Double-sheath mineral insulated heating cable and preparation method and application thereof

Abstract

The invention relates to the technical field of petroleum and natural gas development, and discloses a double-sheath mineral insulated heating cable and a preparation method and application thereof. The double-sheath mineral insulation heating cable sequentially comprises a core body, an insulation heat conduction 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 is greater than or equal to 520 MPa, the ductility is greater than or equal to 35%, and the hardness 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 is greater than or equal to 520 MPa, the elongation is greater than or equal to 35%, and the hardness is less than or equal to 32 HRC; the thickness of the outer sheath layer is 30%-50% smaller than that of the inner sheath layer, the heat conductivity of the inner sheath layer is not larger than that of the outer sheath layer, and the heat conductivity of the inner sheath layer is not smaller than that of the insulating heat conduction layer. The double-sheath mineral insulation heating cable can be suitable for being applied to deep wells and ultra-deep wells of H2S, CO2 and Cl <->, the use temperature is 350-750 DEG C, and the service life is more than 3 years.

Description

Technical Field

[0001] The 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

[0002] Electric heating technology is a relatively advanced technology in the heavy oil reservoir development technology SAGD auxiliary development technology. It is also an emerging environmental protection technology and has received special attention due to its green environmental protection. By assisting or directly heating the heavy oil in the oil field, the heating efficiency of high-viscosity, high-condensation, and high-wax heavy oil can be improved, and energy consumption and carbon emissions can be reduced. Electric heating technology can also be used in my country's medium and low maturity shale oil in situ conversion, underground coal distillation, oil-rich coal and other resource exploitation pilot tests. It is a technical tool for the clean development of unconventional oil and gas resources. At present, thermal recovery development technology is mainly used for heavy oil, including steam throughput, steam drive, fire burning oil layer, steam-assisted gravity drainage (SAGD), etc. However, traditional downhole heating technology has the problems of high pollution and high heat energy loss.

[0003] 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 exploitation is more suitable for domestic storage characteristics. The 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 service underground, and high reliability requirements are required. 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 requirements of high temperature, high pressure, high power, and long life operation underground. Summary of the invention

[0004] The purpose of the present invention is to overcome the problems of low power per meter, low strength, poor corrosion resistance, etc. of the heating cable in the prior art, and to provide a double-sheath mineral insulated heating cable and its preparation method and application. The double-sheath mineral insulated heating cable of the present invention has the characteristics of higher strength, greater vertical depth of the well, longer length of the horizontal well, stronger corrosion resistance, higher temperature resistance, longer service life, etc. The method of the present invention greatly improves the production efficiency.

[0005] In order to achieve the above-mentioned purpose, the first aspect of the present invention provides a double-sheathed mineral insulated heating cable, which 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 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.

[0006] A second aspect of the present invention provides a method for preparing a double-sheathed mineral insulated heating cable, the method comprising:

[0007] (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;

[0008] (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;

[0009] (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.

[0010] Through the above technical solution, the beneficial technical effects achieved by the present invention are as follows:

[0011] (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.

[0012] (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.

[0013] (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 depth, reduces the weld depression, and has no obvious burrs on the inner wall of the pipe column after welding. The tightness between the inner sheath and the outer sheath is enhanced, the thickness of the sheath can be increased by more than 20%, and the welding speed can be increased by more than 30%.

[0014] (4) During the production of the outer sheath layer, the single-sheath heating cable is synchronously built into the outer sheath pipe. The two are simultaneously advanced through a hot tension reducing rolling process to achieve a double-layer sheath of the heating cable that is tightly fitted, with a reduction rate of more than 40%. This process is used for the first time in the heating cable industry.

[0015] (5) The double-sheath heating cable of the present invention is subjected to medium-frequency induction heat treatment to heat the double-sheath 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.

[0016] (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

[0017] Figure 1 is a schematic structural diagram of a cross section of a double-sheathed mineral insulated heating cable provided by one embodiment of the present invention;

[0018] Figure 2 It is a processing diagram of a single sheath mineral insulated heating cable provided by one embodiment of the present invention;

[0019] Figure 3 This is a processing diagram of a double-sheathed mineral insulated heating cable provided by one embodiment of the present invention;

[0020] Figure 4 This is a relative position diagram of a single sheath mineral insulated heating cable and an outer sheath steel strip provided by one embodiment of the present invention. DETAILED DESCRIPTION

[0021] The endpoints and any values ​​of the ranges disclosed in this article 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 endpoint values ​​of each range, the endpoint values ​​of each range and the 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 as specifically disclosed in this article.

[0022] A first aspect of the present invention provides a double-sheathed mineral insulated heating cable, such as Figure 1 As shown, the double-sheathed mineral insulated heating cable comprises a core 1, an insulating heat-conducting layer 2, an inner sheath layer 3 and an outer sheath layer 4 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.

[0023] In the present invention, the thermal conductivity of the inner and outer sheath layers is not less than the thermal conductivity of the core.

[0024] The inner and outer layers of the double-sheath mineral insulated heating cable of the present invention are both made of metal materials. The mechanical properties of 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 hot tension reducing process, the inner layer of low-carbon microalloy undergoes elastic deformation, and the outer layer of stainless steel and other alloys undergoes plastic deformation, so that the two layers of materials 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 bears the functions of corrosion resistance and wear resistance, and the inner sheath bears the functions of tensile and compressive resistance.

[0025] The present invention provides a dieless drawing and diameter reduction process and a manufacturing method of a bimetallic sheathed mineral insulated heating cable with high production efficiency, longer length, greater strength and stronger corrosion resistance. The present invention solves the problems of low production efficiency, low power per meter, short length, joints and poor stability of heating cables manufactured by the traditional magnesium oxide porcelain column filling and drawing process, and the problems of low power per meter, low strength and poor corrosion resistance of heating cables manufactured by the copper strip longitudinally wrapped argon arc welding continuous rolling process.

[0026] The double-sheathed mineral insulated heating cable of the present invention further expands the application scope 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.

[0027] In the present invention, the material of the inner sheath layer is selected from low-carbon microalloy steel according to the use 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, and can be 316L, 310S, titanium alloy, nickel-based alloy, Hastelloy and other new high-corrosion-resistant alloy materials.

[0028] 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.

[0029] 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.

[0030] 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.

[0031] 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.

[0032] 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.

[0033] In some embodiments of the present invention, the insulating heat-conductive layer has a porosity of less than 5%, an insulation withstand voltage strength of more than 6000V, a thermal conductivity of more than 30W / mk, and a temperature resistance of 350°C-750°C.

[0034] In some embodiments of the present invention, the radial thickness of the insulating heat-conducting layer is 2-5 mm.

[0035] In some embodiments of the present invention, the insulating heat-conducting layer is made of an insulating heat-conducting substance.

[0036] Preferably, the insulating heat-conducting material is magnesium oxide, and its sphericity is greater than 50% and its purity is greater than 99%.

[0037] 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.

[0038] In some embodiments of the present invention, the tensile strength of the core is ≥650 MPa and the elongation is ≥25%.

[0039] In some embodiments of the present invention, the outer diameter of the core is 3-15 mm and the length is 1000-10000 m.

[0040] In some embodiments of the present invention, the core is made of a high-resistance electric heating alloy, preferably Cr20Ni80.

[0041] In the present invention, the core of the heating cable is a high-resistance electric heating alloy such as Cr20Ni80, with a tensile strength of ≥650MPa, an elongation of ≥25%, an outer diameter of Φ3-Φ15mm, and a length of 1000-10000m. The chemical composition of the core 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%.

[0042] In some embodiments of the present invention, the tensile strength of the double-sheathed mineral insulated heating cable is not less than 650 MPa and the elongation is ≥ 25%.

[0043] The double-sheathed mineral insulated heating cable of the present invention has excellent corrosion resistance.

[0044] A second aspect of the present invention provides a method for preparing a double-sheathed mineral insulated heating cable, the method comprising:

[0045] (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;

[0046] (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;

[0047] (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.

[0048] The present invention adopts a dieless drawing and reducing process to prepare a double-sheath mineral insulated heating cable, and the single-sheath 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 reducing + heat treatment process, and is curled into a coil. The outer sheath adopts the "W" forming process, and the produced single sheath heating cable is simultaneously introduced into the outer sheath tube manufacturing process online by "power delivery of holed flat roller" (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 four passes of thermal tension reduction are carried out by a rolling unit; then the whole heating cable is heat treated in a medium frequency induction furnace 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 parent material, so as to improve the comprehensive performance 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 unit to achieve the required outer diameter and straightness of the double sheath mineral insulated heating cable, and then it is coiled onto a special cable reel by a coiler, thereby obtaining a bimetallic sheath mineral insulated heating cable product.

[0049] 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 and tightly fitting the inner and outer sheaths, thereby forming a bimetallic sheath heating cable product.

[0050] The present invention makes the heating cable go down deeper, has stronger corrosion resistance, extends the service life of the heating cable, and has better economy by selecting 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); the production efficiency is further improved by adopting the laser + TIG composite welding process. Compared with the traditional magnesium oxide porcelain column filling and drawing process (single sheath heating cable), it can be produced continuously, 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 thick-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 (welding process limits the sheath thickness to ≤2mm) continuous rolling process.

[0051] Compared with the conventional magnesium oxide porcelain column filling and drawing process and the copper strip longitudinally wrapped argon arc welding continuous rolling process, the method of the invention can reduce the manufacturing cost by more than 30% and improve the processing efficiency by more than 20%.

[0052] In step (1) of the present invention, molten iron is used as the main raw material, alloy elements are added according to the chemical composition of the sheath material alloy, and finally rolled into a hot-rolled plate with a thickness of 2.4-6.35 mm, and then curled after controlled cooling, tempering, and pickling 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 then the multiple steel strips are welded in sequence according to the specified length requirements of the heating cable product.

[0053] In some embodiments of the present invention, the preparation method of the inner sheath steel strip includes: cutting the steel strip according to the designed 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 by non-melting electrode gas shielded welding (TIG) or laser welding to the designed length of the double-sheathed mineral insulated heating cable; after the weld is cooled to ≤150°C, the weld is heated to greater 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.

[0054] 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.

[0055] 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℃ by medium frequency, and the heat preservation is carried out for 1-2min. The weld is quickly cooled by argon gas blowing, and the cooling rate is ≥15℃ / s. If it is 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 by laser-TIG composite welding technology. The groove gap is ≤0.1mm, the misalignment is ≤0.3mm, and the center offset of the weld is ±0.5mm. The "laser-TIG" hybrid welding technology is a process in which the laser beam and the non-melting electrode tungsten arc welding arc are combined as a composite heat source for welding. The welding wire is ER-310 welding wire. When the single-sheath heating cable sheath is welded 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 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, the corresponding welding wire should be selected.

[0056] In some embodiments of the present invention, the first treatment includes: the first treatment includes: heat tension reducing the assembled cable and compacting the insulating material; after heat treatment, cold sizing and straightening are performed to achieve the required outer diameter and straightness of the heating cable.

[0057] In some embodiments of the present invention, the assembly starts from 2-3 m behind the welding point of the inner jacket pipe.

[0058] In some embodiments of the present invention, the single sheath mineral insulated heating cable comprises a core, an insulating heat conductive layer and an inner sheath layer.

[0059] In step (2) of the present invention, starting from about 2 meters behind the welding point of the single sheath heating cable, the single sheath heating cable is assembled by means of vertical built-in core wire + automatic powder filling of magnesium oxide powder. After welding, the two (single sheath 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 thermally tensioned and reduced in diameter 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.

[0060] In step (2) of the present invention, after the single-sheath heating cable is welded and reduced in diameter and the magnesium oxide powder reaches the compaction density requirement, the entire tube body is heat treated by adopting a three-stage medium-frequency induction heating method of "preheating + heating + insulation", the entire tube body is heated to between 900-1100°C, and the insulation is performed for 1-3 minutes. The graphite cold jacket is cooled to below 150°C, water-cooled, cleaned, and rolled into a coil.

[0061] Specifically, after the welding diameter reduction of the single sheath 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 heat tension diameter reduction treatment by two vertical rolling mills (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 sheath heating cable and coiled into a coil.

[0062] In step (3) of the present invention, molten iron is used as the main raw material, and alloy elements are added according to the chemical composition of high-alloy steel, and finally rolled into a hot-rolled plate with a thickness of 2.4-6.35 mm, and then curled after controlled cooling, tempering, and pickling to make 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; and then the multiple steel strips are welded and lengthened in sequence 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 on both sides, and "I-type, V-type or X-type" grooves are opened; the outer sheath steel strip adopts the "W" forming method, and the produced single-sheath heating cable is online built-in in the outer sheath tube (before the outer sheath is formed and closed) by the "hole-type flat roller power delivery" method, and the formed steel strip is longitudinally straight-seam welded by laser-TIG composite welding technology. After the outer sheath is welded by laser-TIG composite welding, the two (single sheath heating cable + outer sheath) move to the next process simultaneously. The single sheath heating cable starts 2 meters after the welding point, and the single sheath 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. The double sheath heating cable is heat-treated and heated to between 900-1100°C using a three-stage medium-frequency induction heating method of "preheating + heating + insulation". The double sheath heating cable is kept warm for 1-3 minutes to adjust the sheath structure and eliminate work hardening. It is cooled to below 150°C through a graphite cooling jacket, cleaned with water, and curled into a coil after cooling.

[0063] In some embodiments of the present invention, the second treatment includes: subjecting the welded cable to a heat tension reduction treatment, and after the heat treatment, cold sizing and straightening to achieve the required outer diameter and straightness of the heating cable.

[0064] In step (3) of the present invention, the double-sheathed heating cable (outer sheathed pipe + built-in single-sheathed heating cable) after forming is locally heated + multiple sets of roller groups (flat rollers + vertical rollers) are heat-reduced, and the pipe diameter is reduced by 10-25%; the single-sheathed heating cable starts from 2 meters after the welding point, and the single-sheathed heating cable is synchronously built into the outer sheathed pipe, and the two move forward synchronously and undergo heat tension reduction to achieve a tight fit of the double-layer sheath of the heating cable. Then the whole pipe body of the double-sheathed mineral insulated heating cable is heat-treated; the double-sheathed heating cable is heat-treated, and the double-sheathed heating cable is heated to between 900-1100°C by a three-stage medium-frequency induction heating method of "preheating + heating + insulation", and is kept warm for 1-3 minutes to adjust the sheath structure and eliminate work hardening, and is cooled to below 150°C through a graphite cooling jacket, washed with water, and curled 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 by a coiler to obtain a double-sheathed mineral insulated heating cable product.

[0065] A third aspect of the present invention provides the use of the aforementioned double-sheath mineral insulated heating cable in deep wells and ultra-deep wells.

[0066] In the present 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 - .

[0067] The double-sheath mineral insulated heating cable of the present invention has excellent corrosion resistance and is suitable for use in H2S, CO2, Cl - It is used in deep wells and ultra-deep wells.

[0068] 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.

[0069] 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:

[0070] (1) Preparation of single sheath mineral insulated heating cable (sheath) steel coil;

[0071] (2) Slitting and butt welding length of single sheathed mineral insulated heating cable (sheath) steel coil;

[0072] (3) Preparation of single sheath mineral insulated heating cable;

[0073] (4) Preparation of steel coils for double-sheathed mineral insulated heating cables (outer sheath);

[0074] (5) Longitudinal sheathing and butt welding of double-sheathed mineral insulated heating cable (outer sheath) steel coils;

[0075] (6) The single sheath heating cable and the outer sheath steel belt enter the forming section simultaneously;

[0076] (7) Longitudinal weld welding of the outer sheath pipe of double sheath heating cable;

[0077] (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%;

[0078] (9) Heat treatment of the entire tube of double-sheathed mineral insulated heating cable;

[0079] (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 coiling machine.

[0080] The step (1) is: 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 curling after controlled cooling, tempering, and pickling to make a coil with a length of more than 400 meters and a width of 1.2-1.5 meters.

[0081] 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.

[0082] In the step (2), when the sheathed steel strip of the single-sheathed heating cable is welded, the end of the steel strip is first processed into a 45° bevel, an I-shaped or V-shaped groove is opened on the bevel, and laser welding is adopted. 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.

[0083] In step (2), after the butt welding is completed, the weld is subjected to solid solution treatment. The inclined weld area is heated to 1050°C-1100°C by medium frequency, kept warm for 1-2 minutes, and the weld is quickly cooled by argon gas blowing, with a cooling rate of ≥15°C / s. For other corrosion-resistant alloys, the corresponding weld heat treatment process is adopted.

[0084] 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 to open an "I-type, V-type or X-type" groove; the steel strip is formed by a "W" forming method, and the formed steel strip is longitudinally straight-seam welded using the "laser-TIG" composite welding technology.

[0085] In the step (3), the groove gap is ≤0.1mm, the misalignment is ≤0.3mm, and the center offset of the weld is ±0.5mm. The "laser-TIG" composite 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, and the welding wire is ER-310 welding wire. When the single-sheath heating cable sheath is welded 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. If it is other corrosion-resistant alloys, select the corresponding welding wire.

[0086] In the step (3), starting from 2 meters behind the welding point of the single sheath heating cable, the single sheath heating cable is assembled by means of vertical built-in core wire + automatic powder filling of magnesium oxide powder. After welding, the two (single sheath 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.

[0087] In the step (3), after the single-sheath 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", the entire pipe body is heated to between 900-1100°C, and the insulation is carried out for 1-3 minutes. The graphite cold jacket is cooled to below 150°C, water-cooled, cleaned, and rolled into a coil.

[0088] The step (4) is: using molten iron as the main raw material, adding alloy elements according to the chemical composition of the high alloy steel, finally rolling into a hot-rolled plate with a thickness of 2.4-6.35 mm, and then curling after controlled cooling, tempering, and pickling to make a coil with a length of more than 400 meters and a width of 1.2-1.5 meters.

[0089] The step (5) is as follows: the prepared outer sheath (high alloy steel) steel coil is longitudinally cut into a plurality of steel strips of 150-400 mm according to the specification requirements 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.

[0090] 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 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) proceed to the next process simultaneously.

[0091] 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 is formed by a "W" forming method, and the formed steel strip is longitudinally straight-seam welded using the "laser-TIG" composite welding technology.

[0092] In the step (8), the single sheath heating cable starts from 2 meters behind the welding point, and the single sheath heating cable is simultaneously built into the outer sheath pipe. The two move forward simultaneously and undergo thermal tension reduction to achieve a tight fit between the double sheaths of the heating cable.

[0093] In the step (9), the double-sheath heating cable is heat-treated by adopting a three-stage medium-frequency induction heating method of "preheating + heating + insulation", and the double-sheath heating cable is heated to between 900-1100°C, and the insulation is carried out for 1-3 minutes to adjust the sheath structure and eliminate work hardening. The cable is cooled to below 150°C through a graphite cooling jacket, washed with water, and curled into a coil after cooling.

[0094] 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 is coiled onto a special cable reel by a coiler, thereby obtaining a bimetallic sheathed mineral insulated heating cable product.

[0095] The present invention will be described in detail below through examples.

[0096] The raw materials used in the following examples and comparative examples were all commercially available.

[0097] Example 1

[0098] A method for preparing a bimetallic sheathed mineral insulated heating cable with an outer diameter of Ф38.1×5mm-310S sheath-2500m (outer sheath material 310S + inner sheath material 310S), specifically comprising the following steps:

[0099] S1. Preparation of single sheath mineral insulated heating cable:

[0100] S11. Using molten iron as the main raw material, according to the chemical composition elements 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, solid solution treatment at 1030-1180℃, pickling and curling, and then finally made into a coil with a wall thickness of 3mm and a length of more than 400 meters. After testing, it is curled into Ф26.8×2mm-310S stainless steel coil by a tension coiling machine for pipe making;

[0101] S12. Cut the stainless steel coil obtained in step S11 longitudinally into 119mm steel strips; then, according to the design length requirements of the cable, process the end of the steel strip into a 45° bevel, open an I-type groove on the bevel, and weld multiple steel strips in sequence to a length of 2500 meters by non-melting 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 it with argon purge. After cooling, the inner sheath steel strip is obtained and rolled into a disk;

[0102] 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;

[0103] S14, such as Figure 2 As shown, starting from 2 meters behind the welding point of the inner sheath tube obtained in step S13, the single sheath heating cable (sheath + core wire + magnesium oxide) is assembled by vertical built-in core wire + automatic filling of magnesium oxide powder. After assembly, the outer diameter of the heating cable is Φ38.1×3mm, which is reduced to Φ31.8×2.3mm by heat tension of 4 sets of rollers (flat rollers + vertical rollers) and compacted with magnesium oxide powder; then, local medium frequency heating is adopted (flowing H2 is passed inside to prevent oxidation of the heating cable sheath), and the diameter is reduced to Φ26.8×2mm by heat tension of 2 vertical roller groups (flat rollers + vertical rollers), and the compacted density of magnesium oxide powder is 2.3g / cm3 , 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 rolled into a coil;

[0104] S2. Preparation of double sheathed mineral insulated heating cable:

[0105] S21. Using molten iron as the main raw material, according to the chemical composition elements 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, solid solution treatment at 1030-1180℃, pickling and curling, and then finally made into a coil with a wall thickness of 4mm and a length of more than 400 meters. After testing, it is curled into Ф38.1×5mm-310S stainless steel coil by a tension coiling machine for pipe making;

[0106] S22, the prepared stainless steel coil is longitudinally cut into 139mm steel strips, and then the ends of the steel strips are processed into 45° bevels according to the design length requirements of the cable, and I-type grooves are opened on the bevels. Multiple steel strips are welded in sequence to a length of 2500 meters by non-melting electrode gas shielded welding (TIG) or laser welding. Before welding, the groove is preheated under the protection of argon atmosphere, and the preheating temperature is 200℃. 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℃, the weld is reheated to 980-1100℃, kept warm for 2-3min, cooled by argon purge, and after cooling, the outer sheath steel strip is obtained and rolled into a disk.

[0107] S23, such as Figure 3 and Figure 4As shown, the outer sheath steel strip obtained in step S22 is milled on both sides, an I-shaped groove is opened, and the "W" forming method is used to control the continuous and stable curling of the steel strip. During the curling, the single-sheath mineral insulated heating cable obtained in step S1 is simultaneously placed online into the curled outer sheath steel strip by "power delivery with hole-type flat roller" (before the outer sheath is formed and closed), and the two are simultaneously advanced to the welding station; after the steel strip is curled, the groove gap at the welding point is ≤0.2mm, the misalignment is ≤0.3mm, and the center offset of the weld 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 stains on the edge of the steel strip. The protective gas is Ar (95%) + N2 (5%), the gas flow rate is 22L / min, the TIG welding current is 180A, the welding voltage is 24V, the wire feeding speed is 200mm / min, the laser defocus amount is +3mm, the power is 1.5Kw, and the welding speed is 3m / min. After assembly by laser-TIG composite welding, the outer diameter of the heating cable is Ф44.5×4mm, and the assembled heating cable (single sheath mineral insulated heating cable + outer sheath) is synchronously advanced to the next process;

[0108] S24, the heating cable assembled in step S23 is heated by local heating (flowing H2 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), and the compacted density of magnesium oxide powder is 2.4-2.6g / cm 3 ;

[0109] S25. The heating cable after the primary diameter reduction and compaction is continuously heated by induction for the second time. The three-stage medium frequency induction heating method of "preheating + heating + insulation" is adopted. H2 is passed through the heating device to prevent the oxidation of the heating cable sheath. The heating cable is heated to between 980-1100℃ and kept warm for 1-3min to achieve the purpose of refining the metallographic structure of the sheath and eliminating work hardening. After that, it is cooled to below 150℃ through a graphite cooling tunnel (with H2 flowing inside to prevent oxidation of the heating cable sheath and accelerate cooling). The cable is cold-diametered to the required outer diameter of Ф38.1×5mm(2+3) and straightness of the heating cable through three vertical roller units (3 flat rollers + 3 vertical rollers arranged alternately), and then coiled onto a special cable drum through a coiler, thereby obtaining a double-sheath mineral insulated heating cable product of Ф38.1×5mm(2+3)-2500m.

[0110] 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 sheath mineral insulated heating cable has an operating temperature of 350℃-750℃ and a service life of more than 3 years.

[0111] Comparative Example 1

[0112] A bimetallic sheathed mineral insulated heating cable was prepared according to the method of Example 1, except that all laser-TIG composite welding was replaced by argon arc welding.

[0113] The welding speed is 80% lower than that of 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 through in a single pass. At the same time, the strength of the thin-walled and thick-sheathed heating cable is reduced. According to the test, the tensile strength of the double-sheathed mineral insulated heating cable (wall thickness less than 2mm) is ≤320MPa. Under certain H2S, CO2, Cl - The service life in the well and at 350℃ is 1-3 months, and the cable sheath is severely corroded.

[0114] Comparative Example 2

[0115] A single sheathed mineral insulated heating cable was prepared according to step S1 of the method of Example 1, without performing step S2.

[0116] Step S1: Prepare a single sheath mineral insulated heating cable. If a low-carbon microalloy 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.

[0117] Step S1: prepare a single sheath mineral insulated heating cable. If a 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-sheath heating cable.

[0118] It can be seen from the above results that the double-sheath mineral insulated heating element prepared by the method of the present invention has a temperature range of 350°C-750°C and a service life of more than 3 years.

[0119] The present invention adopts the "laser welding-TIG" composite welding technology and arc heat source, which expands the heat action range, increases the amount of molten metal, improves the weld quality, increases the welding penetration, and reduces the weld depression relative to the argon arc welding method. After welding, there is no obvious burr on the inner wall of the pipe column. 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" composite 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 resistance of the outer sheath and high strength of the inner sheath), compared with the single sheath heating cable of the same wall thickness pure material, under the condition of meeting the use intensity, the double sheath heating cable saves more alloy cost than the single sheath heating cable of the same wall thickness pure alloy, and can reduce the manufacturing cost by about 30% or more.

[0120] The preferred embodiments of the present invention are described in detail above, but the present invention is not limited thereto. Within the technical concept of the present invention, the technical solution of the present invention can be subjected to a variety of simple modifications, including the combination of various technical features in any other suitable manner, and these simple modifications and combinations should also be regarded as the contents disclosed by the present invention and belong to the protection scope 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, 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.

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, 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.

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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