A kind of high toughness corrosion-resistant ultra-deep well shale gas well pipe steel and heat treatment process for producing shale gas well pipe

Abstract

The application provides a high-toughness corrosion-resistant shale gas well pipe steel for super-deep wells and a heat treatment process for producing shale gas well pipes, and the composition comprises C, Si, Mn, Cr, Mo, Ni, Cu, Al, Ti, B, P, S, N, T.O and Fe. Compared with the prior art, the application produces a shale gas well pipe product for super-deep wells by the composition and a heat treatment process, wherein the yield strength is greater than or equal to 1100 MPa, the tensile strength is greater than or equal to 1200 MPa, the elongation A is greater than or equal to 17%, the reduction of area Z is greater than or equal to 55%, the impact energy KU2 at 0 DEG C is greater than or equal to 100 J, and the corrosion rate in the shale gas environment of the super-deep well is less than or equal to 0.07 mm / a, so that the shale gas well pipe product can meet the more stringent use in deep wells and super-deep wells.

Description

Technical Field

[0001] This invention belongs to the field of alloy steel and relates to a high-strength, high-toughness, corrosion-resistant steel for shale gas well pipes in ultra-deep wells, as well as a heat treatment process for producing shale gas well pipes. The produced shale gas well pipes have a yield strength ≥1100MPa, tensile strength ≥1200MPa, elongation A ≥17%, reduction of area Z ≥55%, and impact energy KU2 ≥100J at 0℃. They also exhibit a corrosion rate ≤0.07mm / a in ultra-deep shale gas environments, meeting the more stringent requirements for use in deep and ultra-deep well environments. Background Technology

[0002] Shale gas is a type of exploitable natural gas resource found in shale formations, and its formation and enrichment have unique characteristics. Some shale gas reservoirs have complex underground structures, strong folds, and large variations in reservoir depth, with most exceeding 5 km and some reaching 8-10 km. Shale gas reservoirs generally exhibit low porosity and low permeability, typically with low permeability and a maximum porosity of only 4%–5%.

[0003] Currently, shale gas extraction primarily employs horizontal well multi-stage perforation fracturing technology to enhance production. This technology places high demands on the strength, toughness, and resistance to internal and external pressure of the casing. Furthermore, during shale gas transportation, the gas contains a large amount of bacteria and carbon dioxide, which are highly corrosive to the pipe materials. Additionally, the acidic and alkaline underground environment further exacerbates the corrosion of the pipes, leading to frequent leaks during shale gas transport. This results in environmental pollution and resource waste.

[0004] Patent CN116145023A, published on May 23, 2023, discloses a high-strength, high-toughness, and high-extrusion-resistant casing and its processing method. Its components and their weight percentages are: C: 0.15–0.30%, Si: 0.15–0.45%, Mn: 0.80–1.50%, Cr: 0.20–0.80%, Mo: 0.16–0.30%, Ni: 0.12–0.30%, Nb: 0.03–0.06%, V: 0.04–0.16%, Ti≤0.04%, B: 0.0008–0.0020%, S≤0.002%, P≤0.012%, with the balance being Fe and unavoidable impurities. This patent's yield strength only reaches 960 MPa, making it unsuitable for shale gas extraction in ultra-deep wells.

[0005] Patent CN116065002A, published on May 5, 2023, discloses a sub-temperature tempering heat treatment method for casing used in rare earth shale gas extraction. Through heat treatment optimization, the method specifically includes: a first complete quenching → a second sub-temperature quenching → high-temperature tempering → high-pressure water descaling → straightening. The first complete quenching includes: placing the rolled steel pipe into a walking beam furnace for a first complete quenching treatment at a quenching temperature of 890℃±10℃ for a holding time of 45 minutes, followed by water cooling to room temperature using an internal spray and external rinsing method. The second sub-temperature quenching includes: ... The steel pipe, after its first complete quenching, is placed in a walking beam furnace for a second sub-critical quenching treatment at 830℃±10℃ for 45 minutes. It is then removed from the furnace and water-cooled to room temperature using an internal spray and external rinsing method. The high-temperature tempering involves placing the casing, after the first complete quenching and the second sub-critical quenching, into a walking beam tempering furnace at 660℃-690℃ for 75 minutes before removal. The high-pressure water descaling involves removing the tempered steel pipe from the furnace and subjecting it to high-pressure water descaling at a pressure maintained at 15-18 MPa. This ensures the yield strength of the rare-earth-containing casing reaches 1035 MPa, meeting the operational requirements for deep shale gas extraction casing. However, this patent focuses on the heat treatment process and does not describe the material composition. Furthermore, the steel has relatively low strength, making it unsuitable for ultra-deep well environments.

[0006] Patent CN217080353U, published on July 29, 2022, discloses a corrosion-resistant oil pipe for shale gas. The pipe includes a main body, a ceramic antibacterial pipe, a corrosion-resistant plastic pipe, a thermal insulation layer, a connecting flange, a sealing ceramic pipe, and bolts. The ceramic antibacterial pipe is located on the inner end face of the main body, and the corrosion-resistant plastic pipe is located inside the ceramic antibacterial pipe. This invention achieves double-layer corrosion resistance by using a ceramic antibacterial pipe and a corrosion-resistant plastic pipe on the inner end face of the main body, resulting in strong corrosion resistance. The outer thermal insulation layer provides insulation against high-temperature environments, increasing the resistance to temperature differences. The sealing ceramic pipe has strong sealing and corrosion resistance, enabling the device to withstand corrosion from shale gas and other oils, with strong temperature difference insulation, reduced leakage, and a longer service life. It solves the problem of short service life of shale gas pipelines, but its corrosion resistance is based on corrosion-resistant plastic pipes, without optimizing the corrosion resistance of the main material of the oil pipe, and the depth of use and the strength of resistance are not described. Summary of the Invention

[0007] The purpose of this invention is to provide a high-strength, tough, and corrosion-resistant steel for shale gas well pipes in ultra-deep wells. Through production design and the matching relationship between components, a steel with high strength, toughness, and corrosion resistance is obtained, which meets the requirements for use in shale gas well pipes in ultra-deep wells.

[0008] Another objective of this invention is to provide a heat treatment process for producing shale gas well pipes. This process utilizes the aforementioned high-strength, high-toughness, corrosion-resistant steel for ultra-deep shale gas well pipes. The resulting shale gas well pipes exhibit a yield strength ≥1100MPa, tensile strength ≥1200MPa, elongation A ≥17%, reduction of area Z ≥55%, and impact energy KU2 ≥100J at 0℃. Furthermore, they possess a corrosion rate ≤0.07mm / a in ultra-deep shale gas environments, meeting the stringent requirements for use in deep and ultra-deep wells.

[0009] The specific technical solution of this invention is as follows:

[0010] A high-strength, tough, corrosion-resistant steel for shale gas well casings in ultra-deep wells, comprising the following components by weight percentage:

[0011] C 0.25%–0.30%, Si 0.20%–0.30%, Mn 1.20%–1.40%, Cr 1.00%–1.20%, Mo 0.30%–0.45%, Ni 0.30%–0.50%, Cu 0.20%–0.40%, Al 0.020%–0.035%, Ti 0.015%–0.025%, B 0.015%–0.025%, P≤0.015%, S≤0.010%, N≤0.0080%, TO≤0.0020%. The remainder is Fe and other unavoidable impurities.

[0012] The steel used for high-strength, tough, and corrosion-resistant ultra-deep shale gas well pipes has the following composition: 170≤Q value≤200;

[0013] Q value

[0014] =(60×%C+35×%Mn+17×%Si+30×%Cr+15×%Ni+10×%Cu+25×%Mo)×(1+23×%Ti+45×%B);

[0015] The steel used for high-strength, tough, and corrosion-resistant ultra-deep shale gas well pipes also meets the following requirement: X value ≥ 30;

[0016] X=33×%Cu+42×%Ni+12×%Cr-2×%Si-9×%Cu×%Ni-5×%Mn.

[0017] The present invention provides a heat treatment process for producing shale gas well pipes, which uses the above-mentioned high-strength, tough, corrosion-resistant steel for ultra-deep well shale gas well pipes for heat treatment. The heat treatment process includes stepped quenching and tempering.

[0018] The stepped quenching process specifically involves: heating the steel pipe to a heating temperature T1 of 880–910°C, holding it at that temperature for t1, and then water cooling it; then heating the steel pipe to a heating temperature T2 of 840–870°C, holding it at that temperature for t2, and then water cooling it.

[0019] The heat preservation time t1 is determined by the steel pipe wall thickness S and the heating temperature T1, 260+2×

[0020] S-T1 / 4≤t1≤270+2×S-T1 / 4;

[0021] The heat preservation time t2 is determined by the steel pipe wall thickness S and the heating temperature T2, 260+2×S-T2 / 4≤t2≤270+2×S-T2 / 4;

[0022] Preferably, heating the steel pipe to a heating temperature T1 880~910℃ means heating it to a temperature T1 880~910℃ at a rate of 10~30℃ / min;

[0023] Preferably, heating the steel pipe to a heating temperature T2 840~870℃ means heating it to a temperature T2 840~870℃ at a rate of 10~30℃ / min;

[0024] The tempering process specifically involves heating the steel pipe to a heating temperature T3 of 500-540°C, holding it at that temperature for t3, and then water-cooling or air-cooling it.

[0025] The heat preservation time t3 is determined by the steel pipe wall thickness S and the heating temperature T3, 350+4×

[0026] S-T3 / 2≤t3≤370+4×S-T3 / 2;

[0027] Preferably, heating the steel pipe to T3 500~540℃ means heating it to T3 500~540℃ at a rate of 10~30℃ / min;

[0028] The units are: t1, t2, t3 are in min, S is in mm, and T1, T2, T3 are in ℃. When calculating the above formulas, simply substitute the data before the units into the formula.

[0029] This invention provides a method for producing shale gas well pipes, utilizing the aforementioned high-strength, tough, corrosion-resistant steel for ultra-deep shale gas well pipes and employing the aforementioned heat treatment process. The specific production method includes the following process flow:

[0030] Smelting → LF furnace refining → RH or VD vacuum degassing → continuous casting → heating → rolling / forging → round bar → tube threading → sizing → heat treatment process → finishing → packaging and warehousing.

[0031] The smelting is carried out using an electric arc furnace or a converter.

[0032] The RH or VD vacuum degassing process requires a pure degassing time of ≥20 minutes to ensure the [H] content after vacuum treatment.

[0033] ≤1.5ppm;

[0034] Continuous casting: The target temperature of molten steel in the ladle is controlled at 10-40℃ above the liquidus temperature, and round billets / square billets are continuously cast.

[0035] The shale gas well pipe produced by this invention has a yield strength ≥1100MPa, tensile strength ≥1200MPa, elongation A ≥17%, reduction of area Z ≥55%, and impact energy KU2 ≥100J at 0℃. It also has a corrosion rate ≤0.07mm / a in ultra-deep well shale gas environments, which can meet the requirements for use in ultra-deep well shale gas extraction.

[0036] The design concept of this invention is as follows:

[0037] Carbon (C): Carbon is the cheapest strengthening element in steel. Each 0.1% increase in dissolved C can increase strength by 400–450 MPa. C forms precipitates with alloying elements in steel, resulting in precipitation strengthening. C significantly improves hardenability, making it easier to obtain martensitic structure in the core during oil well tubing preparation. However, as its content increases, plasticity and toughness decrease, and high C content is detrimental to corrosion resistance. Therefore, the C content is controlled at 0.25%–0.30%.

[0038] Si: Si is an effective solid solution strengthening element in steel, increasing its strength and hardness. Si also acts as a deoxidizer during steelmaking. However, Si tends to segregate at austenite grain boundaries, reducing grain boundary bonding and causing brittleness. Furthermore, Si easily causes elemental segregation in steel. Therefore, the Si content should be controlled between 0.20% and 0.30%.

[0039] Mn: Mn can play a solid solution strengthening role, but its solid solution strengthening ability is weaker than that of Si. Mn is an austenite stabilizing element that can significantly improve the hardenability of steel and reduce decarburization. Mn combined with S can prevent hot brittleness caused by S. However, excessive Mn will reduce the plasticity of steel. Therefore, the Mn content should be controlled between 1.20% and 1.40%.

[0040] Cr: Cr is a carbide-forming element. Cr can improve the hardenability and strength of steel, but it easily causes temper brittleness. Cr can improve the oxidation resistance and corrosion resistance of steel, but excessive Cr content will increase crack susceptibility. The Cr content should be controlled between 1.00% and 1.20%.

[0041] Mo: Mo primarily improves the hardenability and heat resistance of steel. Mo dissolved in the matrix helps maintain high stability of the steel's microstructure during tempering and effectively reduces the segregation of impurity elements such as P, S, and As at grain boundaries, thereby improving the steel's toughness and reducing temper brittleness. Mo reduces the stability of M7C3; when the Mo content is high, acicular Mo2C will form, leading to a reduction in the Mo content in the matrix. Mo can improve the strength of steel through the combined effects of solid solution strengthening and precipitation strengthening, and it can also alter the steel's toughness by changing the precipitation of carbides. Therefore, the Mo content should be controlled between 0.30% and 0.45%.

[0042] Cu: Cu expands the austenite phase region. Elemental Cu can act as a second phase, significantly improving strength and enhancing the tempering stability and strength of the microstructure. However, excessive Cu content will lead to Cu brittleness. Therefore, the Cu content should be controlled between 0.20% and 0.40%.

[0043] Ni: Ni can form an infinitely miscible solid solution with Fe. It is an austenite stabilizing element, expanding the phase region, increasing the stability of supercooled austenite, shifting the C-curve to the right, and improving the hardenability of steel. Ni can refine the width of martensite laths, increasing strength. Ni significantly lowers the ductile-brittle transition temperature of steel and improves low-temperature toughness. The Ni content should be controlled between 0.30% and 0.50%.

[0044] Boron (B): Boron is generally considered a trace element in steel and has a strong hardening effect. Improving hardenability can increase the toughness of steel. However, due to its strong hardenability, the B content in steel should not be too high, and is therefore controlled between 0.015% and 0.025%.

[0045] Al: Al is the main deoxidizer in steelmaking. Al combines with N to form finely dispersed AlN, which maintains a coherent relationship with the matrix. This strengthens and refines the microstructure, increases resistance to fatigue crack initiation and propagation, and thus improves the endurance strength of steel. The Al content is controlled between 0.020% and 0.035%.

[0046] Ti: Ti is a strong C and N compound-forming element. Ti (C, N) is finely dispersed and maintains a coherent relationship with the matrix, thus strengthening and refining the microstructure. Strengthening the matrix increases resistance to fatigue crack initiation and propagation, thereby improving fatigue strength. Nb and V contents are controlled between 0.015% and 0.025%, respectively.

[0047] TO and N: TO forms oxide inclusions in steel, so TO should be controlled to ≤0.0020%; N can form fine precipitates with nitride-forming elements in steel to refine the microstructure, but at the same time, Fe4N is precipitated, which reduces the processing performance. Therefore, N should be controlled to within 0.0080%.

[0048] The strength of steel can be improved by adding beneficial alloying elements, and the toughness of steel can be improved by the effective ratio of elements. In this invention, under this composition system, C is the most important interstitial solid solution strengthening element in steel, affecting both strength and toughness, hence the coefficient is 60; Si is a non-metallic element and also a major solid solution strengthening element in steel, contributing 17 to the steel's performance; Mn can very effectively improve hardenability and strength, hence the coefficient is 35; Cr is a major substitution solid solution element and carbide forming element, contributing 30 to strength; Mo is beneficial for improving tempering stability, thereby improving toughness, and can also form carbide precipitation to strengthen the material matrix, hence the coefficient is 25; Ni and Cu improve the hardenability and strength of steel by changing the lattice morphology through solid solution strengthening, and can also effectively improve and enhance the toughness coefficient of the matrix, hence the coefficients are 15 and 10, respectively; In addition, microalloying elements Ti and B improve the strength of steel through interaction and the formation of a second phase, and can also refine the grains to improve toughness. Furthermore, B can improve the strength of steel by changing the lattice of C, hence the coefficients are 23 and 45, respectively. To ensure the overall performance of steel, strength should not be increased indiscriminately, as high strength can lead to a decrease in ductility and toughness. Let the overall strength-toughness factor of steel be Q, then Q satisfies 170 ≤ Q value ≤ 200. Q value

[0049] =(60×%C+35×%Mn+17×%Si+30×%Cr+15×%Ni+10×%Cu+25×%Mo)×(1+23×%Ti+45×%B).

[0050] To ensure good resistance to shale gas corrosion in steel, the proportions of corrosion-resistant elements such as Si, Mn, Cu, Ni, and Cr need to be optimized. Since Cu and Ni significantly improve corrosion resistance, their coefficients are 33 and 42, respectively. Si and Mn are prone to segregation, causing uneven microstructure and thus reducing corrosion resistance; therefore, their coefficients are -2 and -5, respectively. Cr provides a passivation film that enhances the steel surface, offering some corrosion resistance; hence, its coefficient is 12. Because of the interaction between Cu and Ni, which can negate the individual corrosion resistance of each element, their coefficients are -9. Therefore, the steel's corrosion resistance factor X should satisfy an X value ≥ 30.

[0051] X value=33×%Cu+42×%Ni+12×%Cr-2×%Si-9×%Cu×%Ni-5×%Mn.

[0052] Compared with existing technologies, this invention utilizes a high-strength, high-toughness, and corrosion-resistant steel composition design for ultra-deep shale gas well casings. This steel is used to produce ultra-deep shale gas well casings with a yield strength ≥1100MPa, tensile strength ≥1200MPa, elongation A ≥17%, reduction of area Z ≥55%, and impact energy KU2 ≥100J at 0℃. This allows the product to meet the more demanding environments of deep and ultra-deep wells.

[0053] Instruction manual illustrations

[0054] Figure 1 This is a typical microstructure of the product of this invention after heat treatment. Detailed Implementation

[0055] The present invention will be further described below with reference to embodiments and comparative examples.

[0056] Examples 1-3

[0057] A high-strength, tough, corrosion-resistant steel for shale gas well casings in ultra-deep wells, comprising the following components by weight percentage:

[0058] As shown in Table 1, the balance not shown in Table 1 is for Fe and other unavoidable impurities.

[0059] Comparative Examples 1-3

[0060] A steel for shale gas well casing comprises the following composition by mass percentage as shown in Table 1, where the balance not shown in Table 1 is Fe and other unavoidable impurities.

[0061] Table 1. Steel composition (wt%) for each embodiment and comparison.

[0062]

[0063]

[0064] The method for producing shale gas well pipes using the steel described in the above embodiments and comparative examples, utilizing the aforementioned high-strength, tough, corrosion-resistant steel for ultra-deep shale gas well pipes, specifically includes the following process flow:

[0065] Smelting → LF furnace refining → RH or VD vacuum degassing → continuous casting → heating → rolling / forging → round bar → tube threading → sizing → heat treatment process → finishing → packaging and warehousing.

[0066] Electric arc furnace smelting: oxygen is determined before tapping, and slag discharge is strictly controlled during the tapping process;

[0067] LF furnace: C, Si, Mn, Cr, Mo, Ni, Cu, Ti and other elements are adjusted to the target values;

[0068] Vacuum degassing: Pure degassing time ≥ 20 minutes, ensuring that the [H] content after vacuum treatment is ≤ 1.5 ppm;

[0069] Continuous casting: The target temperature of molten steel in the ladle is controlled at 10-40℃ above the liquidus temperature, and round billets / square billets are continuously cast.

[0070] The heat treatment process includes stepped quenching and tempering.

[0071] The stepped quenching process specifically involves: heating the steel pipe to a heating temperature T1 of 880–910°C, holding it at that temperature for t1, and then water cooling it; then heating the steel pipe to a heating temperature T2 of 840–870°C, holding it at that temperature for t2, and then water cooling it.

[0072] The heat preservation time t1 is determined by the steel pipe wall thickness S and the heating temperature T1, 260+2×

[0073] S-T1 / 4≤t1≤270+2×S-T1 / 4;

[0074] The heat preservation time t2 is determined by the steel pipe wall thickness S and the heating temperature T2, 260+2×S-T2 / 4≤t2≤270+2×S-T2 / 4;

[0075] Preferably, heating the steel pipe to a heating temperature T1 880~910℃ means heating it to a temperature T1 880~910℃ at a rate of 10~30℃ / min;

[0076] Preferably, heating the steel pipe to a heating temperature T2 840~870℃ means heating it to a temperature T2 840~870℃ at a rate of 10~30℃ / min;

[0077] The tempering process specifically involves heating the steel pipe to a heating temperature T3 of 500-540°C, holding it at that temperature for t3, and then water-cooling or air-cooling it.

[0078] The heat preservation time t3 is determined by the steel pipe wall thickness S and the heating temperature T3, 350+4×

[0079] S-T3 / 2≤t3≤370+4×S-T3 / 2;

[0080] Preferably, heating the steel pipe to T3 500~540℃ means heating it to T3 500~540℃ at a rate of 10~30℃ / min;

[0081] The units are: t1, t2, t3 in min, S in mm, and T1, T2, T3 in °C. When calculating the above formulas, simply substitute the data before the units into the formulas.

[0082] The heat treatment process parameters for each embodiment and comparative example are shown in Table 2.

[0083] Table 2 Heat treatment processes of embodiments and comparative examples of the present invention

[0084]

[0085] The performance testing methods are as follows:

[0086] Organization: After heat treatment of the tube blank, samples are taken from the finished product for metallographic and grain size analysis.

[0087] Performance: After heat treatment, tensile and impact tests, as well as corrosion tests, are performed on the finished tube blanks. The heat treatment process is shown in Table 2, and the performance is shown in Table 3. Mechanical property tests are conducted according to GB / T 228.1 and GB / T 229.

[0088] Table 3. List of mechanical property test results for embodiments and comparative examples of the present invention.

[0089]

[0090] The items underlined above do not meet the requirements of this invention.

[0091] The chemical composition and production methods of the steels in Examples 1-3 were appropriately controlled, resulting in good strength, plasticity, toughness, and service life. Comparative Example 1 had a reasonable composition design, but its improper heat treatment process led to insufficient material strength and toughness. Comparative Examples 2 and 3 suffered from unsuitable chemistry, resulting in excessively low material strength, insufficient plasticity, toughness, and corrosion resistance; improper heat treatment processes led to unsatisfactory overall performance.

Claims

1. A high-strength, tough, corrosion-resistant steel for shale gas well casings in ultra-deep wells, characterized in that, The high-strength, tough, and corrosion-resistant steel for shale gas well pipes in ultra-deep wells comprises the following components by weight percentage: C 0.25%-0.30%, Si 0.20%-0.30%, Mn 1.20%-1.40%, Cr 1.00%-1.20%, Mo 0.30%-0.45%, Ni 0.30%-0.50%, Cu 0.20-0.40%, Al 0.020%-0.035%, Ti 0.015-0.025%, B 0.015-0.025%, P≤0.015%, S≤0.010%, N≤0.0080%, TO≤0.0020%, with the remainder being Fe and other unavoidable impurities; The high-strength, tough, corrosion-resistant steel for shale gas well pipes in ultra-deep wells has the following composition: 170≤Q value≤200; Q value=(60×%C+35×%Mn+17×%Si+30×%Cr+15×%Ni+10×%Cu+25×%Mo)´(1+23×%Ti+45×%B); The high-strength, tough, corrosion-resistant steel for shale gas well pipes in ultra-deep wells also meets the following composition requirements: X value ≥ 30; X=33×%Cu+42×%Ni+12×%Cr-2×%Si-9×%Cu×%Ni-5×%Mn.

2. A heat treatment process for producing shale gas well pipes, characterized in that, The steel used for high-strength, tough, and corrosion-resistant ultra-deep shale gas well pipes as described in claim 1 is produced by heat treatment, wherein the heat treatment process includes stepped quenching and tempering.

3. The heat treatment process according to claim 2, characterized in that, The stepped quenching process specifically involves: heating the steel pipe to a heating temperature T1 of 880-910℃, holding it for t1, and then water cooling; then heating the steel pipe to a heating temperature T2 of 840-870℃, holding it for t2, and then water cooling.

4. The heat treatment process according to claim 3, characterized in that, The heat preservation time t1 is determined by the steel pipe wall thickness S and the heating temperature T1, and 260+2×S-T1 / 4≤t1≤270+2×S-T1 / 4.

5. The heat treatment process according to claim 3 or 4, characterized in that, The heat preservation time t2 is determined by the steel pipe wall thickness S and the heating temperature T2, and 260+2×S-T2 / 4≤t2≤270+2×S-T2 / 4.

6. The heat treatment process according to claim 2, characterized in that, The tempering process specifically involves heating the steel pipe to T3500-540℃, holding it at that temperature for t3, and then water-cooling or air-cooling it.

7. The heat treatment process according to claim 6, characterized in that, The heat preservation time t3 is determined by the steel pipe wall thickness S and the heating temperature T3, and 350+4×S-T3 / 2≤t3≤370+4×S-T3 / 2.

8. The heat treatment process according to any one of claims 2, 3, 4, 6 or 7, characterized in that, The produced shale gas well pipes have a yield strength ≥1100MPa, tensile strength ≥1200MPa, elongation A ≥17%, reduction of area Z ≥55%, and impact energy KU2 ≥100J at 0℃. They also have a corrosion rate ≤0.07mm / a in ultra-deep shale gas environments.

9. The heat treatment process according to claim 5, characterized in that, The produced shale gas well pipes have a yield strength ≥1100MPa, tensile strength ≥1200MPa, elongation A ≥17%, reduction of area Z ≥55%, and impact energy KU2 ≥100J at 0℃. They also have a corrosion rate ≤0.07mm / a in ultra-deep shale gas environments.

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