Method for evaluating applicability of geothermal buried straight welded pipe

Abstract

The invention discloses a geothermal buried straight weld pipe applicability evaluation method, which belongs to the technical field of geothermal engineering, and comprises the steps of service environment base material and weld corrosion behavior evaluation, post-corrosion residual external pressure collapse resistance strength evaluation and heat conduction efficiency evaluation. The service environment base material and weld corrosion behavior evaluation comprises the steps of determining a corrosion environment and a medium, detecting corrosion morphology and corrosion rate of a base material and a weld, and detecting a groove corrosion result of the weld under an electrochemical corrosion condition; the evaluation of the residual external pressure collapse resistance after corrosion comprises the evaluation of the collapse resistance after uniform corrosion of the base metal and corrosion of the weld groove; the heat conduction efficiency evaluation comprises testing of the heat conductivity coefficient of the material. According to the method, the applicability evaluation selection method based on the use performance is achieved, the multiple purposes of preferably selecting the corrosion-resistant pipe meeting the requirement for external pressure collapse resistance of the geothermal well, reducing the pipe cost and improving the underground heat exchange efficiency are achieved, and a scientific, comprehensive and better solution is provided for selection of the geothermal buried straight weld pipe.

Description

Technical Field

[0001] The present invention mainly relates to the field of geothermal engineering technology, and in particular to a method for evaluating the applicability of geothermal buried straight welded pipes. Background Art

[0002] Medium-deep geothermal buried pipes are mainly used for heat exchange medium heat extraction. At present, the production casing steel for medium-deep geothermal buried pipes adopts API 5CT standard petroleum industry carbon steel J55, N80 and other seamless pipes. The cost is higher than the welded pipe. In addition, the main problems of welded pipes are oxygen corrosion on the surface of the casing base material and the weld, and the high mineralization of chloride ions (Cl - ) and carbon dioxide corrosion. In the existing technology, epoxy powder coating or alloy addition is used to form corrosion-resistant alloy to achieve anti-corrosion effect. These anti-corrosion treatment technologies mainly have the following problems in application: high coating anti-corrosion cost and poor high-temperature resistance (the coating cost of 7-inch casing is 130-150 yuan per meter, and the cost of 1 casing is increased by about 1,400 yuan based on 10 meters, and the coating's high-temperature resistance does not exceed 120°C, and its service life is short in high-temperature environment); high cost of corrosion-resistant alloying anti-corrosion treatment (the main technical means is to add alloy elements such as chromium, nickel, and molybdenum to steel to improve the corrosion resistance of steel. However, the addition of these alloy elements greatly increases the material cost. For example, the use of 3Cr to increase the mass percentage of alloy increases the cost of carbon steel by more than 2,000 yuan per ton); decreased thermal conductivity (the current medium and deep geothermal buried pipes only focus on anti-corrosion performance and ignore their thermal conductivity. During the anti-corrosion treatment process, the thermal conductivity of the medium and deep geothermal buried pipes decreases).

[0003] Due to these issues, existing mid- to deep-layer geothermal buried pipes have high construction costs and low heat exchange efficiency during use. Selecting suitable welded pipes for geothermal buried pipes can reduce construction costs and improve heat exchange efficiency to a certain extent. However, the existing technology lacks a suitable evaluation method for geothermal buried welded pipes that meets these requirements.

[0004] In addition, the existing technology for evaluating the corrosion resistance of pipes only focuses on direct parameters such as the corrosion morphology and corrosion rate of the material, without considering the performance of the material after corrosion, such as the external pressure crushing strength, resulting in an incomplete evaluation of the corrosion resistance of the material. Summary of the Invention

[0005] The technical solution of the present invention addresses the technical problem that the existing technical solutions are too single, and provides a solution that is significantly different from the existing technology. It mainly provides a method for evaluating the applicability of geothermal buried straight welded pipes to select low-cost, corrosion-resistant and high-thermal conductivity pipes, thereby reducing the construction cost of medium and deep geothermal buried pipes and improving their heat exchange efficiency during application.

[0006] The technical solution adopted by the present invention to solve the above technical problems is:

[0007] A geothermal buried welded pipe suitability evaluation method includes evaluation of the corrosion behavior of the base metal and weld in the service environment, evaluation of the residual anti-external pressure collapse strength after corrosion, and evaluation of the heat conduction efficiency. Ultimately, the applicability of the geothermal buried straight welded pipe is determined by combining the three evaluations.

[0008] The evaluation of the corrosion behavior of the base metal and weld in the service environment includes four steps: determining the corrosion environment and medium, selecting factors affecting the material's corrosion resistance, detecting the corrosion morphology and corrosion rate of the base metal and weld, and detecting the groove corrosion results of the weld under electrochemical corrosion conditions.

[0009] The evaluation of the residual anti-external pressure collapse strength after corrosion includes the evaluation of the collapse strength after uniform corrosion of the base material and corrosion of the weld groove;

[0010] The heat conduction efficiency evaluation includes testing the thermal conductivity of materials.

[0011] Furthermore, the corrosive environment and media are specifically determined as follows: geothermal buried pipes are used to stabilize underground wellbores and heat surface injection water. Therefore, the corrosive environment temperature is between 60°C and 100°C, and the corrosive media primarily include dissolved oxygen, carbon dioxide, chloride ions, flow rate, and pH value in the water. The clear determination of the corrosive environment temperature and media provides a foundation for selecting pipe materials for corrosion resistance and further verifies the pipe's suitability for environmental corrosion resistance.

[0012] Furthermore, factors influencing material corrosion resistance are specifically selected as follows: Mechanical property testing, metallographic microstructure analysis, and residual stress testing based on material mechanical property standards are conducted for the proposed geothermal buried pipe material. Coupon corrosion rate evaluation tests are conducted based on the actual operating conditions of the geothermal buried pipe, taking into account temperature, corrosive media, and flow rate. The test duration can be determined based on significant changes in the specimen's geometric dimensions. Material suitability is determined by changes in corrosion rate, and the degree of corrosion is assessed according to relevant standards.

[0013] Furthermore, during the corrosion morphology and corrosion rate testing of the parent metal and weld, a suitability evaluation method based on the actual downhole corrosion environment can be established to assess whether the corrosion resistance of the pipe meets the requirements for downhole use. Sampling of the weld and parent metal is along the length of the pipe, with weld corrosion specimens sampled along the center of the weld fusion line. Coupon corrosion rate evaluation tests are conducted, and the corrosion morphology is observed.

[0014] Furthermore, an evaluation of weld groove corrosion was conducted, and the groove corrosion sensitivity coefficient was tested using an electrochemical workstation. The groove corrosion test applied a constant potential electrochemical polarization method using a three-electrode system. A constant potential of -550mV was applied to the specimen, and the specimen was in an anodic polarization state. The electrochemical driving forces of the weld and the base material were different. After polarization for 144 hours, the geometric parameters of the corrosion groove were measured, and the groove corrosion sensitivity coefficient was calculated. The groove corrosion sensitivity coefficient α was used as an evaluation index and is defined as α=h2 / h1, where h2 and h1 are the depth from the original surface to the bottom of the corrosion groove and the corrosion depth of the base material before the corrosion test, respectively.

[0015] Furthermore, two types of post-corrosion residual compressive collapse strength evaluations were conducted. The first was uniform corrosion of the base metal and welds; the second was weld groove corrosion. Using actual external pressure collapse and theoretical simulation analysis, the reduction in actual collapse strength under different corrosion levels was compared. The change in collapse mechanism was the primary cause of the significant decrease in collapse strength, and a sensitivity coefficient for groove corrosion against external pressure collapse was established. In the evaluation of residual post-corrosion compressive collapse strength, based on corrosion environment testing of full-scale actual pipes, the residual wall thickness of the base metal due to uniform corrosion and the depth of weld groove corrosion were measured. A comparative analysis of the residual collapse strength was conducted to establish a groove corrosion evaluation sensitivity coefficient.

[0016] Furthermore, factors influencing material thermal conductivity are specifically selected: chemical composition, grain size, temperature environment, and metallographic structure. Physical and chemical properties are used to quantify these factors. Material selection is based on key factors (carbon content in chemical composition, grain size, and applicable ambient temperature) to preliminarily determine the intended pipe material. This allows for the subsequent determination of test materials for thermal conductivity coefficient measurements, allowing for the selection of suitable geothermal buried pipe materials.

[0017] Furthermore, the evaluation of the thermal conductivity coefficient is specifically as follows: GB / T 3651-2008 Metal High-Temperature Thermal Conductivity Measurement Method Standard is used to select the preferred geothermal buried pipe material based on the factors affecting the thermal conductivity of the material. The thermal conductivity coefficient is tested for the medium-deep temperature of 60-100 °C, and the material with high thermal conductivity is selected to improve the heat exchange efficiency of the geothermal buried pipe material.

[0018] Furthermore, the suitability evaluation method for straight welded geothermal buried pipes is based on the three elements of geothermal buried pipe suitability: evaluation of the corrosion behavior of the base material and weld in the service environment, evaluation of the residual anti-external pressure collapse strength after corrosion, and evaluation of the heat conduction efficiency. Finally, the suitability of the straight welded pipe of the corresponding pipe material as a geothermal buried pipe is determined by comprehensively evaluating the three aspects.

[0019] Compared with the prior art, the present invention has the following beneficial effects:

[0020] (1) The present invention selects and evaluates the applicability of geothermal buried pipes from three aspects: corrosion status, residual anti-extrusion strength after corrosion, and thermal conductivity. Compared with the currently used medium-deep geothermal buried pipes (which place too much emphasis on anti-corrosion performance, fail to consider the actual load-bearing performance, and ignore their thermal conductivity, resulting in excessively high anti-corrosion treatment costs and decreased thermal conductivity of medium-deep geothermal buried pipes), the present invention is based on the actual performance of the pipes, uses the residual anti-external pressure strength after corrosion as a criterion, evaluates the corrosion resistance requirements, and focuses on thermal conductivity efficiency, selecting pipes with high thermal conductivity efficiency, which effectively reduces the anti-corrosion cost and ultimately provides better thermal conductivity efficiency. This corrosion resistance applicability evaluation system based on actual performance can comprehensively weigh the various properties of the pipes and select pipes that perform well in key performance. This achieves reasonable corrosion resistance requirements, effectively reduces pipe costs, and improves the multiple goals of underground heat exchange efficiency. The present invention provides a more scientific, comprehensive, and advantageous solution for the applicability evaluation of geothermal buried straight welded pipes.

[0021] (2) The evaluation of corrosion resistance in the present invention is based on the residual compressive strength after two types of corrosion, namely uniform corrosion and weld groove corrosion, which occur in welded pipes. At the same time, the material's own factors, including the chemical composition of alloy elements, metallographic structure, grain size, impact energy, yield strength, hardness, and residual stress, are taken into account. Compared with the existing technology (the existing selection factors mainly include: material structure, hardness, chemical composition, and do not consider the effect of residual stress on corrosion), the present invention fully considers the effect of residual stress distribution and size on corrosion during the pipe manufacturing process, and incorporates it into the key considerations for corrosion resistance evaluation, forming a more comprehensive and accurate evaluation system, so that the corrosion resistance of pipes can be analyzed more comprehensively and deeply. The residual compressive strength groove corrosion evaluation coefficient after corrosion was experimentally studied, and the residual stress distribution size of the material structure of two materials J55 and N80Q with two different production processes was microscopically detected by blind hole method. The corrosion rate was compared under the same corrosion environment. It was found that residual stress is the key factor causing welded pipe corrosion. Therefore, based on the residual stress distribution and size of the corrosion factor, the normalized state structure ferrite + pearlite was selected instead of the tempered bainite structure considered in the traditional production process.

[0022] The present invention will be explained in detail below with reference to the accompanying drawings and specific embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] FIG1 is a metallographic microstructure diagram of two materials, J55 and N80Q, in an embodiment of the present invention; FIG1 (a) is a metallographic microstructure diagram of the HFW weld of J55; FIG1 (b) is a metallographic microstructure diagram of the HFW weld of N80Q; FIG1 (c) is a metallographic microstructure diagram of the HFW base material of J55; FIG1 (d) is a metallographic microstructure diagram of the HFW base material of N80Q;

[0024] Figure 2 This is a diagram showing the position of the pipe during the residual stress test using the blind hole method according to an embodiment of the present invention;

[0025] FIG3 is a surface morphology of the J55 and N80Q material samples after corrosion in an embodiment of the present invention; FIG3 is a surface morphology of the J55 weld; FIG3 is a surface morphology of the N80Q weld; FIG3 is a surface morphology of the J55 base material; FIG3 is a surface morphology of the N80Q base material; FIG3 is a surface morphology of the J55 base material; FIG3 is a surface morphology of the N80Q base material;

[0026] FIG4 is a microscopic image of the corrosion morphology of the welds of J55 and N80Q materials in an embodiment of the present invention, wherein FIG (a) is a microscopic feature image of the morphology of the J55 weld; FIG (b) is a microscopic feature image of the morphology of the N80Q weld;

[0027] FIG5 is a microscopic image of the groove corrosion morphology of the welds of J55 and N80Q materials in an embodiment of the present invention, wherein FIG55 (a) is a microscopic feature image of the morphology of the J55 weld; FIG50 (b) is a microscopic feature image of the morphology of the N80Q weld;

[0028] Figure 6 This is a morphology of the pipe crushed by external pressure after uniform corrosion of N80Q material and electrochemical groove corrosion of the weld in an embodiment of the present invention;

[0029] Figure 7 It is a step block diagram of the present invention. DETAILED DESCRIPTION

[0030] To facilitate understanding of the present invention, the present invention will be described more comprehensively below with reference to the relevant drawings. Several embodiments of the present invention are given in the drawings. However, the present invention can be implemented in different forms and is not limited to the embodiments described in the text. On the contrary, these embodiments are provided to make the content disclosed in the present invention more thorough and comprehensive.

[0031] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly used by those skilled in the art to which the present invention pertains. The terms used in the specification of the present invention are for the purpose of describing specific embodiments only and are not intended to limit the present invention. The term "and / or" used herein includes any and all combinations of one or more of the related listed items.

[0032] Example: Please refer to the attached Figure 7 A method for evaluating the suitability of a geothermal buried straight welded pipe comprises the following steps:

[0033] 1. Evaluation of corrosion behavior of base metal and weld in service environment

[0034] (1) Determination of corrosion environment and medium

[0035] The corrosion environment is primarily determined by ambient temperature, chloride ion concentration, pH value, carbon dioxide content in the water, and circulation rate. Geothermal buried pipes are used underground in underground wells. The geotechnical composition of the strata varies significantly depending on the location (for example, the Xi'an area and the Loess Plateau in northern Shaanxi are completely different), resulting in completely different corrosive media. Differences in chloride ion content in the rock and soil after water injection lead to differences in chloride ion concentration in the water. This includes pH value, geothermal gradient, carbon dioxide partial pressure, and the injection and production flow rate of each well. Therefore, corrosion resistance evaluation requires consideration of the specific underground environment.

[0036] In this embodiment, the underground working environment for investigating geothermal buried pipes is: Cl - Concentration: 33687 mg / L; pH: 6.0; water circulation flow rate: 2.0 m / s; CO2 partial pressure: 2.0 MPa; ambient temperature: 80°C.

[0037] The determination of the corrosive environment temperature and medium is clear, which provides a basis for the selection of the corrosion resistance of the pipe and can further verify the environmental corrosion resistance applicability of the pipe.

[0038] (2) Selection of factors affecting material corrosion resistance

[0039] Corrosion factors include material factors such as alloying element composition, metallographic structure, grain size, impact energy, yield strength, hardness, and residual stress; and environmental factors such as temperature, corrosive medium composition, concentration, and partial pressure, and fluid flow rate. By quantifying these factors, a suitability evaluation method based on measured downhole corrosion environments can be established to assess whether the uniform corrosion resistance of the tubing meets requirements for downhole use.

[0040] An analysis and comparison of J55 and N80Q pipes was conducted to examine factors significantly influencing corrosion. Chemical composition data is shown in Table 1. Tensile mechanical properties are shown in Table 2. Metallographic structures are shown in Table 3 and Figure 1 (based on the microscopic examination method for the rating chart in accordance with GBT 10561-2023, Determination of Non-Metallic Inclusions in Steel). Impact fracture toughness is shown in Table 4. Residual stress testing results using the blind hole method are shown in Table 5.

[0041] Table 1 Chemical composition of materials

[0042]

[0043] Table 2 Tensile mechanical properties

[0044]

[0045] Table 3 Metallographic structure and grain size

[0046]

[0047] Note: In Table 3, A: sulfide; B: alumina; C: silicate; D: spherical oxide; F: ferrite; P: pearlite; S: tempered troostite.

[0048] Table 4 Impact fracture toughness #timg# Table 5 Residual stress test results of blind hole method (MPa)

[0049] Note: The various positions of the pipes in Table 5 are as follows Figure 2 shown.

[0050] (3) Corrosion morphology and corrosion rate detection of base metal and weld

[0051] Based on the actual operating conditions of geothermal buried pipes, coupon corrosion rate evaluation tests are conducted based on temperature, corrosive media, and flow rate. The test duration can be determined based on significant changes in the sample geometry. The suitability of the material is determined by the change in corrosion rate, and the degree of corrosion is evaluated according to relevant standards.

[0052] The blind hole method, a localized microscopic residual stress test method, was used to examine the residual stresses generated by different manufacturing processes in two materials (J55 and N80Q). Corrosion tests were conducted on small coupons cut from the sample, effectively preserving the residual stresses caused by the manufacturing process. The small coupon corrosion tests also effectively and comprehensively evaluated the impact of the residual stress distribution and magnitude on corrosion, and found that residual stress plays a significant role in chloride ion corrosion.

[0053] Corrosive medium environment: Cl - Concentration: 33687 mg / L, pH value: 6.0, medium flow rate: 2.0 m / s, CO2 partial pressure: 2.0 MPa, temperature 80 ℃.

[0054] The sampling direction of the weld and the base material is along the length of the pipe body, and the corrosion sample at the weld is sampled along the center of the weld fusion line.

[0055] A corrosion high-temperature autoclave was used to simultaneously control the pressure, temperature, and flow rate. The coupons were placed in the solution for corrosion, with a running time of 240 h. The solution was changed every 24 h. The samples were taken out after ten days to observe the corrosion morphology of the samples and the morphology of the corrosion products.

[0056] The corrosion weight loss results of the base material and welds of J55 and N80Q pipes are shown in Table 6, the corrosion morphologies are shown in Figures 3 and 4, and the corrosion degree evaluation criteria are shown in Table 7. The Metal Corrosion Protection Handbook edited by the China Corrosion and Protection Society divides the corrosion resistance of metal materials into 10 levels.

[0057] Table 6 Corrosion weight loss results

[0058]

[0059] Table 7 10-level standard for uniform corrosion

[0060]

[0061] (4) Detection of groove corrosion results of welds under electrochemical corrosion conditions

[0062] The specimens used for electrochemical testing were Φ15 × 5 mm in size. The specimens were prepared by soldering a copper wire to one end, leaving the other end bare, and sealing the remaining ends with epoxy resin, leaving only the test surface. After preparation, the specimens were polished using 400#, 600#, and 800# grit metallographic sandpaper, degreased with acetone, and dried with cold air to prepare the electrochemical specimens for testing.

[0063] The experimental instruments used were an EG&GPar M273A potentiostat and an M5210 lock-in amplifier to test open-circuit potential, polarization potential, and AC impedance. This device can be controlled either from the instrument panel or by corrosion analysis software. A 1-liter glass electrolytic cell with a three-electrode system was used, with a graphite electrode as the auxiliary electrode, a saturated calomel electrode as the reference electrode, and the sample as the working electrode. The experimental conditions were: a 3.5% NaCl neutral aqueous solution heated in a water bath at 30°C, with CO2 gas introduced. Dynamic polarization scans were performed on the sample at a scan rate of 0.3 mV / s to examine the corrosion potential of the pipe under different polarization conditions.

[0064] The trench corrosion test utilizes a constant potential electrochemical polarization method using a three-electrode system. A constant potential of -550 mV is applied to the specimen, placing it in an anodic polarization state. The weld and base metal exhibit different electrochemical driving forces. After 144 hours of polarization, the geometric parameters of the corrosion trench are measured, and the trench corrosion sensitivity coefficient is calculated. The trench corrosion sensitivity coefficient, α, serves as an evaluation indicator and is defined as α = h2 / h1, where h2 and h1 are the depth from the original surface to the bottom of the trench before the corrosion test and the corrosion depth of the base metal, respectively.

[0065] The open circuit potential test results for N80Q and J55 casing steels show that N80Q steel has the lowest open circuit potential, at -0.653 V, followed by J55 steel at -0.562 V. The corrosion potential also indicates that N80Q steel has the highest corrosion rate, followed by J55 steel. This is consistent with the weight loss test results, further verifying the accuracy and validity of the test results.

[0066] The groove corrosion morphologies of the welds of the two materials are shown in Figure 5, and the test results are shown in Table 8.

[0067] Table 8 Groove corrosion sensitivity coefficient

[0068]

[0069] The geometric parameters of the weld fusion zone and heat-affected zone have a significant impact on the width and extent of groove corrosion. The weld geometric parameters are shown in Table 9.

[0070] Table 9 Welding parameter test results

[0071]

[0072] By comparing the groove corrosion test data and weld parameters, we know that if the groove corrosion sensitivity coefficient is 1.3, the weld parameters should be determined as the width of the weld heat affected zone is: h o ≈h i ≈1.5~2.2hn;h n ≤t / 10; fusion line deflection or bending width S≤t / 10.

[0073] 2. Evaluation of residual anti-external pressure collapse strength after corrosion

[0074] (1) Determination of the main influencing factors of residual collapse strength

[0075] Based on the morphology of the corrosion test results, it can be seen that corrosion of the base metal affects its collapse strength and can be considered uniform corrosion. Corrosion in the weld and heat-affected zone is V-shaped and can be considered localized corrosion. By comparing the collapse strength of the intact casing with the uniform corrosion and localized groove corrosion, the main factors affecting the residual collapse strength were determined.

[0076] The comparative analysis of the four types of crushing strength is shown in Table 10.

[0077] Table 10 Comparison of four types of crush strength

[0078]

[0079] It can be seen from Table 10 that: 1) the main factor determining the residual collapse strength is the uniform corrosion of the inner wall of the casing; 2) the influence of the weld groove corrosion depth on collapse is much smaller than the uniform wall thickness corrosion of the inner wall.

[0080] (2) Analysis of collapse strength with different groove corrosion sensitivity coefficients

[0081] Based on the uniform corrosion wall thickness, the influence of different groove depths on its collapse strength was studied. The collapse pressure of groove depths with different groove corrosion sensitivity coefficients after uniform corrosion of the inner wall is shown in Table 11.

[0082] The morphology of the pipe crushed by external pressure after uniform corrosion of N80Q material and electrochemical groove corrosion of weld seam is as follows Figure 6 shown.

[0083] Table 11 Collapse strength of different groove corrosion sensitivity coefficients

[0084]

[0085] From the above analysis and comparison, we can see that:

[0086] 1) When the groove depth is 2.5 times the corrosion depth of the base material, the collapse failure mechanism undergoes a qualitative change to collapse due to local plastic deformation;

[0087] 2) The change of collapse mechanism is the main reason for the serious decrease in collapse strength.

[0088] 3) Determination of groove corrosion sensitivity coefficient: Considering the safety of actual use, 2.0 is taken as the critical value. Referring to the casing design collapse safety factor of 1.2, 1.6 is taken as the weld groove corrosion sensitivity coefficient.

[0089] The following conclusions can be drawn: 1) Uniform corrosion of the wall thickness is the main factor determining the collapse of the casing;

[0090] 2) Under certain wall thickness corrosion conditions, the groove corrosion sensitivity coefficient can be determined to be 1.6 based on anti-extrusion analysis.

[0091] 3. Selection and evaluation of factors affecting material heat conduction

[0092] Based on the chemical composition, metallographic structure and grain size analysis of the commonly used pipes N80Q and J55, it can be seen that the content of C, a key element affecting thermal conductivity, is basically the same, the grain size is close, and the structure has significant differences.

[0093] The test was conducted using the national standard method (GB / T 3651-2008 Measurement method of thermal conductivity of metals at high temperatures); the test results at an operating temperature of 80°C are shown in Table 12.

[0094] Table 12 Thermal conductivity (test temperature 80 ℃)

[0095]

[0096] Materials with high thermal conductivity are preferred to improve the heat exchange efficiency of geothermal buried pipe materials.

[0097] 4. Selection of geothermal buried pipes

[0098] Based on the three factors for selecting geothermal buried pipes: evaluation of the corrosion behavior of the base material and weld in the service environment, evaluation of the remaining anti-external pressure collapse strength after corrosion, and evaluation of the heat conduction efficiency, materials with good corrosion resistance and good corrosion resistance are preferred as geothermal buried straight welded pipe materials.

[0099] From the above analysis, we can see that:

[0100] 1) Corrosion Resistance: In the normalized J55 steel, the secondary phase in the steel primarily dissolves into the ferrite matrix as a solid solution. This reduces the number of corrosion microcells and reduces the corrosion rate of the matrix. For N80Q steel, the quenched microstructure consists of martensite, retained austenite, and granular carbides. After quenching, alloying elements dissolve into the matrix as a solid solution. Due to the rapid cooling, many alloying elements solidify before precipitation. To reduce retained austenite, stabilize the microstructure, relieve stress, and improve red hardness, N80Q steel undergoes high-temperature tempering. Tempering increases hardness and strength, but reduces corrosion resistance. Furthermore, this increased strength results in higher hoop residual tensile stress (350 MPa) in N80Q, 1.75 times that of J55 (200 MPa), a significant factor contributing to the pipe's poor corrosion resistance.

[0101] 2) Evaluation of the residual anti-external pressure collapse strength after corrosion. Under the condition of constant wall thickness corrosion, the groove corrosion sensitivity coefficient is based on the external pressure collapse applicable coefficient of 1.6. It can be seen that: J55

[0102] The groove corrosion coefficient of N80Q meets the service requirements. Therefore, uniform corrosion in the service environment is the main factor determining the collapse of the casing. The corrosion weight loss results in Table 6 show that J55 has the lowest corrosion rate and high residual collapse strength after corrosion.

[0103] 3) High thermal conductivity: The thermal conductivity of the two materials J55 and N80Q was basically the same under the operating environment temperature of 80°C, with no significant difference.

[0104] Based on the above test evaluation, it is recommended to give priority to the use of low yield strength material J55 normalized state structure of straight weld electric resistance welded steel pipe, with grain size ≥8.5 level uniform distribution (P+F) structure and low hoop residual tensile stress ≤200MPa; the width of the weld heat affected zone is: h o ≈h i ≈1.5~2.2hn;h n ≤t / 10; fusion line deflection or bend width S ≤t / 10; groove corrosion sensitivity coefficient 1.34 ≤ 1.6; annual corrosion rate of parent metal weld ≤ 1.972mm / a; suitable for underground use and significantly reducing pipe costs. The use of J55 welded pipe with an outer diameter of 177.8mm and a wall thickness of 9.19mm significantly reduced the cost of geothermal buried pipes, achieving a 20% reduction compared to seamless pipes and a 30% reduction compared to N80Q seamless pipes.

[0105] If 80 yield strength material must be used, it is recommended to use N80-1 type normalized state structure, which can improve the corrosion resistance of geothermal water extraction; in addition, high temperature greater than 650 ℃ tempering heat treatment to eliminate residual stress can slow down corrosion and reduce costs (compared with N80Q quenched and tempered state structure).

[0106] The above description of the present invention is exemplified in conjunction with the accompanying drawings. It is obvious that the specific implementation of the present invention is not limited to the above-mentioned method. As long as such non-substantial improvements are made using the method concept and technical solution of the present invention, or the concept and technical solution of the present invention are directly applied to other occasions without improvement, they are all within the scope of protection of the present invention.

Claims

1. A method for evaluating the suitability of geothermal buried straight welded pipes, characterized by: include Evaluation of the corrosion behavior of the base metal and welds in the service environment, evaluation of the residual anti-external pressure collapse strength after corrosion, and evaluation of the heat conduction efficiency are finally combined to determine the applicability of the geothermal buried straight welded pipe; The evaluation of the corrosion behavior of the base metal and weld in the service environment includes four steps: determining the corrosion environment and medium, selecting factors affecting the material's corrosion resistance, detecting the corrosion morphology and corrosion rate of the base metal and weld, and detecting the groove corrosion results of the weld under electrochemical corrosion conditions. The evaluation of the residual anti-external pressure collapse strength after corrosion includes the evaluation of the collapse strength after uniform corrosion of the base material and corrosion of the weld groove; The heat conduction efficiency evaluation includes testing the thermal conductivity of the material.

2. The method for evaluating the suitability of a geothermal buried straight welded pipe according to claim 1, wherein: In the step of determining the corrosive environment and medium, the corrosive environment mainly considers the ambient temperature to be between 60°C and 100°C, and the corrosive medium mainly includes carbon dioxide, chloride ions, flow rate and pH value.

3. The method for evaluating the suitability of a geothermal buried straight welded pipe according to claim 1, wherein: In the corrosion morphology and corrosion rate detection steps of the base material and weld, the sampling direction of the weld and base material is along the length direction of the pipe body, among which the corrosion sample at the weld is sampled along the center of the weld fusion line; the coupon corrosion rate evaluation test is carried out and the corrosion morphology is observed.

4. The method for evaluating the suitability of a geothermal buried straight welded pipe according to claim 1, wherein: In the groove corrosion result detection step of the weld under electrochemical corrosion conditions, the groove corrosion test applies the constant potential electrochemical polarization method. A three-electrode system is used to apply a constant potential of -550mV to the specimen. The specimen is in an anodic polarization state. The geometric parameters of the corrosion groove are measured after polarization for 144 hours, and the sensitivity coefficient of the groove corrosion is calculated; and the geometric parameters of the weld fusion zone and heat-affected zone are detected.

5. The method for evaluating the suitability of a geothermal buried straight welded pipe according to claim 1, wherein: In the evaluation of the residual anti-external pressure collapse strength after corrosion, based on the corrosion environment test of full-size physical pipes, the residual wall thickness of the uniformly corroded parent material and the depth of weld groove corrosion are measured, and a comparative analysis of the residual anti-collapse strength is carried out to establish the groove corrosion evaluation sensitivity coefficient.

6. The method for evaluating the suitability of a geothermal buried straight welded pipe according to claim 1, wherein: In the step of testing the thermal conductivity, the thermal conductivity of the pipe to be used is tested and the material with high thermal conductivity is selected.

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