A segmented counterweight system for the inner tube of a coaxial casing in a geothermal well

Abstract

This invention discloses a segmented counterweight system for the inner tube of a coaxial casing in a geothermal well, comprising an outer tube, an inner tube, and multiple counterweight tubes. The outer tube extends into and is fixed within the geothermal well, and the inner tube is located within the outer tube. The inner tube is composed of multiple segments of different sections connected together, and the multiple counterweight tubes are respectively fitted onto the different sections of the inner tube. This invention, through segmented counterweighting, ensures balanced stress on the material, resulting in less stress compared to a single-use bottom-hole counterweight, avoiding excessive localized stress, and improving the service life of the material.

Description

Technical Field

[0001] This invention relates to geothermal wells, and more specifically to a segmented counterweight system for the inner tube of a coaxial casing in a geothermal well. Background Technology

[0002] Global geothermal resources are abundant, approximately 170 million times the calorific value of global coal and 2,000 times the total calorific value of all currently produced coal worldwide. Even considering a 1% utilization rate, geothermal energy within 3 km of the surface is equivalent to 2.9 trillion tons of standard coal equivalent. With adjustments to energy industry policies and increasing human demand for energy, the energy structure has gradually shifted from being dominated by traditional energy sources to one primarily based on new, clean, and renewable energy. The energy strategy of "adhering to green and low-carbon development, strengthening the clean energy industry, and promoting high-quality development" has propelled the development of geothermal energy application technologies to the forefront. The comprehensive development and utilization of geothermal resources yields significant social, economic, and environmental benefits, demonstrating an increasingly important role in national economic development.

[0003] For medium-deep coaxial casing geothermal well heat exchange technology, due to the long inner tube length and the material density generally being lower than the heat exchange medium density, the bottom temperature in geothermal anomaly zones can generally reach 90~110℃, which places high demands on parameters such as material thermal stability, expansion rate and tensile strength under high temperature conditions.

[0004] Currently, there are still some problems in addressing the buoyancy issue of the inner pipe. The conventional method involves calculating the buoyancy difference and adding a one-time counterweight at the bottom of the well. In deep geothermal wells reaching depths of 3000m, this counterweight requires 20m or more of the bottom length. The lower part, with its high temperature, could serve as a good heat extraction section. However, if this section is used as a counterweight, under current technology where heat is extracted but not water, a large portion of the bottom pipe cannot exchange heat, resulting in significant waste and failing to fully realize the economic advantages of geothermal pipes. Secondly, the large weight at the bottom of the well places higher demands on the tensile strength of the materials under high-temperature environments, posing a significant risk to stable and safe operation. In medium-deep wells, the great depth and complex conditions mean that material cracking or fracture will greatly affect the heat extraction effect, and in severe cases, may lead to heat exchange failure.

[0005] Chinese utility model patent CN215632769U discloses a non-interference-free inner casing counterweight device for medium-deep geothermal wells. This device comprises an inner casing connector connected in series from top to bottom, multiple counterweights, connecting steel wires, and a guide. While this patent plays a role in counterweighting, it still has problems. The steel wire fixing method used in the patent is prone to instability and rusting and detachment underwater; the counterweights lack hydraulic drag reduction measures, resulting in excessive resistance to the heat exchange medium; and concentrating the counterweight on a single section leads to excessive load on the area near the counterweight end. These issues need to be addressed. In summary, the existing technology fails to effectively solve the problems of central tube counterweight stability and reducing heat exchange medium resistance. Summary of the Invention

[0006] To address the shortcomings of existing technologies, this invention provides a segmented counterweight system for the inner tube of a coaxial casing in a geothermal well.

[0007] A coaxial casing inner tube segmented counterweight system for a geothermal well includes an outer tube, an inner tube, and multiple counterweight tubes. The outer tube extends into and is fixed in the geothermal well, and the inner tube is located inside the outer tube. The inner tube is composed of multiple segments of different sections connected together, and the multiple counterweight tubes are respectively sleeved on the different segments of the segments.

[0008] Optionally, the inner diameter of the counterweight tube is 1-20 mm larger than the outer diameter of the inner tube; the outer diameter of the counterweight tube is 20-40 mm smaller than the inner diameter of the outer tube; a convex ring supporting the counterweight tube is formed at the junction of different sections of the tube; the tubes are connected by heat fusion, and the convex ring is formed by flanging; chamfers are formed at both ends of the counterweight tube; an assembly cavity is formed between the inner wall of the counterweight tube and the outer wall of the inner tube, and an L-shaped rubber sealing ring is provided in the assembly cavity located at both ends of the counterweight tube; the L-shaped rubber sealing ring includes a washer, and an annular protrusion is formed on the washer, the inner diameter of the washer is equal to the inner diameter of the annular protrusion, and the outer diameter of the washer is larger than the outer diameter of the annular protrusion; the annular protrusion is frustum-shaped, and the diameter of the frustum gradually increases from top to bottom; an exhaust port is provided on the side wall of the counterweight tube.

[0009] The beneficial effects of this invention are as follows: This process design prevents the inner tube from breaking due to buoyancy issues, and also provides conditions for the safe lowering of the inner tube with a centralizer; the segmented counterweight makes the material more evenly stressed, resulting in less stress than a single-stage bottom counterweight, avoiding excessive local stress and improving the material's service life; the segmented counterweight and the high temperature at the bottom significantly reduce the tensile strength of the material, providing more options for optimizing the selection of inner tube materials, which helps to reduce the cost of the inner tube, effectively solves the problem of excessive initial investment in geothermal wells, and promotes the development and utilization of clean and efficient geothermal energy. Attached Figure Description

[0010] Figure 1 This is a schematic diagram of the segmented counterweight system;

[0011] Figure 2 for Figure 1 Enlarged view of part A in the middle;

[0012] Figure 3 for Figure 1 Enlarged view of part B in the middle;

[0013] Figure 4 This is a schematic diagram of the structure of an L-shaped rubber sealing ring. Detailed Implementation

[0014] To make the above-mentioned objects, features, and advantages of the present invention more apparent and understandable, the specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings, so that the above and other objects, features, and advantages of the present invention will become clearer. In all the drawings, the same reference numerals indicate the same parts. The drawings are not intentionally drawn to scale; the focus is on illustrating the main points of the invention.

[0015] The terms and words used in the following description and claims are not limited to their literal meaning, but are intended solely by the inventors to provide a clear and consistent understanding of the invention. Therefore, it will be apparent to those skilled in the art that the following description, which provides various embodiments of the invention, is for illustrative purposes only and not for limiting the invention as defined by the appended claims and their equivalents.

[0016] It should be understood that the singular forms “a,” “an,” and “the” include plural objects unless the context explicitly indicates otherwise. Thus, for example, referring to a “module” includes referring to one or more such modules. The advantages and features of the invention, as well as methods of implementing the invention, can be more readily understood by referring to the detailed description and accompanying drawings of the embodiments below. However, the invention can be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that the invention will be thorough and complete, and fully convey the concept of the invention to those skilled in the art.

[0017] like Figure 1 As shown, the geothermal well coaxial casing inner tube segmented counterweight system of the present invention includes an outer tube 1 and an inner tube 2. The outer tube 1 extends into and is fixed in the geothermal well. It can be made of metal and can support the inner wall of the geothermal well to prevent the well wall from collapsing. The inner tube 2 is located inside the outer tube 1 and is mainly used to realize heat exchange. The inner tube 2 can be made of modified PE or PPR substrate, which has characteristics such as high thermal resistance and low coefficient of thermal expansion.

[0018] The inner pipe 2 is composed of multiple connected branch pipes, which can have the same or different lengths, ranging from a few meters to tens of meters. These branch pipes are connected to form an inner pipe 2 with a total length exceeding one kilometer. A counterweight pipe 3 is fitted onto each branch pipe. The counterweight pipe 3 can be made of tubular metal with a certain wall thickness, possessing characteristics such as high density and corrosion resistance. The inner diameter of the counterweight pipe 3 is larger than the outer diameter of the inner pipe 2, making it easier to fit onto the inner pipe 2. However, it should not be too large, as this would cause the counterweight pipe 3 to wobble relative to the inner pipe 2, potentially damaging it. Therefore, the inner diameter of the counterweight pipe 3 should ideally be 1-20 mm larger than the outer diameter of the inner pipe 2. However, the outer diameter of the counterweight pipe 3 should be smaller than the inner diameter of the outer pipe 1. This allows the counterweight pipe 3 to be lowered into the geothermal well along with the inner pipe 2. Furthermore, considering the length of the inner pipe 2, it is prone to significant wobble within the outer pipe 1, and the inner pipe 2 may tilt during lowering. It is advisable that the outer diameter of the counterweight tube 3 is 20-40mm smaller than the inner diameter of the outer tube 1, so that the counterweight tube 3 can play a role in straightening the inner tube 2 and prevent the inner tube 2 from shaking or tilting significantly in the outer tube 1.

[0019] See Figure 2 A convex ring 6 is formed at the junction of one branch pipe and another to support the counterweight pipe 3. For example, the branch pipes can be joined by heat fusion and then the convex ring 6 is formed by flanging. The outer diameter of the convex ring 6 is larger than the inner diameter of the counterweight pipe 3. In this way, the convex ring 6 can provide stable support for the counterweight pipe 3 and prevent the counterweight pipe 3 from sliding down from the inner pipe 2 due to gravity.

[0020] The counterweight pipe 3 can be fitted onto multiple different branch pipes. The interval between adjacent counterweight pipes 3 can be calculated based on the actual project requirements and material properties. It should be further noted that it is not required that every branch pipe segment form a convex ring 6. The convex ring 6 can be formed only on the branch pipes where the counterweight pipe 3 is required. This can reduce unnecessary hot-melt flanging processes, thereby improving construction efficiency.

[0021] Chamfers 4, preferably 45 degrees, are formed at both ends of the counterweight tube 3. Specifically, the chamfer at the lower end of the counterweight tube 3 relative to the axis of the inner tube 2 forms an angle of 45 degrees with the axis, while the chamfer at the upper end of the counterweight tube 3 relative to the axis of the inner tube 2 forms an angle of 135 degrees with the axis. By chamfering both ends of the counterweight tube 3, the resistance loss of the heat exchange medium can be reduced.

[0022] See also Figure 2 , Figure 3 and Figure 4An assembly cavity 5 is formed between the inner wall of the counterweight tube 3 and the outer wall of the inner tube 2. Due to the presence of the assembly cavity 5, on the one hand, the counterweight tube 3 will shake during construction, which will impact the inner tube 2 and the convex ring 6, thus reducing the service life of the inner tube 2; on the other hand, during operation, the counterweight tube 3 will shear and compress the convex ring 6 of the inner tube under the action of gravity and the flow of the heat exchange medium. To address this, the present invention provides L-shaped rubber sealing rings 4 in the assembly cavities 5 located at both ends of the counterweight tube 3. The L-shaped rubber sealing rings 4 are fitted onto the inner tube 2. By tightly inserting the L-shaped rubber sealing rings 4 into the assembly cavity 5, the counterweight tube 3 can be stabilized, thereby eliminating the shaking that occurs during construction. Furthermore, the L-shaped rubber sealing rings 4 seal the assembly cavity 5, reducing the shear and compression caused by the flow of the heat exchange medium and improving its service life.

[0023] like Figure 4 As shown, the L-shaped rubber sealing ring 4 has good heat resistance, pressure resistance, and flexibility. It includes a washer 10 with an annular protrusion 9 formed on it. The inner diameter of the washer 10 is equal to the inner diameter of the annular protrusion 9, and the outer diameter of the washer 10 is larger than the outer diameter of the annular protrusion 9. During installation, the annular protrusion 9 is inserted into the assembly cavity 5, and the washer 10 abuts against the upper or lower end of the counterweight tube 3. Preferably, the annular protrusion 9 is frustum-shaped, with the diameter of the frustum gradually increasing from top to bottom (with the direction of the washer as the bottom). This frustum structure facilitates the insertion of the annular protrusion 9 into the assembly cavity 5 while also providing a good and stable seal.

[0024] Furthermore, an exhaust port 7 is provided on the side wall of the counterweight tube 3. Multiple exhaust ports 7 can be provided. Air in the assembly cavity 5 can be discharged through the exhaust ports 7, thereby preventing the L-shaped rubber sealing ring 8 from being pushed out of the assembly cavity 5 due to excessive pressure in the assembly cavity 5.

[0025] The following section provides a detailed description of the specific construction process, with reference to the accompanying drawings.

[0026] The number of sections and the weight of each section of counterweight tube 3 need to be calculated based on buoyancy, inner tube length, etc. Buoyancy can be calculated using the buoyancy formula. The segment interval can be calculated based on the actual project needs and material properties.

[0027] After the inner tube 2 is joined by heat fusion, an outer convex ring 6 is formed by flanging. By calculating the weight of the material of a single section of the tube and the buoyancy at different depths, and in combination with the size of the inner tube 2, a commonly used standard specification corrosion-resistant counterweight tube 3 is selected. By calculating and selecting the appropriate length, an L-shaped rubber sealing ring 8 is first put on the inner tube 2 where it is joined by heat fusion, and then the counterweight tube 3 is put in to prevent the counterweight tube 3 from squeezing and shearing the convex ring 6 at the heat fusion connection. An L-shaped rubber sealing ring 8 is also put on the upper end of the counterweight tube 3 for compaction and sealing.

[0028] Construction process:

[0029] The first step is to determine parameters such as the inner and outer diameters of the pipe, the pipe length, and the density of the heat exchange medium, and to calculate the buoyancy of the inner pipe 2 and the shear force of the counterweight pipe 3 on the convex ring 6. The calculation is carried out in segments based on the buoyancy, the pipe length, and considering the bearing capacity threshold of the convex ring 6 of the inner pipe 2.

[0030] The second step is to determine the weight and segment spacing of the counterweight tube 3, prefabricate the counterweight tube 3, and perform chamfering treatment 4 and through-venting 7 on the counterweight tube 3.

[0031] The third step is to run the outer casing 1 into the well and cement it in place, and then run the inner casing 2. The inner casing is connected by heat fusion, which ensures that the convex ring 6 of the inner casing 2 protrudes outward to a certain size.

[0032] The fourth step is to arrange the counterweight tube 3 according to the calculated counterweight and spacing. L-shaped rubber sealing rings 8 are arranged at the upper and lower ends of the counterweight tube 3 and the inner tube 2.

[0033] Fifth, repeat step four until all counterweight tubes 3 are completed.

[0034] This invention prevents the inner tube from breaking due to buoyancy issues and also provides conditions for the safe lowering of the inner tube with a centralizer. The segmented counterweight ensures balanced material stress, resulting in less stress compared to a single-stage bottom counterweight, avoiding excessive local stress and extending material lifespan. The segmented counterweight and high-temperature conditions at the bottom significantly reduce the tensile strength of the material, providing more options for optimizing inner tube material selection, reducing inner tube costs, effectively addressing the problem of excessive initial investment in geothermal wells, and promoting the development and utilization of clean and efficient geothermal energy.

[0035] Example application:

[0036] A medium-deep coaxial casing geothermal well is approximately 3000m deep, with an inner tube density of 0.9g / cm³. 3 It is lower than the density of the heat exchange medium by 1.0 g / cm³. 3 Lowering the inner pipe will generate buoyancy. The outer diameter of the inner pipe is 110mm, and the inner diameter is 90mm. The inner pipe is counterweighted according to the high-temperature and high-pressure resistant modified random copolymer polypropylene type deep geothermal jacking pipe: F_buoyancy = 0.785 × (0.11) 2 -0.09 2 )×3000×(1000-900)kg=942kg. Considering the natural frictional resistance between the inner tube string and the casing and the frictional force caused by local well deviation during the geothermal well construction, the counterweight mass of the inner tube is determined to be 1200kg.

[0037] Considering the impact of the counterweight on the pipe load and the issues of well inclination and pipe lowering resistance, a counterweight of 540 kg is needed at the bottom of the central pipe to ensure the lowering power of the central pipe. At the same time, a section of counterweight of 1200-540=660 kg is placed on the central pipe to reduce the load on the pipe. One section of counterweight steel pipe is set every 100 meters, with a weight of 660÷29=22.8 kg. The length of the steel pipe is taken as 1.97 m, the steel pipe model is φ121×4, the single weight is 11.54 kg / m, and the thickness is ≤4 mm. The steel pipe is placed outside the central pipe (the outer diameter of the central pipe is 110 mm and it is made of flexible material). The steel pipe can be easily inserted into the flexible inner pipe.

[0038] While the technology has been described and illustrated with respect to one or more embodiments, changes and / or modifications may be made to the illustrated examples without departing from the spirit and scope of the appended claims. In particular, with respect to the various functions performed by the aforementioned components or structures (components, devices, circuits, systems, etc.), the terminology used to describe such components (including references to “apparatus”) is intended to correspond to any component or structure performing the specified function of the described component (e.g., functionally equivalent), even if structurally not equivalent to the disclosed structure performing the function of the illustrated embodiments described herein, unless otherwise specified. Furthermore, while a particular feature may have been disclosed with respect to one of several embodiments, such feature may be combined with one or more other features in other embodiments as may be desired and advantageous for any given or particular application. Moreover, with regard to the use of the terms “comprising,” “including,” “having,” “containing,” “comprising,” or variations thereof in the detailed description or claims, such terms are intended to be inclusive in a manner similar to the term “comprising.”

Claims

1. A segmented counterweight system for the inner tube of a coaxial casing in a geothermal well, characterized in that, It includes an outer pipe, an inner pipe, and multiple counterweight pipes. The outer pipe extends into and is fixed in the geothermal well. The inner pipe is located inside the outer pipe. The inner pipe is composed of multiple sections of different sub-pipes connected together. The multiple counterweight pipes are respectively sleeved on the different sections of the sub-pipes. A convex ring is formed at the junction of the different sections of the tube to support the counterweight tube; The branch pipes are connected by heat fusion, and the aforementioned convex ring is formed by flanging. An assembly cavity is formed between the inner wall of the counterweight tube and the outer wall of the inner tube, and an L-shaped rubber sealing ring is provided in the assembly cavity located at both ends of the counterweight tube. The L-shaped rubber sealing ring includes a washer with an annular protrusion formed on it. The inner diameter of the washer is equal to the inner diameter of the annular protrusion, and the outer diameter of the washer is larger than the outer diameter of the annular protrusion. The annular protrusion is shaped like a frustum, and the diameter of the frustum gradually increases from top to bottom; The inner diameter of the counterweight tube is 1-20 mm larger than the outer diameter of the inner tube; The outer diameter of the counterweight tube is 20-40 mm smaller than the inner diameter of the outer tube; Chamfers are formed at both ends of the counterweight tube; An exhaust port is provided on the side wall of the counterweight tube.

Images