A method for cathodic protection of the connection between an oilfield string tubing and a coupling

Abstract

This invention discloses a cathodic protection method for the connection between oilfield tubing and couplings. The method comprises: Step 1, coupling pretreatment; Step 2, cladding material preparation; Step 3, laser cladding construction; Step 4, cathodic protection range verification; and Step 5, post-treatment. Beneficial effects include: strong targeted protection; high bonding strength; excellent corrosion resistance; convenient construction; high safety, and broad industrial application prospects.

Description

Technical Field

[0001] This invention relates to a cathodic protection method, and more particularly to a cathodic protection method for the connection between the oil pipe and the coupling in an oilfield tubing string. Background Technology

[0002] Currently, oil well tubing is a core component in the crude oil or oil and gas lift process, and the couplings connecting the downhole tubing are high-risk areas for corrosion failure. In complex downhole conditions, corrosion at these connections primarily stems from the following factors:

[0003] Flow field induced corrosion: The flow field changes abruptly in the oil pipe joint area, which easily forms eddies, accelerating the transfer and scouring of corrosive components, leading to increased local corrosion.

[0004] Crevice corrosion: The narrow gap formed between the coupling and the oil pipe becomes an electrolyte enrichment area. Combined with the effects of CO2, O2, and H2S, it promotes the formation and expansion of pitting corrosion, eventually leading to corrosion perforation.

[0005] Stress-induced corrosion: Stress concentration at the joint, combined with the high temperature and high pressure environment downhole, accelerates the corrosion process and leads to leakage failure.

[0006] Multi-media corrosion: Anthropogenic oil displacement media, Cl⁻, CO2, H2S and other media in formation water cause electrochemical corrosion of pipes, especially in high-salt and high-sulfur environments, the corrosion rate is significantly increased.

[0007] Currently, anti-corrosion measures for oil well pipe connections mainly include thread grease sealing and internal coatings. However, thread grease can only provide short-term isolation and cannot block electrochemical corrosion. Internal coatings are easily damaged at points of abrupt changes in flow field, thus losing their protective function. Therefore, developing a cathodic protection method with high bonding strength, excellent corrosion resistance, and targeted protection for connection points has become the key to improving the service life of oil well pipes. Summary of the Invention

[0008] The main objective of this invention is to address the numerous problems existing in the current anti-corrosion measures for oil well pipe connections, and to provide a cathodic protection method for the connection between the oil pipe and the coupling in oilfield tubing.

[0009] The cathodic protection method for the connection between the oilfield tubing and coupling provided by this invention includes the following steps:

[0010] Step 1, Coupling pretreatment: Select the oil pipe coupling to be treated, and machine an annular groove at the empty cut position in the middle of its inner hole; the width of the annular groove is the original width of the empty cut, the depth is 3mm, and the groove shape is adapted to the empty cut position in the inner hole of the coupling to ensure a smooth and full transition between the cladding layer and the substrate.

[0011] Step 2, Preparation of cladding material: Purchase zinc-aluminum alloy powder with specified content from the market. The mass percentage composition is: Al 10%-15%, Zn 80%-88%; the impurity content is ≤0.5%, the powder particle size is 150-300 mesh, and the flowability is ≥9s / 50g.

[0012] Step 3, Laser Cladding Construction: The zinc-aluminum alloy powder prepared in Step 2 is clad using laser cladding technology. The cladding parameters are controlled as follows: laser power 1.2-1.8kW, scanning speed 8-12mm / s, spot diameter 3-5mm, protective gas is argon, flow rate 10-15L / min; after cladding, a zinc-aluminum alloy cladding layer with a thickness matching the groove depth of 3mm and a width matching the original width of the empty blade is formed. The cladding layer and the coupling substrate achieve metallurgical bonding without defects such as cracks and pores.

[0013] Step 4, Cathodic Protection Range Verification: Using the zinc-aluminum alloy cladding layer prepared in Step 3 as the sacrificial anode, it forms an electrical couple with the tubing and coupling. In the downhole electrolyte environment, the cladding layer preferentially dissolves and releases electrons, forming cathodic protection for the connection area between the tubing and coupling. The effective protection range is: within a 200mm radius on both sides of the cladding layer along the tubing axis, covering the joint of the tubing coupling, the root of the thread, and the area of ​​abrupt change in the flow field. When the relative saturated copper sulfate reference electrode CSE potential of the protected object is stable between -0.85V and -1.2V, the cathodic protection is deemed effective.

[0014] Step 5, Post-processing: After the cladding is completed, the cladding part of the inner hole of the mating ring is re-processed according to API standards.

[0015] In step 1, the coupling steel grades are J55, N80, and P110.

[0016] In step 3, laser cladding ensures that the thickness of the cladding layer is precisely controlled to be greater than or equal to 3mm and that the cladding is full.

[0017] The above methods can be applied to corrosion protection of well pipe connections in high-temperature and high-pressure oil and gas wells, sulfur-containing oil and gas wells, and high-salinity oil and gas wells.

[0018] The core technical principle of this invention is as follows: Utilizing the more negative electrode potential (-1.05V to -1.20V vs. CSE) of the zinc-aluminum alloy compared to the tubing and coupling substrates (J55, N80, P110 approximately -0.85V vs. CSE), the zinc-aluminum alloy is clad onto the hollow area in the inner hole of the tubing coupling, forming a sacrificial anode layer. When the tubing and coupling are assembled and lowered into the well, the cladding layer forms an electrical couple with the tubing and coupling in the electrolyte environment. The cladding layer preferentially dissolves and oxidizes, releasing electrons that flow to the connection area between the tubing and coupling, placing this area in a cathodic polarized state. This inhibits electrochemical corrosion reactions and achieves cathodic protection. Simultaneously, laser cladding technology enables a metallurgical bond between the zinc-aluminum alloy and the coupling substrate, ensuring that it does not detach or crack under high temperature, high pressure, and scouring conditions in the well.

[0019] The beneficial effects of this invention are:

[0020] The protection is highly targeted: the cladding layer is precisely set at the hollow part of the coupling's inner hole, directly covering the high-corrosion area (flow field change zone, crevice zone, stress concentration zone) where the coupling connects to the oil pipe. It effectively solves the local corrosion problem mentioned in the background technology. The protection range is within 200mm on both sides of the oil pipe axis, covering the entire connection area (as shown in Figure 3). It can comprehensively suppress the electrochemical corrosion in this area, which is consistent with the technical objectives proposed in the invention.

[0021] High bonding strength: Laser cladding achieves metallurgical bonding between zinc-aluminum alloy and coupling substrate, with a bonding strength ≥400MPa. It does not fall off or crack under high pressure and scouring conditions downhole, and extends service life by more than 5 times.

[0022] Excellent corrosion resistance: The zinc-aluminum alloy cladding layer combines the cathodic protection of a sacrificial anode with its own corrosion resistance, making it suitable for complex downhole media such as CO2, Cl⁻, and H2S. The corrosion rate is ≤0.02mm / year, making it suitable for various complex downhole working conditions.

[0023] Convenient construction: It only requires machining a groove at the empty hole of the coupling and performing local cladding, without changing the overall structure of the coupling or affecting the subsequent assembly accuracy with the tubing. It can be adapted to the existing production and assembly process of oil well pipes and couplings, without the need for additional special equipment.

[0024] High safety: The cladding layer is tightly bonded to the substrate, eliminating the risk of coating peeling off. It avoids tubing failure caused by impurities clogging the well casing and pump sticking, ensuring well integrity and production safety, and reducing well workover risks.

[0025] ⑥ Industrial Applicability: The method of this invention can be widely applied to corrosion protection engineering of downhole pipe and pipe connection couplings in oilfield production, oil and gas, strata gas, coalbed methane, and water injection wells, especially suitable for oil and gas wells under complex conditions such as high temperature, high pressure, high salinity, and high sulfur content. Field tests have verified that well pipe connections treated with this method show no corrosion perforation or leakage during their service life, significantly extending pump inspection cycles and substantially reducing well workover costs, thereby improving the economic efficiency and safety of oil and gas field development. Furthermore, this method is simple, highly operable, and can be mass-produced for coupling components of oilfield lift tubing, demonstrating broad industrial application prospects. Attached Figure Description

[0026] Figure 1 is a schematic diagram of the annular groove structure at the empty cutter position of the inner hole of the tubing coupling. The middle position of the inner hole of the coupling body 1 is provided with the empty cutter position 2. An annular groove 3 is machined at this position. The width of the annular groove 3 is the original width of the empty cutter, and the depth is 3mm. The groove wall is smooth and burr-free. The groove shape is adapted to the empty cutter position of the inner hole of the coupling to ensure a smooth transition between the subsequent cladding layer and the substrate and avoid stress concentration.

[0027] Figure 2 is a schematic diagram of the inner hole structure of the tubing coupling after laser cladding of zinc-aluminum alloy. The annular groove 3 at the empty cutter is filled and clad with a zinc-aluminum alloy cladding layer 4. The thickness of the zinc-aluminum alloy cladding layer 4 is consistent with the groove depth (3mm), and the width is the original width of the empty cutter. It achieves metallurgical bonding with the coupling substrate, with a smooth surface and no defects such as cracks or pores. The edge transition of the cladding layer is smooth and does not affect the subsequent assembly and fit with the tubing.

[0028] Figure 3 is a schematic diagram of the connection between the tubing and the coupling and the cathodic protection range. After pretreatment and laser cladding, the coupling is connected to the tubing body 5 through the internal thread. The zinc-aluminum alloy cladding layer 4 is located in the middle of the inner hole of the coupling. The cathodic protection range 6 extends 200mm to both sides along the tubing axis with the zinc-aluminum alloy cladding layer 4 as the center. This range completely covers the connection between the coupling and the tubing, the thread root, and the flow field change zone, which can completely block the electrochemical corrosion in this area, consistent with the protection range specified in the claims.

[0029] The annotations in the above figure are as follows:

[0030] 1. Coupling body; 2. Hole opening in the inner hole of the coupling; 3. Annular groove; 4. Zinc-aluminum alloy cladding layer.

[0031] 5. Oil pipe body 6. Cathodic protection range. Detailed Implementation

[0032] Please see Figures 1 to 3 As shown:

[0033] Example 1:

[0034] The cathodic protection method for the connection between the oilfield tubing and coupling provided by this invention includes the following steps:

[0035] Step 1, Coupling pretreatment: Select API 5CT J55 steel grade, specification 2-7 / 8 (outer diameter 73.02mm, wall thickness 5.51mm) oil pipe coupling, and machine an annular groove 3 with a width equal to the original width of the empty cutter and a depth of 3mm at the empty cutter position in the middle of its inner hole. The groove wall is smooth and burr-free.

[0036] Step 2, Preparation of cladding material: Purchase suitable zinc-aluminum alloy powder from the market. The powder composition is Al 12%, Zn 87.5%, impurities 0.5%, powder particle size 200 mesh, flowability 9.2s / 50g, and the powder is free of lumps and impurities to ensure cladding quality.

[0037] Step 3, Laser Cladding Construction: A dedicated laser cladding machine is used. The parameters are set as follows: laser power 1.5kW, scanning speed 10mm / s, spot diameter 4mm, and argon gas protection flow rate 12L / min. After cladding, a zinc-aluminum alloy cladding layer 4 with a thickness of 3mm and a width equal to the original width of the blank blade is formed (as shown in Figure 2). After testing, the cladding layer and the substrate are free of cracks and pores, and the bonding strength is ≥400MPa.

[0038] Step 4, verification of cathodic protection range 6: After assembling the treated coupling with the API 5CT J55 steel grade tubing (outer diameter 73.02mm, wall thickness 5.51mm), it was lowered into a simulated formation water environment containing CO2 and Cl⁻ (conductivity 2000μS / cm, temperature 80℃). The potential was tested using a reference electrode: the potential in the central region of the cladding layer was -1.15V vs. CSE, and the potential in the region extending 180mm to both sides along the tubing axis was stable between -0.90V and -1.15V vs. CSE. The cathodic protection range 6 completely covered the flow field abrupt change area at the coupling connection.

[0039] Step 5, Post-processing: After the cladding is completed, the cladding layer of the inner hole of the coupling is re-processed according to the API 5CT standard requirements to ensure the accuracy of the fit with the tubing.

[0040] Example 2:

[0041] The cathodic protection method for the connection between the oilfield tubing and coupling provided by this invention includes the following steps:

[0042] Step 1, Coupling Pretreatment: Select an API 5CT N80 steel grade, specification 3-1 / 2 (outer diameter 88.9mm, wall thickness 6.45mm) oil pipe coupling, and machine an annular groove 3 with a width equal to the original width of the empty cutter and a depth of 3mm at the empty cutter position in the middle of its inner hole. The groove wall is smooth.

[0043] Step 2, Preparation of cladding materials: Purchase appropriate zinc-aluminum alloy powder with a mass composition of Al 10%, Zn 89.8%, impurities 0.2%, powder particle size of 150 mesh, and flowability of 8.8s / 50g, which meets the requirements of laser cladding construction.

[0044] Step 3, Laser Cladding Construction: A dedicated laser cladding machine is used. Parameter settings: laser power 1.8kW, scanning speed 8mm / s, spot diameter 5mm, argon gas protection flow rate 15L / min; after cladding, a cladding layer with a thickness of 3.2mm (meeting the requirement of ≥3mm) and a width equal to the original width of the empty blade is formed, with a bonding strength ≥450MPa.

[0045] Step 4, Cathode Protection Range 6 Verification: The assembled oil pipe and coupling were placed in a simulated high-sulfur environment (containing H2S, Cl⁻, conductivity 3000μS / cm, temperature 120℃). The potential was tested using a reference electrode: the potential in the 170mm area on both sides of the cladding layer was stable between -0.95V and -1.18V vs. CSE, which meets the effective protection potential requirements, effectively covering the connection between the coupling and the oil pipe, and the cathodic protection effect is good.

[0046] Step 5, Post-processing: After the cladding is completed, the cladding layer of the inner hole of the mating ring is reprocessed according to the requirements of API 5CT standard.

Claims

1. A method for cathodic protection at the connection between the oilfield tubing and the coupling, characterized in that: The method includes the following steps: Step 1, Coupling pretreatment: Select the oil pipe coupling to be treated, and machine an annular groove at the empty cut position in the middle of its inner hole; the width of the annular groove is the original width of the empty cut, the depth is 3mm, and the groove shape is adapted to the empty cut position in the inner hole of the coupling to ensure a smooth and full transition between the cladding layer and the substrate. Step 2, Preparation of cladding material: Purchase zinc-aluminum alloy powder with a specified content from the market. The mass percentage composition is: Al 10%-15%, Zn 80%-88%. The impurity content is ≤0.5%, the powder particle size is 150-300 mesh, and the flowability is ≥9s / 50g; Step 3, Laser Cladding Construction: The zinc-aluminum alloy powder prepared in Step 2 is clad using laser cladding technology. The cladding parameters are controlled as follows: laser power 1.2-1.8kW, scanning speed 8-12mm / s, spot diameter 3-5mm, protective gas is argon, flow rate 10-15L / min; after cladding, a zinc-aluminum alloy cladding layer with a thickness matching the groove depth of 3mm and a width matching the original width of the empty blade is formed. The cladding layer and the coupling substrate achieve metallurgical bonding without defects such as cracks and pores. Step 4, Cathodic Protection Range Verification: Using the zinc-aluminum alloy cladding layer prepared in Step 3 as the sacrificial anode, it forms an electrical couple with the tubing and coupling. In the downhole electrolyte environment, the cladding layer preferentially dissolves and releases electrons, forming cathodic protection for the connection area between the tubing and coupling. The effective protection range is: within a 200mm radius on both sides of the cladding layer along the tubing axis, covering the joint of the tubing coupling, the root of the thread, and the area of ​​abrupt change in the flow field. When the relative saturated copper sulfate reference electrode CSE potential of the protected object is stable between -0.85V and -1.2V, the cathodic protection is deemed effective. Step 5, Post-processing: After the cladding is completed, the cladding part of the inner hole of the mating ring is re-processed according to API standards.

2. The cathodic protection method for the connection between the oilfield tubing and the coupling according to claim 1, characterized in that... In step 1, the coupling steel grades are J55, N80, and P110.

3. The cathodic protection method for the connection between the oilfield tubing and the coupling according to claim 1, characterized in that... In step 3, laser cladding ensures that the thickness of the cladding layer is precisely controlled to be greater than or equal to 3 mm and that the cladding is full.

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