High-strength Anti-non-uniform-load oil casing and manufacturing method therefor

Abstract

Disclosed in the present invention is an oil casing. In addition to Fe and inevitable impurities, the oil casing further comprises the following chemical elements in percentage by mass: C: 0.05-0.15%, Si: 0.4-0.8%, Mn: 3-5%, Ni: 0.2-0.5%, Ca: 0.0005-0.005%, Ti: 0.01-0.04%, Al: 0.01-0.05%, and Nb: 0.02-0.04%. The microstructure of the oil casing is ferrite + granular bainite + austenite. Further disclosed in the present invention is a method for manufacturing the oil casing.

Description

High-strength non-uniform load resistant petroleum casing and manufacturing method thereof Technical Field

[0001] The present invention relates to a pipe and a manufacturing method thereof, in particular to a petroleum casing and a manufacturing method thereof. Background Art

[0002] Shale gas is a clean, recoverable natural gas energy source found in shale and mud layers. Compared to conventional natural gas extraction, shale gas extraction faces challenges such as complex formations and complicated fracturing and perforation processes. Currently, shale gas development utilizes a specialized process combining horizontal wells with multi-stage fracturing. The horizontal section is up to 3,000 meters long, with 15-30 perforations performed, followed by large-scale hydraulic fracturing. High formation pressures (90-120 MPa) are associated with fracturing, leading to the development of shale fractures and formation creep. The high-strength casing used is highly susceptible to deformation, with an average deformation rate exceeding 30%. This type of casing deformation is typically characterized by radial deformation due to uneven formation loading, rather than collapse. As multi-stage fracturing progresses, the degree of casing deformation increases, making it difficult for bridge plugs used in fracturing operations to pass through the deformed areas, hindering hydraulic fracturing and severely impacting shale gas recovery efficiency.

[0003] A Chinese patent document with publication number CN101532113A, published on September 16, 2009, and titled "A Collapse-Resistant Petroleum Casing and Its Manufacturing Method," discloses a technical solution for achieving ultra-high collapse resistance and excellent performance of collapse-resistant casing through texture and microstructure design and control.

[0004] The Chinese patent document, publication number CN102296239A, published on December 28, 2011, and titled "A high-strength, collapse-resistant oil casing string and its manufacturing method," uses a string with an outer diameter of φ141.3mm to φ142.7mm and a wall thickness of 10.7mm to 12.9mm. However, the document does not mention the performance of resisting non-uniform external loads.

[0005] The Chinese patent document, CN104651723A, published on May 27, 2015, and titled "A rare earth-containing, hydrogen sulfide-resistant, high-collapse-resistant oil casing and its production method," describes oil casing with a yield strength of 780-870 MPa and a transverse impact value of ≥70 J / cm at 0°C. 2 , with high resistance to hydrogen sulfide corrosion and high anti-collapse performance.

[0006] Although the oil casings mentioned in the above patent documents all have high anti-collapse performance, they are limited to studying the anti-collapse performance of the casing under uniform load, and do not solve the technical problem that the oil casing has poor anti-deformation ability when subjected to uneven load.

[0007] Summary of the Invention

[0008] One objective of the present invention is to provide a high-strength oil casing with a high work-hardening index and toughness, a low yield strength ratio, and strong resistance to deformation under non-uniform loads. When the casing is deformed by formation forces, work-hardening significantly increases the material's strength, thereby improving the casing's resistance to deformation and ensuring the passage of bridge plugs for smooth fracturing operations.

[0009] In order to achieve the above object, the present invention provides a petroleum casing, which, in addition to Fe and inevitable impurities, also contains the following chemical elements in the following mass percentages:

[0010] C: 0.05-0.15%; Si: 0.4-0.8%; Mn: 3-5%; Ni: 0.2-0.5%; Ca: 0.0005-0.005%; Ti: 0.01-0.04%; Al: 0.01-0.05%; Nb: 0.02-0.04%;

[0011] The microstructure of oil casing is ferrite + granular bainite + austenite.

[0012] Specifically, in the petroleum casing of the present invention, the austenite is retained austenite.

[0013] Preferably, the present invention provides a petroleum casing containing the following chemical elements in percentage by mass:

[0014] C: 0.05-0.15%; Si: 0.4-0.8%; Mn: 3-5%; Ni: 0.2-0.5%; Ca: 0.0005-0.005%; Ti: 0.01-0.04%; Al: 0.01-0.05%; Nb: 0.02-0.04%; the balance is Fe and unavoidable impurities.

[0015] The petroleum casing of the present invention is controlled to have a mixed structure of ferrite, granular bainite and austenite through reasonable composition design and optimal combination with heat treatment process, thereby improving the work hardening index n, significantly enhancing the deformation resistance of the casing under non-uniform load, and obtaining good strength and toughness.

[0016] Specifically, the design principles of the chemical elements in the oil casing of the present invention are:

[0017] C: In the oil casing described herein, C is a precipitate-forming element that increases the strength of the steel. However, it should be noted that when the C content is less than 0.05%, the hardenability of the steel is reduced, making it difficult to maintain its strength. When the C content exceeds 0.15%, it inhibits the formation of ferrite, significantly reducing the toughness of the steel. Furthermore, C forms a large number of coarse precipitates with Cr and Mo, significantly exacerbating the segregation of the steel and leading to a significant reduction in toughness. Therefore, in the oil casing described herein, the mass percentage of C is limited to between 0.05% and 0.15%.

[0018] Si: In the oil casing described herein, Si promotes the diffusion of carbon into austenite, inhibiting the precipitation of cementite within the austenite. This not only provides solid solution strengthening but also promotes carbon enrichment within the austenite, thereby improving austenite stability. However, it is important to note that excessive Si addition to steel should not be excessive, as this degrades the steel's workability and toughness. Furthermore, Si addition should not be too low. When the Si content falls below 0.4% by mass, the effect on improving austenite stability is insignificant. Therefore, in the oil casing described herein, the Si content is controlled between 0.4% and 0.8% by mass.

[0019] Mn: In the oil casing described in the present invention, Mn is an austenite-forming element. It not only stabilizes the austenite in the steel but also slows the transformation rate of ferrite and reduces the diffusion of carbon from ferrite into austenite. This increases the amount of austenite, which helps enhance the material's work-hardening effect. In the present invention, to achieve the desired microstructure, the Mn content in the steel pipe must be controlled to above 3%. It should be noted that a Mn content exceeding 5% significantly increases structural segregation in the steel, significantly impacting the uniformity and impact resistance of the hot-rolled structure. Therefore, in the high-strength, non-uniformly load-resistant oil casing described in the present invention, the mass percentage of Mn is limited to between 3% and 5%.

[0020] Ni: In the oil casing described herein, Ni promotes the formation of austenite, ensuring austenite in the steel after rolling, which helps improve the material's work hardening index. However, it's important to note that a Ni content above 0.5% results in high costs and a limited improvement in the austenite content. Meanwhile, a Ni content below 0.2% fails to effectively increase the austenite content. Therefore, in the oil casing described herein, the mass percentage of Ni is controlled between 0.2% and 0.5%.

[0021] Ca: Ca can purify molten steel, promote MnS spheroidization, and improve impact toughness. However, excessive levels can easily form coarse non-metallic inclusions. Therefore, the mass percentage of Ca in the oil casing of the present invention is controlled to 0.0005-0.005%.

[0022] Ti: In the oil casing described herein, Ti is a strong carbonitride-forming element, significantly refining austenite grains and compensating for the strength loss caused by reduced carbon content. However, it should be noted that when the Ti content exceeds 0.04%, coarse TiN tends to form, significantly reducing the toughness of the material. Therefore, in the oil casing described herein, the mass percentage of Ti is controlled between 0.01% and 0.04%.

[0023] Al: In the oil casing described herein, Al acts as a deoxidizing and nitrogen-fixing element, contributing to grain refinement. However, it's important to note that excessive Al content should be avoided, as it significantly reduces the material's yield strength ratio. Therefore, the mass percentage of Al in the oil casing described herein is limited to between 0.01% and 0.05%.

[0024] Nb: In the oil casing described herein, Nb acts as a grain-refining and precipitation-strengthening element, compensating for the strength loss caused by reduced carbon content. When the Nb content exceeds 0.04%, coarse Nb(CN) forms, reducing toughness. Meanwhile, when the Nb content is below 0.02%, the strengthening effect is insignificant. Therefore, in the oil casing described herein, the mass percentage of Nb is limited to between 0.02% and 0.04%.

[0025] Preferably, the petroleum casing of the present invention further satisfies at least one of the following: 0<Cr≤1%; 0<Mo≤0.5%; 0<V≤0.15%.

[0026] In a preferred embodiment of the present invention, the oil casing may further be added with at least one of Cr, Mo, and V.

[0027] Cr: Cr is an element that strongly improves hardenability and is also a strong precipitate forming element. It can precipitate precipitates during the cooling phase transformation process and improve the strength of steel. It should be noted that when the Cr content is higher than 1%, it is easy to precipitate coarse M at the grain boundary. 23 Therefore, in the oil casing of the present invention, the mass percentage of Cr element is preferably limited to 0<Cr≤1%.

[0028] Mo: Mo primarily improves steel strength and stability during cooling phase transformations through precipitation and solid solution strengthening. The oil casing of the present invention has a low carbon content. Mo content above 0.5% has little effect on improving steel strength, resulting in alloy waste and increased manufacturing costs. Therefore, in the oil casing of the present invention, the mass percentage of Mo is preferably limited to 0 < Mo ≤ 0.5%.

[0029] V: V is a typical precipitation-strengthening element, compensating for the decrease in strength caused by a reduction in carbon content. However, when the V content in steel exceeds 0.15%, coarse V (CN) particles tend to form, reducing toughness. Therefore, in the oil casing described herein, the mass percentage of V is preferably limited to 0 < V ≤ 0.15%.

[0030] Preferably, in the high-strength non-uniform load resistant petroleum casing of the present invention, among the inevitable impurities, P≤0.015%; S≤0.005%.

[0031] In the oil casing of the present invention, the main impurity elements are P and S. Under the condition that technical conditions permit, it is desirable that their contents are as low as possible. In some embodiments, it is preferred to control P≤0.015% and S≤0.005%.

[0032] Preferably, in the petroleum casing of the present invention, the volume fraction of granular bainite is 6-10%.

[0033] Preferably, in the petroleum casing of the present invention, the volume fraction of austenite is 8-20%.

[0034] Preferably, the performance of the petroleum casing described in the present invention meets at least one of the following: yield strength of 758-965MPa; tensile strength ≥930MPa; yield strength ratio ≤0.85; elongation ≥35%; uniform elongation ≥20%; 0°C transverse Charpy impact energy ≥60J; work hardening index n ≥0.15; and the yield strength of the material after 5% deformation is increased by more than 30% compared with the undeformed material.

[0035] Another object of the present invention is to provide a method for manufacturing oil casing. The resulting oil casing exhibits high strength, toughness, uniform elongation, and work hardening index, while maintaining low process costs. This method addresses the existing problem of oil casing's poor deformation resistance under non-uniform loads, improves the casing's pressure resistance during downhole use, and meets the demand for high-strength oil casing in oilfield production.

[0036] In order to achieve the above object, the present invention provides a method for manufacturing the above-mentioned oil casing, which comprises the following steps performed in sequence:

[0037] (1) smelting and continuous casting to obtain round billets;

[0038] (2) perforation;

[0039] (3) rolling and tension reducing to obtain steel pipe;

[0040] (4) Two-phase zone normalizing heat treatment: Heat the steel pipe to the austenitizing temperature Ac1+60℃ to Ac3-60℃, keep warm for 30-60min and then air cool; where Ac1 = 723-10.7Mn-16.9Ni+29.1Si+16.9Cr; Ac3 = 910-203℃ 1 / 2 -15.2Ni+44.7Si+104V+31.5Mo; in the formula, substitute the values ​​of each chemical element before the percentage sign for its mass percentage;

[0041] (5) Thermal straightening.

[0042] The method described in the present invention obtains a mixed structure of ferrite, granular bainite and austenite by adopting a specific two-phase zone normalizing heat treatment process adapted to the above-mentioned steel pipe element composition content range, thereby improving the work hardening index n and enhancing the deformation resistance of the oil casing under non-uniform loads.

[0043] Preferably, in the method of the present invention, in step (1), the continuous casting speed is 1.6-2.0 m / min.

[0044] Preferably, in the method of the present invention, in step (2), the round billet is homogenized at 1220-1270°C and then pierced, and the piercing temperature is 1150-1200°C.

[0045] More preferably, in the method of the present invention, in step (3), the final rolling temperature is 920-980°C.

[0046] Preferably, in the method of the present invention, in step (3), the temperature of the stretch reducing is 900-950°C.

[0047] The high-strength non-uniform load resistant petroleum casing and the manufacturing method thereof of the present invention have the following advantages and beneficial effects:

[0048] Through rational composition design, the present invention achieves high strength, toughness, uniform elongation, and work hardening index for its oil casing at a low cost. This addresses the existing problem of poor deformation resistance under non-uniform loads in oil casing, improves the casing's pressure resistance during downhole use, and meets the demand for high-strength oil casing in oilfield production.

[0049] In addition, the manufacturing process of the present invention adopts a specific two-phase zone normalizing heat treatment adapted to the elemental composition content, thereby obtaining a mixed structure of ferrite, granular bainite and austenite, improving the work hardening index n, and ultimately improving the deformation resistance of the oil casing under non-uniform loads, while achieving a good strength-toughness match.

[0050] In some embodiments, the oil casing described in the present invention has a yield strength of 758-965 MPa, a tensile strength ≥930 MPa, a yield strength ratio ≤0.85, an elongation ≥35%, a uniform elongation ≥20%, a 0°C transverse Charpy impact energy ≥60 J, a work hardening index n ≥0.15, and the yield strength of the material after 5% deformation is increased by more than 30% compared with the undeformed material. DETAILED DESCRIPTION

[0051] The high-strength non-uniform load resistant petroleum casing and the manufacturing method thereof of the present invention will be further explained and illustrated below in conjunction with specific embodiments. However, such explanation and illustration do not constitute an undue limitation to the technical solution of the present invention.

[0052] Examples 1-6 and Comparative Examples 1-8

[0053] The oil casing of Examples 1-6 was prepared by the following steps:

[0054] (1) Smelting and continuous casting: Smelting and continuous casting, controlling the continuous casting speed to 1.6-2.0 m / min, to obtain round billets. In each embodiment and comparative example, smelting was carried out according to the chemical composition shown in Table 1.

[0055] (2) Piercing: The round billet is heated at 1220-1270℃ and then pierced. The piercing temperature is 1150-1200℃.

[0056] (3) Rolling and tension reducing: the final rolling temperature is 920-980℃, and the tension reducing is carried out after the final rolling at a temperature of 900-950℃;

[0057] (4) Two-phase zone normalizing heat treatment: Heat the steel pipe to the austenitizing temperature, which is between Ac1+60℃ and Ac3-60℃, keep it warm for 30-60min and then air cool it; where Ac1 = 723-10.7Mn-16.9Ni+29.1Si+16.9Cr; Ac3 = 910-203℃ 1 / 2 -15.2Ni+44.7Si+104V+31.5Mo; substitute the mass percentage of each chemical element before the percentage sign. For example, if the Mn content in the steel pipe is 3.5%, substitute 3.5 for the calculation.

[0058] (5) Thermal straightening.

[0059] It should be noted that the chemical composition and related process parameters of the oil casing of Examples 1-6 of the present invention all meet the requirements of the present invention. However, the oil casing of Comparative Examples 1-8 was produced using a process similar to the above process, but the chemical composition and / or manufacturing process parameters of the oil casing of Comparative Examples 1-8 did not meet the design requirements of the present invention.

[0060] Table 1 lists the mass percentages of chemical elements in the oil casings of Examples 1-6 and Comparative Examples 1-8.

[0061] Table 1. (wt%, the balance is Fe and other inevitable impurities except P and S)

[0062] Table 2 lists the specific process parameters of the oil casing of Examples 1-6 and Comparative Examples 1-8.

[0063] Table 2.

[0064] The oil casings of Examples 1-6 and Comparative Examples 1-8 finally obtained were sampled respectively. After polishing, the samples were observed for microstructure using an optical microscope. The test results are listed in Table 3.

[0065] Table 3.

[0066] The oil casings of Examples 1-6 and Comparative Examples 1-8 were sampled and tested for performance. The test results are listed in Table 4.

[0067] (1) Yield strength, tensile strength, elongation, and uniform elongation are measured in accordance with GB / T 228.1-2000.

[0068] (2) The transverse Charpy impact energy at 0°C is measured according to GB / T 229-2007 “Charpy pendulum impact test method for metallic materials”.

[0069] (3) The yield strength and work hardening index n of the petroleum casing after 5% deformation are measured according to GB / T228.1-2000 standard.

[0070] Table 4 lists the performance test results of the oil casing of Examples 1-6 and Comparative Examples 1-8.

[0071] Table 4.

[0072] Table 4 shows that the oil casing of each embodiment of the present invention meets the following performance requirements: yield strength of 760-930 MPa, tensile strength ≥ 950 MPa, yield strength ratio < 0.85, elongation ≥ 37%, uniform elongation ≥ 21.5%, 0°C transverse Charpy impact energy > 76 J, and yield strength increased by more than 30% after 5% deformation. This demonstrates that the oil casing of each embodiment of the present invention has high strength, toughness, uniform elongation, and work hardening index, making it suitable for use under non-uniform loads and highly resistant to deformation.

[0073] However, the Mn content in Comparative Examples 1 and 2 exceeded the range specified in the present invention, the C and Si contents in Comparative Example 3 exceeded the range specified in the present invention, the C content in Comparative Example 4 exceeded the range specified in the present invention, the Ni content in Comparative Examples 5 and 6 exceeded the range specified in the present invention, and the two-phase region normalizing heat treatment temperature in Comparative Examples 7 and 8 exceeded the range specified in the present invention. As a result, at least one mechanical property of the oil casing of Comparative Examples 1-8 failed to meet the required range of the oil casing of the present invention, and the overall performance was inferior to the high-strength non-uniform load resistant oil casing of each embodiment of the present invention.

[0074] It should be noted that the existing technology within the protection scope of the present invention is not limited to the embodiments given in this application document.

[0075] The combination of the various technical features in this case is not limited to the combination described in the claims of this case or the combination described in the specific embodiments. All technical features recorded in this case can be freely combined or combined in any way unless there is a contradiction between them.

[0076] It should also be noted that the above examples are merely specific embodiments of the present invention. Obviously, the present invention is not limited to the above examples, and many similar variations are possible. Any variations that can be directly derived or conceived by those skilled in the art from the present disclosure are intended to fall within the scope of protection of the present invention.

Claims

1. A petroleum casing, in addition to Fe and inevitable impurities, also contains the following chemical elements in mass percentage: C: 0.05-0.15%; Si: 0.4-0.8%; Mn: 3-5%; Ni: 0.2-0.5%; Ca: 0.0005-0.005%; Ti: 0.01-0.04%; Al: 0.01-0.05%; Nb: 0.02-0.04%; The microstructure of the oil casing is ferrite+granular bainite+austenite.

2. The oil casing according to claim 1, characterized in that: The oil casing contains the following chemical elements in mass percentage: C: 0.05-0.15%; Si: 0.4-0.8%; Mn: 3-5%; Ni: 0.2-0.5%; Ca: 0.0005-0.005%; Ti: 0.01-0.04%; Al: 0.01-0.05%; Nb: 0.02-0.04%; the balance being Fe and inevitable impurities.

3. The oil casing according to claim 1 or 2, characterized in that: The petroleum casing also satisfies at least one of the following: 0<Cr≤1%; 0<Mo≤0.5%; 0<V≤0.15%.

4. The petroleum casing according to claim 1 or 2, characterized in that: Among the unavoidable impurities, P≤0.015% and S≤0.005%.

5. The oil casing according to claim 1 or 2, characterized in that: The volume fraction of the granular bainite is 6-10%.

6. The oil casing according to claim 1 or 2, characterized in that: The volume fraction of austenite is 8-20%.

7. The oil casing according to claim 1 or 2, characterized in that: The performance of the oil casing meets at least one of the following: yield strength is 758-965MPa; tensile strength is ≥930MPa; yield strength ratio is ≤0.85; elongation is ≥35%; uniform elongation is ≥20%; 0°C transverse Charpy impact energy is ≥60J; work hardening index n is ≥0.15; the yield strength of the material after 5% deformation is increased by more than 30% compared with that of the material without deformation.

8. A method for manufacturing a petroleum casing according to any one of claims 1 to 7, characterized in that: The method comprises the following steps performed in sequence: (1) smelting and continuous casting to obtain round billets; (2) Perforation; (3) rolling and tension reducing to obtain a steel pipe; (4) Two-phase normalizing heat treatment: Heat the steel pipe to the austenitizing temperature Ac1+60℃ to Ac3-60℃, keep warm for 30-60min and then air cool; where Ac1=723-10.7Mn-16.9Ni+29.1Si+16.9Cr; Ac3=910-203C 1 / 2 -15.2Ni+44.7Si+104V+31.5Mo; each chemical element in the formula is substituted with the value before the percentage sign of its mass percentage content; (5) Thermal straightening.

9. The method according to claim 8, characterized in that In step (1), the continuous casting speed is 1.6-2.0 m / min.

10. The method according to claim 8, characterized in that In step (2), the round billet is heated at 1220-1270°C and then pierced at a piercing temperature of 1150-1200°C.

11. The method according to claim 8, characterized in that In step (3), the final rolling temperature is 920-980°C.

12. The method according to claim 8, characterized in that In step (3), the temperature of the stretch reduction is 900-950°C.

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