Downhole liquid nitrogen electromagnetic heating device and downhole liquid nitrogen electromagnetic heating control method

Abstract

The application discloses a downhole liquid nitrogen electromagnetic heating device and a control method thereof, and belongs to the technical field of coalbed methane resource heat injection. In view of the problems of complex structure, high safety hidden danger and low heat conduction efficiency caused by the independent heating element of the existing downhole liquid nitrogen heating technology, the technical scheme of combining the pressure-bearing function of the conveying pipeline with the electromagnetic induction heating function is provided. The device comprises a liquid nitrogen heating well, a nitrogen conveying pipeline and an electromagnetic heating system sleeved on the outer wall of the nitrogen conveying pipeline. The electromagnetic heating system generates an alternating magnetic field to make the nitrogen conveying pipeline wall generate eddy current and spontaneously heat, so as to indirectly heat the liquid nitrogen in the pipeline by taking the pipeline wall as a heating body. A grading PID power regulation strategy is adopted. The heat efficiency of the device can reach more than 90%, the heating speed is increased by 30-50% compared with the traditional resistance heating, the safety hidden danger of exposed high-temperature electric heating wire is eliminated, the downhole equipment structure is simplified, and the device is suitable for large-scale nitrogen injection operation for coalbed methane exploitation.

Description

Technical Field

[0001] This invention relates to the field of thermal extraction technology for coalbed methane resources, specifically to an underground liquid nitrogen electromagnetic heating device and its control method. Background Technology

[0002] Thermal nitrogen injection technology is essentially a physical or chemical enhancement process based on a hot nitrogen medium. Due to its high safety, environmental friendliness, and strong adaptability, it has been widely applied in various fields such as oil and gas extraction and industrial equipment maintenance. Among these applications, thermal nitrogen injection is a key auxiliary technology in coalbed methane development, often used in conjunction with fracturing technology to address the problem of poor coalbed methane extraction efficiency in my country, and it has significant engineering value in promoting the efficient development of coalbed methane.

[0003] While the traditional method of generating nitrogen from the ground, heating it, and then transporting it to the target formation via pipeline is convenient, it suffers from significant heat and pressure losses during transport, low mass flow rate, and high operating costs, making it unsuitable for long-distance heated nitrogen injection. To address these issues, downhole heating can be employed.

[0004] In the prior art, patent document CN104775801A discloses a vacuum spiral tube nitrogen heater for in-situ underground conversion of oil shale, which uses combustion heat exchange to heat room temperature nitrogen (gaseous state) for in-situ extraction of oil shale layers. This technology is not designed for heating liquid nitrogen and has issues with combustion safety and limited thermal efficiency.

[0005] Patent document CN218671600U discloses a liquid nitrogen heating system and a liquid nitrogen pump truck, which uses a boiler to burn and heat the circulating medium (water or antifreeze), and then indirectly heats the liquid nitrogen through a water bath evaporator. Although it improves the heat conduction efficiency compared with the traditional direct-fired evaporator, it still belongs to the technical route of heating liquid nitrogen through multiple intermediate media by an independent heat source. It is bulky and cannot achieve in-situ preparation of hot nitrogen at the target layer.

[0006] Furthermore, the technologies disclosed in patent documents CN117189115A and CN107762473A both utilize the low-temperature characteristics (-196°C) of liquid nitrogen to freeze and fracture coal seams. Liquid nitrogen is used as a cold source rather than a heated object, and its technical purpose is completely different from that of thermal nitrogen injection mining.

[0007] The aforementioned existing technologies involving liquid nitrogen heating (such as water bath heating and resistance heating represented by CN218671600U) use independent heating elements (such as heating wires and heating furnaces) to heat liquid nitrogen through multi-stage heat conduction, which has the following fundamental defects: 1. The drawbacks of high heat loss and low thermal conductivity. Existing technologies use independent heating elements to generate heat, which then needs to be conducted through multiple stages of intermediate media (such as heating liquid, insulation layer, and pipe wall) to reach liquid nitrogen. This results in high thermal resistance, high energy loss, difficulty in improving thermal efficiency, and limited heating speed.

[0008] 2. High safety risks and structural complexity. Independent heating elements (such as exposed high-temperature heating wires and open flame burners) are susceptible to oxidation, melting, short circuits, or explosions. Furthermore, additional supporting structures, insulation protection structures, and intermediate heat exchange devices are required, resulting in complex equipment structures and making in-situ preparation of hot nitrogen at the target layer impossible.

[0009] 3. The defect of separating heating and insulation functions. Existing technology does not consider the different heat flux density requirements of the liquid nitrogen delivery section, heating section, and hot nitrogen output section, and mostly adopts a uniform insulation design, resulting in poor insulation effect or material waste. In addition, the heating section does not form an integrated space to accommodate the heating equipment, and fails to achieve synergy between insulation function and equipment installation.

[0010] 4. Insufficient precision in liquid nitrogen phase change control. Existing technologies do not have specific control strategies designed for the large hysteresis and nonlinear characteristics of the liquid nitrogen phase change (liquid to gas) process, which can easily lead to temperature overshoot or control oscillation. It is difficult to balance rapid heating and precise temperature control, thus affecting heating quality and safety.

[0011] 5. Limitations in heating scale and insufficient system reliability. Existing technologies mostly adopt an integrated design, which makes it difficult to flexibly adjust the heating scale according to changes in well depth and nitrogen injection volume. Furthermore, single-point failures can easily lead to overall shutdown. Modular redundancy design cannot improve system reliability, and large-scale nitrogen injection is also difficult to achieve.

[0012] 6. Insufficient adaptability to extremely low temperature environments. Existing technologies have not adapted materials for the extreme conditions of liquid nitrogen at -196°C. Conventional materials may become brittle, experience a decrease in magnetic properties, or fail mechanically at extremely low temperatures, making it difficult for the heating system to operate stably in extreme environments.

[0013] 7. Defects of equipment interference and safety hazards. Existing technology places heating equipment and extraction equipment in the same wellbore or in a similar space. The heating operation and coalbed methane extraction operation interfere with each other, which poses risks of mechanical damage and safety hazards, and lacks independent space for equipment installation and maintenance.

[0014] In this field, when introducing downhole heating technology to replace surface heating, the technical approach is to use new technologies (downhole combustion heat exchange, water bath heating, resistance heating, etc.) to implement the old method (the independent heat source approach of surface heating). Heat is generated through independent heating elements and then transferred to liquid nitrogen through multi-stage heat conduction (such as heating wire → insulation layer → pipe wall → liquid nitrogen, or combustion → heating liquid → liquid nitrogen). The heating purpose is achieved by utilizing the correlation between "independent heat source - heat conduction - liquid nitrogen heating". However, this approach fails to break through the traditional separate thinking of "independent heat source heating container", does not recognize that electromagnetic induction can be used to heat the delivery pipeline itself to indirectly heat the liquid nitrogen, and does not consider the coordinated optimization of insulation design with equipment installation space, control strategy and phase change characteristics, system layout and modular expansion.

[0015] Existing technologies focus on the thermal conduction relationship between "heat generation by independent heating elements" and "liquid nitrogen heating". However, due to the technical understanding that "liquid nitrogen is an extremely low temperature, non-conductive, and non-magnetic medium that cannot be directly heated by an electromagnetic field", electromagnetic induction heating paths are generally not considered, and systematic adaptations have not been made to the physical characteristics of the liquid nitrogen phase change process and the material characteristics of the extremely low temperature environment.

[0016] Correspondingly, this patent application opens up a technical route for indirect electromagnetic induction heating using the pipe wall of a nitrogen pipeline as the heating element. It adapts the energy conversion mechanism of the "electromagnetic induction eddy current thermal effect"—the physical phenomenon of eddy current self-heating in austenitic stainless steel pipe wall under extremely low temperature conditions—to electromagnetic induction technology. At the same time, it achieves the integration of pressure bearing and heating functions through structural innovation, the synergy of thermal and structural functions through segmented insulation, the precise control of the phase change process through graded PID, the flexible expansion of the system through modular design, the reliable operation in extremely low temperature environments through material adaptation, and the physical isolation of the equipment through independent wellbores.

[0017] Therefore, there is an urgent need for a technology that combines the pressure-bearing function of downhole pipelines with the electromagnetic induction heating function to indirectly heat the fluid inside the pipeline. This technology would overcome the shortcomings of existing technologies, such as complex structure, high safety risks, and low heat conduction efficiency caused by the use of independent heating elements. It would also address the technical deficiencies in areas such as insulation and structural coordination, precise phase change control, modular system expansion, adaptation to extremely low temperature environments, and equipment isolation. Summary of the Invention

[0018] The purpose of this invention is to provide a technology that combines the pressure-bearing function of an underground pipeline with the electromagnetic induction heating function to indirectly heat the fluid inside the pipeline, thereby overcoming the defects of the prior art caused by the use of independent heating elements, such as complex structure, high safety risks and low heat conduction efficiency.

[0019] To achieve the above objectives, the present invention provides a downhole liquid nitrogen electromagnetic heating device, comprising: The liquid nitrogen heated well (103) is located within the target formation (107); A nitrogen delivery pipeline (108) is installed inside the liquid nitrogen heating well (103) for transporting liquid nitrogen; An electromagnetic heating system (208) is fitted onto the outer wall of the nitrogen delivery pipe (108). When the electromagnetic heating system (208) is energized, it generates an alternating magnetic field that causes eddy currents in the pipe wall of the nitrogen delivery pipe (108) to generate self-heating. The pipe wall is used as a heating element to indirectly heat the liquid nitrogen inside the pipe, thus achieving a combination of pressure bearing and heating functions. A flow regulating device is installed on the nitrogen delivery pipeline (108), including an inlet valve (104) and an outlet valve (106), for regulating the liquid nitrogen flow rate and preventing pipeline overpressure damage.

[0020] The electromagnetic heating system (208) includes an electromagnetic heating coil (105), a high permeability magnetic core (210), and a coil heat insulation protection layer (211). The magnetic core (210) is made of iron-silicon-aluminum soft magnetic alloy, manganese-zinc ferrite, or permalloy. The coil heat insulation protection layer (211) is located outside the electromagnetic heating coil (105).

[0021] The nitrogen transport pipeline (108) is made of austenitic stainless steel, which has an impact toughness of ≥100J at -196°C.

[0022] The outer wall of the nitrogen delivery pipeline (108) is fitted with a pipeline insulation layer (102). The pipeline insulation layer (102) is divided into sections with different thicknesses according to the heat flux density. The thickness of the heating section is greater than the thickness of the liquid nitrogen delivery section and the hot nitrogen output section. The thickened part of the heating section forms an installation space to accommodate the electromagnetic heating system (208).

[0023] The thickness of the liquid nitrogen delivery section and the hot nitrogen output section is 50–80 mm, and the thickness of the heating section is 80–120 mm.

[0024] It also includes a ground control system (109), which is electrically connected to the electromagnetic heating system (208) via a cable (101). The ground control system (109) includes a power supply system (201), a rectifier module (202), a filter module (203), an inverter module (204), a PLC host (205), a drive module (206), and a temperature and pressure detection module (207).

[0025] The ground control system (109) dynamically adjusts the output power of the electromagnetic heating system (208) based on the temperature feedback from the outlet temperature and pressure sensor (112) using a graded PID power control strategy oriented towards the phase change characteristics of liquid nitrogen. The power is adjusted in four grades according to the deviation between the actual temperature and the set temperature: full power heating when the temperature is below 75%, power reduction to 70%–80% when the temperature is between 75% and 97.5%, fine adjustment of 5%–10% when the temperature is between 97.5% and 102.5%, and main heating is paused and 1%–2% power is maintained for antifreeze when the temperature is above 102.5%.

[0026] The electromagnetic heating systems (208) are arranged in a modular manner with fixed spacing, and each electromagnetic heating system (208) is independently powered and controlled.

[0027] This invention also discloses a downhole liquid nitrogen electromagnetic heating control method, which uses the above-mentioned downhole liquid nitrogen electromagnetic heating device and includes the following steps: Obtain the feedback signal from the outlet temperature and pressure sensor (112); Calculate the deviation between the feedback signal and the set temperature; Based on the deviation value, the output power of the electromagnetic heating system (208) is dynamically adjusted using a graded PID power control strategy; The tiered PID power control strategy includes: When the actual temperature is 75% lower than the set temperature, the electromagnetic heating system (208) is controlled to operate at full power; When the actual temperature is greater than or equal to 75% of the set temperature and less than 97.5%, the electromagnetic heating system (208) is controlled to operate at 70%–80% of the rated power; When the actual temperature is greater than or equal to 97.5% and less than or equal to 102.5% of the set temperature, the electromagnetic heating system (208) is controlled to operate at 5%–10% of the rated power. When the actual temperature is 102.5% higher than the set temperature, the electromagnetic heating system (208) is controlled to suspend the main heating and operate at 1%–2% of the rated power to prevent the pipe from freezing.

[0028] The parameters of the graded PID power control strategy are configured as follows: proportional coefficient P = 0.3–0.8, integral time I = 10–30s, and derivative time D = 2–5s.

[0029] The present invention has the following advantages: I. Advantages of integrated structure combining pressure bearing and heat generation functions and safety assurance.

[0030] This invention combines the pressure-bearing function of the nitrogen pipeline (108) with the electromagnetic induction heating function. It uses electromagnetic induction to make the pipe wall self-heat and indirectly heat the liquid nitrogen through heat conduction. This breaks through the limitations of traditional technology that relies on independent heating elements, eliminates the safety hazards of exposed high-temperature heating wires, simplifies the structure of downhole equipment, and reduces the number of failure points.

[0031] Conventional heating device design follows the separate thinking of "heat source heating container". In contrast, the "pressure-bearing pipeline" is also used as an "electromagnetic induction heating element" and applied to the heating of extremely low temperature fluids. This requires breaking through the conventional understanding that pipelines are only pressure-bearing transport components, which is an innovative development in the application of electromagnetic heating technology.

[0032] II. Advantages of optimized heat conduction path and improved energy efficiency.

[0033] This invention employs electromagnetic induction heating, using the tube wall as the heating element. There is no intermediate medium for heat conduction loss, resulting in a thermal efficiency of over 90%. The heating speed is 30-50% faster than traditional resistance heating. This "endogenous heat conduction" mechanism (direct heating of the tube wall → liquid nitrogen) replaces the traditional "exogenous heat conduction" (heating wire → insulation layer → tube wall → liquid nitrogen), eliminating multiple stages of thermal resistance.

[0034] When applying electromagnetic heating to cryogenic liquid nitrogen scenarios, the conventional approach focuses on how to directly heat the liquid nitrogen or use an intermediate medium to transfer heat, rather than utilizing the self-heating of the pipe's metal wall and direct conduction; such a substantial change in the heat transfer path (removing the intermediate medium) is not easy to conceive.

[0035] III. Advantages of segmented differentiated insulation and multi-functional synergy.

[0036] The present invention adopts a segmented differentiated heat preservation design. The thickness of the liquid nitrogen conveying section and the hot nitrogen output section is 50-80mm, and the thickness of the heating section is 80-120mm. This not only realizes the functional optimization of preventing vaporization, reducing heat loss and preserving temperature, but also forms an installation space to accommodate the electromagnetic heating system (208) in the thickened part of the heating section, realizing the structural synergy of heat preservation function and equipment integration.

[0037] The design of insulation layer thickness is usually based solely on heat flux density calculations. However, this invention combines thickness design with the physical installation space requirements of the electromagnetic heating system, enabling the insulation layer to have both thermal and structural support functions. This is a collaborative optimization design for engineering constraints with limited downhole space.

[0038] IV. Advantages of precise PID control with phase change process stability.

[0039] This invention employs a segmented PID power control strategy tailored to the phase change characteristics of liquid nitrogen. The power is dynamically adjusted in four levels based on the temperature deviation. Combined with the rapid response characteristics of electromagnetic heating, this invention achieves precise control of the liquid nitrogen phase change process, effectively avoiding vaporization or overheating, and overcoming the control difficulties of large hysteresis and nonlinearity in the liquid nitrogen phase change process.

[0040] The reason that is not easily thought of by those skilled in the art is that designing specific threshold values ​​(75%, 97.5%, 102.5%) to match the specific physical characteristics of liquid nitrogen phase change (intense endothermic process of liquid to gas at -196°C) and matching them with the instantaneous response characteristics of electromagnetic heating requires control parameter adaptation based on specific process characteristics, which cannot be covered by general PID parameter tuning.

[0041] V. Advantages of Modular Independent Power Supply and System Expansion This invention adopts a modular independent power supply layout, which supports flexible expansion of the heat injection scale according to the well depth and nitrogen injection volume. Each module is independently powered and controlled, realizing flexible expansion and redundancy backup of the system, and improving the system reliability and adaptability.

[0042] Reasons that are not easily thought of by those skilled in the art: Although modular layout is a conventional method in electrical engineering, in the case of downhole electromagnetic heating, it is necessary to solve specific engineering problems such as electrical insulation in extremely low temperature environments, thermal interference between modules, and limited downhole maintenance space. Adapting the conventional modular concept to the downhole liquid nitrogen heating condition requires specific engineering considerations.

[0043] VI. Advantages of Synergistic Adaptation of Materials in Extremely Low Temperature Environments This invention selects austenitic stainless steel as the nitrogen delivery pipe (108) (impact toughness ≥100J at -196°C), silver-plated copper alloy as the electromagnetic heating coil (105), and iron-silicon-aluminum soft magnetic alloy as the magnetic core (210), ensuring the reliable operation of the system in extremely low temperature environments.

[0044] The reason that is not easily thought of by those skilled in the art is that conventional material selection often considers low-temperature resistance (mechanical) or electromagnetic properties (magnetic permeability) separately. However, this invention achieves a synergistic matching of material functions of "low-temperature mechanical properties + eddy current heating characteristics + magnetic stability" for the specific working conditions of liquid nitrogen. This requires adaptation design for specific working conditions based on the understanding of materials science. Attached Figure Description

[0045] Figure 1 A schematic diagram of the overall structure of the downhole liquid nitrogen electromagnetic heating device; Figure 2 This is a block diagram illustrating the principles of the ground control system and the electromagnetic heating system.

[0046] In the diagram: 101. Cable; 102. Pipe insulation layer; 103. Liquid nitrogen heating well; 104. Inlet valve; 105. Electromagnetic heating coil; 106. Outlet valve; 107. Target formation; 108. Nitrogen pipeline; 109. Ground control system; 110. Liquid nitrogen input interface; 111. Inlet temperature and pressure sensor; 112. Outlet temperature and pressure sensor; 113. Hot nitrogen output interface; 201. Power supply system; 202. Rectifier module; 203. Filter module; 204. Inverter module; 205. PLC host; 206. Drive module; 207. Temperature and pressure detection module; 208. Electromagnetic heating system; 210. Magnetic core; 211. Coil heat insulation protection layer; 212. PID algorithm module. Detailed Implementation

[0047] like Figures 1 to 2 As shown, the downhole liquid nitrogen electromagnetic heating device of the present invention mainly includes a liquid nitrogen heating well 103, a nitrogen delivery pipeline 108, an electromagnetic heating system 208, a surface control system 109, and related auxiliary devices.

[0048] A liquid nitrogen heating well 103 is drilled separately next to the production well. The liquid nitrogen heating well 103 is located in the target formation 107 and is connected to the production well through directional drilling. The bottom end of the liquid nitrogen heating well 103 is connected to the production well, which physically isolates the electromagnetic heating equipment from the production equipment to avoid mutual interference.

[0049] A nitrogen delivery pipeline 108 is installed inside the liquid nitrogen heating well 103. The nitrogen delivery pipeline 108 is made of austenitic stainless steel, with an impact toughness ≥100J at -196°C, ensuring pressure resistance and resistance to brittle fracture under extremely low temperature conditions. The nitrogen delivery pipeline 108 is equipped with a liquid nitrogen inlet 110 and a hot nitrogen outlet 113, located at the inlet and outlet ends of the liquid nitrogen heating well 103, respectively. A liquid nitrogen storage tank (not shown) is a conventional technical device; it is connected to the nitrogen delivery pipeline 108 via the liquid nitrogen inlet 110 to supply liquid nitrogen during use and disconnected when not in use; the liquid nitrogen storage tank is located on the ground next to the liquid nitrogen heating well 103.

[0050] The nitrogen delivery pipeline 108 is externally fitted with a pipeline insulation layer 102. The pipeline insulation layer 102 uses a wide-temperature-range insulation material, typically a nano-aerogel composite felt or a vacuum multilayer insulation board, with a thermal conductivity ≤0.02W / (m·K) and an applicable temperature range of -200°C to +250°C. The pipeline insulation layer 102 has differentiated thicknesses in sections according to heat flux density: the thickness of the liquid nitrogen delivery section and the hot nitrogen output section is 50–80mm, mainly to prevent liquid nitrogen vaporization and reduce heat loss from hot nitrogen; the thickness of the heating section is 80–120mm. This thickened portion not only reduces heat loss to the wellbore but, more importantly, creates installation space to accommodate the electromagnetic heating system 208, achieving structural synergy between insulation function and equipment integration. The heating section corresponds to the nitrogen delivery pipeline section between the inlet valve 104 and the outlet valve 106.

[0051] A flow regulation device is installed on the nitrogen pipeline 108, including an inlet valve 104, an outlet valve 106, and other flow control valves that work together, to regulate the liquid nitrogen flow rate and prevent the rapid expansion of liquid nitrogen from damaging the pipeline.

[0052] The electromagnetic heating system 208 is fitted onto the outer wall of the nitrogen supply pipeline 108, and its coverage length is equal to the pipeline length between the inlet valve 104 and the outlet valve 106. The electromagnetic heating system 208 is integrated from an electromagnetic heating coil 105, a high-permeability magnetic core 210, and a coil heat insulation protection layer 211.

[0053] The electromagnetic heating coil 105 is made of silver-plated copper alloy to reduce resistance loss and adapt to high-frequency operating conditions. The magnetic core 210 is made of iron-silicon-aluminum (FeSiAl) soft magnetic alloy, manganese-zinc ferrite, or permalloy to enhance the alternating magnetic field strength and ensure the stability of magnetic performance at extremely low temperatures of -196°C. The coil thermal insulation layer 211 is located outside the electromagnetic heating coil 105 and uses a ceramic fiber braided sleeve with a long-term operating temperature ≥300°C. It protects the coil from high-temperature damage and reduces heat loss to the wellbore.

[0054] The electromagnetic heating system 208 is arranged in a modular manner with fixed spacing. Each electromagnetic heating system 208 is independently powered and controlled, and supports flexible expansion of the heat injection scale according to the well depth and nitrogen injection volume.

[0055] The electromagnetic heating system 208 works as follows: when energized, it generates an alternating magnetic field (enhanced by the magnetic core 210), causing eddy currents in the metal wall of the nitrogen delivery pipe 108, which then generates heat. Using the pipe wall as the heating element, it indirectly heats the liquid nitrogen inside the pipe through heat conduction, thus combining pressure bearing and heating functions. This "endogenous heating" mechanism eliminates the multi-stage heat conduction process of heating wire → insulation layer → pipe wall in traditional resistance heating, achieving a thermal efficiency of over 90%.

[0056] A surface control system 109 is installed on the ground next to the liquid nitrogen heating well 103. The surface control system 109 is connected to various devices downhole (electromagnetic heating coil 105, inlet valve 104, outlet valve 106, inlet temperature and pressure sensor 111, outlet temperature and pressure sensor 112) via cable 101 to achieve unified control of power supply and signal transmission.

[0057] The ground control system 109 integrates a power supply system 201, a rectifier module 202, a filter module 203, and an inverter module 204 to convert industrial frequency AC power into DC power and high-frequency AC power. This high-frequency AC power is used to excite the electromagnetic heating coil 105 to generate an alternating magnetic field. The ground control system 109 also includes a PLC host 205, a drive module 206, a temperature and pressure detection module 207, and a PID algorithm module 212. The drive module 206 amplifies the PLC signal and controls the operating frequency and duty cycle of the inverter module 204. The temperature and pressure detection module 207 receives detection signals from the inlet temperature and pressure sensor 111 and the outlet temperature and pressure sensor 112.

[0058] An inlet temperature and pressure sensor 111 and an outlet temperature and pressure sensor 112 are respectively installed at the inlet and outlet of the heating device to monitor the temperature and pressure of liquid nitrogen and hot nitrogen flowing into and out of the heating device in real time.

[0059] The ground control system 109 dynamically adjusts the output power of the electromagnetic heating system 208 based on the temperature feedback from the outlet temperature and pressure sensor 112, employing a graded PID power control strategy tailored to the liquid nitrogen phase change characteristics. The graded PID power control strategy adjusts the power in four levels according to the deviation between the actual temperature and the set temperature: In the first setting, when the actual temperature is 75% lower than the set temperature, the electromagnetic heating system 208 is controlled to operate at full power to achieve rapid heating. In the second setting, when the actual temperature is greater than or equal to 75% of the set temperature but less than 97.5%, the electromagnetic heating system 208 is controlled to operate at 70%–80% of its rated power (including both values, the same below), reducing the heating rate. In the third setting, when the actual temperature is greater than or equal to 97.5% of the set temperature and less than or equal to 102.5%, the electromagnetic heating system 208 is controlled to operate at 5%–10% of the rated power for fine-tuning. In the fourth setting, when the actual temperature is 102.5% higher than the set temperature, the electromagnetic heating system 208 is controlled to suspend the main heating and operate at 1%–2% of the rated power to prevent the pipes from freezing.

[0060] The parameter configuration of the tiered PID power control strategy is as follows: proportional coefficient P = 0.3–0.8, integral time I = 10–30 s, and derivative time D = 2–5 s. These PID parameters are suitable for liquid nitrogen phase change heating control scenarios with large time lag and nonlinear thermal inertia characteristics, and are compatible with conventional nitrogen injection mining scenarios of 5–20 m³ / h.

[0061] The above-mentioned graded PID power control strategy is designed based on the large hysteresis and nonlinear characteristics of the liquid nitrogen phase change process. The temperature control is divided into a rapid heating zone, a transition adjustment zone, a phase change critical fine-tuning zone, and an overheat protection zone. Each zone adopts a different power adjustment range (full power, medium-high power, low power, anti-freeze maintenance) and is adapted to the rapid response characteristics of the electromagnetic heating system (208). The above design idea is unique to this invention and can realize a smooth switching and stable control from rapid heating to precise temperature control, effectively avoiding temperature overshoot or control oscillation during the liquid nitrogen phase change process.

[0062] The following is in conjunction with the appendix Figure 1 To be continued Figure 2 The working process of this invention is described in detail below: 1. Liquid nitrogen transport and pre-insulation stage.

[0063] Liquid nitrogen enters the nitrogen transport pipeline (108) from the liquid nitrogen storage tank located on the ground through the liquid nitrogen input interface (110), and first flows through the liquid nitrogen transport section. In this stage, the pipeline insulation layer (102) adopts a thickness of 50-80mm. Compared with the thickened design of the heating section, this thickness configuration effectively prevents the liquid nitrogen from vaporizing prematurely during the transport process, while optimizing material costs and well space occupation. More importantly, this segmented differentiated insulation strategy reserves a space for the subsequent structural thickening of the heating section, realizing the unity of functional zoning and structural adaptation.

[0064] 2. Electromagnetic induction heating stage.

[0065] When liquid nitrogen flows into the heating section, the ground control system (109) starts the heating program. The power supply system (201), rectifier module (202), filter module (203) and inverter module (204) work together to convert the power frequency AC to high frequency AC, which is then amplified by the drive module (206) and sent to the electromagnetic heating coil (105). Under the magnetic focusing effect of the high permeability magnetic core (210), a high-intensity alternating magnetic field is generated; this magnetic field penetrates the coil heat insulation protection layer (211), which is made of ceramic fiber braided sleeve, which protects the coil from high temperature damage and reduces heat loss to the well environment, and directly acts on the austenitic stainless steel pipe wall of the nitrogen delivery pipeline (108).

[0066] Based on the physical principle of electromagnetic induction eddy current effect, the pipe wall of the nitrogen transport pipeline (108) generates eddy currents by cutting alternating magnetic field lines, thus generating heat. This design innovates the traditional external heat conduction, i.e., the multi-stage thermal resistance loss path of heating wire, insulation layer, pipe wall, and liquid nitrogen, into an endogenous heat conduction mechanism: the pipe wall itself becomes the heating element, and heat is directly generated on the metal surface in contact with the liquid nitrogen, indirectly heating the liquid nitrogen inside the pipe through heat conduction. This structural innovation that combines pressure bearing and heating functions completely eliminates the risks of oxidation, melting, and exposed high temperature hazards of independent heating elements in traditional resistance heating. At the same time, because the intermediate heat transfer medium is eliminated, the thermal efficiency can reach more than 90%, and the heating speed is increased by 30-50% compared with traditional resistance heating, achieving a triple technological breakthrough of high efficiency, safety, and structural simplification.

[0067] 3. Stage of graded PID intelligent control.

[0068] During this process, the inlet temperature and pressure sensor (111) and the outlet temperature and pressure sensor (112) collect the temperature and pressure parameters of liquid nitrogen in real time, and the signals are transmitted to the temperature and pressure detection module (207) of the ground control system (109) via cable (101). The PLC host (205) calls the PID algorithm module (212) to perform calculations based on the graded PID power control strategy for liquid nitrogen phase change characteristics, making full use of the instantaneous response advantage of electromagnetic heating compared with traditional resistance heating, and overcoming the control difficulties of large lag and nonlinearity in the liquid nitrogen phase change process: The rapid heating mode controls the electromagnetic heating system (208) to operate at full power when the actual temperature is 75% lower than the set temperature, utilizing the rapid response characteristics of electromagnetic heating to quickly raise the temperature; the power reduction mode automatically reduces the rated power to 70-80% when the temperature enters the 75%–97.5% range of the set temperature to prevent temperature overshoot; the fine-tuning mode switches to 5-10% of the rated power for fine-tuning when the temperature is in the phase change critical zone of 97.5-102.5% to achieve precise constant temperature control; the antifreeze protection mode immediately suspends the main heating and maintains 1-2% of the antifreeze power if the temperature abnormally exceeds 102.5% of the set value to prevent damage to the pipeline due to freezing at extremely low temperatures.

[0069] 4. Heat injection and output stage.

[0070] The heated nitrogen flows through the hot nitrogen output section, which uses a 50–80 mm thick pipe insulation layer (102) to effectively prevent heat loss and ensure that the nitrogen temperature reaching the production well meets the injection requirements. The inlet valve (104) and outlet valve (106) on the nitrogen delivery pipeline (108) dynamically adjust the liquid nitrogen flow rate according to temperature and pressure feedback to prevent pipeline overpressure damage caused by rapid expansion of liquid nitrogen. Finally, the high-temperature and high-pressure hot nitrogen is injected into the production well from the hot nitrogen output interface (113) to complete the injection operation into the target formation (107).

[0071] 5. Modular expansion and engineering implementation.

[0072] Thanks to the modular and independent power supply design of the electromagnetic heating system (208), operators can add heating modules at fixed intervals according to the well depth and nitrogen injection volume requirements. Each module operates independently without affecting the others, which not only realizes the flexible expansion of the system, but also improves the overall reliability through redundant design. At the same time, the independently arranged liquid nitrogen heating well (103) structure is connected to the production well through directional drilling and is connected at the bottom, which physically isolates the heating equipment from the production equipment, completely avoiding mutual interference between the heat injection operation and the coalbed methane production operation, and providing an independent engineering space for the safe implementation of the main heating technology.

[0073] The above embodiments are for illustrative purposes only and are not intended to limit the technical solutions of the present invention. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the present invention without departing from the spirit and scope of the present invention. Any modifications or partial substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A downhole liquid nitrogen electromagnetic heating device, characterized in that: include: The liquid nitrogen heated well (103) is located within the target formation (107); A nitrogen delivery pipeline (108) is installed inside the liquid nitrogen heating well (103) for transporting liquid nitrogen; An electromagnetic heating system (208) is fitted onto the outer wall of the nitrogen delivery pipe (108). When the electromagnetic heating system (208) is energized, it generates an alternating magnetic field that causes eddy currents in the pipe wall of the nitrogen delivery pipe (108) to generate self-heating. The pipe wall is used as a heating element to indirectly heat the liquid nitrogen inside the pipe, thus achieving a combination of pressure bearing and heating functions. A flow regulating device is installed on the nitrogen delivery pipeline (108), including an inlet valve (104) and an outlet valve (106), for regulating the liquid nitrogen flow rate and preventing pipeline overpressure damage.

2. The downhole liquid nitrogen electromagnetic heating device according to claim 1, characterized in that: The electromagnetic heating system (208) includes an electromagnetic heating coil (105), a high permeability magnetic core (210), and a coil heat insulation protection layer (211). The magnetic core (210) is made of iron-silicon-aluminum soft magnetic alloy, manganese-zinc ferrite, or permalloy. The coil heat insulation protection layer (211) is located outside the electromagnetic heating coil (105).

3. The downhole liquid nitrogen electromagnetic heating device according to claim 1 or 2, characterized in that: The nitrogen transport pipeline (108) is made of austenitic stainless steel, which has an impact toughness of ≥100J at -196°C.

4. The downhole liquid nitrogen electromagnetic heating device according to claim 1, characterized in that: The outer wall of the nitrogen delivery pipeline (108) is fitted with a pipeline insulation layer (102). The pipeline insulation layer (102) is divided into sections with different thicknesses according to the heat flux density. The thickness of the heating section is greater than the thickness of the liquid nitrogen delivery section and the hot nitrogen output section. The thickened part of the heating section forms an installation space to accommodate the electromagnetic heating system (208).

5. The downhole liquid nitrogen electromagnetic heating device according to claim 4, characterized in that: The thickness of the liquid nitrogen delivery section and the hot nitrogen output section is 50–80 mm, and the thickness of the heating section is 80–120 mm.

6. The downhole liquid nitrogen electromagnetic heating device according to claim 1, characterized in that: It also includes a ground control system (109), which is electrically connected to the electromagnetic heating system (208) via a cable (101). The ground control system (109) includes a power supply system (201), a rectifier module (202), a filter module (203), an inverter module (204), a PLC host (205), a drive module (206), and a temperature and pressure detection module (207).

7. The downhole liquid nitrogen electromagnetic heating device according to claim 6, characterized in that: The ground control system (109) dynamically adjusts the output power of the electromagnetic heating system (208) based on the temperature feedback from the outlet temperature and pressure sensor (112) using a graded PID power control strategy oriented towards the phase change characteristics of liquid nitrogen. The power is adjusted in four grades according to the deviation between the actual temperature and the set temperature: full power heating when the temperature is below 75%, power reduction to 70%–80% when the temperature is between 75% and 97.5%, fine adjustment of 5%–10% when the temperature is between 97.5% and 102.5%, and main heating is paused and 1%–2% power is maintained for antifreeze when the temperature is above 102.5%.

8. The downhole liquid nitrogen electromagnetic heating device according to claim 1, characterized in that: The electromagnetic heating systems (208) are arranged in a modular manner with fixed spacing, and each electromagnetic heating system (208) is independently powered and controlled.

9. A method for controlling downhole liquid nitrogen electromagnetic heating, using the downhole liquid nitrogen electromagnetic heating device according to any one of claims 1 to 8, characterized in that: include: Obtain the feedback signal from the outlet temperature and pressure sensor (112); Calculate the deviation between the feedback signal and the set temperature; Based on the deviation value, the output power of the electromagnetic heating system (208) is dynamically adjusted using a graded PID power control strategy; The tiered PID power control strategy includes: When the actual temperature is 75% lower than the set temperature, the electromagnetic heating system (208) is controlled to operate at full power; When the actual temperature is greater than or equal to 75% of the set temperature and less than 97.5%, the electromagnetic heating system (208) is controlled to operate at 70%–80% of the rated power; When the actual temperature is greater than or equal to 97.5% and less than or equal to 102.5% of the set temperature, the electromagnetic heating system (208) is controlled to operate at 5%–10% of the rated power. When the actual temperature is 102.5% higher than the set temperature, the electromagnetic heating system (208) is controlled to suspend the main heating and operate at 1%–2% of the rated power to prevent the pipe from freezing.

10. The method according to claim 9, characterized in that: The parameters of the graded PID power control strategy are configured as follows: proportional coefficient P = 0.3–0.8, integral time I = 10–30s, and derivative time D = 2–5s.

Images