and complex working conditions;

[0043] Figure 2 It is a schematic flow diagram of step S1;

[0044] Figure 3 It is a schematic flow diagram of step S3;

[0045] Figure 4 It is a contour map of the temperature field distribution of the entire wellbore in the ultra-high temperature oil and gas well in the embodiment;

[0046] Figure 5 It is a three-dimensional solid model diagram of a certain thread in the embodiment;

[0047] Figure 6 It is a schematic diagram of the three-dimensional finite element model and load application of a certain thread in the embodiment;

[0048] Figure 7 It is a stress contour map of a certain thread under the action of the optimal make-up torque and the working condition of 200 °C in the embodiment;

[0049] Figure 8 It is a schematic diagram of the sealing performance of a certain thread under the action of 350 °C and the simultaneous action of 350 °C and 120 tons of axial tensile load in the embodiment;

[0050] Figure 9 It is the change of the safety margin of a certain thread under ultra-high temperature and complex working conditions in the embodiment. Detailed implementation mode

[0051] The principles and features of the present invention will be described below with reference to the accompanying drawings. The examples given are only used to explain the present invention and are not intended to limit the scope of the present invention. Those not specified in the examples are carried out according to conventional conditions or conditions recommended by the manufacturer. Those reagents or instruments not specified by the manufacturer can be obtained as conventional products through commercial purchase.

[0052] As Figure 1 shown, a wellbore seal integrity evaluation method under ultra-high temperature and complex working conditions includes the following steps:

[0053] S1. According to the actual wellbore structure and production parameters, establish a finite element model of the thermal-solid coupling wellbore temperature and stress field of the wellbore string-cement sheath-formation under ultra-high temperature and complex working conditions, and obtain the temperature field and stress field results at the thread position (see Figure 4 ); the production parameters include the wellbore string size and the cement sheath return depth;

[0054] Preferably, the step S1 of establishing a finite element model of the thermal-solid coupling wellbore temperature and stress field of the wellbore string-cement sheath-formation under ultra-high temperature and complex working conditions according to the actual wellbore structure and production parameters includes the following steps:

[0055] S1.1. According to the actual wellbore structure, the finite element model of the casing-cement sheath-formation coupling under ultra-high temperature and complex loads is meshed respectively according to different wellbore string sizes and cement ring return depths, and material properties are assigned to each component after meshing; the material properties include elastic modulus, Poisson's ratio and heat transfer coefficient.

[0056] Preferably, all components in the finite element model of the casing-cement sheath-formation coupling under ultra-high temperature and complex loads in step S1.1 adopt hexahedral mesh form.

[0057] S1.2. Using adaptive meshing, combined with the characteristics of string lowering, cementing and production operations, control the sealing effect generated between the wellbore cement sheath and the casing at different stages in the contact setting.

[0058] Specifically, the adaptive mesh is ALE adaptive mesh. The ALE adaptive mesh keeps a relatively good state during the whole analysis process without huge distortion and deformation. The adaptive method of the ALE adaptive mesh enables the mesh to flow independently of the material, which can improve the mesh condition and keep the mesh in a relatively good state during the whole analysis process. The ALE mesh adaptive method will not change the topological structure of the mesh. The ALE adaptive mesh technology is adopted in the contact module of the model to restore the forces between the cement sheath and the casing before and after cementing.

[0059] S1.3. Set the loads: Apply the self-weight to each stage of the wellbore string during the lowering stage, and apply friction at the lower part of the wellbore string at the same time.

[0060] S1.4. Set the constraints and boundary conditions: Apply constraints to the top axial direction (Y direction) of each stage of the wellbore string; Constrain the lower part of the string to the current position; During the cementing stage, apply constraints to the upper and lower parts of the formation in the U2 direction, and apply temperature to the whole formation according to the geothermal gradient; During the production stage, apply the wellbore temperature to the wellbore fluid.

[0061] S1.5. Mesh generation: The mesh generation follows the trend of gradually densifying from the distal formation to the wellbore. The mesh adopts temperature-displacement coupled mesh to complete the establishment of the finite element model of the thermal-solid coupling of the wellbore string-cement sheath-formation temperature and stress field under ultra-high temperature and complex working conditions.

[0062] Specifically, the mesh generation follows the trend of gradually densifying from the distal formation to the wellbore, which can ensure the accuracy of the calculation results.

[0063] As Figure 2 shown, S2. Establish a three-dimensional solid model of the thread according to the structural parameters of the thread (see Figure 5 ); The thread includes internal thread and external thread.

[0064] Preferably, in step S2, a three-dimensional solid model of the thread is established by using a sketch-based parameter optimization design method.

[0065] As Figure 3 shown, S3. Import the three-dimensional solid model of the thread into finite element analysis software. When analyzing, combine the temperature field and stress field results at the thread position in step 1 to establish a three-dimensional finite element model for the thread sealing performance of the wellbore string (see Figures 6 - 9 shown);

[0066] Preferably, after importing the three-dimensional solid model of the thread into the finite element analysis software in step S3, material properties are assigned to the internal thread and the external thread respectively.

[0067] Preferably, in step S3, 3 analysis steps are set during analysis. The 3 analysis steps are a static analysis step, a first temperature-displacement coupling analysis step, and a second temperature-displacement coupling analysis step; the static analysis step is a simulation of the make-up torque pre-tightening analysis step, the first temperature-displacement coupling analysis step is an analysis step for the internal thread and the external thread to bear internal and external pressures and temperature loads, and the second temperature-displacement coupling analysis step is an analysis step for the internal thread to bear axial external loads and temperature loads.

[0068] Preferably, after setting the 3 analysis steps in step S3, boundary conditions are set. Specifically:

[0069] The internal thread remains fully constrained in all 3 analysis steps;

[0070] For the external thread, the displacement of the three-dimensional solid model of the thread in the axial direction is constrained in the static analysis step to simulate the make-up process of the thread, and it is not constrained in the subsequent analysis steps; the temperature boundary condition is applied to the whole of the three-dimensional solid model of the thread; no external load needs to be set in the analysis step for simulating the make-up process;

[0071] In the first temperature-displacement coupling analysis step, a pressure load is applied to the inner surfaces of the internal thread component and the external thread component;

[0072] In the second temperature-displacement coupling analysis step, an axial tensile or compressive load is applied to the reference point of the external thread.

[0073] Specifically, the application of the load is flexible, and it is possible to select whether to enable an analysis step, add an analysis step, or adjust the magnitude and direction of the load according to the actual working conditions.

[0074] Preferably, mesh encryption is performed on the shoulder and thread contact parts of the three-dimensional solid model of the thread.

[0075] Specifically, the interference fit is used to simulate the make-up torque of the target thread, which is simple, efficient, and easy to converge under the premise of ensuring the make-up torque. During the mesh generation process, the non-linear geometric profile areas of the internal and external thread contacts (shoulders, sealing surfaces, thread engagement areas) are encrypted, and a sensitivity analysis of the number of meshes is carried out. Finally, a suitable mesh density is obtained to save computer resources while ensuring accurate calculation results.

[0076] S4. After the analysis is completed, the stress, strain, and contact pressure of the three-dimensional finite element model of the thread sealing performance of the wellbore string are obtained;

[0077] Specifically, after the analysis is completed, the output results of the Mises stress, equivalent plastic strain, and contact pressure of the three-dimensional finite element model of the thread sealing performance of the wellbore string are extracted.

[0078] S5. Calculate the thread critical sealing performance index and the thread sealing performance index under the effective sealing contact length. According to the comparison relationship between the thread critical sealing performance index and the thread sealing performance index under the effective sealing contact length, complete the evaluation and analysis of the thread sealing performance.

[0079] Preferably, the thread critical sealing performance index W ac has the following calculation formula where: p gas is the atmospheric pressure, MPa; p atm is the internal pressure of the pipe to be sealed by the thread, MPa.

[0080] Preferably, the calculation formula for the thread sealing performance index under the effective sealing contact length is: where: L es is the effective sealing contact length, mm; P is the contact pressure.

[0081] Specifically, calculate the area integral of the contact length and contact pressure on the thread shoulder surface, sealing surface, and each thread tooth engagement surface as the thread sealing performance index W ac under the effective sealing contact length, providing comparison data for the subsequent quantitative evaluation of the thread sealing performance.

[0082] Preferably, when the thread sealing performance index under the effective sealing contact length is greater than the thread critical sealing performance index, it indicates that the thread provides sufficient sealing ability. The connection sealing ability of the thread under the temperature of this working condition can be accurately evaluated and analyzed.

[0083] Preferably, the effective sealing contact length includes three effective contact lengths, namely: the contact length between the shoulder and the sealing surface, half of the contact length between the shoulder and the thread engagement area, and the overall contact length between the shoulder and the thread engagement area.

[0084] Specifically, the sealing performance of the threads under three effective contact lengths is used to gain a clearer understanding of the sealing conditions at various positions of the special threads.

[0085] The present invention uses the finite element method to accurately calculate the downhole temperature and pressure fields and the sealing performance of the joint threads, breaking through the highest temperature limit given by the indoor test standard for thread sealing performance; combined with the sealing performance evaluation index and the critical sealing performance index of the threads under the three effective contact lengths mentioned above, the leakage conditions and safety margins at various positions of the thread connection under ultra-high temperature complex working conditions can be determined, rather than simply obtaining a conclusion of "leakage" or "no leakage". Taking the calculation result of the sealing performance of half of the contact length of the shoulder and thread engagement area as an example, when the internal pressure is 25 MPa, the critical sealing index is 10588 mm·MPa 1.4 , and at this time, the sealing performance evaluation index calculated for half of the contact length of the shoulder and thread engagement area is 37316 mm·MPa 1.4 , then the safety margin at this time is 1.78. The evaluation results are more intuitive and accurate.

[0086] The present invention is particularly suitable for ultra-high temperature wellbore environments and can also be widely promoted in fields such as thermal recovery wells and geothermal wells. For completed oil and gas wells, the present invention can guide the completion design and risk identification problems without the need to lower a leakage detection tool for detection.

[0087] In the description of the present invention, it should be understood that if the orientation or positional relationship indicated by terms etc. is based on the orientation or positional relationship shown in the drawings, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, so the terms describing the positional relationship in the drawings are only for illustrative purposes and cannot be understood as a limitation of the present invention.

[0088] The above examples are only illustrative of the present invention and do not constitute a limitation on the protection scope of the present invention. Any design identical or similar to the present invention falls within the protection scope of the present invention.

Claims

1. A method for evaluating the wellbore seal integrity under ultra-high temperature and complex working conditions, characterized in that: The steps are as follows: S1. According to the actual wellbore structure and production parameters, establish a finite element model of the thermal-solid coupling wellbore temperature and stress field of the wellbore string - cement sheath - formation under ultra-high temperature and complex working conditions, and obtain the temperature field and stress field results at the thread position; the production parameters include the wellbore string size and the cement sheath return depth. S2. According to the structural parameters of the thread, establish a three-dimensional solid model of the thread; the thread includes internal threads and external threads. S3. Import the three-dimensional solid model of the thread into the finite element analysis software, and when analyzing, combine the temperature field and stress field results at the thread position in step 1 to establish a three-dimensional finite element model of the thread sealing performance of the wellbore string. S4. After the analysis is completed, obtain the stress, strain and contact pressure of the three-dimensional finite element model of the thread sealing performance of the wellbore string. S5. Calculate the thread critical sealing performance index and the thread sealing performance index under the effective sealing contact length, and complete the thread sealing performance evaluation analysis according to the comparison relationship between the thread critical sealing performance index and the thread sealing performance index under the effective sealing contact length.

2. The method for evaluating the wellbore seal integrity under ultra-high temperature and complex working conditions according to claim 1, wherein: The step S1 of establishing a finite element model of the thermal-solid coupling wellbore temperature and stress field of the wellbore string - cement sheath - formation under ultra-high temperature and complex working conditions according to the actual wellbore structure and production parameters includes the following steps: S1.

1. According to the actual wellbore structure, respectively divide the finite element model of the casing - cement sheath - formation coupling under ultra-high temperature and complex loads according to different wellbore string sizes and cement sheath return depths, and assign material properties to each component after division; the material properties include elastic modulus, Poisson's ratio and heat transfer coefficient. S1.

2. Use adaptive meshing, and combine the characteristics of pipe string lowering, cementing and production operations to control the sealing effect generated between the wellbore cement sheath and the casing at different stages in the contact setting. S1.

3. Set the loads: Apply the self-weight to each stage of the wellbore string during the lowering stage, and at the same time apply friction resistance to the lower part of the wellbore string. S1.

4. Set the constraints and boundary conditions: Apply axial constraints to the top of each stage of the wellbore string; constrain the lower part of the pipe string to the current position; during the cementing stage, apply constraints to the upper and lower parts of the formation in the U2 direction, and apply temperature to the whole formation according to the geothermal gradient; during the production stage, apply the wellbore temperature to the wellbore fluid. S1.

5. Mesh generation: The mesh generation follows the trend of gradually densifying from the distal formation to the wellbore. The mesh uses a temperature-displacement coupled mesh to complete the establishment of the finite element model of the thermal-solid coupling wellbore temperature and stress field of the wellbore string - cement sheath - formation under ultra-high temperature and complex working conditions.

3. The method for evaluating the wellbore seal integrity under ultra-high temperature and complex working conditions according to claim 2, wherein: In the finite element model of the casing - cement sheath - formation coupling under ultra-high temperature and complex loads in step S1.1, all components adopt a hexahedron mesh form.

4. The method for evaluating the wellbore seal integrity under ultra-high temperature and complex working conditions according to claim 1, characterized in that: The step S2 uses a parameter optimization design method based on sketches to establish a three-dimensional solid model of the thread.

5. The method for evaluating the wellbore seal integrity under ultra-high temperature and complex working conditions according to claim 1, wherein: After importing the three-dimensional solid model of the thread into the finite element analysis software in step S3, assign material properties to the internal threads and external threads respectively.

6. The method for evaluating the wellbore seal integrity under ultra-high temperature and complex working conditions according to claim 1, wherein: In the analysis in step S3, three analysis steps are set, which are a static analysis step, a first temperature-displacement coupling analysis step, and a second temperature-displacement coupling analysis step; the static analysis step is a simulation of the pre-tightening analysis step of the make-up torque, the first temperature-displacement coupling analysis step is an analysis step for the internal and external threads to bear internal and external pressures and temperature loads, and the second temperature-displacement coupling analysis step is an analysis step for the internal thread to bear axial external loads and temperature loads.

7. The method for evaluating the wellbore seal integrity under ultra-high temperature and complex working conditions according to claim 6, wherein: After setting the three analysis steps in step S3, boundary conditions are set, specifically: The internal thread remains fully constrained in all three analysis steps; For the external thread, the displacement in the axial direction of the three-dimensional solid model of the thread is constrained in the static analysis step to simulate the make-up process of the thread, and it is not constrained in the subsequent analysis steps; the temperature boundary condition is applied to the whole three-dimensional solid model of the thread; no external load needs to be set in the analysis step for simulating the make-up process; In the first temperature-displacement coupling analysis step, a pressure load is applied to the inner surfaces of the internal thread component and the external thread component; In the second temperature-displacement coupling analysis step, an axial tensile or compressive load is applied to the reference point of the external thread.

8. The method for evaluating the wellbore seal integrity under ultra-high temperature and complex working conditions according to claim 1, wherein: The critical sealing performance index W of the thread ac is calculated by the formula where: p gas is the atmospheric pressure, MPa; p atm is the internal pressure of the pipe to be sealed by the thread, MPa.

9. The method for evaluating the wellbore seal integrity under ultra-high temperature and complex working conditions according to claim 1, wherein: The calculation formula for the thread sealing performance index under the effective sealing contact length is as follows: Where: L es is the effective sealing contact length, in mm; P is the contact pressure.

10. The method for evaluating the wellbore seal integrity under ultra-high temperature and complex working conditions according to claim 9, wherein: The effective sealing contact length includes three types of effective contact lengths, namely: the contact length between the shoulder and the sealing surface, half of the contact length between the shoulder and the thread engagement zone, and the overall contact length between the shoulder and the thread engagement zone.

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