Segmented middle-deep geothermal heat exchanger gradient sealing connection structure and method

Abstract

The present application relates to geothermal energy development technical field, especially segment type middle deep layer geothermal heat exchanger gradient sealing connection structure and method, including: at least two heat exchange pipe sections, heat exchange pipe section includes inner tube and outer sleeve, be used for connecting two adjacent heat exchange pipe sections to form complete wellbore gradient sealing connection assembly, including one coaxial sleeve is set to the sealing sleeve of two heat exchange pipe sections butt joint end outside, and multiple dynamic sealing mechanism is arranged between sealing sleeve and heat exchange pipe section, it is configured as: when the temperature, pressure change under the well causes heat exchange pipe section to produce axial expansion, still can maintain the static sealing strength and dynamic sealing followability of the connection; the present application is adapted to geological gradient by segmentation, multiple dynamic sealing and rigid structure connection are combined, the problem of low efficiency, poor reliability, high cost in the field of middle deep layer geothermal heat exchanger for a long time is solved, and reliable technical equipment support is provided for realizing the efficient large-scale development of geothermal energy.

Description

Technical Field

[0001] This invention relates to the field of geothermal energy development technology, and in particular to a gradient sealing connection structure and method for a segmented medium-deep geothermal heat exchanger. Background Technology

[0002] Geothermal energy is a renewable and clean energy source with abundant reserves and stable and reliable operation. Coaxial shell-and-tube heat exchangers (also known as "shell-and-tube underground heat exchangers") are core equipment for medium-deep geothermal development (shallower than 2000-3000 meters). They exchange heat with the rock strata through a circulating working fluid enclosed within the wellbore, achieving non-intrusive development by "extracting heat without extracting water," and possessing advantages such as environmental friendliness and wide applicability. However, existing coaxial heat exchanger technology has significant drawbacks, limiting its heat exchange efficiency, long-term reliability, and economy. Existing heat exchangers typically use a single material and uniform wall thickness casing extending from the wellhead to the bottom. However, in reality, the thermal conductivity of underground rock formations is not uniform, and the in-situ stress increases with depth in an approximately linear gradient. A single-structure heat exchanger cannot adapt to this gradient change, resulting in excessive heat loss in shallow sections due to excessively high pipe wall thermal conductivity, while deeper sections face the risk of deformation due to insufficient pipe wall strength or thickness, or excessively conservative designs leading to high material costs. Furthermore, after the circulating working fluid acquires heat at depth, its return to the surface... During the process, the casing will pass through shallow, low-temperature zones. If the shallow section of the casing has good thermal conductivity, a large amount of heat will be lost to the surrounding low-temperature rock layers, resulting in a reduction in effective heat output and a decrease in system efficiency. Furthermore, if a segmented design is adopted, the core challenge lies in the connection between the segments. The downhole environment is harsh, with high temperature, high pressure, corrosive formation fluids, and thermal expansion and contraction effects. Traditional welding or threaded connection methods are difficult to guarantee absolute sealing under such complex dynamic loads for a long time. Once a leak occurs, it will lead to the failure of the entire system, resulting in extremely high maintenance costs. Currently, there is a lack of an effective connection solution that can simultaneously meet the requirements of high strength, high sealing performance, fatigue resistance, and adaptability to axial expansion and contraction of the casing.

[0003] Therefore, to address the above problems, a segmented gradient sealing connection structure and method for medium-deep geothermal heat exchangers is proposed. By combining segmented adaptation to geological gradients, multiple dynamic seals, and rigid structural connections, the long-standing problems of low efficiency, poor reliability, and high cost in the field of medium-deep geothermal heat exchangers are solved, providing reliable technical equipment support for the efficient and large-scale development of geothermal energy. Summary of the Invention

[0004] To overcome the problems of large shallow heat loss, low efficiency, and insufficient sealing reliability caused by the failure to consider the gradient differences of geological conditions in the application of existing coaxial heat exchangers.

[0005] The technical solution of this invention is: a segmented medium-deep geothermal heat exchanger gradient sealing connection structure, comprising: At least two heat exchange tube segments made of different materials and / or different wall thicknesses are sequentially arranged along the depth direction of the geothermal well. The material selection and structural parameters of the heat exchange tube segments are designed to be compatible with the thermal conductivity of the rock strata and the geostress gradient in their respective depth ranges. Each heat exchange tube segment includes an inner tube and an outer tube arranged coaxially. The outer diameter of the inner tube is slightly smaller than the inner diameter of the outer tube, and the outer wall of the inner tube is in close contact with the inner wall of the outer tube. A gradient sealing connection assembly is used to connect two adjacent heat exchange tube segments to form a complete wellbore. The gradient sealing connection assembly includes a sealing sleeve coaxially sleeved on the outside of the docking end of the two heat exchange tube segments, and a multi-dynamic sealing mechanism disposed between the sealing sleeve and the heat exchange tube segment. The multiple dynamic sealing mechanism is configured to maintain the static sealing strength and dynamic sealing follow-up at the connection point when the heat exchange tube section undergoes axial expansion and contraction due to changes in downhole temperature and pressure. The inner tube filling material is filled in the inner tube of the heat exchange tube section, and its thermal conductivity is configured to work in synergy with the outer tube material of the corresponding depth section.

[0006] Preferably, in this invention, at least two heat exchange tube segments made of different materials and / or different wall thicknesses are sequentially arranged along the depth direction of the geothermal well. The material selection and structural parameters of the heat exchange tube segments are designed for compatibility based on detailed geological survey data of the thermal conductivity of the rock strata and the geostress gradient in their respective depth range, thereby optimizing cost and performance while ensuring safety. A gradient sealing connection assembly is used to connect two adjacent heat exchange tube segments to form a complete and continuous pressure boundary. This assembly is a sealing sleeve coaxially fitted onto the outside of the mating ends of the two heat exchange tube segments. Within the annular space between the sealing sleeve and the heat exchange tube segments, Multiple dynamic sealing mechanisms are installed. These mechanisms are specifically configured to not only provide static sealing strength in the initial state, but also maintain effective contact of the sealing surface when changes in downhole temperature and pressure cause axial expansion and contraction of the heat exchange tube section. This dynamic sealing capability is crucial for ensuring long-term sealing. The inner tube filling material is filled in the inner tube of the heat exchange tube section. Its thermal conductivity is selected to create a synergistic effect with the outer tube material of the corresponding depth section. High thermal conductivity filling material is used in the deep section to enhance heat exchange, while low thermal conductivity filling material is used in the shallow section to assist in insulation. Together, they optimize the heat exchange efficiency throughout the entire well depth range and reduce ineffective heat loss.

[0007] Preferably, the at least two heat exchange tube sections include a shallow section, a medium-deep section, and a deep section; The outer casing of the shallow section (0-500m) is made of alloy steel with low thermal conductivity, and its wall thickness is determined based on shallow geological pressure and corrosion protection requirements; its core function is heat preservation to reduce heat loss of the upward working fluid. The outer casing of the medium-deep section (500-1500m) is made of alloy steel with medium thermal conductivity and high strength. Its wall thickness is reinforced based on the gradient stress in this depth range. It is optimized through finite element analysis to ensure that it can withstand huge external pressure without increasing unnecessary cost and weight due to over-design. The outer casing of the deep section (1500-2000m and below) is made of special alloy steel with high thermal conductivity, ultra-high strength and high corrosion resistance. Its wall thickness is determined based on the maximum ground stress and high temperature creep performance. Its inner and outer walls can be coated with high-performance anti-corrosion coatings, and the annular gap between the inner and outer pipes can be filled with a high thermal conductivity medium for efficient and safe geothermal absorption in extreme environments.

[0008] Preferably, the gradient sealing connection assembly is specifically disposed between the shallow section and the medium-deep section, and between the medium-deep section and the deep section; The thermal expansion coefficient of the sealing sleeve is between that of the heat exchange tube sections connected to its two ends, and is used to form a thermal stress transition zone. When the temperature changes, it can reduce the shear stress generated at the connection due to the different expansion of pipe sections of different materials, thereby improving the fatigue life of the connection.

[0009] Preferably, the multiple dynamic sealing mechanism comprises the following components arranged sequentially from the inside to the outside: A metal sealing ring, set within an annular groove structure at the connection end face of two heat exchange tube sections, is made of a highly ductile metal material. The metal sealing ring is configured to undergo plastic deformation under axial preload, forming a first static metal seal. During installation, the ring undergoes plastic deformation by applying a large axial preload, filling all microscopic unevenness and achieving a static metal seal. The metal seal is high-temperature resistant, anti-aging, and exhibits stable performance. An elastic polymer sealing ring is disposed on the outside of the metal sealing ring to provide a dynamic seal that follows the expansion and contraction of the pipe body. Its function is that when the metal sealing ring undergoes slight displacement due to temperature changes or small gaps are generated due to pressure fluctuations, the elastomer automatically expands under the medium pressure or rebounds by its own elastic force to tightly fit the sealing surface, compensate for the gap, and ensure the continuity of the seal. The hot-melt sealant layer, which fills the annular space between the sealing sleeve and the heat exchange tube section, is configured to melt and solidify under downhole temperature to form a third all-around sealing layer. During the welding process, the heat provided by the welding heat source melts the hot melt sealant layer, turning it into a fluid that penetrates into any minor defects that may exist in the first two seals. After cooling, it solidifies to form a third, all-around, seamless encapsulation seal, which greatly improves the reliability and safety of the seal.

[0010] Preferably, the inner wall of the sealing sleeve and the outer wall of the heat exchange tube section are provided with interlocking limiting step structures to withstand the axial load generated by the downhole pressure and convert part of the axial force into radial force.

[0011] Preferably, the contact surface between the sealing sleeve and the heat exchange tube section is metallurgically bonded by welding.

[0012] The main function of the limiting step structure is mechanical bearing, used to withstand the axial load generated by the huge external pressure downhole, to prevent the pipe section from being squeezed and deformed inward, thereby protecting the aforementioned sealing material from being crushed; the contact surface between the sealing sleeve and the heat exchange pipe section is metallurgically bonded by welding, forming a permanent connection with strength equivalent to the base material, solidifying the entire component into a whole.

[0013] A gradient sealing connection method for a segmented medium-deep geothermal heat exchanger, the method being applied to the aforementioned connection structure, includes the following steps: S1: Based on the geological exploration data of the target geothermal well, determine the thermal conductivity and geostress gradient of the rock strata at different depths, and design and prepare heat exchange tube sections with different materials, wall thicknesses and structural parameters accordingly. S2: Process the ends of two adjacent heat exchange tube sections to be connected to form a clean and flat mating surface and a groove for accommodating the sealing mechanism, and place the sealing sleeve on the end of one of the tube sections. S3: Install the components of the multiple dynamic sealing mechanism into the pre-treated docking area in sequence; S4: Move the sealing sleeve to the docking position, apply axial preload to cause the metal sealing ring to undergo plastic deformation, and complete the initial sealing connection; S5: Hot melt sealant is injected into the annular space through the pre-set injection hole on the sealing sleeve, and then the sealing sleeve and the heat exchange tube section are welded together by the ring welding process to form a sealing structure. S6: Perform localized annealing heat treatment on the welded joints to eliminate residual welding stress.

[0014] Preferably, step S3 specifically includes: First, press the metal sealing ring into the groove at the joint end of the heat exchange tube section; Then, the elastic polymer sealing ring is installed in the annular gap outside the metal sealing ring; Finally, the solid hot melt sealant preform is placed in the designated annular receiving groove inside the sealing sleeve.

[0015] Preferably, the ring welding process in step S5 adopts narrow gap heat input control welding technology. During the welding process, the pre-assembled elastic polymer sealing ring and hot melt sealant are actively cooled and protected to prevent them from failing at high temperature.

[0016] Preferably, step S1 further includes: Different thermal conductivity filling materials are selected for the inner tubes of heat exchange tube sections at different depths, with the thermal conductivity of the filling material in the deeper sections being higher than that in the shallower sections.

[0017] The beneficial effects of this invention are: 1. The gradient thermal resistance structure of the present invention matches the thermal-mechanical characteristics of the heat exchanger at different depths with the local rock strata; the low thermal conductivity material in the shallow section effectively reduces heat loss during the upward flow of heat flow; the high thermal conductivity material and high thermal conductivity filler in the deep section enhance the heat extraction capacity; the internal and external synergy improves the net heat extraction efficiency of the system. 2. This invention provides a combination of multiple dynamic sealing mechanisms, interlocking and welding connection structures, including metal plastic sealing, elastomer dynamic sealing, hot melt adhesive all-round sealing, mechanical step pressure resistance, and metallurgical welding solid sealing. This allows the connection to withstand extreme static pressure and adapt to alternating stresses caused by downhole thermal circulation. The sealing reliability and structural life are higher than traditional single connection methods, avoiding leakage risks. 3. Segmented differentiated design avoids the "overconsumption" or "insufficient performance" of materials; lower-cost materials are used in non-critical segments, while high-performance materials are used in critical segments, achieving optimized material allocation and reducing initial investment; at the same time, higher efficiency and longer mean time between failures reduce operating costs and improve the overall economic efficiency of the project. 4. The connection method provided by this invention decomposes the complex sealing and connection process into a standardized process that can be precisely controlled, ensuring the consistency and high quality of on-site construction, and is suitable for large-scale promotion and application. Attached Figure Description

[0018] Figure 1 The diagram shown is a three-dimensional structural schematic of the gradient sealing connection structure of the segmented medium-deep geothermal heat exchanger of the present invention. Figure 2 The diagram shown is a three-dimensional cross-sectional view of the gradient sealing connection structure of the segmented medium-deep geothermal heat exchanger of the present invention. Figure 3 The diagram shown is a cross-sectional plan view of the gradient sealing connection structure of the segmented medium-deep geothermal heat exchanger of the present invention. Figure 4The diagram shown is a partial structural diagram of the first part of the gradient sealing connection structure of the segmented medium-deep geothermal heat exchanger of the present invention. Figure 5 The diagram shown is a second partial structural schematic of the segmented medium-deep geothermal heat exchanger gradient sealing connection structure of the present invention. Explanation of reference numerals in the attached drawings: 1. Heat exchange tube section; 2. Gradient sealing connection assembly; 21. Sealing sleeve; 22. Multiple dynamic sealing mechanism; 2201. Metal sealing ring; 2202. Elastic polymer sealing ring; 2203. Hot melt sealant; 3. Inner tube filling material; 4. Limiting step structure. Detailed Implementation

[0019] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0020] Please see Figure 1 , Figure 2 , Figure 3 , Figure 4 and Figure 5 This invention provides an embodiment of a segmented medium-deep geothermal heat exchanger gradient sealing connection structure, comprising: At least two heat exchange tube segments 1 made of different materials and / or different wall thicknesses are arranged sequentially along the depth direction of the geothermal well. The material selection and structural parameters of the heat exchange tube segment 1 are designed to be compatible with the thermal conductivity of the rock strata and the geostress gradient in its depth range. The heat exchange tube segment 1 includes an inner tube and an outer tube arranged coaxially. The outer diameter of the inner tube is slightly smaller than the inner diameter of the outer tube, and the outer wall of the inner tube is in close contact with the inner wall of the outer tube. The gradient sealing connection assembly 2 is used to connect two adjacent heat exchange tube segments 1 to form a complete wellbore. The gradient sealing connection assembly 2 includes a sealing sleeve 21 coaxially sleeved on the outside of the docking end of the two heat exchange tube segments 1, and a multi-dynamic sealing mechanism 22 disposed between the sealing sleeve 21 and the heat exchange tube segment 1. The multiple dynamic sealing mechanism 22 is configured to maintain the static sealing strength and dynamic sealing follow-up at the connection when the heat exchange tube section 1 undergoes axial expansion and contraction due to changes in downhole temperature and pressure. The inner tube filling material 3 is filled in the inner tube of the heat exchange tube section 1, and its thermal conductivity is configured to work in synergy with the outer tube material of the corresponding depth section.

[0021] In this invention, at least two heat exchange tube segments 1, made of different materials and / or with different wall thicknesses, are sequentially arranged along the depth direction of the geothermal well. The material selection and structural parameters of the heat exchange tube segments 1 are designed for compatibility based on detailed geological survey data of the thermal conductivity of the rock strata and the stress gradient in their respective depth range, thereby optimizing cost and performance while ensuring safety. The gradient sealing connection assembly 2 is used to connect two adjacent heat exchange tube segments 1 to form a complete and continuous pressure boundary. This assembly is a sealing sleeve 21 coaxially sleeved on the outside of the mating end of the two heat exchange tube segments 1. Within the annular space between the sealing sleeve 21 and the heat exchange tube segment 1, a... Multiple dynamic sealing mechanisms 22 are provided; these mechanisms are specifically configured to not only provide static sealing strength in the initial state, but also maintain effective contact of the sealing surface when the heat exchange tube section 1 undergoes axial expansion and contraction due to changes in downhole temperature and pressure, i.e., they have dynamic sealing following capability, which is the key to ensuring long-term sealing; the inner tube filling material 3 is filled in the inner tube of the heat exchange tube section 1; its thermal conductivity is selected to form a synergistic effect with the outer tube material of the corresponding depth section. High thermal conductivity filling material is used in the deep section to enhance heat exchange, and low thermal conductivity filling material is used in the shallow section to assist in heat preservation, together optimizing the heat exchange efficiency throughout the entire well depth range and reducing ineffective heat loss.

[0022] Furthermore, at least two heat exchange tube sections 1 include a shallow section, a medium-deep section, and a deep section; The outer casing of the shallow section (0-500m) is made of alloy steel with low thermal conductivity, and its wall thickness is determined based on the shallow geological pressure and corrosion protection requirements; its core function is heat preservation to reduce heat loss of the upward working fluid. The outer casing of the medium-deep section (500-1500m) is made of alloy steel with medium thermal conductivity and high strength. Its wall thickness is reinforced based on the gradient stress in this depth range. It is optimized through finite element analysis to ensure that it can withstand huge external pressure without adding unnecessary cost and weight due to over-design. The outer casing of the deep section (1500-2000m and below) is made of special alloy steel with high thermal conductivity, ultra-high strength and high corrosion resistance. Its wall thickness is determined based on the maximum ground stress and high temperature creep performance. Its inner and outer walls can be coated with high-performance anti-corrosion coatings, and the annular gap between the inner and outer pipes can be filled with a high thermal conductivity medium for efficient and safe geothermal absorption in extreme environments.

[0023] Furthermore, the gradient sealing connection assembly 2 is specifically disposed between the shallow section and the intermediate-deep section, and between the intermediate-deep section and the deep section; The thermal expansion coefficient of the sealing sleeve 21 is between that of the heat exchange tube section 1 connected to its two ends, and is used to form a thermal stress transition zone. When the temperature changes, it can reduce the shear stress generated at the connection due to the different expansion of pipe sections of different materials, thereby improving the fatigue life of the connection.

[0024] Furthermore, the multiple dynamic sealing mechanism 22 includes the following components arranged sequentially from the inside to the outside: The metal sealing ring 2201 is set in the annular groove structure opened at the connection end face of the two heat exchange tube sections 1. It is made of a metal material with excellent plasticity. The metal sealing ring 2201 is configured to: undergo plastic deformation under the action of axial preload to form the first static metal seal; during installation, the ring undergoes plastic deformation by applying a huge axial preload to fill all micro-uneven areas and achieve static metal seal; the metal seal is resistant to high temperature, anti-aging, and has stable performance. The elastic polymer sealing ring 2202 is disposed outside the metal sealing ring 2201 to provide a dynamic seal that follows the expansion and contraction of the pipe body. Its function is that when the metal sealing ring 2201 undergoes slight displacement due to temperature changes or generates small gaps due to pressure fluctuations, the elastomer automatically expands under the medium pressure or rebounds by its own elastic force to tightly fit the sealing surface, compensate for the gaps, and ensure the continuity of the seal. The hot melt sealant 2203 layer, which fills the annular space between the sealing sleeve 21 and the heat exchange tube section 1, is configured to melt and cure under the action of downhole temperature to form a third all-round encapsulation seal. During the welding process, the heat provided by the welding heat source melts the 2203 layer of hot melt sealant, turning it into a fluid that penetrates into any minor defects that may exist in the first two seals. After cooling, it solidifies to form a third, all-around, seamless encapsulation seal, which greatly improves the reliability and safety of the seal.

[0025] Furthermore, the inner wall of the sealing sleeve 21 and the outer wall of the heat exchange tube section 1 are both provided with interlocking limiting step structures 4, which are used to withstand the axial load generated by the downhole pressure and convert part of the axial force into radial force.

[0026] Furthermore, the contact surface between the sealing sleeve 21 and the heat exchange tube section 1 is metallurgically bonded by welding.

[0027] The main function of the limiting step structure 4 is mechanical bearing, which is used to withstand the axial load generated by the huge external pressure downhole, prevent the pipe section from being squeezed and deformed inward, and thus protect the aforementioned sealing material from being crushed; the contact surface between the sealing sleeve 21 and the heat exchange pipe section 1 is metallurgically bonded by welding to form a permanent connection with the strength of the base material, thus solidifying the entire component into a whole.

[0028] A gradient sealing connection method for segmented medium-deep geothermal heat exchangers, the method being applied to the aforementioned connection structure, includes the following steps: S1: Based on the geological exploration data of the target geothermal well, determine the thermal conductivity and geostress gradient of the rock strata in different depth ranges, and design and prepare heat exchange tube sections 1 with different materials, wall thicknesses and structural parameters accordingly. S2: The ends of the two adjacent heat exchange tube sections 1 to be connected are processed to form a clean and flat mating surface and a groove for accommodating the sealing mechanism, and the sealing sleeve 21 is placed on the end of one of the tube sections. S3: Install the components of the multi-dynamic sealing mechanism 22 into the pre-treated docking area in sequence; S4: Move the sealing sleeve 21 to the docking position, apply axial preload to cause the metal sealing ring 2201 to undergo plastic deformation, and complete the initial sealing connection; S5: Hot melt sealant 2203 is injected into the annular space through the pre-set injection hole on the sealing sleeve 21. Then, the sealing sleeve 21 and the heat exchange tube section 1 are welded together by the ring welding process to form a sealed structure. S6: Perform localized annealing heat treatment on the welded joints to eliminate residual welding stress.

[0029] Furthermore, step S3 specifically includes: First, press a metal sealing ring 2201 into the groove at the joint end of heat exchange tube section 1; Then, an elastic polymer sealing ring 2202 is installed in the annular gap outside the metal sealing ring 2201; Finally, the solid hot melt sealant 2203 preform is placed in the designated annular receiving groove inside the sealing sleeve 21.

[0030] Furthermore, in step S5, the ring welding process adopts narrow gap heat input control welding technology. During the welding process, the pre-assembled elastic polymer sealing ring 2202 and hot melt sealant 2203 are actively cooled and protected to prevent them from failing at high temperature.

[0031] Furthermore, step S1 also includes: Different thermal conductivity filling materials are selected for the inner tubes of heat exchange tube sections 1 at different depths, with the thermal conductivity of the inner tube filling material 3 in the deep section being higher than that in the shallow section.

[0032] During operation, the specific implementation of the gradient sealing connection structure of this segmented medium-deep geothermal heat exchanger needs to be based on the geological exploration data of the target geothermal well. First, according to the thermal conductivity of the rock strata and the geostress gradient in different depth ranges (such as shallow section 0-500m, medium-deep section 500-1500m, and deep section 1500m and below), a suitable heat exchange tube section 1 is designed and manufactured. The outer casing of the shallow section is made of low thermal conductivity alloy steel, and the wall thickness meets the requirements of geological pressure and corrosion resistance. The medium-deep section uses medium thermal conductivity high... The high-strength alloy steel is used, and the wall thickness is optimized through finite element analysis to resist gradient stress. The deep section uses special alloy steel with high thermal conductivity, ultra-high strength and corrosion resistance, and the wall thickness is determined by combining high temperature creep performance. The inner tube of each section is filled with an outer tube material with coordinated thermal conductivity. For example, the deep section is filled with a high thermal conductivity material to enhance heat extraction, and the shallow section is filled with a low thermal conductivity material to reduce heat loss. Subsequently, the ends of adjacent heat exchange tube sections 1 are cleaned, leveled and machined with sealing grooves. The sealing sleeve 21 is placed at the end of a tube section to lay the foundation for subsequent sealing connection. The installation of the gradient sealing connection assembly 2 must strictly follow the assembly sequence of the multiple dynamic sealing mechanism 22. First, a plastic metal sealing ring 2201 is pressed into the groove at the docking end of the heat exchange tube section 1. The axial preload causes it to undergo plastic deformation, forming the first static metal seal. Next, an elastic polymer sealing ring 2202 is installed in the annular gap outside the metal sealing ring 2201. Its elastic deformation capability adapts to the axial expansion and contraction of the tube body, achieving dynamic sealing compensation. Finally, a pre-made solid hot melt sealant 2203 is placed in the annular receiving groove inside the sealing sleeve 21. After moving the sealing sleeve 21 to the docking position, the hot melt sealant 2203 is injected through the injection hole. The welding heat source melts it and penetrates into the minor defects of the first two seals. After cooling, it solidifies to form the third all-round encapsulation seal. The limiting step structure 4 of the inner wall of the sealing sleeve 21 and the outer wall of the heat exchange tube section 1 can withstand axial loads and convert them into radial forces. Combined with the ring welding process, it forms a metallurgically bonded integral structure, ensuring the mechanical strength and sealing reliability of the connection. The entire connection method requires the system to execute the entire process from design to verification. Step S1 determines the parameters of each depth range based on geological data, prepares the heat exchange tube section 1, and selects the inner tube filling material 3. Step S2 completes the pretreatment of the tube section end and the pre-installation of the sealing sleeve 21. Step S3 assembles the multiple dynamic sealing mechanism 22 components in sequence. Step S4 deforms the metal sealing ring 2201 through axial pre-tightening force to achieve preliminary sealing. Step S5 adopts narrow gap heat input control welding technology, actively cooling and protecting the elastic polymer sealing ring 2202 and hot melt sealant 2203 during the welding process to prevent high temperature failure, and welds the sealing sleeve 21 and the tube section into a whole through circumferential welding. Step S6 performs local annealing heat treatment on the welded parts to eliminate residual stress. The final connection structure not only meets the static sealing strength requirements, but also maintains dynamic sealing follow-up in the tube expansion and contraction caused by downhole temperature and pressure changes, ensuring the long-term efficient and reliable operation of the medium-deep geothermal heat exchanger.

[0033] Through the above steps, the gradient thermal resistance structure of this invention matches the thermal-mechanical characteristics of the heat exchanger at different depths with the local rock formations; the low thermal conductivity material in the shallow section effectively reduces heat loss during the upward flow; the high thermal conductivity material and high thermal conductivity filler in the deep section enhance the heat extraction capacity; the internal and external synergy improves the net heat extraction efficiency of the system; this invention provides a combination of metal plastic sealing, elastomer dynamic sealing, hot melt adhesive all-round sealing, mechanical step pressure resistance, and metallurgical welding solidification through multiple dynamic sealing mechanisms 22 combined with interlocking and welded connection structures; enabling the connection to not only withstand extreme static pressure, but also adapt to the heat exchange caused by downhole thermal circulation. The variable stress design offers superior sealing reliability and structural lifespan compared to traditional single-connection methods, avoiding leakage risks. The segmented, differentiated design prevents material overuse or underperformance. Utilizing lower-cost materials in non-critical sections and high-performance materials in critical sections optimizes material allocation and reduces initial investment. Simultaneously, higher efficiency and longer mean time between failures (MTBF) lower operating costs and improve overall project economics. The connection method provided by this invention breaks down complex sealing and connection processes into precisely controllable, standardized procedures, ensuring consistency and high quality in on-site construction, making it suitable for large-scale application.

[0034] The embodiments of the present invention have been described in detail above with reference to the accompanying drawings. However, the present invention is not limited to the above embodiments. Within the scope of knowledge possessed by those skilled in the art, various changes can be made without departing from the spirit of the present invention.

Claims

1. A segmented, medium-deep geothermal heat exchanger gradient sealing connection structure, characterized in that: include: At least two heat exchange tube segments (1) made of different materials and / or different wall thicknesses are arranged sequentially along the depth direction of the geothermal well. The material selection and structural parameters of the heat exchange tube segments (1) are designed to be compatible with the thermal conductivity of the rock strata and the geostress gradient in the depth range where they are located. The heat exchange tube segment (1) includes an inner tube and an outer tube arranged coaxially. The outer diameter of the inner tube is slightly smaller than the inner diameter of the outer tube, and the outer wall of the inner tube is in close contact with the inner wall of the outer tube. The gradient sealing connection assembly (2) is used to connect two adjacent heat exchange tube segments (1) to form a complete wellbore. The gradient sealing connection assembly (2) includes a sealing sleeve (21) coaxially sleeved on the outside of the docking end of the two heat exchange tube segments (1), and a multi-dynamic sealing mechanism (22) disposed between the sealing sleeve (21) and the heat exchange tube segment (1). The multiple dynamic sealing mechanism (22) is configured to maintain the static sealing strength and dynamic sealing follow-up at the connection when the heat exchange tube section (1) undergoes axial expansion and contraction due to changes in downhole temperature and pressure. The inner tube filling material (3) is filled in the inner tube of the heat exchange tube section (1), and its thermal conductivity is configured to be compatible with the outer tube material of the corresponding depth section. The multiple dynamic sealing mechanism (22) includes the following components arranged sequentially from the inside to the outside: The metal sealing ring (2201) is set in the annular groove structure opened on the connecting end face of the two heat exchange tube sections (1), and is made of a metal material with excellent plasticity. The metal sealing ring (2201) is configured to be able to undergo plastic deformation under the action of axial preload to form the first static metal seal. An elastic polymer sealing ring (2202), disposed outside the metal sealing ring (2201), is used to provide a dynamic seal that follows the expansion and contraction of the tube body; The hot melt sealant (2203) layer, which fills the annular space between the sealing sleeve (21) and the heat exchange tube section (1), is configured to melt and solidify under the action of downhole temperature to form a third all-round sealing layer.

2. The segmented medium-deep geothermal heat exchanger gradient sealing connection structure according to claim 1, characterized in that: The at least two heat exchange tube sections (1) include a shallow section, a medium-deep section, and a deep section; The outer casing of the shallow section is made of alloy steel with low thermal conductivity, and its wall thickness is determined based on the shallow geological pressure and corrosion protection requirements; the shallow section is 0-500m; The outer casing of the mid-deep section (500-1500m) is made of alloy steel with medium thermal conductivity and high strength, and its wall thickness is reinforced based on the gradient stress in this depth range; the mid-deep section is 500-1500m. The outer casing of the deep section (1500-2000m) is made of special alloy steel with high thermal conductivity, ultra-high strength and high corrosion resistance. The wall thickness is determined based on the maximum ground stress and high-temperature creep performance. The deep section is 1500-2000m.

3. The segmented medium-deep geothermal heat exchanger gradient sealing connection structure according to claim 2, characterized in that: The gradient sealing connection assembly (2) is specifically disposed between the shallow section and the medium-deep section, and between the medium-deep section and the deep section; The thermal expansion coefficient of the sealing sleeve (21) is between that of the heat exchange tube section (1) connected to its two ends, and is used to form a thermal stress transition zone.

4. The segmented medium-deep geothermal heat exchanger gradient sealing connection structure according to claim 1, characterized in that: The inner wall of the sealing sleeve (21) and the outer wall of the heat exchange tube section (1) are both provided with interlocking limiting step structures (4) to bear the axial load generated by the downhole pressure and convert part of the axial force into radial force.

5. The segmented medium-deep geothermal heat exchanger gradient sealing connection structure according to claim 1, characterized in that: The contact surface between the sealing sleeve (21) and the heat exchange tube section (1) is metallurgically bonded by welding.

6. A gradient sealing connection method for a segmented medium-deep geothermal heat exchanger, characterized in that, The method, when applied to the connection structure as described in any one of claims 1-5, includes the following steps: S1: Based on the geological exploration data of the target geothermal well, the thermal conductivity of the rock strata and the geostress gradient in different depth ranges are determined, and heat exchange tube sections with different materials, wall thicknesses and structural parameters are designed and prepared accordingly (1). S2: Process the ends of the two adjacent heat exchange tube sections (1) to be connected to form a clean and flat mating surface and a groove for accommodating the sealing mechanism, and put the sealing sleeve (21) on the end of one of the tube sections. S3: Install the components of the multi-dynamic sealing mechanism (22) into the pre-treated docking area in sequence; S4: Move the sealing sleeve (21) to the docking position, apply axial preload to cause the metal sealing ring (2201) to undergo plastic deformation, and complete the initial sealing connection; S5: Hot melt sealant (2203) is injected into the annular space through the pre-set injection hole on the sealing sleeve (21), and then the sealing sleeve (21) and the heat exchange tube section (1) are welded together by the ring welding process to form a sealing structure; S6: Perform localized annealing heat treatment on the welded joints to eliminate residual welding stress.

7. The gradient sealing connection method for a segmented medium-deep geothermal heat exchanger according to claim 6, characterized in that: Step S3 specifically includes: First, press the metal sealing ring (2201) into the groove at the joint end of the heat exchange tube section (1). Then the elastic polymer sealing ring (2202) is installed in the annular gap outside the metal sealing ring (2201). Finally, the solid hot melt sealant (2203) preform is placed in the designated annular receiving groove inside the sealing sleeve (21).

8. The gradient sealing connection method for a segmented medium-deep geothermal heat exchanger according to claim 6, characterized in that: The ring welding process described in step S5 adopts narrow gap heat input control welding technology. During the welding process, the pre-assembled elastic polymer sealing ring (2202) and hot melt sealant (2203) are actively cooled and protected.

9. The gradient sealing connection method for a segmented medium-deep geothermal heat exchanger according to claim 6, characterized in that: Step S1 also includes: Different thermal conductivity filling materials are selected for the inner tubes of heat exchange tube sections (1) at different depths, among which the thermal conductivity of the inner tube filling material (3) in the deep section is higher than that in the shallow section.

Images