Double-sheath mineral insulated heating cable, preparation method therefor and use thereof

Abstract

Disclosed in the present invention are a double-sheath mineral insulated heating cable, a preparation method therefor and the use thereof. The double-sheath mineral insulated heating cable sequentially comprises a core, an insulating thermally-conductive layer, an inner sheath layer and an outer sheath layer from inside to outside, wherein the yield strength of the inner sheath layer is greater than or equal to 205 MPa, the tensile strength thereof is greater than or equal to 520 MPa, the elongation thereof is greater than or equal to 35％, and the hardness thereof is less than or equal to 32 HRC; the yield strength of the outer sheath layer is greater than or equal to 205 MPa, the tensile strength thereof is greater than or equal to 520 MPa, the elongation thereof is greater than or equal to 35％, and the hardness thereof is less than or equal to 32 HRC; and the outer sheath layer is 30％-50％ thinner than the inner sheath layer, the thermal conductivity of the inner sheath layer is not greater than the thermal conductivity of the outer sheath layer, and the thermal conductivity of the inner sheath layer is not less than the thermal conductivity of the insulating thermally-conductive layer. The double-sheath mineral insulated heating cable of the present invention may be suitable for being used in an H2S deep well and ultra-deep well, a CO2 deep well and ultra-deep well, and a Cl- deep well and ultra-deep well, the usage temperature is 350ºC-750ºC, and the service life is three years or more.

Description

Double-sheathed mineral insulated heating cable and its preparation method and application

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims the benefit of Chinese patent application No. 202311118486.4 filed on August 31, 2023, the contents of which are incorporated herein by reference. Technical Field

[0003] The present invention relates to the technical field of oil and natural gas development, and in particular to a double-sheathed mineral insulated heating cable and a preparation method and application thereof. Background Art

[0004] Electric heating technology is a relatively advanced technology in heavy oil reservoir development, including SAGD-assisted development technology. It is also an emerging environmentally friendly technology, attracting particular attention due to its green and environmentally friendly nature. By assisting or directly heating heavy oil in the oil field, the heating efficiency of high-viscosity, high-condensation, and high-wax heavy oil recovery can be improved, reducing energy consumption and carbon emissions. Electric heating technology can also be used in pilot trials for in-situ conversion of medium- and low-maturity shale oil, underground coal retorting, and oil-rich coal extraction in my country, representing a technological advantage in the clean development of unconventional oil and gas resources. Currently, thermal recovery is the main method for heavy oil development, including steam stimulation, steam flooding, oil layer combustion, and steam-assisted gravity drainage (SAGD). However, traditional downhole heating technologies suffer from high pollution and high heat energy loss.

[0005] my country is rich in shale oil resources, and has great potential for in-situ conversion of medium- and low-maturity shale oil. Studies have shown that the use of downhole electric heating technology for in-situ shale oil extraction is more suitable for domestic storage characteristics. Electric heating technology for underground in-situ conversion of shale oil has obvious advantages in terms of pollutant emissions and surface environmental protection. However, mineral insulated heating cables need to withstand high temperatures and high pressures underground, as well as corrosion from media such as chloride ions and hydrogen sulfide during long-term underground service, and their reliability is very high. The corrosion resistance, high temperature resistance and precise control of heating temperature of mineral insulated heating cables are the key to stable operation. The existing single-sheath mineral insulated heating cables in China have poor reliability, low strength, and joints, which cannot meet the high temperature, high pressure, high power and long life requirements of underground operation.

[0006] Summary of the Invention

[0007] The present invention aims to overcome the problems of low power per meter, low strength, and poor corrosion resistance in existing heating cables by providing a double-sheathed mineral-insulated heating cable, its preparation method, and its application. The double-sheathed mineral-insulated heating cable of the present invention offers advantages such as increased strength, greater vertical depth downhole capability, longer horizontal downhole reach, enhanced corrosion resistance, higher temperature resistance, and a longer service life. This method significantly improves production efficiency.

[0008] In order to achieve the above-mentioned objectives, the first aspect of the present invention provides a double-sheathed mineral insulated heating cable, which comprises, from the inside to the outside, a core, an insulating heat-conducting layer, an inner sheath layer and an outer sheath layer; wherein the yield strength of the inner sheath layer is ≥205MPa, the tensile strength is ≥520MPa, the elongation is ≥35%, and the hardness is ≤32HRC; the yield strength of the outer sheath layer is ≥205MPa, the tensile strength is ≥520MPa, the elongation is ≥35%, and the hardness is ≤32HRC; and the thickness of the outer sheath layer is 30%-50% thinner than the thickness of the inner sheath layer, the thermal conductivity of the inner sheath layer is not greater than the thermal conductivity of the outer sheath layer, and the thermal conductivity of the inner sheath layer is not less than the thermal conductivity of the insulating heat-conducting layer.

[0009] A second aspect of the present invention provides a method for preparing a double-sheathed mineral insulated heating cable, the method comprising:

[0010] (1) Milling the edges of both sides of the inner sheath steel strip, opening an I-shaped, V-shaped or X-shaped groove, and forming it using a W-forming method; then longitudinally welding the formed steel strip using a laser-TIG hybrid welding method to obtain an inner sheath pipe;

[0011] (2) assembling by vertically built-in core and automatically filling insulation material, and then undergoing a first treatment to obtain a single-sheath mineral insulated heating cable, and winding it into a coil;

[0012] (3) Milling the edges of both sides of the outer sheath steel strip, opening an I-type, V-type or X-type groove, and forming the outer sheath tube by a W-forming method; before the outer sheath tube is formed and closed, the single-sheath mineral insulated heating cable is placed online into the outer sheath tube by a flat roller with holes for power delivery, and the two are simultaneously advanced to the welding station, and then the formed steel strip is longitudinally welded by laser-TIG composite welding, and then subjected to a second treatment to obtain a double-sheath mineral insulated heating cable, which is then rolled into a coil.

[0013] Through the above technical solution, the beneficial technical effects achieved by the present invention are as follows:

[0014] (1) The tensile strength of the double sheathed mineral insulated heating cable of the present invention is not less than 635MPa and the elongation is ≥25%. It has excellent corrosion resistance and is suitable for use in environments containing a certain amount of H2S, CO2, Cl - It is used in deep wells and ultra-deep wells, with an operating temperature of 350℃-750℃ and a service life of more than 3 years.

[0015] (2) The double-sheathed mineral insulated heating cable of the present invention has the characteristics of higher strength, greater vertical depth in the well, longer length in the horizontal well, stronger corrosion resistance, higher temperature resistance, and longer service life.

[0016] (3) The present invention adopts laser-TIG composite welding technology and arc heat source, which expands the range of heat action, increases the amount of molten metal, improves the quality of the weld, increases the welding penetration, reduces the weld concavity, and leaves no obvious burrs on the inner wall of the pipe column after welding. It enhances the tightness of the fit between the inner and outer sheaths, can increase the sheath thickness by more than 20%, and increase the welding speed by more than 30%.

[0017] (4) During the production of the outer sheath layer, the single-sheath heating cable is synchronously built into the outer sheath pipe. The two move forward synchronously and undergo a hot tension reducing rolling process to achieve a double-layer sheath of the heating cable that is tightly fitted, with a reducing rate of more than 40%. This process is used for the first time in the heating cable industry.

[0018] (5) The double-sheathed heating cable of the present invention is subjected to medium-frequency induction heat treatment to heat the double-sheathed heating cable to between 900-1100°C (depending on the heat treatment temperature requirements of the outer sheath material). On the one hand, the outer sheath structure is adjusted and work hardening is eliminated; at the same time, protective gases such as H2 are passed through the heating and cooling tunnel to prevent oxidation of the heating cable surface during the heating process, thereby reducing corrosion resistance.

[0019] (6) The present invention solves the problems of low power consumption, low strength and poor corrosion resistance of heating cables manufactured by the copper strip longitudinally wrapped argon arc welding continuous rolling process (automatic canning method), and compared with the traditional magnesium oxide porcelain column filling and drawing process and the copper strip longitudinally wrapped argon arc welding continuous rolling process, the present invention reduces the manufacturing cost by more than 30% and improves the production efficiency by more than 20%. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] FIG1 is a schematic structural diagram of a cross section of a double-sheathed mineral insulated heating cable provided in one embodiment of the present invention;

[0021] FIG2 is a processing diagram of a single-sheathed mineral insulated heating cable provided by one embodiment of the present invention;

[0022] FIG3 is a processing diagram of a double-sheathed mineral insulated heating cable provided by one embodiment of the present invention;

[0023] FIG4 is a diagram showing the relative positions of a single-sheathed mineral insulated heating cable and an outer sheathed steel strip according to an embodiment of the present invention. [0023.1] [Corrected 04.01.2024 according to Rule 91] Explanation of the reference numerals 1 - core 2 - insulating heat-conducting layer 3 - inner sheath layer 4 - outer sheath layer 21 - automatic powder filling device 22 - laser welding gun 23 - argon arc welding gun 24 - welding point 25 - medium frequency heat treatment 26 - vertical roller 27 - flat roller 28 - single sheath heating cable coiled into a drum 31 - single sheath heating cable coiled 32 vertical roller 33 - flat roller 34 - welding point 35 - laser welding gun 36 - argon arc welding gun 37 - medium frequency heat treatment 38 - medium frequency heat treatment 39 - graphite cooling tunnel I - heating cable production line direction II - heating cable production line direction DETAILED DESCRIPTION

[0024] The endpoints of the ranges and any values ​​disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoints of each range, the endpoints of each range and individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered to be specifically disclosed herein.

[0025] A first aspect of the present invention provides a double-sheathed mineral insulated heating cable, as shown in Figure 1. The double-sheathed mineral insulated heating cable comprises, from the inside to the outside, a core 1, an insulating heat-conducting layer 2, an inner sheath layer 3 and an outer sheath layer 4; wherein, the yield strength of the inner sheath layer is ≥205MPa, the tensile strength is ≥520MPa, the elongation is ≥35%, and the hardness is ≤32HRC; the yield strength of the outer sheath layer is ≥205MPa, the tensile strength is ≥520MPa, the elongation is ≥35%, and the hardness is ≤32HRC; and the thickness of the outer sheath layer is 30%-50% thinner than the thickness of the inner sheath layer, the thermal conductivity of the inner sheath layer is not greater than the thermal conductivity of the outer sheath layer, and the thermal conductivity of the inner sheath layer is not less than the thermal conductivity of the insulating heat-conducting layer.

[0026] In the present invention, the thermal conductivity of the inner and outer sheath layers is not less than the thermal conductivity of the core.

[0027] The inner and outer layers of the double-sheathed mineral insulated heating cable of the present invention are both made of metal materials. The mechanical properties of the two different materials are utilized, that is, the elastic deformation range of the outer sheath (high alloy such as stainless steel) is utilized, and the low yield strength of the inner sheath (low-carbon microalloy) is utilized. Under the thermal tension reducing process, the inner layer of low-carbon microalloy undergoes elastic deformation, and the outer layer of stainless steel or other alloy undergoes plastic deformation, so that the two layers of material are tightly embedded in the composite molding, and the bonding force of the inner and outer sheaths is 0.5-1.0MPa. The outer sheath is responsible for corrosion resistance and wear resistance, while the inner sheath is responsible for tensile and compressive resistance.

[0028] The present invention provides a dieless drawing and reducing process and manufacturing method for a bimetallic sheathed mineral insulated heating cable, which boasts high production efficiency, longer length, greater strength, and enhanced corrosion resistance. The present invention addresses the problems of low production efficiency, low power per meter, short length, joints, and poor stability associated with heating cables manufactured using the traditional magnesium oxide ceramic column filling and drawing process, as well as the low power per meter, low strength, and poor corrosion resistance associated with heating cables manufactured using the copper strip longitudinally wrapped argon arc welding and continuous rolling process.

[0029] The double-sheathed mineral insulated heating cable of the present invention further expands the application range of the cable, provides equipment support for improving the recovery rate of heavy oil and shale oil development, and provides a new type of electric heating product for heavy oil reservoir development SAGD development technology and shale oil in-situ conversion technology.

[0030] In the present invention, the material of the inner sheath layer is selected from low-carbon microalloy steel according to the working conditions; the material of the outer sheath layer is selected from different corrosion-resistant alloys according to the flow and pressure requirements of the conveying medium and the service conditions of the product, which can be new high-corrosion-resistant alloy materials such as 316L, 310S, titanium alloy, nickel-based alloy, Hastelloy, etc.

[0031] In some embodiments of the present invention, the material of the inner sheath layer is selected from 310S austenitic stainless steel, titanium alloy, 825 nickel-based alloy or high-temperature alloy.

[0032] In some embodiments of the present invention, the outer diameter of the inner jacket layer is 20-70 mm, preferably 25.4-60.3 mm; the radial thickness is 2-5 mm, preferably 2.5-4.4 mm; and the length is 1000-10000 m, preferably 2000-8000 m.

[0033] In some embodiments of the present invention, the material of the outer sheath layer is selected from 310S austenitic stainless steel, titanium alloy, 825 nickel-based alloy or high-temperature alloy.

[0034] In some embodiments of the present invention, the outer diameter of the outer sheath layer is 30-90 mm, preferably 31.8-88.9 mm; the radial thickness is 2-7 mm, preferably 2.5-6.5 mm; and the length is 1000-10000 m, preferably 2000-8000 m.

[0035] In the present invention, 310S austenitic stainless steel includes the following chemical components: C:≤0.08, Si:≤1.500, Mn:≤2.00, P:≤0.035, S:≤0.030, Ni:≤19.00-22.00, Cr:≤24.00-26.00.

[0036] In some embodiments of the present invention, the insulating and heat-conductive layer has a porosity of less than 5%, an insulation withstand voltage strength of greater than 6000V, a thermal conductivity coefficient of greater than 30W / mk, and a temperature resistance of 350°C-750°C.

[0037] In some embodiments of the present invention, the radial thickness of the insulating heat-conducting layer is 2-5 mm.

[0038] In some embodiments of the present invention, the insulating heat-conducting layer is made of an insulating heat-conducting substance.

[0039] Preferably, the insulating heat-conducting material is magnesium oxide, with a sphericity greater than 50% and a purity greater than 99%.

[0040] In the present invention, the insulating and heat-conducting material filled in the heating cable is an insulating and heat-conducting material such as high-purity high-temperature resistant magnesium oxide powder.

[0041] In some embodiments of the present invention, the tensile strength of the core is ≥650 MPa and the elongation is ≥25%.

[0042] In some embodiments of the present invention, the outer diameter of the core is 3-15 mm and the length is 1000-10000 m.

[0043] In some embodiments of the present invention, the core is made of a high-resistance electric heating alloy, preferably Cr20Ni80.

[0044] In the present invention, the core of the heating cable is made of a high-resistance electric heating alloy such as Cr20Ni80, with a tensile strength of ≥650 MPa, an elongation of ≥25%, an outer diameter of Φ3-Φ15 mm, and a length of 1,000-10,000 m. The core's chemical composition includes: C ≤ 0.08%; Mn ≤ 0.6%; P ≤ 0.02%; S ≤ 0.015%; Si: 0.75-1.60%; Cr: 20.0-23.0%; Ni: balance; Fe ≤ 1.0%; Al ≤ 0.5%.

[0045] In some embodiments of the present invention, the double-sheathed mineral insulated heating cable has a tensile strength of not less than 650 MPa and an elongation of ≥25%.

[0046] The double-sheathed mineral insulated heating cable of the present invention has excellent corrosion resistance.

[0047] A second aspect of the present invention provides a method for preparing a double-sheathed mineral insulated heating cable, the method comprising:

[0048] (1) Milling the edges of both sides of the inner sheath steel strip, opening an I-shaped, V-shaped or X-shaped groove, and forming it using a W-forming method; then longitudinally welding the formed steel strip using a laser-TIG hybrid welding method to obtain an inner sheath pipe;

[0049] (2) assembling by vertically built-in core and automatically filling insulation material, and then undergoing a first treatment to obtain a single-sheath mineral insulated heating cable, and winding it into a coil;

[0050] (3) Milling the edges of both sides of the outer sheath steel strip, opening an I-type, V-type or X-type groove, and forming the outer sheath tube by a W-forming method; before the outer sheath tube is formed and closed, the single-sheath mineral insulated heating cable is placed online into the outer sheath tube by a flat roller with holes for power delivery, and the two are simultaneously advanced to the welding station, and then the formed steel strip is longitudinally welded by laser-TIG composite welding, and then subjected to a second treatment to obtain a double-sheath mineral insulated heating cable, which is then rolled into a coil.

[0051] The present invention adopts a dieless drawing and diameter reducing process to prepare a double-sheathed mineral insulated heating cable, and the single-sheathed heating cable adopts a "W" forming + built-in heating core wire + automatic filling of magnesium oxide powder + laser-TIG composite welding + heat treatment + multiple batches of thermal tension diameter reducing + heat treatment process, and is curled into a coil. The outer sheath adopts the "W" forming process, and the single sheath heating cable that has been produced is simultaneously introduced into the outer sheath tube manufacturing process online by the "hole-type flat roller power delivery" method (before the outer sheath is formed and closed). After the outer sheath is welded by laser-TIG composite welding, the double sheath heating cable is continuously and rapidly heated in a medium frequency induction furnace, and is subjected to four passes of hot tension reduction by a rolling mill; then the full heating cable is subjected to medium frequency induction furnace heat treatment to reduce the work hardening of the outer sheath tube during the diameter reduction process, release the processing stress, and further eliminate the microstructure difference between the weld and the base material, so as to achieve the comprehensive performance improvement of the heating cable sheath, such as plasticity, toughness, and corrosion resistance; after heat treatment, it is cooled to room temperature, and finally passed through three passes of micro-cold sizing and straightening by the rolling mill to achieve the required outer diameter and straightness of the double sheath mineral insulated heating cable, and is coiled onto a special cable reel by a coiler, thereby obtaining a bimetallic sheathed mineral insulated heating cable product.

[0052] The present invention adopts a single-sheath heating cable and an outer sheath to undergo multiple heat tension reductions to achieve the purpose of mechanically compounding the inner and outer sheaths and making them tightly fitted, thereby forming a bimetallic sheath heating cable product.

[0053] The present invention selects the strength and corrosion resistance of the inner and outer sheath materials (high-strength steel for the inner sheath and corrosion-resistant steel for the outer sheath), so that the heating cable can be lowered deeper into the well, has stronger corrosion resistance, and has a longer service life, and is more economical. The laser + TIG composite welding process is adopted to further improve production efficiency. Compared with the traditional magnesium oxide porcelain column filling and drawing process (single-sheath heating cable), continuous production is possible, eliminating the problems of many joints, low temperature resistance, low efficiency, and short service life. Compared with the continuous rolling process of copper strip longitudinal wrapped argon arc welding, it can produce large-walled bimetallic composite sheathed cables, eliminating the problems of low strength, poor corrosion resistance, and short service life of the traditional copper strip longitudinal wrapped argon arc welding (the welding process limits the sheath thickness to ≤2mm) continuous rolling process.

[0054] Compared with the traditional magnesium oxide porcelain column filling and drawing process and the copper strip longitudinal wrapped argon arc welding continuous rolling process, the method of the present invention can reduce the manufacturing cost by more than 30% and improve the processing efficiency by more than 20%.

[0055] In step (1) of the present invention, molten iron is used as the main raw material, alloying elements are added according to the chemical composition of the sheath material alloy, and the steel is finally rolled into a hot-rolled plate with a thickness of 2.4-6.35 mm. After controlled cooling, tempering, and pickling, the steel is coiled to form a steel coil with a length of more than 400 meters and a width of 1.2-1.5 meters. The prepared steel coil is longitudinally cut into multiple steel strips with a width of 120-400 mm according to the specifications of the heating cable; and the multiple steel strips are then butt-welded to length according to the specified length requirements of the heating cable product.

[0056] In some embodiments of the present invention, the preparation method of the inner sheath steel strip includes: cutting the steel strip according to the design size, processing the end of the steel strip into a 45° bevel, opening an I-type or V-type groove on the bevel, and welding multiple steel strips in sequence to the design length of the double-sheathed mineral insulated heating cable using a non-metallic electrode gas shielded welding (TIG) or laser welding method; after the weld is cooled to ≤150°C, the weld is reheated to greater than 1000°C for insulation, and after cooling with argon gas, it is coiled into a coil to obtain the inner sheath steel strip.

[0057] In some embodiments of the present invention, before welding, the groove is preheated under the protection of an argon atmosphere; during welding, the protective gas is a mixture of Ar and N2.

[0058] In the present invention, laser welding is used for butt welding. During welding, the shielding gas is Ar (99.99%), the power is 1.2-1.5KW, the welding speed is 1-1.5m / min, the gas flow rate is 18-25L / min, and the weld back protection device is turned on. After the butt welding is completed, the weld is subjected to solid solution treatment. The inclined weld area is heated to 1050℃-1100℃ using a medium frequency, kept warm for 1-2 minutes, and the weld is quickly cooled by argon gas blowing. The cooling rate is ≥15℃ / s. For other corrosion-resistant alloys, the corresponding weld heat treatment process is adopted. According to the sheath wall thickness requirements of the single-sheath heating cable, the present invention mills the edges of the extended steel strip on both sides and opens an "I-type, V-type or X-type" groove; the steel strip adopts the "W" forming method, and the formed steel strip is welded longitudinally with a straight seam using laser-TIG composite welding technology. The groove gap is ≤0.1mm, the misalignment is ≤0.3mm, and the weld center offset is ±0.5mm. "Laser-TIG" hybrid welding technology combines a laser beam and a non-melting electrode tungsten inert gas arc as a composite heat source. ER-310 welding wire is used. For longitudinal welding of single-sheathed heating cable sheaths, the shielding gas is Ar (95%) + N2 (5%). The TIG welding current is 150-240A, the voltage is 22-32V, the laser defocus is -5 to +5mm, the laser power is 1kW-5kW, the welding speed is 1.5-5m / min, and the gas flow rate is 18-25L / min. For other corrosion-resistant alloys, corresponding welding wire should be selected.

[0059] In some embodiments of the present invention, the first treatment includes: the first treatment includes: performing heat tension reduction on the assembled cable and compacting the insulating material; after heat treatment, cold sizing and straightening to achieve the required outer diameter and straightness of the heating cable.

[0060] In some embodiments of the present invention, the assembly starts 2-3 m after the welding point of the inner jacket pipe.

[0061] In some embodiments of the present invention, the single-sheath mineral insulated heating cable includes a core, an insulating heat-conducting layer and an inner sheath layer.

[0062] In step (2) of the present invention, starting from about 2 meters after the welding point of the single-sheathed heating cable, the assembly of the single-sheathed heating cable is completed by adopting a vertical built-in core wire + automatic powder filling magnesium oxide powder. After welding, the two (single-sheathed heating cable + outer sheath) move forward synchronously to the heating induction coil, and the outer sheath is continuously heated by local induction heating. The cable is subjected to thermal tension reduction by a vertical 4-set roller group (flat roller + vertical roller), and the compaction density requirement of the magnesium oxide powder is met at the same time.

[0063] In step (2) of the present invention, after the welding diameter reduction of the single-sheathed heating cable is completed and the magnesium oxide powder reaches the compaction density requirement, the entire pipe body is heat treated by adopting a three-stage medium-frequency induction heating method of "preheating + heating + insulation" to heat the entire pipe body to between 900-1100°C, insulation for 1-3 minutes, and the graphite cold jacket is cooled to below 150°C, water-cooled, cleaned, and coiled into a coil.

[0064] Specifically, after the welding diameter reduction of the single-sheathed heating cable is completed and the magnesium oxide powder reaches the compaction density requirement, local medium-frequency heating is adopted (H2 flows inside to prevent oxidation of the heating cable sheath), and the cable is subjected to thermal tension diameter reduction treatment by two vertical roller units (flat roller + vertical roller). The cable is cooled to below 150°C through a graphite cooling tunnel (H2 flows inside to prevent oxidation of the heating cable sheath and accelerate cooling), cleaned, and finally prepared into a Φ26.8×2mm-2500m single metal sheathed heating cable and coiled into a coil.

[0065] In step (3) of the present invention, molten iron is used as the main raw material, and alloying elements are added according to the chemical composition of high-alloy steel. The steel is finally rolled into a hot-rolled plate with a thickness of 2.4-6.35 mm. After controlled cooling, tempering, and pickling, it is coiled to produce a steel coil with a length of more than 400 meters and a width of 1.2-1.5 meters. The prepared outer sheath (high-alloy steel) steel coil is longitudinally cut into multiple steel strips of 150-400 mm according to the specifications of the heating cable; the multiple steel strips are then butt-welded to length according to the specified length requirements of the heating cable. According to the sheath wall thickness requirements of the single-sheath (corrosion-resistant alloy) heating cable, the edges of the extended steel strips are milled and "I-shaped, V-shaped, or X-shaped" grooves are formed. The outer sheath steel strip is formed using a "W" forming method. The produced single-sheath heating cable is installed online in the outer sheath tube using a "perforated flat roller power delivery" method (before the outer sheath is formed and closed). The formed steel strips are longitudinally straight-seam welded using laser-TIG hybrid welding technology. After the outer sheath is welded using laser-TIG hybrid welding, the two (single-sheath heating cable + outer sheath) proceed simultaneously to the next process. The single-sheath heating cable begins 2 meters after the welding point and is simultaneously built into the outer sheath pipe. The two proceed simultaneously and undergo thermal tension reduction to ensure a tight fit between the double-layer sheath of the heating cable. The double-sheath heating cable undergoes heat treatment using a three-stage medium-frequency induction heating method (preheating + heating + insulation). The double-sheath heating cable is heated to between 900-1100°C and held at this temperature for 1-3 minutes to adjust the sheath structure and eliminate work hardening. It is then cooled to below 150°C using a graphite cooling jacket, cleaned with water, and coiled into a coil after cooling.

[0066] In some embodiments of the present invention, the second treatment includes: subjecting the welded cable to a heat tension reducing treatment, and after the heat treatment, cold sizing and straightening to achieve the required outer diameter and straightness of the heating cable.

[0067] In step (3) of the present invention, the formed double-sheathed heating cable (outer sheathed pipe + built-in single-sheathed heating cable) is locally heated and heat-reduced by multiple roller groups (flat rollers + vertical rollers) to reduce the pipe diameter by 10-25%. Starting from 2 meters behind the welding point, the single-sheathed heating cable is simultaneously built into the outer sheathed pipe. The two are moved forward synchronously and reduced by heat tension to achieve a tight fit between the double-layer sheath of the heating cable. Then, the entire pipe body of the double-sheathed mineral insulated heating cable is heat-treated. After heat treatment, the double-sheathed heating cable adopts a three-stage medium-frequency induction heating method of "preheating + heating + insulation" to heat the double-sheathed heating cable to between 900-1100°C, insulation for 1-3 minutes, adjust the sheath structure and eliminate work hardening, cool to below 150°C through a graphite cooling jacket, water-cooled and cleaned, and then coiled into a coil after cooling. The double-sheathed heating cable undergoes full-body heat treatment, then cools to room temperature, and passes through 5 groups of rollers (flat rollers + vertical rollers) for diameter reduction and straightening to achieve the required outer diameter and straightness of the heating cable. It is then coiled onto a special cable reel via a coiler to obtain a double-sheathed mineral insulated heating cable product.

[0068] A third aspect of the present invention provides the use of the aforementioned double-sheathed mineral insulated heating cable in deep wells and ultra-deep wells.

[0069] In this invention, deep well refers to a well with a drilling depth of 4500m-6000m; ultra-deep well refers to a well with a drilling depth of more than 6000m, and the deep well and ultra-deep well contain H2S, CO2, Cl - .

[0070] The double sheathed mineral insulated heating cable of the present invention has excellent corrosion resistance and is suitable for use in H2S, CO2, Cl - Application in deep and ultra-deep wells.

[0071] In some embodiments of the present invention, the operating temperature of the double-sheathed mineral insulated heating cable is 350° C.-750° C., and the service life is more than 3 years.

[0072] According to a particularly preferred embodiment of the present invention, a method for manufacturing a double-sheathed mineral insulated heating cable comprises the following steps:

[0073] (1) Preparation of single sheathed mineral insulated heating cable (sheath) steel coil;

[0074] (2) Slitting and butt welding of single sheathed mineral insulated heating cable (sheath) steel coils;

[0075] (3) Preparation of single sheathed mineral insulated heating cables;

[0076] (4) Preparation of double-sheathed mineral insulated heating cable (outer sheath) steel coil;

[0077] (5) Slitting and butt welding of double-sheathed mineral insulated heating cable (outer sheath) steel coils;

[0078] (6) The single sheathed heating cable and the outer sheathed steel strip enter the forming section simultaneously;

[0079] (7) Longitudinal weld of outer sheath pipe of double sheath heating cable;

[0080] (8) Double-sheathed heating cable (outer sheathed pipe + internal single-sheathed heating cable) local heating + multiple roller groups (flat rollers + vertical rollers) heat reduction, reducing the pipe diameter by 10-25%;

[0081] (9) Heat treatment of the entire tube of double-sheathed mineral insulated heating cable;

[0082] (10) After secondary diameter reduction and straightening, the double-sheathed mineral insulated heating cable reaches the required product straightness and outer diameter, and then is coiled onto a special cable reel via a winding machine.

[0083] The step (1) comprises: using molten iron as the main raw material, adding alloy elements according to the chemical composition of the sheath material alloy, finally rolling into a hot-rolled plate with a thickness of 2.4-6.35 mm, and then coiling after controlled cooling, tempering, and pickling to produce a coil with a length of more than 400 meters and a width of 1.2-1.5 meters.

[0084] The step (2) is as follows: the prepared single-sheath heating cable sheath steel coil is longitudinally cut into a plurality of steel strips with a width of 120-400 mm according to the specifications of the heating cable; and the plurality of steel strips are sequentially butt-welded to length according to the specified length requirements of the heating cable product.

[0085] In step (2), when the sheathed steel strips of the single-sheathed heating cable are butt-welded, the ends of the steel strips are first processed into 45° bevels, and an I-shaped or V-shaped groove is formed on the bevels. Laser welding is then performed. During welding, the shielding gas is Ar (99.99%), the power is 1.2-1.5 kW, the welding speed is 1-1.5 m / min, the gas flow rate is 18-25 L / min, and the weld backside protection device is activated.

[0086] In step (2), after the butt welding is completed, the weld is subjected to a solid solution treatment. The inclined weld area is heated to 1050°C-1100°C using a medium frequency heating method, kept at this temperature for 1-2 minutes, and the weld is rapidly cooled using argon gas blowing at a cooling rate of ≥15°C / s. For other corrosion-resistant alloys, the corresponding weld heat treatment process is adopted.

[0087] The step (3) is as follows: according to the sheath wall thickness requirement of the single-sheath heating cable, the edges of both sides of the extended steel strip are milled and an "I-type, V-type or X-type" groove is opened; the steel strip adopts a "W" forming method and utilizes "laser-TIG" composite welding technology to perform longitudinal straight seam welding on the formed steel strip.

[0088] In step (3), the groove gap is ≤0.1mm, the misalignment is ≤0.3mm, and the weld center offset is ±0.5mm. "Laser-TIG" hybrid welding technology is a process in which a laser beam and a non-melting electrode tungsten inert gas arc are combined as a composite heat source for welding. ER-310 welding wire is used. When welding the sheath of a single-sheathed heating cable longitudinally, the shielding gas is Ar (95%) + N2 (5%), the TIG welding current is 150-240A, the TIG welding voltage is 22-32V, the laser defocus is -5-+5mm, the laser power is 1Kw-5Kw, the welding speed is 1.5-5m / min, and the gas flow rate is 18-25L / min. For other corrosion-resistant alloys, corresponding welding wire is selected.

[0089] In the step (3), starting from 2 meters behind the welding point of the single-sheathed heating cable, the single-sheathed heating cable is assembled by adopting a vertical built-in core wire + automatic powder filling magnesium oxide powder. After welding, the two (single-sheathed heating cable + outer sheath) move forward synchronously to the heating induction coil, and the outer sheath is continuously heated by local induction heating. The cable is subjected to thermal tension reduction by a vertical 4-set roller group (flat roller + vertical roller), and the compaction density requirement of the magnesium oxide powder is met at the same time.

[0090] In the step (3), after the single-sheathed heating cable is welded and reduced in diameter and the magnesium oxide powder reaches the compaction density requirement, the entire pipe body is heat treated by adopting a three-stage medium-frequency induction heating method of "preheating + heating + insulation" to heat the entire pipe body to between 900-1100°C, insulation for 1-3 minutes, and the graphite cold jacket is cooled to below 150°C, water-cooled, cleaned, and coiled into a coil.

[0091] The step (4) is as follows: using molten iron as the main raw material, adding alloy elements according to the chemical composition of the above-mentioned high-alloy steel, finally rolling into a hot-rolled plate with a thickness of 2.4-6.35 mm, and then coiling after controlled cooling, tempering, and pickling to produce a coil with a length of more than 400 meters and a width of 1.2-1.5 meters.

[0092] The step (5) is as follows: the prepared outer sheath (high alloy steel) steel coil is longitudinally cut into multiple steel strips of 150-400 mm according to the specifications of the heating cable; and the multiple steel strips are sequentially butt-welded to length according to the specified length requirements of the heating cable.

[0093] The step (6) is as follows: the outer sheath is produced by adopting the "W" forming process, and the produced single sheath heating cable is online built into the outer sheath tube by using the "hole-type flat roller power delivery" method (before the outer sheath is formed and closed), and the outer sheath is welded by "laser-TIG" composite welding, and then the two (single sheath heating cable + outer sheath) are synchronously advanced to the next process.

[0094] The step (7) is as follows: according to the sheath wall thickness requirement of the single-sheath (corrosion-resistant alloy) heating cable, the edges of both sides of the extended steel strip are milled to open an "I-type, V-type or X-type" groove; the steel strip adopts a "W" forming method and utilizes "laser-TIG" composite welding technology to perform longitudinal straight seam welding on the formed steel strip.

[0095] In the step (8), the single-sheathed heating cable starts from 2 meters behind the welding point, and the single-sheathed heating cable is synchronously built into the outer sheath pipe. The two move forward synchronously and undergo thermal tension reduction to achieve a tight fit of the double-layer sheath of the heating cable.

[0096] In the step (9), the double-sheathed heating cable is heat-treated by adopting a three-stage medium-frequency induction heating method of "preheating + heating + insulation", heating the double-sheathed heating cable to between 900-1100°C, insulation for 1-3 minutes, adjusting the sheath structure and eliminating work hardening, cooling to below 150°C through a graphite cooling jacket, water-cooling cleaning, and curling into a coil after cooling.

[0097] In the step (10), the double-sheathed heating cable is subjected to full-body heat treatment, cooled to room temperature after the heat treatment, and is reduced and straightened by 5 groups of rollers (flat rollers + vertical rollers) to achieve the required outer diameter and straightness of the heating cable, and then wound onto a special cable reel by a coiler, thereby obtaining a double-metal sheathed mineral insulated heating cable product.

[0098] The present invention will be described in detail below through examples.

[0099] The raw materials used in the following examples and comparative examples were all commercially available.

[0100] Example 1

[0101] A method for preparing a bimetallic sheathed mineral insulated heating cable (outer sheath material 310S + inner sheath material 310S) with an outer diameter of Ф38.1×5mm and a 310S sheath and a length of 2500m, specifically comprising the following steps:

[0102] S1. Preparation of single sheathed mineral insulated heating cable:

[0103] S11. Using molten iron as the main raw material, according to the chemical composition of 310S stainless steel, the chemical composition is as follows: C: ≤ 0.08, Si: ≤ 1.500, Mn: ≤ 2.00, P: ≤ 0.035, S: ≤ 0.030, Ni: ≤ 19.00-22.00, Cr: ≤ 24.00-26.00. It is solution treated at 1030-1180°C, pickled, and then coiled. The final product is a 3mm thick coil with a length of more than 400 meters. After testing, it is coiled into 26.8×2mm 310S stainless steel coils on a tension coiler for pipe making.

[0104] S12. Cut the stainless steel coil obtained in step S11 into 119mm steel strips longitudinally; then, according to the design length requirements of the cable, process the ends of the steel strips into 45° bevels, open I-type grooves on the bevels, and weld multiple steel strips in sequence to a length of 2500 meters using non-metallic electrode gas shielded welding (TIG) or laser welding. Before welding, preheat the groove under the protection of argon atmosphere, and the preheating temperature is 200°C. During welding, the protective gas is Ar (95%) + N2 (5%) gas, the welding current is 230A, the welding voltage is 30V, the welding speed is 260mm / min, and the gas flow rate is 20L / min. After the weld is cooled to ≤150°C, reheat the weld to 1020°C, keep warm for 3 minutes, and cool with argon purge. After cooling, the inner sheath steel strip is obtained and rolled into a disk;

[0105] S13. Mill the edges of both sides of the inner sheath steel strip obtained in step S12 and open an I-shaped groove; use the "W" forming method to control the continuous and stable forming of the steel strip; after the steel strip is formed, the groove gap is ≤0.2mm, the misalignment is ≤0.3mm, and the weld center offset is ±0.6mm. Then use the laser-TIG composite welding technology to longitudinally weld the formed inner sheath steel strip. During welding, the groove is first preheated to 150°C, the protective gas is Ar (95%) + N2 (5%), the TIG welding current is 180A, and the welding voltage is 24V; the laser defocus amount is +3mm, the power is 1.5Kw, the welding speed is 3m / min, and the gas flow rate is 22L / min to obtain the inner sheath tube;

[0106] S14, as shown in Figure 2, starting from 2 meters behind the inner sheath pipe welding point obtained in step S13, a vertical built-in core wire + automatic filling of magnesium oxide powder is used to complete the assembly of a single sheath heating cable (sheath + core wire + magnesium oxide). After assembly, the outer diameter of the heating cable is Φ38.1×3mm, which is reduced to Φ31.8×2.3mm by heat tensioning by 4 sets of rollers (flat rollers + vertical rollers) and compacted with magnesium oxide powder; then, local medium frequency heating is adopted (with flowing H2 inside to prevent oxidation of the heating cable sheath), and the cable is reduced to Φ26.8×2mm by heat tensioning by 2 vertical roller groups (flat rollers + vertical rollers). The compacted density of magnesium oxide powder is 2.3g / cm 3, cooled to below 150°C through a graphite cooling tunnel (with flowing H2 inside to prevent oxidation of the heating cable sheath and accelerate cooling), cleaned, and finally prepared into a Φ26.8×2mm-2500m single-sheath mineral insulated heating cable, which was coiled into a coil;

[0107] S2. Preparation of double sheathed mineral insulated heating cable:

[0108] S21. Using molten iron as the main raw material, according to the chemical composition of 310S stainless steel, the chemical composition is as follows: C: ≤ 0.08, Si: ≤ 1.500, Mn: ≤ 2.00, P: ≤ 0.035, S: ≤ 0.030, Ni: ≤ 19.00-22.00, Cr: ≤ 24.00-26.00. It is solution treated at 1030-1180°C, pickled, and then coiled. Finally, it is made into a coil with a wall thickness of 4mm and a length of more than 400 meters. After testing, it is coiled into 310S stainless steel coils with a diameter of 38.1×5mm in a tension coiler for use in pipe making.

[0109] S22. The prepared stainless steel coil is longitudinally cut into 139mm steel strips. Then, according to the design length requirements of the cable, the ends of the steel strips are processed into 45° bevels, and I-shaped grooves are cut on the bevels. Multiple steel strips are welded sequentially to a length of 2500 meters using non-metallic electrode gas shielded welding (TIG) or laser welding. Before welding, the grooves are preheated under argon atmosphere protection at a preheating temperature of 200°C. During welding, the shielding gas is Ar (95%) + N2 (5%) gas, the welding current is 230A, the welding voltage is 30V, the welding speed is 260mm / min, and the gas flow rate is 20L / min. After the weld cools to ≤150°C, the weld is reheated to 980-1100°C, kept at this temperature for 2-3 minutes, and cooled by argon purge. After cooling, the outer sheath steel strip is obtained and coiled into a coil.

[0110] S23. As shown in Figures 3 and 4, the outer sheath steel strip produced in step S22 is milled on both sides to create an I-shaped groove. A "W" forming method is used to control the continuous and stable curling of the steel strip. During the curling process, the single-sheathed mineral insulated heating cable produced in step S1 is simultaneously placed online into the curled outer sheath steel strip using a "perforated flat roller power delivery" method (before the outer sheath is formed and closed). The two are then simultaneously advanced to the welding station. After the steel strip is curled, the groove gap at the weld is ≤0.2mm, the misalignment is ≤0.3mm, and the weld center offset is ±0.6mm. The formed outer sheath steel strip is longitudinally welded using laser-TIG hybrid welding technology. During welding, the groove is preheated to 150°C to remove moisture or oil from the edge of the steel strip. The shielding gas is Ar (95%) + N2 (5%), the gas flow rate is 22 L / min, the TIG welding current is 180A, the welding voltage is 24V, the wire feed speed is 200 mm / min, the laser defocus distance is +3mm, the power is 1.5 kW, and the welding speed is 3 m / min. After assembly using laser-TIG hybrid welding, the outer diameter of the heating cable is Ø44.5 × 4 mm. The assembled heating cable (single-sheathed mineral insulated heating cable + outer sheath) is simultaneously advanced to the next process.

[0111] S24: The heating cable assembled in step S23 is heated by local heating (with H2 flowing inside to prevent oxidation of the cable sheath during the heating process), and then reduced to Φ40.5×3.2mm by thermal tension through 4 sets of rollers (4 flat rollers + 4 vertical rollers arranged alternately). The compacted density of the magnesium oxide powder is 2.4-2.6g / cm 3 ;

[0112] S25, the heating cable after the first diameter reduction and compaction is continuously heated by secondary induction, using the "preheating + heating + insulation" three-stage medium frequency induction heating method, with H2 flowing inside the heating device to prevent the heating cable sheath from oxidizing. The heating cable is heated to between 980-1100℃ and kept warm for 1-3 minutes to achieve the purpose of refining the sheath metallographic structure and eliminating work hardening. 2， To prevent the heating cable sheath from oxidizing and accelerating cooling) it is cooled to below 150°C, and the cable is secondary cold-sized to the outer diameter Ф38.1×5mm(2+3) and straightness required by the heating cable through three vertical roller units (3 flat rollers + 3 vertical rollers arranged alternately), and then it is coiled onto a special cable reel through a coiler to obtain a double-sheathed mineral insulated heating cable product of Ф38.1×5mm(2+3)-2500m.

[0113] After testing, the tensile strength of the double sheathed mineral insulated heating cable is not less than 650MPa, the elongation is ≥25%, and it has excellent corrosion resistance and is suitable for H2S, CO2, Cl -The double sheathed mineral insulated heating cable has an operating temperature of 350℃-750℃ and a service life of more than 3 years.

[0114] Comparative Example 1

[0115] A bimetallic sheathed mineral insulated heating cable was prepared according to the method of Example 1, except that all laser-TIG hybrid welding was replaced by argon arc welding.

[0116] The welding speed is reduced by 80% compared with laser-TIG hybrid welding. When the sheath with a wall thickness of more than 2mm is not grooved, the argon arc welding may not be able to penetrate the wall in a single pass. At the same time, the strength of the thin-walled sheathed heating cable is reduced. According to the test, the tensile strength of the double-sheathed mineral insulated heating cable (with a wall thickness of less than 2mm) is ≤320MPa. Under certain conditions of H2S, CO2, Cl - The service life in the well and at 350℃ is 1-3 months, and the cable sheath is severely corroded.

[0117] Comparative Example 2

[0118] A single-sheathed mineral insulated heating cable was prepared according to step S1 of the method of Example 1, without performing step S2.

[0119] Step S1: prepare a single sheath mineral insulated heating cable. If a low carbon micro alloy material is used as the sheath, the corrosion resistance is poor. - The service life in the well and at 350℃ is 3-6 months, and the cable sheath is severely corroded.

[0120] Step S1: prepare a single sheath mineral insulated heating cable. If corrosion-resistant alloy is used as the sheath, the strength of the single sheath heating cable is low. - The service life in the well and at 350℃ is 10-12 months, the cable sheath suffers from severe stress corrosion, and the manufacturing cost is more than 20% higher than that of double-sheathed heating cable.

[0121] It can be seen from the above results that the double-sheathed mineral insulated heating element prepared by the method of the present invention has a temperature range of 350°C to 750°C and a service life of more than 3 years.

[0122] The present invention adopts "laser welding-TIG" hybrid welding technology and an arc heat source, which expands the range of heat action, increases the amount of molten metal, improves the quality of the weld, and increases the weld penetration. Compared with the argon arc welding method, the weld concavity is reduced, and there is no obvious burr on the inner wall of the pipe column after welding. The thermal expansion and diameter reduction technology is used to enhance the tightness of the fit between the inner sheath and the outer sheath. At the same time, the production efficiency is improved by more than 20% (the welding speed of the "laser welding-TIG" hybrid welding technology is 1-3m / min, and the welding speed of the argon arc welding is 0.2-0.4m / min). Through the combination of double sheath materials (corrosion-resistant outer sheath and high-strength inner sheath), compared with a single-sheath heating cable of pure material with the same wall thickness, under the condition of meeting the use intensity, the double-sheath heating cable saves more alloy cost than a single-sheath heating cable of pure alloy with the same wall thickness, and can reduce the manufacturing cost by about 30% or more.

[0123] The preferred embodiments of the present invention have been described in detail above, but the present invention is not limited thereto. Within the technical concept of the present invention, various simple variations of the technical solution of the present invention may be made, including combining the various technical features in any other appropriate manner. These simple variations and combinations should also be regarded as disclosed in the present invention and fall within the scope of protection of the present invention.

Claims

1. A double sheathed mineral insulated heating cable, characterized in that: The double-sheathed mineral insulated heating cable comprises a core, an insulating heat-conducting layer, an inner sheath layer and an outer sheath layer from the inside to the outside; wherein, the yield strength of the inner sheath layer is ≥205MPa, the tensile strength is ≥520MPa, the elongation is ≥35%, and the hardness is ≤32HRC; the yield strength of the outer sheath layer is ≥205MPa, the tensile strength is ≥520MPa, the elongation is ≥35%, and the hardness is ≤32HRC; and the thickness of the outer sheath layer is 30%-50% thinner than that of the inner sheath layer, the thermal conductivity of the inner sheath layer is not greater than the thermal conductivity of the outer sheath layer, and the thermal conductivity of the inner sheath layer is not less than the thermal conductivity of the insulating heat-conducting layer.

2. The double-sheathed mineral insulated heating cable according to claim 1, wherein: The outer diameter of the inner sheath layer is 20-70 mm, the radial thickness is 2-5 mm, and the length is 1000-10000 m.

3. The double sheathed mineral insulated heating cable according to claim 1, wherein: The outer diameter of the outer sheath layer is 30-90 mm, the radial thickness is 2-7 mm, and the length is 1000-10000 m.

4. The double-sheathed mineral insulated heating cable according to claim 1, wherein: The insulating and heat-conducting layer has a porosity of <5%, a dielectric withstand voltage strength of >6000V, a thermal conductivity of >30W / mk, and a temperature resistance of 350°C-750°C; And / or, the radial thickness of the insulating heat-conducting layer is 2-5 mm.

5. The double-sheathed mineral insulated heating cable according to claim 4, wherein: The insulating heat-conducting layer is made of an insulating heat-conducting material; Preferably, the insulating heat-conducting material is magnesium oxide, and its sphericity is greater than 50% and its purity is greater than 99%.

6. The double-sheathed mineral insulated heating cable according to claim 1, wherein: The tensile strength of the core is ≥650MPa and the elongation is ≥25%; And / or, the outer diameter of the core is 3-15 mm and the length is 1000-10000 m.

7. The double-sheathed mineral insulated heating cable according to claim 6, wherein: The material of the core is a high-resistance electric heating alloy.

8. The double-sheathed mineral insulated heating cable according to claim 1, wherein: The tensile strength of the double-sheathed mineral insulated heating cable is not less than 650 MPa and the elongation is ≥25%.

9. A method for preparing a double-sheathed mineral insulated heating cable, characterized in that: The preparation method comprises: (1) Milling the edges of both sides of the inner sheath steel strip, opening an I-shaped, V-shaped or X-shaped groove, and forming it by a W-forming method; then longitudinally welding the formed steel strip by laser-TIG hybrid welding to obtain an inner sheath tube; (2) Assembling by vertically built-in core and automatically filling insulation material, and then undergoing a first treatment to obtain a single-sheath mineral insulated heating cable, and winding it into a coil; (3) Milling the edges of both sides of the outer sheath steel strip, opening an I-type, V-type or X-type groove, and forming the outer sheath tube by a W-forming method; before the outer sheath tube is formed and closed, the single sheath mineral insulated heating cable is placed in the outer sheath tube online by a hole-type flat roller power delivery method, and the two are simultaneously advanced to the welding station, and then the formed steel strip is longitudinally welded by a laser-TIG composite welding method, and then a double sheath mineral insulated heating cable is obtained after a second treatment, and then rolled into a coil.

10. The preparation method according to claim 9, wherein: The preparation method of the inner sheath steel strip comprises: cutting the steel strip according to the design size, processing the end of the steel strip into a 45° bevel, opening an I-shaped groove on the bevel, and welding a plurality of steel strips in sequence to the design length of the double-sheath mineral insulated heating cable by non-melting electrode gas shielded welding or laser welding; after the weld is cooled to ≤150°C, the weld is heated to more than 1000°C for insulation, and after cooling by argon gas purging, it is curled into a coil to obtain the inner sheath steel strip.

11. The preparation method according to claim 10, wherein: Before welding, the groove is preheated under the protection of argon atmosphere; during welding, the protective gas is a mixture of Ar and N2.

12. The preparation method according to claim 9, wherein: The first treatment includes: performing heat tension reduction on the assembled cable and compacting the insulating material; and performing cold sizing and straightening after the heat treatment.

13. The preparation method according to claim 12, wherein: The assembly starts from 2-3 m behind the welding point of the inner sheath pipe.

14. The preparation method according to claim 12, wherein: The single-sheath mineral insulated heating cable comprises a core, an insulating heat-conducting layer and a sheath layer.

15. The preparation method according to claim 9, wherein: The second treatment includes: subjecting the welded cable to a heat tension reduction treatment, and after the heat treatment, performing secondary cooling sizing and straightening.

16. The double-sheathed mineral insulated heating cable according to any one of claims 1 to 8 or the double-sheathed mineral insulated heating cable produced by the production method according to any one of claims 9 to 15 is used in deep wells and ultra-deep wells.

17. The use according to claim 16, wherein: The operating temperature of the double-sheath mineral insulated heating cable is 350°C-750°C, and the service life is more than 3 years.

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