Double-jacketed mineral insulated heating cable and method of manufacture and use thereof

Abstract

The application relates to the technical field of oil and gas development, and discloses a double-sheath mineral insulation heating cable as well as a preparation method and application thereof. The double-sheath mineral insulation heating cable comprises, from inside to outside, a core body, an insulation heat conduction layer, an inner sheath layer and an outer sheath layer; 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 elongation 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% thinner than that of the inner sheath layer, the thermal conductivity of the inner sheath layer is not greater than that of the outer sheath layer, and the thermal conductivity of the inner sheath layer is not less than that of the insulation heat conduction layer. The double-sheath mineral insulation heating cable can be applied in deep wells and ultra-deep wells containing H2S, CO2 and Cl ‑ , and has a service temperature of 350-750 DEG C and a service life of more than 3 years.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of oil and gas development, in particular to a double-sheath mineral insulated heating cable and a preparation method and application thereof. BACKGROUND

[0002] The electric heating technology is a more advanced technology in the SAGD auxiliary development technology of the heavy oil reservoir development technology, and is also a new environmental protection technology, which is particularly concerned due to its green and environmental protection. Through the auxiliary or direct heating of the downhole heavy oil of the oil field, the heating efficiency of the high viscosity, high condensation and high wax content heavy oil mining can be improved, and the energy consumption and carbon emission can be reduced. The electric heating technology can also be used for the pilot test of in-situ conversion of domestic low-maturity shale oil, underground dry distillation of coal, and rich oil coal resources, and is a technical weapon for clean development of unconventional oil and gas resources. At present, the heavy oil mainly adopts thermal recovery development technology, including steam huff and puff, steam flooding, fire flooding, steam assisted gravity drainage (SAGD), etc. However, the traditional downhole heating technology has the problems of serious pollution and high heat energy loss.

[0003] China is rich in shale oil resources, and the in-situ conversion potential of low-maturity shale oil is huge. Research shows that the downhole electric heating technology is more suitable for the domestic storage characteristics for in-situ mining of shale oil. The electric heating technology for in-situ conversion of shale oil underground has obvious advantages in pollutant emission and surface environmental protection. However, the mineral insulated heating cable needs to withstand high temperature, high pressure, chlorine ion, hydrogen sulfide and other medium corrosion in the long-term service in the downhole, and has high reliability requirements. The corrosion resistance, high temperature resistance and heating temperature accurate control of the mineral insulated heating cable are the key to stable operation. The existing single-sheath mineral insulated heating cable in China has poor reliability, low strength, joints and other problems, which cannot meet the requirements of high temperature, high pressure, high power and long service life in the downhole. SUMMARY

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

[0005] To achieve the above objectives, the first aspect of the present invention provides a double-sheathed mineral-insulated heating cable, wherein the double-sheathed mineral-insulated heating cable comprises, from the inside out, a core, an insulating and thermally conductive 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 and thermally conductive 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) Mill the edges on both sides of the inner sheath steel strip, open I-type, V-type or X-type bevels, and form it using the W forming method; then use laser-TIG composite welding to longitudinally weld the formed steel strip to obtain the inner sheath tube.

[0008] (2) The cable is assembled using a vertical built-in core and automatic filling of insulation material, and then undergoes a first treatment to obtain a single-sheath mineral-insulated heating cable, which is then coiled into a reel.

[0009] (3) Mill the edges on both sides of the outer sheath steel strip and open I-type, V-type or X-type bevels. Use the W forming method to form the outer sheath tube. Before the outer sheath tube is formed and closed, the single sheath mineral insulated heating cable is placed into the outer sheath tube online by a perforated flat roller power delivery method. The two advance to the welding station simultaneously. Then, the formed steel strip is longitudinally welded by laser-TIG composite welding. After the second processing, a double sheath mineral insulated heating cable is obtained and coiled into a coil.

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

[0011] (1) The double-sheathed mineral-insulated heating cable of this invention has a tensile strength of not less than 635 MPa and an elongation of ≥25%. It possesses excellent corrosion resistance and is suitable for applications involving certain amounts of H2S, CO2, and Cl-. - It is used in deep 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 wells, longer horizontal length in wells, stronger corrosion resistance, higher temperature resistance, and longer service life.

[0013] (3) The present invention adopts laser-TIG composite welding technology and electric arc heat source, which expands the heat action range, increases the amount of molten metal, improves the weld quality, increases the welding penetration, reduces weld depression, and leaves no obvious burrs on the inner wall of the pipe after welding. It also enhances the tightness of the fit between the inner and outer sheaths, which can increase the sheath thickness by more than 20% and increase the welding speed by more than 30%.

[0014] (4) During the production of the outer sheath layer, the single-sheath heating cable is simultaneously built into the outer sheath tube. The two advance simultaneously and undergo the hot tension reduction rolling process to achieve a tight fit between the double sheaths of the heating cable, with a reduction rate of more than 40%. This process is the first of its kind to be used in the heating cable industry.

[0015] (5) The double-sheathed heating cable of the present invention undergoes medium-frequency induction heat treatment to heat the double-sheathed heating cable to between 900-1100℃ (according to the heat treatment temperature requirements of the outer sheath material). On the one hand, it adjusts the structure of the outer sheath and eliminates work hardening; on the other hand, protective gases such as H2 are introduced into the heating and cooling tunnel to prevent oxidation of the surface of the heating cable during the heating process and reduce corrosion resistance.

[0016] (6) This invention solves the problems of low power per meter, low strength and poor corrosion resistance of heating cables manufactured by the copper strip longitudinal wrapping argon arc welding continuous rolling process (automatic filling method). Compared with the traditional magnesium oxide ceramic column filling and drawing process and the copper strip longitudinal wrapping argon arc welding continuous rolling process, it reduces the manufacturing cost by more than 30% and increases the production efficiency by more than 20%. Attached Figure Description

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

[0018] Figure 2 This is a process diagram of a single-sheathed mineral-insulated heating cable provided in one embodiment of the present invention;

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

[0020] Figure 4 This is a diagram showing the relative positions of a single-sheathed mineral-insulated heating cable and an outer sheath steel strip, according to one embodiment of the present invention. Detailed Implementation

[0021] The endpoints and any values ​​of the ranges 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 endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0022] The first aspect of this invention provides a double-sheathed mineral-insulated heating cable, such as... Figure 1 As shown, the double-sheathed mineral-insulated heating cable comprises, from the inside out, a core 1, an insulating and thermally conductive layer 2, an inner sheath layer 3, and an outer sheath layer 4. The inner sheath layer has a yield strength ≥205MPa, tensile strength ≥520MPa, elongation ≥35%, and hardness ≤32HRC. The outer sheath layer has a yield strength ≥205MPa, tensile strength ≥520MPa, elongation ≥35%, and hardness ≤32HRC. Furthermore, 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 that of the outer sheath layer, and the thermal conductivity of the inner sheath layer is not less than that of the insulating and thermally conductive layer.

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

[0024] This invention relates to a double-sheathed mineral-insulated heating cable, in which both the inner and outer layers are made of metallic materials. It utilizes the mechanical properties of two different materials: the elastic deformation range of the outer sheath (high alloys such as stainless steel) and the low yield strength of the inner sheath (low-carbon microalloy). Under a thermal tension reduction process, the inner low-carbon microalloy undergoes elastic deformation, while the outer stainless steel or other alloy undergoes plastic deformation, resulting in a tight, composite molding of the two layers. The bonding force between the inner and outer sheaths is 0.5-1.0 MPa. The outer sheath provides corrosion and wear resistance, while the inner sheath withstands tensile and compressive forces.

[0025] This invention provides a dieless drawing and diameter reduction process and manufacturing method for bimetallic sheathed mineral-insulated heating cables, resulting in higher production efficiency, longer length, greater strength, and stronger corrosion resistance. This invention solves the problems of low production efficiency, low power per meter, short length, joints, and poor stability in heating cables manufactured using the traditional magnesium oxide ceramic column filling and drawing process, as well as the problems of low power per meter, low strength, and poor corrosion resistance in heating cables manufactured using the copper strip longitudinal wrapping argon arc welding continuous rolling process.

[0026] The double-sheathed mineral-insulated heating cable of the present invention further expands the application range of cables, provides equipment support for improving heavy oil recovery and shale oil development, and provides a new type of electric heating product for SAGD development technology in heavy oil reservoirs and in-situ conversion technology in shale oil.

[0027] In this invention, the material of the inner sheath layer is selected from low-carbon microalloy steel, etc., according to the operating 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. It can be new high corrosion-resistant alloy materials such as 316L, 310S, titanium alloy, nickel-based alloy, Hastelloy, etc.

[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 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.

[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 this invention, the 310S austenitic stainless steel comprises the following chemical composition: 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 porosity of the insulating and thermally conductive layer is <5%, the insulation withstand voltage is >6000V, the thermal conductivity is >30W / mk, and the temperature resistance is 350℃-750℃.

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

[0035] In some embodiments of the present invention, the insulating and thermally conductive layer is made of an insulating and thermally conductive material.

[0036] Preferably, the insulating and thermally conductive material is magnesium oxide, with a sphericity > 50% and a purity > 99%.

[0037] In this invention, the insulating and heat-conducting material filling the heating cable is a high-purity, high-temperature resistant magnesium oxide powder or other insulating and heat-conducting materials.

[0038] In some embodiments of the present invention, the core has a tensile strength ≥650MPa and an elongation ≥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 heating alloy, preferably Cr20Ni80.

[0041] In this invention, the core of the heating cable is a high-resistance heating alloy such as Cr20Ni80, with a tensile strength ≥650MPa, elongation ≥25%, outer diameter Φ3-Φ15mm, and length 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) Mill the edges on both sides of the inner sheath steel strip, open I-type, V-type or X-type bevels, and form it using the W forming method; then use laser-TIG composite welding to longitudinally weld the formed steel strip to obtain the inner sheath tube.

[0046] (2) The cable is assembled using a vertical built-in core and automatic filling of insulation material, and then undergoes a first treatment to obtain a single-sheath mineral-insulated heating cable, which is then coiled into a reel.

[0047] (3) Mill the edges on both sides of the outer sheath steel strip and open I-type, V-type or X-type bevels. Use the W forming method to form the outer sheath tube. Before the outer sheath tube is formed and closed, the single sheath mineral insulated heating cable is placed into the outer sheath tube online by a perforated flat roller power delivery method. The two advance to the welding station simultaneously. Then, the formed steel strip is longitudinally welded by laser-TIG composite welding. After the second processing, a double sheath mineral insulated heating cable is obtained and coiled into a coil.

[0048] This invention uses a dieless drawing and diameter reduction process to prepare double-sheathed mineral-insulated heating cables. Single-sheathed heating cables are prepared by a process of “W” forming + built-in heating core wire + automatic filling of magnesium oxide powder + laser-TIG composite welding + heat treatment + multiple thermal tension diameter reduction + heat treatment, and then rolled into a coil. The outer sheath adopts a "W" forming process. Simultaneously, the pre-produced single-sheath heating cable is introduced online into the outer sheath tube manufacturing process (before the outer sheath is formed and joined) using a "perforated flat roller power delivery" method. After the outer sheath is laser-TIG composite welded, the double-sheath heating cable is continuously and rapidly heated in a medium-frequency induction furnace and undergoes four passes of thermal tension reduction through a rolling mill. Then, the entire heating cable undergoes medium-frequency induction furnace heat treatment to reduce work hardening during the outer sheath tube reduction process, release processing stress, and further eliminate the differences in microstructure between the weld and the base material, thereby improving the comprehensive performance of the heating cable sheath in terms of plasticity, toughness, and corrosion resistance. After heat treatment, it is cooled to room temperature and finally undergoes three passes of micro-cold sizing and straightening through a rolling mill to achieve the required outer diameter and straightness for the double-sheath mineral-insulated heating cable. It is then wound onto a cable-specific reel by a winding machine to obtain the bimetallic sheath mineral-insulated heating cable product.

[0049] This invention employs a single-sheath heating cable and an outer sheath that undergo multiple thermal tension reduction processes to achieve mechanical bonding and tight fit between the inner and outer sheaths, thus forming a bimetallic sheath heating cable product.

[0050] This invention, through the selection of inner and outer sheath materials based on strength and corrosion resistance (high-strength steel for the inner sheath + corrosion-resistant steel for the outer sheath), enables heating cables to be lowered into wells to greater depths, exhibit stronger corrosion resistance, extend their lifespan, and improve economic efficiency. The use of laser + TIG composite welding further enhances production efficiency. Compared to the traditional magnesium oxide ceramic column filling and drawing process (single-sheath heating cables), continuous production is possible, eliminating problems such as numerous joints, low temperature resistance, low efficiency, and short service life. Compared to the copper strip longitudinal wrapping argon arc welding continuous rolling process, it can produce thick-walled bimetallic composite sheathed cables, eliminating the problems of low cable strength, poor corrosion resistance, and short service life associated with the traditional copper strip longitudinal wrapping argon arc welding (welding process limits sheath thickness to ≤2mm) continuous rolling process.

[0051] Compared with the traditional magnesium oxide ceramic column filling and drawing process and the copper strip longitudinal wrapping argon arc welding continuous rolling process, the method of the present invention can reduce manufacturing costs by more than 30% and increase processing efficiency by more than 20%.

[0052] In step (1) of this invention, molten iron is used as the main raw material, and alloying elements are added according to the alloy chemical composition of the sheath material. The resulting hot-rolled plate is 2.4-6.35 mm thick, then cooled, tempered, and pickled before being coiled to produce steel coils with a length exceeding 400 meters and a width of 1.2-1.5 meters. The prepared steel coils are then longitudinally cut into multiple steel strips with a width of 120-400 mm according to the specifications of the heating cable. Finally, according to the length requirements specified for the heating cable product, the multiple steel strips are sequentially butt-welded together.

[0053] In some embodiments of the present invention, the method for preparing the inner sheath steel strip includes: cutting the steel strip according to the design dimensions, processing the end of the steel strip into a 45° bevel, opening an I-shaped or V-shaped bevel on the bevel, and sequentially welding multiple steel strips to the design length of the double-sheathed mineral-insulated heating cable using a non-consumable 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 heat preservation, and after being cooled by argon purging, it is rolled into a coil to obtain the inner sheath steel strip.

[0054] In some embodiments of the present invention, the bevel is preheated under an argon atmosphere before welding; during welding, the protective gas is a mixture of Ar and N2.

[0055] In this 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, and the gas flow rate is 18-25L / min. The back protection device of the weld is turned on. After the butt welding is completed, the weld is solution treated. The oblique weld area is heated to 1050℃-1100℃ using medium frequency heating and held for 1-2 minutes. The weld is then rapidly cooled using argon gas counter-blowing, with a cooling rate ≥15℃ / s. For other corrosion-resistant alloys, the corresponding weld heat treatment process is adopted. Based on the sheath wall thickness requirements of single-sheath heating cables, the two sides of the extended steel strip are milled to open "I-type, V-type, or X-type" bevels. The steel strip is formed using a "W" forming method, and longitudinal straight seam welding is performed on the formed steel strip using laser-TIG composite welding technology. The bevel gap should be ≤0.1mm, the misalignment ≤0.3mm, and the weld center offset ±0.5mm. "Laser-TIG" hybrid welding technology combines a laser beam with a non-consumable electrode tungsten inert gas (TIG) welding arc as a composite heat source for welding. 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 TIG welding voltage is 22-32V, the laser defocusing distance 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, select the corresponding welding wire.

[0056] In some embodiments of the present invention, the first process includes: reducing the diameter of the assembled cable by thermal tension and compacting the insulation material; after heat treatment, performing cold sizing and straightening to achieve the required outer diameter and straightness of the heated cable.

[0057] In some embodiments of the present invention, the assembly begins 2-3m after the welding point of the inner sheath tube.

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

[0059] In step (2) of this invention, starting about 2 meters after the welding point of the single-sheath heating cable, the assembly of the single-sheath heating cable is completed by using a vertical built-in core wire + automatic magnesium oxide powder filling method. After welding, the two (single-sheath heating cable + outer sheath) advance synchronously to the heating induction coil. The outer sheath is continuously heated by local induction heating method. The cable thermal tension is reduced by a vertical four-roller group (flat roller + vertical roller) to achieve the required compaction density of magnesium oxide powder.

[0060] In step (2) of this invention, after the single-sheath heating cable is welded and the diameter reduction is completed and the magnesium oxide powder reaches the compaction density requirement, the entire pipe body undergoes heat treatment. The entire pipe body is heated to 900-1100℃ using a three-stage medium-frequency induction heating method of "preheating + heating + heat preservation". The temperature is maintained for 1-3 minutes, the graphite ink cold jacket is cooled to below 150℃, water-cooled, cleaned, and rolled into a coil.

[0061] Specifically, after the welding and diameter reduction of the single-sheathed heating cable is completed and the magnesium oxide powder reaches the required compaction density, local medium-frequency heating (with internal H2 flow to prevent oxidation of the heating cable sheath) is used. The cable undergoes thermal tension reduction treatment through two sets of vertical rolling mills (flat rollers + vertical rollers), and is cooled to below 150°C through a graphite cooling tunnel (with internal H2 flow to prevent oxidation of the heating cable sheath and accelerate cooling). After cleaning, the cable is finally made into a Φ26.8×2mm-2500m single-metal sheathed heating cable and coiled into a reel.

[0062] In step (3) of this 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 final product is rolled into a hot-rolled plate with a thickness of 2.4-6.35mm. After controlled cooling, tempering, pickling, and coiling, a steel coil with a length of more than 400 meters and a width of 1.2-1.5 meters is produced. The prepared outer sheath (high alloy steel) steel coil is longitudinally cut into multiple steel strips of 150-400mm according to the specifications of the heating cable. Then, according to the specified length requirements of the heating cable, the multiple steel strips are welded together in sequence. 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 to open "I-type, V-type or X-type" bevels. The outer sheath steel strip is formed using the "W" forming method. The produced single sheath heating cable is installed online in the outer sheath tube (before the outer sheath is formed and joined) using the "perforated flat roller power delivery" method. The formed steel strip is longitudinally welded using laser-TIG composite welding technology. After the outer sheath is laser-TIG composite welded, both the single-sheath heating cable and the outer sheath advance synchronously to the next process. Starting 2 meters after the welding point, the single-sheath heating cable is simultaneously embedded into the outer sheath tube. Both advance synchronously, undergoing thermal tension reduction to ensure a tight fit between the two sheaths. The double-sheath heating cable undergoes heat treatment using a three-stage medium-frequency induction heating method: preheating, heating, and heat preservation. The cable is heated to 900-1100℃, held for 1-3 minutes to adjust the sheath structure and eliminate work hardening. It is then cooled to below 150℃ using a graphite cooling jacket, water-cooled and cleaned, and finally coiled into a reel after cooling.

[0063] In some embodiments of the present invention, the second process includes: performing thermal tension reduction on the welded cable, followed by cold sizing and straightening after heat treatment to achieve the required outer diameter and straightness of the heated cable.

[0064] In step (3) of this invention, the formed double-sheathed heating cable (outer sheath tube + inner single-sheathed heating cable) is locally heated and then subjected to thermal reduction by multiple roller groups (flat rollers + vertical rollers), reducing the tube diameter by 10-25%. The single-sheathed heating cable is simultaneously installed in the outer sheath tube starting 2 meters after the welding point. The two advance synchronously and undergo thermal tension reduction to achieve a tight fit between the double sheaths of the heating cable. Then, the entire tube of the double-sheathed mineral-insulated heating cable is heat-treated. After heat treatment, the double-sheathed heating cable is heated to 900-1100℃ using a three-stage medium-frequency induction heating method of "preheating + heating + heat preservation". The heat preservation is carried out for 1-3 minutes to adjust the sheath structure and eliminate work hardening. The cable is then cooled to below 150℃ using a graphite cooling jacket, water-cooled and cleaned, and finally rolled into a coil after cooling. The double-sheathed heating cable undergoes full tube heat treatment, followed by cooling to room temperature. After passing through 5 sets of rollers (flat rollers + vertical rollers) for diameter reduction and straightening, it achieves the required outer diameter and straightness for the heating cable. Then, it is wound onto a cable-specific drum by a winding machine to obtain the double-sheathed mineral-insulated heating cable product.

[0065] The third aspect of this invention provides the application of the aforementioned double-sheathed mineral-insulated heating cable in deep and ultra-deep wells.

[0066] In this invention, a deep well refers to a well with a completed drilling depth of 4500m-6000m; an ultra-deep well refers to a well with a completed drilling depth of more than 6000m, and such deep and ultra-deep wells contain H2S, CO2, and Cl. - .

[0067] The double-sheathed mineral-insulated heating cable of this invention has excellent corrosion resistance and is suitable for use in environments containing H2S, CO2, and Cl-. - Applications in deep and ultra-deep wells.

[0068] In some embodiments of the present invention, the double-sheathed mineral-insulated heating cable has an operating temperature of 350℃-750℃ and a service life of 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 steel coils for single-sheathed mineral-insulated heating cables (sheath);

[0071] (2) Longitudinal shearing and butt welding of single-sheath mineral-insulated heating cable (sheath) steel coil;

[0072] (3) Preparation of single-sheathed mineral-insulated heating cables;

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

[0074] (5) Double-sheathed mineral-insulated heating cable (outer sheath) steel coil longitudinally cut and butt-welded length;

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

[0076] (7) Welding of longitudinal seams of outer sheath tubing for double-sheathed heating cables;

[0077] (8) Double-sheathed heating cable (outer sheathed pipe + inner single sheathed heating cable) local heating + multi-roller group (flat roller + vertical roller) thermal reduction, pipe diameter reduced by 10-25%;

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

[0079] (10) After the double-sheathed mineral insulated heating cable is reduced in diameter and straightened in two stages, it achieves the required product straightness and outer diameter, and then is wound onto a cable-specific drum by a winding machine.

[0080] The step (1) is as follows: using molten iron as the main raw material, adding alloying elements according to the alloy chemical composition of the sheath material, finally rolling into a hot-rolled plate with a thickness of 2.4-6.35mm, controlling cooling, tempering, pickling and then rolling it to make a coil with a length of more than 400 meters and a width of 1.2-1.5 meters.

[0081] Step (2) is as follows: the prepared single-sheath heating cable sheath steel coil is longitudinally cut into multiple steel strips with a width of 120-400mm according to the heating cable specifications; then, according to the length requirements specified for the heating cable product, the multiple steel strips are welded together in sequence.

[0082] In step (2), when welding the sheathed steel strip of the single-sheathed heating cable, the end of the steel strip is first processed into a 45° bevel, and an I-shaped or V-shaped bevel is opened on the bevel. Laser welding is then used. 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 back protection device of the weld is turned on.

[0083] In step (2), after the butt welding is completed, the weld is subjected to solution treatment. The oblique weld area is heated to 1050℃-1100℃ using medium frequency heating and held for 1-2 minutes. The weld is then rapidly cooled using argon gas counter-blowing, with a cooling rate ≥15℃ / s. If it is another corrosion-resistant alloy, the corresponding weld heat treatment process is adopted.

[0084] The step (3) is as follows: according to the sheath wall thickness requirements of the single-sheath heating cable, mill the edges on both sides of the extended steel strip and open "I-type, V-type or X-type" bevels; the steel strip adopts the "W" forming method and uses "laser-TIG" composite welding technology to perform longitudinal straight seam welding on the formed steel strip.

[0085] In step (3), the bevel gap is ≤0.1mm, the misalignment is ≤0.3mm, and the weld center offset is ±0.5mm. The "laser-TIG" composite welding technology is a process that combines a laser beam with a non-consumable electrode tungsten inert gas (TIG) welding arc as a composite heat source for welding. 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 TIG welding voltage is 22-32V, the laser defocusing distance 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, select the corresponding welding wire.

[0086] In step (3), starting 2 meters after the welding point of the single-sheath heating cable, the assembly of the single-sheath heating cable is completed by using a vertical built-in core wire + automatic magnesium oxide powder filling method. After welding, the two (single-sheath heating cable + outer sheath) advance synchronously to the heating induction coil. The outer sheath is continuously heated by local induction heating method. The cable diameter is reduced by the thermal tension of the four vertical roller groups (flat roller + vertical roller) while achieving the required compaction density of magnesium oxide powder.

[0087] In step (3), after the single-sheath heating cable is welded and the diameter reduction is completed, and the magnesium oxide powder reaches the compaction density requirement, the entire pipe body undergoes heat treatment. The entire pipe body is heated to 900-1100℃ using a three-stage medium-frequency induction heating method of "preheating + heating + heat preservation". The temperature is maintained for 1-3 minutes, and the graphite ink cold jacket is cooled to below 150℃. The pipe body is then water-cooled, cleaned, and rolled into a coil.

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

[0089] Step (5) is as follows: the prepared outer sheath (high alloy steel) steel coil is longitudinally cut into multiple steel strips of 150-400mm according to the specifications of the heating cable; then, according to the specified length requirements of the heating cable, the multiple steel strips are welded together in sequence.

[0090] The step (6) is as follows: the outer sheath is produced using the “W” forming process. The produced single-sheath heating cable is installed online in the outer sheath tube using the “perforated flat roller power delivery” method (before the outer sheath is formed and closed). After the outer sheath is welded using “laser-TIG” composite welding, 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 requirements of the single-sheath (corrosion-resistant alloy) heating cable, mill the edges on both sides of the extended steel strip and open "I-type, V-type or X-type" bevels; the steel strip adopts the "W" forming method and uses "laser-TIG" composite welding technology to perform longitudinal straight seam welding on the formed steel strip.

[0092] In step (8), starting 2 meters after the welding point, the single-sheathed heating cable is simultaneously installed in the outer sheath tube. The two advance synchronously and pass through the thermal tension reduction to achieve a tight fit between the double sheaths of the heating cable.

[0093] In step (9), the double-sheathed heating cable undergoes heat treatment using a three-stage medium-frequency induction heating method of "preheating + heating + heat preservation" to heat the double-sheathed heating cable to between 900-1100℃, keep it warm for 1-3 minutes, adjust the sheath structure, eliminate work hardening, cool it to below 150℃ through a graphite cooling jacket, water-cooled clean it, and then roll it into a coil after cooling.

[0094] In step (10), the double-sheathed heating cable undergoes full tube heat treatment, is cooled to room temperature after heat treatment, and is then reduced in diameter and straightened by 5 sets of roller groups (flat rollers + vertical rollers) to achieve the required outer diameter and straightness of the heating cable. It is then wound onto a cable-specific drum by a winding machine to obtain the double-metal sheathed mineral-insulated heating cable product.

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

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

[0097] Example 1

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

[0099] S1. Preparation of single-sheathed mineral-insulated heating cables:

[0100] S11. Using molten iron as the main raw material, and based on the chemical composition of 310S stainless steel (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), the material undergoes solution treatment at 1030-1180℃, pickling, and then coiling. Finally, it is produced into coils with a wall thickness of 3mm and a length exceeding 400 meters. After inspection, it is coiled into Ф26.8×2mm 310S stainless steel coils using a tension coiling machine for pipe manufacturing.

[0101] S12. The stainless steel coil obtained in step S11 is longitudinally cut into 119mm steel strips. Then, according to the cable design length requirements, the ends of the steel strips are processed into 45° bevels. An I-bevel is made on the bevel, and multiple steel strips are sequentially welded to a length of up to 2500 meters using gas metal arc welding (TIG) or laser welding. Before welding, the bevel is preheated under an argon atmosphere at a preheating temperature of 200℃. During welding, the shielding gas is Ar (95%) + N2 (5%), 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℃, it is reheated to 1020℃, held for 3 minutes, and then purged with argon for cooling. After cooling, the inner sheath steel strip is obtained and rolled into a coil.

[0102] S13. Mill the edges of both sides of the inner sheath steel strip obtained in step S12 to create an I-shaped bevel. Use the "W" forming method to control the continuous and stable forming of the steel strip. After the steel strip is formed, the bevel gap is ≤0.2mm, the misalignment is ≤0.3mm, and the weld center offset is ±0.6mm. Then, use laser-TIG composite welding technology to perform longitudinal welding on the formed inner sheath steel strip. During welding, the bevel is preheated to 150℃, the shielding gas is Ar (95%) + N2 (5%), the TIG welding current is 180A, the welding voltage is 24V, the laser defocusing distance 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 2 meters after the inner sheath weld point obtained in step S13, the assembly of the single-sheath heating cable (sheath + core wire + magnesium oxide) is completed using a vertical internal core wire + automatic magnesium oxide powder filling method. After assembly, the outer diameter of the heating cable is Ф38.1×3mm. It is then reduced to Ф31.8×2.3mm by thermal tensioning using a group of four rollers (flat rollers + vertical rollers) and compacted with magnesium oxide powder. Then, local medium-frequency heating (with internal H2 flow to prevent oxidation of the heating cable sheath) is used, and the diameter is further reduced to Φ26.8×2mm by thermal tensioning using two sets of vertical roller mills (flat rollers + vertical rollers). The compacted magnesium oxide powder density is 2.3g / cm³.3 The cable is cooled to below 150°C through a graphite cooling tunnel (with H2 flowing inside to prevent oxidation of the heating cable sheath and accelerate cooling), cleaned, and finally made into a single-sheath mineral-insulated heating cable with a diameter of Φ26.8×2mm-2500m, which is then coiled into a reel.

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

[0105] S21. Using molten iron as the main raw material, and based on the chemical composition of 310S stainless steel (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), the mixture undergoes solution treatment at 1030-1180℃, pickling, and then coiling. Finally, it is produced into coils with a wall thickness of 4mm and a length exceeding 400 meters. After inspection, it is coiled into Ф38.1×5mm 310S stainless steel coils using a tension coiling machine for pipe manufacturing.

[0106] S22. The prepared stainless steel coil is longitudinally cut into 139mm steel strips. According to the cable design length requirements, the ends of the steel strips are machined into 45° bevels. An I-bevel is made on the bevel, and multiple steel strips are sequentially welded up to 2500 meters long using gas metal arc welding (TIG) or laser welding. Before welding, the bevel is preheated under argon atmosphere protection at 200℃. During welding, the shielding gas is Ar (95%) + N2 (5%), 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℃, it is reheated to 980-1100℃, held for 2-3 minutes, and then purged with argon for cooling. After cooling, the outer sheath steel strip is obtained and rolled into a coil.

[0107] S23, such as Figure 3 and Figure 4As shown, the outer sheath steel strip obtained in step S22 is milled on both sides to create an I-shaped bevel. The steel strip is then continuously and stably coiled using a "W" forming method. Simultaneously, the single-sheath mineral-insulated heating cable obtained in step S1 is placed online into the coiled outer sheath steel strip using a "perforated flat roller power delivery" method (before the outer sheath is formed and joined). Both are simultaneously advanced to the welding station. After the steel strip is coiled, the bevel 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 then longitudinally welded using laser-TIG composite welding technology. During welding, the bevel is preheated to 150℃ to remove moisture or oil from the steel strip edges. The shielding gas is Ar (95%) + N2 (5%), with a gas flow rate of 22L / min. The TIG welding current is 180A, the welding voltage is 24V, the wire feed speed is 200mm / min, the laser defocusing distance is +3mm, the power is 1.5Kw, and the welding speed is 3m / min. After assembly using laser-TIG composite welding, the outer diameter of the heating cable is Ф44.5×4mm. The assembled heating cable (single-sheathed mineral-insulated heating cable + outer sheath) is then synchronously advanced to the next process.

[0108] S24. The heating cable assembled in step S23 is heated using a localized heating method (with internal H2 flow to prevent oxidation of the cable sheath during the heating process), and then its diameter is reduced to Φ40.5×3.2mm by thermal tension through a four-roll mill (four flat rolls + four vertical rolls arranged alternately), with the magnesium oxide powder compaction density being 2.4-2.6g / cm³. 3 ;

[0109] S25. After the initial diameter reduction and compaction, the heating cable undergoes a second round of continuous induction heating using a three-stage medium-frequency induction heating method: preheating, heating, and heat preservation. H2 flows internally through the heating device to prevent oxidation of the cable sheath. The cable is heated to 980-1100℃ and held for 1-3 minutes to refine the sheath's metallographic structure and eliminate work hardening. It is then cooled to below 150℃ through a graphite cooling tunnel (with internal H2 flow to prevent sheath oxidation and accelerate cooling). The cable is then subjected to a second cold sizing process using three sets of vertical rolling mills (3 horizontal rolls + 3 vertical rolls arranged alternately) to achieve the required outer diameter Ф38.1×5mm (2+3) and straightness. Finally, it is wound onto a cable-specific reel by a winding machine, resulting in a Ф38.1×5mm (2+3)-2500m double-sheathed mineral-insulated heating cable.

[0110] Testing revealed that this double-sheathed mineral-insulated heating cable possesses a tensile strength of no less than 650 MPa and an elongation of ≥25%, exhibiting excellent corrosion resistance and making it suitable for use in environments containing H2S, CO2, and Cl-. -This double-sheathed mineral-insulated heating cable is suitable for use in deep and ultra-deep wells. It has an operating temperature range of 350℃-750℃ and a service life of over 3 years.

[0111] Comparative Example 1

[0112] Bimetallic sheathed mineral-insulated heating cables were prepared according to the method of Example 1, except that all laser-TIG composite welding was replaced with argon arc welding.

[0113] Compared to laser-TIG hybrid welding, the welding speed is reduced by 80%. For sheaths thicker than 2mm without beveling, argon arc welding sometimes fails to achieve full penetration in a single pass. Simultaneously, the strength of thin-walled, thick-sheathed heating cables is reduced. Testing shows that the tensile strength of double-sheathed mineral-insulated heating cables (wall thickness less than 2mm) is ≤320MPa, under certain conditions of H2S, CO2, and Cl... - The cable has a service life of 1-3 months in wells and at 350℃, and the cable sheath is severely corroded.

[0114] Comparative Example 2

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

[0116] Step S1 involves preparing a single-sheathed mineral-insulated heating cable. If a low-carbon microalloyed material is used for the sheath, its corrosion resistance is poor, especially under certain conditions of H2S, CO2, and Cl. - The cable has a service life of 3-6 months in wells and at 350℃, and the cable sheath is severely corroded.

[0117] Step S1 involves preparing a single-sheathed mineral-insulated heating cable. If a corrosion-resistant alloy material is used for the sheath, the single-sheathed heating cable will have lower strength, especially in environments containing H2S, CO2, and Cl. - The service life is 10-12 months in wells and at 350℃. The cable sheath suffers from severe stress corrosion, and the manufacturing cost is more than 20% higher than that of double-sheathed heating cables.

[0118] The results above show that the double-sheathed mineral-insulated heating element prepared by the method of this invention has a service life of more than 3 years at 350℃-750℃.

[0119] This invention employs a laser welding-TIG composite welding technology. The electric arc heat source expands the heat application range, increases the amount of molten metal, improves weld quality, and increases weld penetration. Compared to argon arc welding, weld depression is reduced, and there are no obvious burrs on the inner wall of the welded tube. Thermal expansion and diameter reduction technology enhances the tightness of the fit between the inner and outer sheaths. Simultaneously, production efficiency is increased by more than 20% (laser welding-TIG composite welding technology has a welding speed of 1-3 m / min, while argon arc welding has a welding speed of 0.2-0.4 m / min). Through the combination of dual sheath materials (corrosion-resistant outer sheath and high-strength inner sheath), compared to single-sheath heating cables of the same wall thickness made of pure alloy, the double-sheath heating cable saves more alloy costs compared to a single-sheath heating cable of the same wall thickness, while meeting the required strength requirements, thus reducing manufacturing costs by approximately 30% or more.

[0120] The preferred embodiments of the present invention have been described in detail above; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combinations of various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

Claims

1. A method for preparing a double-sheathed mineral-insulated heating cable, characterized in that, The preparation method includes: (1) Mill the edges on both sides of the inner sheath steel strip, open I-type, V-type or X-type bevels, and form it using the W forming method; then use laser-TIG composite welding to longitudinally weld the formed steel strip to obtain the inner sheath tube. (2) The cable is assembled using a vertical built-in core and automatic filling of insulation material, and then undergoes a first treatment to obtain a single-sheath mineral-insulated heating cable, which is then coiled into a coil; the first treatment includes: thermal tension reduction of the assembled cable and compaction of the insulation material; after heat treatment, cold sizing and straightening are performed; (3) Mill the edges on both sides of the outer sheath steel strip and open I-type, V-type or X-type bevels. Use W forming method to form the outer sheath tube. Before the outer sheath tube is formed and closed, the single sheath mineral insulated heating cable is placed into the outer sheath tube online by using a perforated flat roller power delivery method. The two advance to the welding station simultaneously. Then, the formed steel strip is longitudinally welded by laser-TIG composite welding. After the second treatment, a double sheath mineral insulated heating cable is obtained and coiled into a coil. The second treatment includes: performing hot tension reduction treatment on the welded cable. After heat treatment, the cable is sized and straightened by secondary cold treatment.

2. The preparation method according to claim 1, wherein, The method for preparing the inner sheath steel strip includes: cutting the steel strip according to the design dimensions, processing the ends of the steel strip into 45° bevels, opening an I-shaped bevel on the bevels, and sequentially welding multiple steel strips to the design length of the double-sheathed mineral-insulated heating cable using non-consumable electrode gas shielded welding or laser welding; after the weld is cooled to ≤150°C, the weld is reheated to greater than 1000°C for heat preservation, and after being cooled by argon purging, it is rolled into a coil to obtain the inner sheath steel strip.

3. The preparation method according to claim 2, wherein, Before welding, the groove is preheated under an argon atmosphere; during welding, the shielding gas is a mixture of Ar and N2.

4. The preparation method according to claim 1, wherein, The assembly begins 2-3m after the welding point of the inner sheath tube.

5. The preparation method according to claim 1, wherein, The single-sheathed mineral-insulated heating cable includes a core, an insulating and heat-conducting layer, and a sheath layer.

6. A double-sheathed mineral-insulated heating cable prepared by the preparation method according to any one of claims 1-5, characterized in that, The double-sheathed mineral-insulated heating cable comprises, from the inside out, a core, an insulating and thermally conductive layer, an inner sheath, and an outer sheath. The inner sheath has a yield strength ≥205 MPa, tensile strength ≥520 MPa, elongation ≥35%, and hardness ≤32 HRC. The outer sheath has a yield strength ≥205 MPa, tensile strength ≥520 MPa, elongation ≥35%, and hardness ≤32 HRC. Furthermore, the outer sheath is 30%-50% thinner than the inner sheath, and the thermal conductivity of the inner sheath is not greater than that of the outer sheath, and not less than that of the insulating and thermally conductive layer.

7. The double-sheathed mineral-insulated heating cable according to claim 6, 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.

8. The double-sheathed mineral-insulated heating cable according to claim 7, wherein, The outer diameter of the inner sheath layer is 25.4-60.3 mm; the radial thickness is 2.5-4.4 mm; and the length is 2000-8000 m.

9. The double-sheathed mineral-insulated heating cable according to claim 6, wherein, The outer diameter of the outer sheath is 30-90 mm; the radial thickness is 2-7 mm; and the length is 1000-10000 m.

10. The double-sheathed mineral-insulated heating cable according to claim 9, wherein, The outer diameter of the outer sheath is 31.8-88.9 mm; the radial thickness is 2.5-6.5 mm; and the length is 2000-8000 m.

11. The double-sheathed mineral-insulated heating cable according to claim 6, wherein, The insulating and thermally conductive layer has a porosity of <5%, an insulation withstand voltage of >6000V, a thermal conductivity of >30W / mk, and a temperature resistance of 350℃-750℃. And / or, the radial thickness of the insulating thermally conductive layer is 2-5 mm.

12. The double-sheathed mineral-insulated heating cable according to claim 11, wherein, The insulating and thermally conductive layer is made of an insulating and thermally conductive material.

13. The double-sheathed mineral-insulated heating cable according to claim 12, wherein, The insulating and thermally conductive material is magnesium oxide, with a sphericity >50% and a purity >99%.

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

15. The double-sheathed mineral-insulated heating cable according to claim 14, wherein, The core is made of a high-resistance heating alloy.

16. The double-sheathed mineral-insulated heating cable according to claim 6, wherein, The tensile strength of the double-sheathed mineral-insulated heating cable is not less than 650 MPa and the elongation is ≥25%.

17. The double-sheathed mineral-insulated heating cable prepared by the preparation method according to any one of claims 1-5 or the double-sheathed mineral-insulated heating cable according to any one of claims 6-16 is used in deep wells and ultra-deep wells.

18. The application according to claim 17, wherein, The double-sheathed mineral-insulated heating cable has an operating temperature of 350℃-750℃ and a service life of more than 3 years.

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