A steel for drill collars

Abstract

This invention relates to a steel for drill collars, the composition of which, by mass percentage, is: C: 0.01-0.06%, Mn: 20.3-21.7%, Cr: 18.0-20.0%, Mo: 0.5%-3.5%, Ni: 1.4-4.5%, N: 0.6-2.2%, W: 0.43-0.75%, B: 0.006-0.012%, with the balance being Fe and unavoidable impurities, and W and B satisfying ([W] / 183.8) / ([B] / 10.8)=3.17-6.23, where [W] and [B] represent the mass percentage content of W and B in the steel, respectively.

Description

Technical Field

[0001] This invention relates to a drill collar steel that not only has mechanical properties equal to or higher than those of traditional drill collar steel, but also has excellent SSC resistance, making it particularly suitable for use in drilling and production processes in acidic oil and gas fields. Background Technology

[0002] Drill collars are a major component of the drill string. In oil and gas drilling, magnetic compasses or instruments are typically used for positioning. To avoid interference with positioning, non-ferromagnetic drill collars are required. Simultaneously, to provide drilling pressure to the drill bit and improve the rigidity of the drill string, drill collars need excellent strength and toughness. Drilling operations often encounter electrochemical corrosion and pitting corrosion, requiring drill collar materials to have excellent corrosion resistance. In summary, existing drill collar materials have relatively high costs due to the stringent requirements for various properties; therefore, it is desirable for drill collar materials to have the longest possible service life.

[0003] However, during drilling and production in acidic oil and gas fields, the working environment is accompanied by high concentrations of H2S. Working under high H2S conditions for extended periods will cause sulfide stress cracking (SSC) in the drill collars, often resulting in drill collar cracking, fracture failure, short service life, and high drilling and production costs.

[0004] Based on the above problems, the inventors of this invention aim to provide a drill collar material that can effectively resist sulfide stress cracking, extend its stable and reliable service time in the drilling and production process of acidic oil and gas fields, and reduce drilling and production costs. Summary of the Invention

[0005] This invention provides a drill collar steel that not only has mechanical properties equal to or higher than those of traditional drill collar steel, but also has excellent anti-SSC properties. When used in the drilling and production process of acidic oil and gas fields, it is not easy to crack or break, and has stable working performance and reliable service time, which can effectively reduce drilling and production costs.

[0006] The technical objective of this invention is achieved through the following means.

[0007] The purpose of this invention is to provide a steel for drill collars, the composition of which, by mass percentage, is: C: 0.01-0.06%, Mn: 20.3-21.7%, Cr: 18.0-20.0%, Mo: 0.5%-3.5%, Ni: 1.4-4.5%, N: 0.6-2.2%, W: 0.43-0.75%, B: 0.006-0.012%, with the balance being Fe and unavoidable impurities, and W and B satisfying ([W] / 183.8) / ([B] / 10.8)=3.17-6.23, where [W] and [B] represent the mass percentage content of W and B in the steel, respectively.

[0008] The composition design principle of this invention will be introduced below.

[0009] C: Carbon is one of the most important austenite-forming elements. This invention aims to obtain a fully austenitic structure to ensure the paramagnetism of the material; therefore, a certain carbon content is required. Furthermore, the addition of carbon helps improve the strength of steel, which is crucial for ensuring its strength properties. However, excessive carbon content easily leads to carbide formation, resulting in a decrease in the corrosion resistance of the steel. Therefore, considering the metallographic structure, strength, and corrosion resistance of this invention, the carbon content in the steel is set at 0.01-0.06%.

[0010] Mn: In this invention, manganese is a crucial element. It helps ensure the strength of the steel and stabilizes the austenitic structure. Simultaneously, only a sufficiently high manganese content can ensure adequate nitrogen dissolves into the steel, thereby ensuring a sufficiently high nitrogen content to replace nickel with nitrogen and reduce steel costs. However, excessive manganese content will lead to a deterioration in the corrosion resistance of the steel. Considering the metallographic structure, strength, and corrosion resistance of this invention, the manganese content in the steel is set at 20.3-21.7%.

[0011] Cr: Chromium can stabilize the austenitic structure of steel. On the other hand, its most important role is to ensure that the steel has sufficient corrosion resistance. However, excessive chromium will lead to a decrease in the processing performance of the steel and a deterioration in its toughness. Taking into account the metallographic structure, corrosion resistance, processing performance, toughness and other characteristics of this invention, the chromium content in the steel is set at 18.0-20.0%.

[0012] Mo: Molybdenum is an important element that ensures the excellent corrosion resistance of steel. It also significantly improves strength; however, excessive molybdenum content will deteriorate the processing performance of the steel. Considering both the corrosion resistance and processing performance of this invention, the molybdenum content in the steel is set at 0.5%-3.5%.

[0013] Ni: Nickel is an important element for improving the corrosion resistance of steel and is crucial for maintaining austenitic stability. However, nickel resources are becoming increasingly scarce, and its price remains high. Therefore, this invention uses a high-manganese, high-nitrogen approach to replace nickel. Based on corrosion resistance considerations and cost control, this invention sets the nickel content in the steel to 1.4-4.5%.

[0014] Nitrogen (N): The addition of nitrogen helps maintain austenite stability and is an element that improves strength and ensures corrosion resistance. Increasing the amount of manganese added can increase the solubility of nitrogen in the steel matrix, thus allowing manganese and nitrogen to replace most of the nickel. However, excessive nitrogen content makes addition difficult and easily forms pores in the steel matrix, affecting various properties of the steel. Based on the requirements of metallographic structure, strength, and corrosion resistance, this invention sets the nitrogen content in the steel to 0.6-2.2%.

[0015] W: Tungsten is an element that stabilizes the formation of austenite. The austenite matrix can ensure that the steel is paramagnetic and can effectively improve the strength and corrosion resistance of the steel. However, too high a content will lead to increased costs and deteriorate the plasticity of the steel. Taking into account the structure, non-magnetism, strength, corrosion resistance and plasticity of the steel, this invention sets the tungsten content in the steel to be 0.43-0.75%.

[0016] B: Boron is an element that improves hardenability and promotes austenite formation. It ensures non-magnetism and significantly increases the strength of steel, but excessive amounts can lead to a deterioration in the plasticity and processing performance of the steel. Taking into account the microstructure, strength, and plasticity of the steel of this invention, the boron content in the steel of this invention is set at 0.006-0.012%.

[0017] The values ​​of ([W] / 183.8) and ([B] / 10.8): As described in the background section, existing drill collar materials are easily affected by H2S when working in acidic oil and gas fields, resulting in cracks or even fractures. This leads to a short effective service life, poor operational stability, and high drilling and production costs. Through repeated experiments, the inventors of this invention discovered that by synergistically controlling the content of W and B, the anti-SSC properties of steel can be effectively improved. This prevents sulfide stress cracking caused by high concentrations of H2S when working in acidic oil and gas fields, ensuring excellent service life of the drill collar, extending the time for reliable and stable operation, and effectively reducing drilling and production costs. The inventors of this invention have confirmed that the range of W and B contents that ensures excellent anti-SSC properties in steel is defined as ([W] / 183.8) / ([B] / 10.8) = 3.17-6.23.

[0018] In this invention, unavoidable impurity elements can be listed as such as P, S, O, H, etc. These elements are common elements in steel, introduced by raw materials and cannot be completely removed. The lower the content of impurity elements, the better the performance of the corresponding steel, but the higher the production cost of the steel. Taking into account both performance and cost, the content of P and S in this invention is controlled below 0.01%, and the content of O and H is not higher than 50 ppm.

[0019] By way of non-limiting description, the steel for drill collars of the present invention has a yield strength of 980 MPa or higher, a tensile strength of 1055 MPa or higher, an elongation of not less than 20%, a reduction of area of ​​not less than 50%, a room temperature impact energy of not less than 90 J, and a relative permeability of μ. r ≤1.005.

[0020] The drill collar steel of this invention exhibits excellent SSC resistance. According to Method A of NACE TM0177-2016, a sample of the drill collar steel (φ6.35mm × 25.4mm) was immersed in a test solution. The test solution was prepared by adding acetic acid and sodium acetate to a 5wt.% NaCl aqueous solution to adjust the pH to 3.5, with a solution temperature of 25°C, an atmosphere of CO2: 0.95 atm, and H2S: 0.05 atm. A load of 90% of the yield strength was applied, and no cracks were observed after immersion in the solution for 720 hours.

[0021] As another aspect of the present invention, the present invention also provides the application of the aforementioned drill collar steel in drilling and production in acidic oil and gas fields. Since the drill collar steel of the present invention has excellent mechanical properties in addition to those of traditional drill collar steel, it also has excellent anti-SSC properties and can effectively resist stress cracking caused by H2S under high humidity and high H2S concentration conditions. Therefore, the drill collar steel of the present invention is particularly suitable for drilling and production in acidic oil and gas fields.

[0022] By way of non-limiting description, the method for preparing steel for drill collars of the present invention can be conventional smelting and casting, or it can be prepared by electroslag remelting.

[0023] As a non-limiting description, the steel for drill collars of the present invention preferably includes a primary hot working step after the billet is prepared. The hot working can be rough rolling, forging, etc. The corresponding dimensions are obtained through the primary hot working. Before the primary hot working, a homogenization treatment step is preferably included. The homogenization temperature is 1150-1250°C and the duration is 2-5 hours. After the homogenization treatment, the primary hot working is performed directly.

[0024] As a non-limiting description, after the above-mentioned primary heat treatment, it is preferable to perform a solution treatment at 1000-1150°C for 2-4 hours.

[0025] As a non-limiting description, after the above solution treatment, it is preferable to perform secondary hot working strengthening, such as finishing rolling at 500-800°C with a total reduction of 15-65%, or extrusion or forging at 500-800°C with a total reduction of area of ​​10-70%.

[0026] The beneficial effects of this invention are as follows.

[0027] This invention, through compositional optimization, yields drill collar steel with excellent strength, toughness, and plasticity. Its mechanical properties meet or exceed those of traditional drill collar steel, effectively fulfilling the mechanical performance requirements of drill collars during drilling and production. By controlling its composition, this steel achieves an austenitic matrix structure, is a non-magnetic material, and has a relative permeability μ. r With a concentration ≤1.005, it will not interfere with exploration instruments during drilling and production. In particular, by adding W and B and synergistically controlling their content, the steel for drill collars obtained by this invention has excellent resistance to sulfide stress cracking. It will not crack or fail even after long-term operation under high concentration H2S conditions, and has excellent service life and stable long-term working capability. This makes the steel of this invention particularly suitable for drilling and production in acidic oil and gas fields, and can significantly reduce drilling and production costs. Detailed Implementation

[0028] To enable those skilled in the art to fully understand the technical solution and beneficial effects of the present invention, the following detailed explanation is provided in conjunction with specific experimental examples.

[0029] The steel was smelted according to the design composition and cast into ingots. The contents of P and S were controlled at 0.008±0.001% and the contents of O and H were 40ppm±5ppm respectively. The dimensions of the steel ingots were 1000mm long × 400mm wide × 400mm high. The specific composition is shown in Table 1, where k=([W] / 183.8) / ([B] / 10.8). Two steel ingots of the above dimensions were cast for test serial numbers 1, 2 and 3, and were named test serial numbers 1-1, 1-2, 2-1, 2-2, 3-1 and 3-2 respectively.

[0030] Tests 1-2, 2-2, and 3-2 were prepared as finished products in the form of cast ingots. The remaining test samples were homogenized at 1180℃ for 3 hours, then hot-forged into round bars with a diameter of 300 mm. The bars were then solution-treated at 1080℃ for 2.5 hours, and then air-cooled to 675℃. The round bars were then extruded to reduce the cross-sectional area by 50% to obtain the finished bar products.

[0031] Various performance tests were conducted on the finished bars. The yield strength, tensile strength, elongation, and reduction of area were tested according to GB / T228.1-2021, the impact energy test was conducted according to GB / T229-2020, and the relative permeability test was conducted according to GB / T 35690-2017. The SSC resistance was tested according to Method A in NACE TM0177-2016. The sample (φ6.35mm×25.4mm) was immersed in the test solution, which was prepared by adding acetic acid and sodium acetate to a 5wt.% NaCl aqueous solution to adjust the pH of the test solution to 3.5, the liquid temperature was 25℃, and the atmosphere was CO2: 0.95atm and H2S: 0.05atm. 90% of the yield strength (based on the yield strength obtained from sampling and testing on the same bar) was used as the load stress. After immersion in the impregnation solution for 720 hours, the presence of cracks was observed.

[0032] Table 1: Composition of various steels, %, balance is Fe.

[0033]

[0034] The results of various tests on the steel samples numbered 1 to 12 above are shown in Table 2.

[0035] Table 2: Properties of various steels.

[0036]

[0037] The above-mentioned inventive examples and comparative examples will be further analyzed and explained below with reference to Tables 1-2.

[0038] All test numbers 1-6 in Table 1 meet the composition requirements of this invention; therefore, test numbers 1-6 are inventive examples of this invention. The test results clearly show that the steel with the compositions of test numbers 1-6 meets the requirements of this invention in terms of yield strength, tensile strength, elongation, reduction of area, impact energy, and relative permeability. In particular, no cracks were found after testing according to NACE TM0177-2016, confirming that the steel with the compositions of this invention has excellent SSC resistance.

[0039] In Table 2, test numbers 1-2, 2-2, and 3-2 represent performance tested under as-cast conditions, while test numbers 1-1, 2-1, and 3-1, with the same composition, represent samples after strengthening deformation. It is evident that their yield strength and tensile strength have increased, while elongation, reduction of area, and toughness have decreased to some extent, but remain within the range required by this invention. The tests confirm that steel with the composition of this invention, whether in the as-cast or strengthened deformation state, can meet all the performance requirements of this invention; the only difference is the shift in performance focus between strength and ductility / toughness. Users can choose between the as-cast or strengthened deformation state based on their actual performance needs.

[0040] Tests 7, 8, 9, and 10 adjusted the W and B content of tests 1, 2, 3, and 4, respectively. Although the adjusted W and B content still met the requirements of this invention, the value of k [([W] / 183.8) / ([B] / 10.8)] was not within the range required by the invention. The test results showed that although the strength, plasticity, toughness, and magnetic properties of tests 7, 8, 9, and 10 met the requirements of this invention, cracks appeared after testing according to NACE TM0177-2016, confirming that the above steel has poor SSC resistance and cannot meet the requirements for long-term stable operation in acidic oil and gas field conditions.

[0041] Tests 11 and 12 omitted B and W from tests 5 and 6, respectively. The results showed that the yield strength and tensile strength decreased, failing to meet the requirements of the present invention of a yield strength of 980 MPa or more and a tensile strength of 1055 MPa or more. Furthermore, due to the lack of B or W, the synergistic effect of W and B could not be utilized. Finally, after testing, its anti-SSC properties were found to be poor, failing to meet the requirements of the present invention.

[0042] It is easy to see from the above invention examples and comparative examples that by synergistically adding W and B and controlling the content relationship of W and B within the corresponding range, drill collar steel with the required strength, plasticity, toughness and relative magnetic permeability can be obtained. Most importantly, this steel has excellent anti-SSC properties, which can meet the requirements of long-term stable operation under acidic oil and gas field conditions, has excellent service life, and can effectively control drilling and production costs.

[0043] The above description of the embodiments is only for the purpose of helping to understand the method and core ideas of the present invention. It should be noted that those skilled in the art can make several improvements and modifications to the present invention without departing from the principles of the present invention, and these improvements and modifications also fall within the protection scope of the claims of the present invention.

[0044] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A type of steel for drill collars, characterized in that, The steel used for the drill collar has the following composition by mass percentage: C: 0.01-0.06%, Mn: 20.3-21.7%, Cr: 18.0-20.0%, Mo: 0.5%-3.5%, Ni: 1.4-4.5%, N: 0.6-2.2%, W: 0.43-0.75%, B: 0.006-0.012%, with the balance being Fe and unavoidable impurities. Furthermore, W and B satisfy ([W] / 183.8) / ([B] / 10.8) = 3.17-6.23, where [W] and [B] represent the mass percentages of W and B in the steel, respectively.

2. The drill collar steel according to claim 1, characterized in that, The yield strength of the steel used for the drill collar is above 980 MPa.

3. The drill collar steel according to any one of claims 1-2, characterized in that, The tensile strength of the steel used for the drill collar is above 1055 MPa.

4. The drill collar steel according to any one of claims 1-2, characterized in that, The elongation of the steel used for the drill collar is not less than 20%.

5. The drill collar steel according to any one of claims 1-2, characterized in that, The section reduction rate of the steel used for the drill collar is not less than 50%.

6. The drill collar steel according to any one of claims 1-2, characterized in that, The room temperature impact energy of the steel used for the drill collar is not less than 90J.

7. The drill collar steel according to any one of claims 1-2, characterized in that, The relative magnetic permeability μ of the steel used for the drill collar r ≤1.

005.

8. The use of the drill collar steel according to any one of claims 1-7, characterized in that, The steel used for drill collars is used in drilling and production in acidic oil and gas fields.

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