A downhole electric heater based on small resistivity heat conductor and method of manufacture

Abstract

This invention relates to the field of downhole heating equipment technology, and particularly to a downhole electric heater based on a low-resistivity heating conductor and its manufacturing method. The technical solution is as follows: A downhole electric heater based on a low-resistivity heating conductor includes an outer alloy sheath, which is a continuous tubular structure. The outer alloy sheath is made of nickel-based alloy or stainless steel strip laser-welded. A cold section conductor and a heating section conductor are coaxially arranged inside the outer alloy sheath. This invention uses a copper-nickel alloy as the heating section conductor material. The resistivity of copper-nickel alloy is much lower than that of traditional nickel-chromium alloy. Under the same power requirements, the conductor diameter can be significantly reduced, thereby significantly reducing the overall outer diameter of the heater. This allows it to be adapted to small-sized tubing, solving the installation problem of heating equipment in old well retrofitting and new narrow-bore wells, and leaving sufficient space for the parallel installation of other tools downhole, improving the flexibility and efficiency of single-well operations.

Description

Technical Field

[0001] This invention relates to the field of downhole heating equipment technology, and in particular to a downhole electric heater based on a low resistivity heating conductor and its manufacturing method. Background Technology

[0002] In unconventional energy extraction, in-situ downhole heating is a key technology for reducing oil and gas viscosity, improving wellbore fluid flow, and increasing recovery rate.

[0003] Existing downhole electric heaters generally use nickel-chromium alloy as the conductor material for the heating section, which has a high resistivity. When a large heating power is required downhole, the cross-sectional area of ​​the conductor needs to be increased to meet the power demand. This results in an excessively large overall outer diameter of the heater, making it difficult to adapt to small-sized tubing. In old well stimulation or narrow-bore new well operations, the well diameter is strictly limited, making it impossible to smoothly run the heater. It also cannot be installed in parallel with other downhole tools, which seriously restricts the efficiency of single-well operations and the implementation of heavy oil thermal recovery technology. Furthermore, the high resistivity conductor fluctuates significantly under long-term high-temperature operation, affecting the stability of the heating power and increasing the difficulty of downhole temperature control. Summary of the Invention

[0004] To overcome the problem that existing downhole electric heaters generally use nickel-chromium alloy as the conductor material for the heating section, which has high resistivity, and when a large heating power is required downhole, it is necessary to increase the cross-sectional area of ​​the conductor to meet the power demand, resulting in an excessively large overall outer diameter of the heater, making it difficult to adapt to small-sized oil pipes.

[0005] The technical solution of the present invention is as follows: a downhole electric heater based on a low resistivity heating conductor, comprising an outer alloy sheath, the outer alloy sheath being a continuous tubular structure, the outer alloy sheath being made of nickel-based alloy or stainless steel strip laser welded, and a cold section conductor and a heating section conductor being coaxially arranged inside the outer alloy sheath, with three sets of both the cold section conductor and the heating section conductor. One end of the cold section conductor is equipped with a power supply cable, and the other end of the cold section conductor is connected to the heating section conductor by a cold pressure welding process. The conductor of the heating section is made of a solid copper-nickel alloy rod that has been solution strengthened. An insulating layer is filled between the cold section conductor, the heated section conductor, and the outer alloy sheath. The tail section of the heating conductor is formed with a star junction by argon arc welding.

[0006] Preferably, the cold section conductor is made of T2 copper solid rod, and the diameter of the cold section conductor is equal to the diameter of the heating section conductor to ensure low-loss transmission of power supply.

[0007] Preferably, the outer alloy sheath is made of nickel-based alloy or nickel-iron-molybdenum multiphase stainless steel.

[0008] As a preferred option, the connection ends of the cold section conductor and the heating section conductor are sanded to remove the oxide film, and then connected by a cold pressure welding process.

[0009] Preferably, the conductors of the heating section converge and connect at the tail of the heater, and form a star junction by argon arc welding, with the weld point wrapped with a high-temperature resistant insulating sleeve.

[0010] Preferably, a method for manufacturing a downhole electric heater based on a low resistivity heating conductor includes the following steps: S11: The outer alloy sheath strip is cut at 45° at both ends, and after mechanical grinding and cleaning with anhydrous ethanol, it is butt-welded using fiber laser with a power of 2.0~3.0 kW and a speed of 1.5~2 m / min to form a continuous strip. S12: The cold section copper rod and the heated section copper-nickel alloy rod are straightened with a straightness error of ≤0.5 mm / m. After the ends are ground, they are butt welded by cold pressure and then stress-relieved annealed at 450~650℃. S13: The continuous strip is fed into a roll forming machine and rolled into a tube shape. The long side is welded by fiber laser welding with a weld reinforcement height of ≤0.5 mm. Argon gas protection is used to form a seamless outer sheath. S14: Three conductors are inserted into the positioning channel of the core tube through the guide wheel and arranged in an equilateral triangle. Magnesium oxide powder is filled synchronously through the powder filling channel, and the vibration frequency is 0.5~1 Hz for initial compaction. S15: After filling, the sheath is reduced in diameter by multiple sets of vertical rolling mills, the magnesium oxide is compacted, and then it is turned 90° by guide wheels to enter the horizontal straightening process; S16: Induction annealing at 800℃ and water cooling to eliminate cold working stress; S17: Reduced to the target size by a horizontal rolling mill, compacted density ≥2.8 g / cm³, then annealed twice at 600℃ and water-cooled; S18: After flaw detection to ensure no cracks or pores are found, laser diameter and length measurement is performed. Qualified products are then wound up and sealed with epoxy resin.

[0011] Preferably, in S14, the core tube has a double-layer structure, with the inner layer used for conductor positioning and the outer layer forming a magnesium oxide-filled channel between it and the outer sheath.

[0012] Preferably, in S16 and S17, the two online annealing temperatures are 800℃ and 600℃ respectively, and the cooling method is water cooling.

[0013] Preferably, a method for manufacturing a downhole electric heater based on a low-resistivity heating conductor further includes the following steps when the downhole electric heater based on the low-resistivity heating conductor is subjected to thermal condition simulation testing: S21: Place the electric heater sample inside the test furnace, and connect the conductor and outer sheath to the test electrode according to the test items; S22: Select the power supply according to the test type and turn on the oil cooler, circulating pump, thermocouple recorder and infrared thermal imager; S23: Set the heating temperature, heating rate, holding time and cooling method to simulate high-temperature working conditions downhole; S24: Set the target temperature and control accuracy, start the closed-loop temperature control system, and record the temperature, resistance, and voltage curves in real time; S25: After the furnace temperature stabilizes to the target value, connect the insulation resistance tester and the withstand voltage tester; S26: Apply 1.1 times the rated voltage, with a voltage increase rate of 150 V / s, and apply the voltage for 60 seconds. Record the leakage current. The judgment criteria are no breakdown and leakage current ≤0.2mA. S27: Apply the corresponding voltage level, apply pressure for 60 seconds, record the insulation resistance, and the judgment criterion is that it is not lower than the standard value; S28: Continuous measurement mode, records resistance fluctuation, with a judgment criterion of fluctuation ≤2%; S29: Input insulation layer thickness, temperature, voltage and current parameters to calculate heat transfer efficiency and thermal resistance; S210: Set the current and time, monitor the sample temperature, current and voltage, and judge the criteria as no melting, no short circuit and stable temperature; S211: Turn off the power in sequence, cool to room temperature, disconnect the connecting wires, and check the status of the sample and equipment.

[0014] As a preferred option, in S26-S210, all test data are automatically collected by the system software and a test report is generated, including temperature curves, resistance change curves, infrared thermal images, leakage current values, insulation resistance values, and current-carrying stability parameters, forming a complete and traceable test archive.

[0015] The beneficial effects of this invention are: 1. Existing downhole electric heaters generally use nickel-chromium alloy as the conductor material for the heating section, which has high resistivity. When a large heating power is required downhole, the conductor cross-sectional area needs to be increased to meet the power demand, resulting in an excessively large overall outer diameter of the heater. This makes it difficult to adapt to small-sized tubing. In old well stimulation or narrow-bore new well operations, the well diameter is strictly limited, preventing the heater from being smoothly lowered. Furthermore, it cannot be installed in parallel with other downhole tools, severely restricting single-well operating efficiency and the implementation of heavy oil thermal recovery processes. Moreover, the high resistivity conductor exhibits significant resistance fluctuations under prolonged high-temperature operation, affecting the stability of the heating power and increasing the difficulty of downhole temperature control. This solution... By using copper-nickel alloy as the conductor material for the heating section, the resistivity of copper-nickel alloy is much lower than that of traditional nickel-chromium alloy. Under the same power requirements, the conductor diameter can be significantly reduced, thereby significantly reducing the overall outer diameter of the heater. This allows it to be adapted to small-sized tubing, solving the problem of heating equipment installation in old well refurbishment and new narrow-bore wells. It also leaves sufficient space for the parallel installation of other tools downhole, improving the flexibility and efficiency of single-well operations. At the same time, the resistivity of copper-nickel alloy changes little at operating temperature, resulting in more stable heating power output, high temperature control precision, and good oxidation resistance, effectively extending the service life of the heater and reducing the maintenance costs of frequent equipment replacement. 2. Existing downhole electric heaters are mostly manufactured using segmented circumferential welding, connecting short heating units to the required length via circumferential welding. This often results in numerous connection points, increasing the risk of leakage and breakage. Furthermore, the weld heat-affected zone is prone to stress concentration and material degradation, leading to a decline in the overall mechanical properties of the heater and making it unable to withstand the enormous tensile forces of wellhead suspension. Simultaneously, the segmented structure results in uneven insulation layer filling, easily forming local voids, affecting insulation performance and thermal conductivity, and impacting the heater's safe operation. This solution employs a continuous tubular integrated design and manufacturing method. Fiber laser welding is used to extend and roll the sheath strip, cold-press welding connects the cold and heating conductors, and high-purity magnesium oxide insulation is simultaneously filled. Vertical and horizontal gradient rolling and two online annealing processes then create a continuous integral structure for the conductor, insulation layer, and outer sheath. This significantly reduces connection points, eliminates stress concentration and performance weaknesses caused by circumferential welding, and significantly improves the overall tensile strength and sealing of the heater, enabling it to stably withstand deep well suspension loads while reducing maintenance and replacement frequency. Attached Figure Description

[0016] Figure 1 The diagram shown is a three-dimensional structural schematic of a downhole electric heater based on a low resistivity heating conductor according to the present invention. Figure 2 The diagram shown is a cross-sectional schematic of a downhole electric heater based on a low resistivity heating conductor according to the present invention. Figure 3The diagram shown is a partial cross-sectional schematic of an AA-type downhole electric heater based on a low resistivity heating conductor according to the present invention. Figure 4 The diagram shown is a schematic diagram of the BB part of a downhole electric heater based on a low resistivity heating conductor according to the present invention. Explanation of reference numerals in the attached diagram: 1. Power supply cable; 2. Cold section conductor; 3. Heating section conductor; 4. Outer alloy sheath; 5. Insulation layer; 6. Star contact. Detailed Implementation

[0017] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0018] Please see Figure 1-4 The present invention provides an embodiment of an underground electric heater based on a low resistivity heating conductor, comprising an outer alloy sheath 4, the outer alloy sheath 4 being a continuous tubular structure, the outer alloy sheath 4 being made of nickel-based alloy or stainless steel strip laser welded, and a cold section conductor 2 and a heating section conductor 3 being coaxially arranged inside the outer alloy sheath 4, with three sets of each of the cold section conductor 2 and the heating section conductor 3. One end of the cold section conductor 2 is equipped with a power supply cable 1, and the other end of the cold section conductor 2 is connected to the heating section conductor 3 by a cold pressure welding process. The conductor 3 in the heating section is made of a solid copper-nickel alloy rod that has undergone solid solution strengthening. The rod has a copper content of 38%~42%, a nickel content of 54%~60%, and a resistivity of ≤0.5×10⁻⁶. -6 Ω·m; An insulating layer 5 is filled between the cold section conductor 2, the heating section conductor 3 and the outer alloy sheath 4. The insulating layer 5 is made of high-purity magnesium oxide, with a compaction density ≥2.8 g / cm³, a thermal conductivity ≥3.5 W / (m·K), and a withstand voltage rating ≥3000V. The tail section of the heating section conductor 3 is welded with argon arc welding to form a star contact 6, which is used to form a three-phase three-wire power supply structure.

[0019] Preferably, the cold section conductor 2 is made of T2 copper solid rod with a copper content of ≥99.9% and a conductivity of ≥58MS / m. The diameter of the cold section conductor 2 is equal to the diameter of the heating section conductor 3 to ensure low-loss transmission in the power supply section. The diameter of the heating section conductor 3 is reduced by 40%~50% compared to the diameter of traditional nickel-chromium alloy conductors under the same power conditions, which reduces the overall outer diameter of the electric heater by 30%~40% and is suitable for small-sized oil pipes ≤φ60.3mm.

[0020] Preferably, the outer alloy sheath 4 is made of Incoloy 825 nickel-based alloy or nickel-iron-molybdenum multiphase stainless steel, with a wall thickness ≥2.4 mm, tensile strength ≥520 MPa, and elongation after fracture ≥40%. The outer alloy sheath 4 is used in corrosive downhole environments containing CO2 or H2S.

[0021] As a preferred option, the connection ends of the cold section conductor 2 and the heating section conductor 3 are sanded to remove the oxide film, and then connected by a cold pressure welding process. The joint is then subjected to stress-relief annealing at 450~650℃ to ensure that the mechanical properties of the joint are not lower than those of the base material.

[0022] Preferably, the heating section conductors 3 converge and connect at the tail of the heater, and form a star junction 6 by argon arc welding. The weld is wrapped with a high-temperature resistant insulating sleeve to ensure the stability and insulation safety of the three-phase electrical connection.

[0023] Example 1 An underground electric heater based on a low resistivity heating conductor includes an outer alloy sheath 4, inside which are arranged three cold section conductors 2 and three heating section conductors 3; one end of the cold section conductor 2 is connected to one end of a power supply cable 1, and the other end of the cold section conductor 2 is connected to the heating section conductor 3; an insulating layer 5 is filled between the cold section conductor 2, the heating section conductor 3 and the outer alloy sheath 4. In this embodiment, the insulating layer 5 is made of high-purity (≥99.5%) magnesium oxide powder, with a compaction density ≥2.8g / cm3, a thermal conductivity ≥3.5W / (mK) in the compacted state, and a withstand voltage rating ≥3000V at 400℃. The outer alloy sheath 4 has a thickness ≥ 2.4 mm and a diameter of 31.8 mm. In this embodiment, nickel-iron-molybdenum multiphase stainless steel is used, which has a tensile strength ≥ 520 MPa and an elongation after fracture ≥ 40% after solution strengthening. The cold section conductor 2 is made of a solid rod containing more than 99% copper, and the heating section conductor 3 is made of a solid copper-nickel alloy rod that has been strengthened by solid solution, wherein the copper content is 38%~42% and the nickel content is 54%~60%. Using this material as the heating section can reduce the conductor diameter by 40%~50% while maintaining the same length and resistance per meter. The cold section conductor 2 and the heating section conductor 3 are connected by cold pressure welding. The three conductors are arranged in an equilateral triangle. The tails of the heating section conductor 3 are gathered together and connected by argon arc welding. By adopting an integrated synchronous assembly manufacturing process for conductors, insulating materials, and outer alloy sheaths, the electric heaters manufactured have a tensile strength more than twice that of the outer alloy sheath base material, ensuring that even with a reduced electric heater volume, they can still meet the weight requirements of hanging at wellheads thousands of meters deep.

[0024] Example 2 The method for manufacturing a downhole electric heater as described in Example 1 includes the following steps: S31: Cut both ends of the outer alloy sheath strip to be joined at a 45° angle. Remove oil, scale, and burrs from the ends by mechanical grinding and wiping with anhydrous ethanol. Weld the ends of the strip using fiber laser welding at a welding power of 2.0~3.0kW and a welding speed of 1.5~2m / min to form a continuous strip that meets the processing length requirements. Send the joined strip into an ultrasonic cleaning machine to remove oil, metal powder, and other impurities from the surface of the strip. S32: Mechanically straighten the cold section copper rod and the heated section copper-nickel alloy rod respectively to ensure that the straightness error is ≤0.5mm / m. Use sandpaper to polish the ends of the conductors to be welded to remove the surface oxide film. Use cold pressure welding process to accurately butt together, then grind to remove burrs, indentations, etc. from the joint, and perform stress relief annealing at 450-650℃. S33: The continuous strip after docking is fed into a roller forming machine and gradually formed into a tubular structure through multiple passes; the long side of the steel strip is continuously welded by fiber laser welding, and the weld reinforcement is controlled to be ≤0.5mm. Argon gas protection is used during the welding process to avoid weld oxidation, forming a continuous seamless outer sheath tube. S34: Three conductors are guided by guide wheels to enter the positioning channel inside the core tube, ensuring that the conductors are arranged in an equilateral triangle; magnesium oxide enters the channel between the inner and outer tube walls of the core tube through the powder filling channel at the same time, and the vibration device is turned on to vibrate synchronously at a frequency of 0.5~1Hz to achieve the initial compaction of magnesium oxide powder; S35: After magnesium oxide and conductor are synchronously filled into the welded outer sheath, it is rolled by multiple sets of vertical rolling mills to reduce the diameter and gradually increase the compaction density of magnesium oxide. Then, the outer sheath passes through multiple sets of guide wheels to turn from a vertical state to a horizontal state by 90°, and then enters the horizontal straightening rollers for straightening. S36: After straightening, the product enters a tubular induction annealing furnace and is heated to 800℃ for online annealing. Then it enters a water-cooled cooling tank for rapid cooling to eliminate the cold working deformation stress caused by vertical rolling and turning, reduce the surface hardness of the material, and facilitate the rolling and diameter reduction process again. S37: After cooling, it enters a horizontal rolling mill for re-rolling to reduce the product diameter to the target size, further increasing the compacted density of magnesium oxide to ≥2.8 g / cm³. 3 This ensures that the product meets insulation performance requirements; subsequently, the product undergoes a second online annealing at 600℃, followed by water cooling to eliminate cold working stress and facilitate subsequent winding. S38: After cooling, the product is inspected for defects by eddy current testing to ensure that the sheath is free of cracks, pores and other defects; the outer diameter and length of the product are measured by laser diameter gauge and length gauge respectively, and qualified products are wound into a roll. The ends of the product are sealed with epoxy resin to prevent the magnesium oxide insulation layer from absorbing moisture and failing. S39: Conduct high-temperature insulation withstand voltage, mechanical properties, fatigue performance, waterproof performance, and corrosion resistance tests on electric heater products to ensure that the products meet the requirements of downhole working conditions.

[0025] Example 3 This embodiment provides a three-core integrated continuous tubular downhole electric heater with an outer diameter of 31.8 mm. The difference from Embodiment 1 is that the outer alloy sheath is made of Incoloy 825 nickel-based alloy with a wall thickness of 3.0 mm. This alloy has excellent corrosion resistance and can be used in CO2 or sulfur-containing environments. The three internal conductors are arranged in an equilateral triangle. The cold section is a 6.1 mm diameter T2 pure copper rod, and the heating section is a 6.1 mm diameter copper-nickel alloy rod. The insulating magnesium oxide powder has a density of 3.0 g / cm³ after manufacturing and compaction. The heating section conductors are connected in a star shape by welding at the tail. The heating section of this electric heater is designed to be 800 meters long. Under the conditions of rated voltage 3000V and three-phase power supply, it can output approximately 600 kW of power and can be used in heavy oil thermal recovery and other scenarios.

[0026] Example 4 This embodiment focuses on the electric heater sample described in Embodiment 1, employing the aforementioned hot-stage operating condition simulation testing method to conduct comprehensive hot-state operating condition testing. It also clarifies the pass / fail criteria to ensure rigorous and traceable test results. Specific steps include: S41: Place the prepared electric heater sample inside the test furnace and reliably connect the sample to the test electrodes using a dedicated connecting wire. When performing insulation resistance, withstand voltage, and leakage current tests, fix one set of electrodes to the outer alloy sheath of the electric heater and fix the other set of electrodes to the internal conductor. When performing the current flow test, connect the electrodes to the conductors at both ends of the sample through the connecting wire and ensure that the joints are insulated from the outer alloy sheath. All connecting wires are isolated and fixed to prevent them from contacting the sheath, the inner wall of the furnace, and the internal heating wire. S42: Select the corresponding power supply circuit according to the test conditions. Close the power switch of the electric furnace for DC resistance, insulation resistance, withstand voltage and leakage current test. Close the AC power switch for current flow test. Turn on the main power switch of the oil cooler and the heating furnace in sequence. After the equipment initialization is completed, start the oil cooler compressor and circulation pump. Observe the operating parameters of the oil cooler to confirm that the cooling system circulation is normal. Simultaneously turn on the thermocouple recorder and infrared thermal imager, start the temperature recording function, and collect the temperature data, temperature change curve and infrared thermal image spectrum of the sample conductor and sheath in real time. S43: On the furnace temperature control system interface, set the heating temperature to 400℃, the heating rate to 10℃ / min, the holding time to 0.5h, and the cooling method to natural cooling. After confirming that the parameter settings are correct, press the "Start" button. The furnace will slowly heat up according to the set curve. Monitor the temperature change in the furnace in real time to ensure that the heating is stable and there is no sudden rise or fall in temperature. S44: Enter the test operation interface of the test platform, set the target temperature to 400℃, the control accuracy to ±5℃, start the temperature acquisition and closed-loop control function, the system feeds back the sample temperature signal in real time through infrared thermal imager and thermocouple, automatically adjusts the furnace heating power, so that the sample temperature is stable within the range of 400℃±5℃, and continuously records the operating curves of temperature, resistance, voltage etc. S45: After the furnace is turned on for heating and the system parameters are set, connect the rear ends of the electrodes outside the furnace to the HD2705B insulation resistance tester and the HDGT-10kV withstand voltage tester respectively. The DC resistance test, heat transfer test and current flow test are built into the system. When the furnace temperature reaches 400℃, perform insulation resistance test, withstand voltage and leakage current test, DC resistance test and heat transfer test on the sample. S46: Enter the insulation withstand voltage test module, check the connection of all test leads and ground wires of the withstand voltage tester and the sample, and connect the withstand voltage insulation tester to the power supply; apply the test voltage between the conductor and the outer alloy sheath. The product's rated voltage is 3000V, set the test voltage to 1.1 times the rated voltage, the minimum voltage increase rate to 150V / s, and the voltage application time to 60s. Turn on the instrument, and the timer will start simultaneously; observe whether there are any flashing or breakdown phenomena during the test, and record the measured leakage current value simultaneously; S47: Enter the insulation resistance test module. When the sample temperature stabilizes at 400℃, check the connection between the insulation resistance tester and all test lines and ground wires of the sample. Connect the tester to the power supply. The product's rated voltage is 3000V. Select the 2500V range to apply DC voltage for measurement. The voltage application time is 60s. When the reading stabilizes, record the measured insulation resistance value. S48: Enter the built-in DC resistance test module of the system, select the appropriate test range according to the expected resistance range of the sample, set the measurement mode to continuous measurement to ensure comprehensive data acquisition; after confirming the parameters, the tester automatically applies the test current and measures the resistance value in real time, and records the statistical information of the measurement results; S49: Enter the built-in heat transfer test module of the system, input the sample magnesium oxide insulation layer thickness of 5mm, initial temperature of 25℃, preset voltage of 3000V, current of 25A and the heat transfer efficiency calculation formula, the formula is: K= (core wire temperature (T1) - outer sheath temperature (T2) / thickness) ), after the test is started, the system collects the conductor temperature and outer sheath temperature in real time, and automatically calculates the heat transfer efficiency, thermal resistance and heat transfer coefficient according to the temperature difference and insulation layer thickness; S410: Enter the current flow test module and confirm that the AC power switch is closed; set the current flow to 25A and the current flow time to 30 days, and match the minimum current rise rate with the voltage rise standard. Simultaneously turn on the thermal imager and thermocouple to monitor the sample temperature and current and voltage fluctuations in real time; strictly control the parameters to keep them stable during the test and avoid sudden rises and falls. Record the output current, measured voltage, measured resistance, power per meter and current density in real time through the current flow software. Observe the sample throughout the process to ensure there are no abnormalities such as overheating, damage, short circuit and leakage. S411: After all test items are completed, turn off the electrode power supply, furnace heating power supply and zone heating switch in sequence, and keep the oil cooler and cooling system running until the sample and furnace cool down to room temperature naturally. Then disconnect the test connection line, check the appearance of the sample, and check for any deformation or damage. Reset, clean and check the status of the test equipment to confirm that there are no abnormalities in the equipment, and complete the entire hot stage working condition simulation test process.

[0027] Example 5 This embodiment is a downhole electric heater based on a low resistivity heating conductor, used for wax removal operations in wellbores with high wax content. It adopts a segmented temperature control design to achieve precise temperature control throughout the wellbore and prevent wax precipitation and deposition.

[0028] The electric heater includes an outer alloy sheath 4 made of Incoloy 825 nickel-based alloy with a wall thickness of 3.0 mm and an outer diameter of 34.0 mm, suitable for φ60.3 mm oil pipes. The outer alloy sheath 4 has three sets of conductors coaxially arranged inside, including a cold section conductor 2 and a heating section conductor 3. The cold section conductor 2 is made of T2 copper with a diameter of 6.5 mm, and the heating section conductor 3 is made of a solution-strengthened copper-nickel alloy (containing 40% copper and 58% nickel) with the same diameter as the cold section, ensuring low-loss transmission of power supply.

[0029] One end of the cold section conductor 2 is connected to the power supply cable 1, and the other end is connected to the heating section conductor 3 by cold pressure welding. The joint is subjected to stress relief annealing at 550℃ to ensure that the joint strength is not lower than that of the base material. The heating section conductor 3 forms a star junction 6 at the tail end by argon arc welding. The weld is wrapped with a high-temperature resistant insulating sleeve to form a three-phase three-wire power supply structure.

[0030] The insulation layer 5 is made of high-purity magnesium oxide powder with a compaction density ≥2.9 g / cm³, thermal conductivity ≥3.8 W / (m·K), and withstand voltage rating ≥3000V. The heating section is designed to be 600 meters long. Under the rated voltage of 3000V, the output power is about 450kW, which can stably control the wellbore temperature above 70℃ and effectively inhibit wax precipitation.

[0031] Example 6 This embodiment is a downhole electric heater based on a low resistivity heating conductor, suitable for highly corrosive gas wells containing H2S and CO2. It adopts a combination of corrosion-resistant materials and an integrated continuous tube structure to ensure safe and reliable long-term operation.

[0032] The electric heater includes an outer alloy sheath 4, made of nickel-iron-molybdenum multiphase stainless steel, with a wall thickness of 3.2 mm and an outer diameter of 32.5 mm, suitable for φ62.0 mm oil pipes. Three sets of conductors are coaxially arranged inside the outer alloy sheath 4, including a cold section conductor 2 and a heating section conductor 3. The cold section conductor 2 is made of T2 copper with a diameter of 6.0 mm, while the heating section conductor 3 is made of a solution-strengthened copper-nickel alloy (containing 42% copper and 55% nickel), exhibiting good resistance to hydrogen sulfide corrosion.

[0033] One end of the cold section conductor 2 is connected to the power supply cable 1, and the other end is connected to the heating section conductor 3 by cold pressure welding. The joint is subjected to stress relief annealing at 600℃ to ensure the stability of the joint's mechanical properties. The heating section conductor 3 forms a star junction 6 at its tail end by argon arc welding. The weld point is wrapped with multiple layers of high-temperature resistant insulating sleeve to ensure the long-term stability of the electrical connection in corrosive media.

[0034] The insulation layer 5 is made of high-purity magnesium oxide powder with a compaction density ≥3.0 g / cm³, thermal conductivity ≥4.0 W / (m·K), and a pressure rating ≥3500V. The heating section is designed to be 500 meters long. Under a rated voltage of 3300V, the output power is about 550kW, which can stably control the wellbore temperature above 150℃. It is suitable for corrosion-resistant heating operations in high-sulfur gas wells.

[0035] The embodiments of the present invention have been described in detail above with reference to the accompanying drawings. However, the present invention is not limited to the above embodiments. Within the scope of knowledge possessed by those skilled in the art, various changes can be made without departing from the spirit of the present invention.

Claims

1. A downhole electric heater based on a low resistivity heating conductor; characterized in that: It includes an outer alloy sheath (4), which is a continuous tubular structure. The material of the outer alloy sheath (4) is a nickel-based alloy or stainless steel strip laser welded together. The outer alloy sheath (4) has a cold section conductor (2) and a heating section conductor (3) coaxially arranged inside. Both the cold section conductor (2) and the heating section conductor (3) are provided with three sets. A power supply cable (1) is provided at one end of the cold section conductor (2), and the other end of the cold section conductor (2) is connected to the heating section conductor (3) by a cold pressure welding process; The conductor (3) of the heating section is made of a solid copper-nickel alloy rod that has been strengthened by solid solution. An insulating layer (5) is filled between the cold section conductor (2), the heating section conductor (3) and the outer alloy sheath (4); The tail section of the heating section conductor (3) is welded with argon arc welding to form a star contact (6), which is used to form a three-phase three-wire power supply structure.

2. The downhole electric heater based on a low resistivity heating conductor according to claim 1, characterized in that: The cold section conductor (2) is made of T2 copper solid rod, and the diameter of the cold section conductor (2) is equal to the diameter of the heating section conductor (3).

3. A downhole electric heater based on a low resistivity heating conductor according to claim 1, characterized in that: The outer alloy sheath (4) is made of nickel-based alloy or nickel-iron-molybdenum multiphase stainless steel.

4. A downhole electric heater based on a low resistivity heating conductor according to claim 1, characterized in that: The cold section conductor (2) and the heating section conductor (3) are connected by a cold pressure welding process.

5. A downhole electric heater based on a low resistivity heating conductor according to claim 1, characterized in that: The heating section conductor (3) converges and connects at the tail of the heater, and forms a star junction (6) by argon arc welding. The weld is wrapped with a high-temperature resistant insulating sleeve.

6. A method for manufacturing a downhole electric heater based on a low resistivity heating conductor, characterized in that: The method for manufacturing a downhole electric heater based on a low resistivity heating conductor as described in any one of claims 1-5 comprises the following steps: S11: Cut the two ends of the outer alloy sheath (4) strip at 45°, and after mechanical grinding and cleaning with anhydrous ethanol, use fiber laser welding to butt together with a power of 2.0~3.0 kW and a speed of 1.5~2 m / min to form a continuous strip; S12: The cold section copper rod and the heated section copper-nickel alloy rod are straightened with a straightness error of ≤0.5 mm / m. After the ends are ground, they are butt welded by cold pressure and then stress-relieved annealed at 450~650℃. S13: The continuous strip is fed into a roll forming machine and rolled into a tube shape. The long side is welded by fiber laser welding with a weld reinforcement height of ≤0.5 mm. Argon gas protection is used to form a seamless outer sheath. S14: Three conductors are inserted into the positioning channel of the core tube through the guide wheel and arranged in an equilateral triangle. Magnesium oxide powder is filled synchronously through the powder filling channel, and the vibration frequency is 0.5~1 Hz for initial compaction. S15: After filling, the sheath is reduced in diameter by multiple sets of vertical rolling mills, the magnesium oxide is compacted, and then it is turned 90° by guide wheels to enter the horizontal straightening process; S16: Induction annealing at 800℃ and water cooling to eliminate cold working stress; S17: Reduced to the target size by a horizontal rolling mill, compacted density ≥2.8 g / cm³, then annealed twice at 600℃ and water-cooled; S18: After flaw detection to ensure no cracks or pores are found, laser diameter and length measurement is performed. Qualified products are then wound up and sealed with epoxy resin.

7. A method for manufacturing a downhole electric heater based on a low resistivity heating conductor according to claim 6, characterized in that: In S14, the core tube has a double-layer structure. The inner layer is used for conductor positioning, and the outer layer forms a magnesium oxide-filled channel between itself and the outer sheath.

8. A method for manufacturing a downhole electric heater based on a low resistivity heating conductor according to claim 6, characterized in that: In S16 and S17, the online annealing temperatures were 800℃ and 600℃ respectively, and the cooling method was water cooling.

9. A method for manufacturing a downhole electric heater based on a low resistivity heating conductor according to claim 6, characterized in that: It also includes a downhole electric heater based on a low resistivity heating conductor, which, during thermal condition simulation testing, includes the following steps: S21: Place the electric heater sample inside the test furnace, and connect the conductor and outer sheath to the test electrode according to the test items; S22: Select the power supply according to the test type and turn on the oil cooler, circulating pump, thermocouple recorder and infrared thermal imager; S23: Set the heating temperature, heating rate, holding time and cooling method to simulate high-temperature working conditions downhole; S24: Set the target temperature and control accuracy, start the closed-loop temperature control system, and record the temperature, resistance, and voltage curves in real time; S25: After the furnace temperature stabilizes to the target value, connect the insulation resistance tester and the withstand voltage tester; S26: Apply 1.1 times the rated voltage, with a voltage increase rate of 150 V / s, and apply the voltage for 60 seconds. Record the leakage current. The judgment criteria are no breakdown and leakage current ≤0.2mA. S27: Apply the corresponding voltage level, apply pressure for 60 seconds, record the insulation resistance, and the judgment criterion is that it is not lower than the standard value; S28: Continuous measurement mode, records resistance fluctuation, with a judgment criterion of fluctuation ≤2%; S29: Input insulation layer thickness, temperature, voltage and current parameters to calculate heat transfer efficiency and thermal resistance; S210: Set the current and time, monitor the sample temperature, current and voltage, and judge the criteria as no melting, no short circuit and stable temperature; S211: Turn off the power in sequence, cool to room temperature, disconnect the connecting wires, and check the status of the sample and equipment.

10. A method for manufacturing a downhole electric heater based on a low resistivity heating conductor according to claim 9, characterized in that: In S26-S210, all test data are automatically collected by the system software and test reports are generated, including temperature curves, resistance change curves, infrared thermal images, leakage current values, insulation resistance values, and current-carrying stability parameters, forming a complete and traceable test archive.

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