Middle-deep layer geothermal heat exchange system based on multi-branch shaft

Abstract

The invention provides a medium-deep layer geothermal heat exchange system based on a multi-branch shaft. The medium-deep layer geothermal heat exchange system comprises a main shaft subsystem, branch shaft subsystems located in a geothermal layer below the ground and a ground surface heat exchange subsystem located above the ground. The main shaft subsystem comprises four main shafts which are vertically arranged, the lower ends of the four main shafts stretch into a geothermal layer in the vertical direction, the upper ends of two main shafts communicate with each other through the ground surface heat exchange subsystem, and the upper ends of the remaining two main shafts communicate with each other through the ground surface heat exchange subsystem. The branch shaft subsystem comprises a plurality of branch shafts which are horizontally arranged, and the left ends and the right ends of the branch shafts communicate with the lower ends of the corresponding main shafts correspondingly. By the adoption of the system, self-circulation of the working fluid can be achieved through thermosyphon of the working fluid, and therefore efficient utilization of geothermal energy is achieved.

Description

Technical Field

[0001] The present invention relates to the technical field of geothermal energy development and utilization, and in particular to a mid-deep geothermal heat exchange system based on a multi-branch wellbore. Background Art

[0002] As a sustainable energy source, geothermal energy has long been valued for its environmentally friendly and renewable characteristics. Traditional geothermal energy technology usually relies on natural geothermal resources, such as underground hot water and steam, to obtain underground heat energy by drilling wells. However, this method has many technical and economic limitations and challenges. First, traditional geothermal power generation relies on specific geographical locations with rich geothermal resources, which limits its application scope and development potential. Secondly, the drilling and mining process may cause environmental problems such as groundwater pollution and surface subsidence. Thirdly, high drilling costs and maintenance costs are also important factors restricting its widespread application. In addition, the development and utilization of geothermal energy involves complex technologies such as drilling of geothermal wells, extraction and conversion of heat energy, and final application of heat energy. It requires not only a high level of technical knowledge and equipment, but also continuous technological innovation to deal with various problems encountered during the development process. In addition, since the production capacity of geothermal wells may decrease over time, especially when the geothermal wells are not fully managed and restored, the problem is more serious. Therefore, the sustainable development of geothermal energy requires effective geothermal resource management and utilization strategies to ensure that geothermal resources will not be quickly depleted due to over-exploitation. Furthermore, the energy conversion efficiency of geothermal energy is limited by many factors such as the temperature of geothermal resources and the flow rate of geothermal fluid. In some areas where geothermal resources are relatively scarce, even if geothermal energy can be mined, low temperatures may lead to low energy conversion efficiency of geothermal energy, affecting the economic benefits and practicality of geothermal energy.

[0003] Traditional geothermal systems mainly rely on a single wellbore to exchange heat with underground heat source rocks. Since the total contact area between the wellbore and the rock directly determines the rate of heat extraction, a major disadvantage of this type of system is the limited heat exchange area, resulting in low heat extraction efficiency. Due to the limitations of wellbore length and diameter, the energy output of this type of system is often limited, especially when the formation conditions are not ideal, it is difficult to achieve efficient geothermal energy development.

[0004] In traditional geothermal systems, the working fluid is often mixed with the formation fluid, which can cause changes in formation pressure and waste of resources. In addition, due to the permeability of the formation, the working fluid may leak into the surrounding formation, resulting in heat loss and reduced system heat extraction efficiency.

[0005] Traditional geothermal systems usually rely on external pumping equipment to maintain the circulation of the working fluid, which not only increases the cost of equipment and energy, but also requires regular maintenance and replacement of equipment, increasing the complexity and uncertainty of system operation. Summary of the invention

[0006] In view of this, an object of the present invention is to provide a mid-deep geothermal heat exchange system based on multi-branch wellbores to alleviate the above-mentioned problems existing in traditional geothermal systems.

[0007] An embodiment of the present invention provides a mid-deep geothermal heat exchange system based on a multi-branch wellbore, comprising a main wellbore subsystem, a branch wellbore subsystem and a surface heat exchange subsystem located above the ground; the working fluid is driven to circulate in the main wellbore subsystem and the branch wellbore subsystem by utilizing thermal siphoning of the working fluid; the main wellbore subsystem comprises a first main wellbore, a second main wellbore, a third main wellbore and a fourth main wellbore which are vertically arranged; the upper end of the first main wellbore is connected to the upper end of the second main wellbore through the surface heat exchange subsystem, the upper end of the third main wellbore is connected to the upper end of the fourth main wellbore through the surface heat exchange subsystem, and the working fluid flowing from the second main wellbore to the first main wellbore and the working fluid flowing from the fourth main wellbore to the first main wellbore are regulated by the surface heat exchange subsystem. heat is exchanged with the working fluid of the third main wellbore; the lower ends of the first main wellbore, the second main wellbore, the third main wellbore and the fourth main wellbore all extend into the geothermal layer below the ground in the vertical direction; the branch wellbore subsystem is located inside the geothermal layer; the branch wellbore subsystem includes a plurality of branch wellbores arranged horizontally; the plurality of branch wellbores include at least one first branch wellbore and at least one second branch wellbore, the left end of each of the first branch wellbore is connected to the lower end of the first main wellbore, and the right end of each of the first branch wellbore is connected to the lower end of the fourth main wellbore; the left end of each of the second branch wellbore is connected to the lower end of the second main wellbore, and the right end of each of the second branch wellbore is connected to the lower end of the third main wellbore.

[0008] As a possible implementation, the branch wellbore subsystem also includes a horizontally arranged first auxiliary wellbore; the left end of the first auxiliary wellbore is connected to the lower end of the first main wellbore, and the right end of the first auxiliary wellbore is connected to the lower end of the fourth main wellbore; the left end and the right end of each of the first branch wellbores are respectively connected to different positions on the side wall of the first auxiliary wellbore.

[0009] As a possible implementation, the branch wellbore subsystem also includes a horizontally arranged second auxiliary wellbore; the left end of the second auxiliary wellbore is connected to the second main wellbore, and the right end of the second auxiliary wellbore is connected to the lower end of the third main wellbore; the left end and the right end of each of the second branch wellbore are respectively connected to different positions on the side wall of the second auxiliary wellbore.

[0010] As a possible implementation, the vertical distance between the lower end of the first main shaft, the lower end of the second main shaft, the lower end of the third main shaft and the lower end of the fourth main shaft and the ground is not less than 2000m.

[0011] As a possible implementation, the vertical distance between the lower end of the first main shaft, the lower end of the second main shaft, the lower end of the third main shaft and the lower end of the fourth main shaft and the ground is not more than 5000m.

[0012] As a possible implementation, the surface heat exchange subsystem includes a first heat exchanger and a second heat exchanger; the upper end of the first main wellbore is connected to the upper end of the second main wellbore through the first heat exchanger, and the working fluid flowing from the second main wellbore to the first main wellbore is heat exchanged through the first heat exchanger; the upper end of the third main wellbore is connected to the upper end of the fourth main wellbore through the second heat exchanger, and the working fluid flowing from the fourth main wellbore to the third main wellbore is heat exchanged through the second heat exchanger.

[0013] As a possible implementation, it also includes a data acquisition subsystem and a regulation subsystem; the data acquisition subsystem is used to collect the respective operating data of the main wellbore subsystem and the branch wellbore subsystem; the regulation subsystem is used to regulate the circulation process of the working fluid in the main wellbore subsystem and the branch wellbore subsystem.

[0014] As a possible implementation, it also includes a control subsystem, which is connected to the data acquisition subsystem and the regulation subsystem respectively, and is used to control the regulation subsystem to regulate the circulation process of the working fluid in the main wellbore subsystem and the branch wellbore subsystem based on the operating condition data.

[0015] As a possible implementation, the operating condition data includes temperature data, pressure data and flow data; the data acquisition subsystem includes a temperature sensor, a pressure sensor and a flow sensor; the temperature sensor is used to collect the temperature data; the pressure sensor is used to collect the pressure data; the flow sensor is used to collect the flow data; the control subsystem is also used to determine the pressure loss of the medium-deep geothermal heat exchange system based on the pipeline parameters of the medium-deep geothermal heat exchange system and the flow parameters of the working fluid.

[0016] As a possible implementation, the regulation subsystem includes a variable speed pump and a flow control valve; the control subsystem is also used to control the respective working states of the variable speed pump and the flow control valve based on the temperature data, the pressure data, the flow data and the pressure loss, so as to regulate the circulation process of the working fluid in the main wellbore subsystem and the branch wellbore subsystem through the variable speed pump and the flow control valve.

[0017] The embodiment of the present invention provides a mid-deep geothermal heat exchange system based on a multi-branch wellbore. By introducing multiple branch wellbores, the total contact area between the wellbore and the underground heat source rock is significantly increased compared with the traditional geothermal system, thereby significantly improving the extraction efficiency of heat energy. Since the working fluid is circulated in a closed wellbore, the permeation loss and environmental pollution risk caused by the mixing of the working fluid and the formation fluid in the traditional system are effectively avoided. Moreover, the working fluid is heated up and returned to the ground after heat exchange with the underground hot rock in the closed wellbore, so the heat loss is low, and the temperature and chemical properties of the working fluid can be maintained. Stability, thereby improving the extraction efficiency of heat energy and the long-term stability of the system; by utilizing the thermal siphon of the working fluid to drive the circulation of the working fluid, the dependence on external pumping equipment is eliminated, and the thermal siphon effect can be established within a few minutes after the system is started and can continuously maintain the stable operation of the system. The system can quickly resume operation even after a long shutdown; since the temperature and density of the working fluid inside the system are dynamically changing, the thermal siphon effect can automatically adjust the circulation rate of the working fluid according to the real-time temperature change, thereby realizing efficient utilization of geothermal energy.

[0018] Other features and advantages of the present invention will be described in the following description, and partly become apparent from the description, or understood by practicing the present invention. The purpose and other advantages of the present invention are realized and obtained by the structures particularly pointed out in the description, claims and drawings.

[0019] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, preferred embodiments are given below and described in detail with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] In order to more clearly illustrate the specific implementation methods of the present invention or the technical solutions in the prior art, the drawings required for use in the specific implementation methods or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some implementation methods of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0021] Figure 1It is a structural schematic diagram of a mid-deep geothermal heat exchange system based on a multi-branch wellbore in an embodiment of the present invention;

[0022] Figure 2 This is a structural example diagram of a mid-deep geothermal heat exchange system based on a multi-branch wellbore in an embodiment of the present invention;

[0023] Figure 3 An exemplary diagram of the working fluid circulation in an embodiment of the present invention;

[0024] Figure 4 This is an example diagram of the partial structure of another medium-deep geothermal heat exchange system based on a multi-branch wellbore in an embodiment of the present invention.

[0025] Icons: 100-main wellbore subsystem; 101-first main wellbore; 102-second main wellbore; 103-third main wellbore; 104-fourth main wellbore; 200-geothermal layer; 300-branch wellbore subsystem; 301-first auxiliary wellbore; 3011-first branch wellbore; 302-second auxiliary wellbore; 3021-second branch wellbore; 400-surface heat exchange subsystem; 401-first heat exchanger; 402-second heat exchanger; 500-working fluid; 600-data acquisition subsystem; 700-regulation subsystem; 800-control subsystem. DETAILED DESCRIPTION

[0026] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution of the present invention will be clearly and completely described in combination with the embodiments below. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0027] The embodiment of the present invention provides a mid-deep geothermal heat exchange system based on a multi-branch wellbore, see Figures 1 to 3 As shown, the mid-deep geothermal heat exchange system may include a main wellbore subsystem 100 and a branch wellbore subsystem 300 and a surface heat exchange subsystem 400 located above the ground; the working fluid 500 may be driven to circulate in the main wellbore subsystem 100 and the branch wellbore subsystem 300 by thermal siphoning.

[0028] See also Figures 1 to 3As shown, the main wellbore subsystem 100 may include a first main wellbore 101, a second main wellbore 102, a third main wellbore 103 and a fourth main wellbore 104 which are vertically arranged; the upper end of the first main wellbore 101 is connected to the upper end of the second main wellbore 102 through a surface heat exchange subsystem 400, and the upper end of the third main wellbore 103 is connected to the upper end of the fourth main wellbore 104 through a surface heat exchange subsystem 400, and the working fluid flowing from the second main wellbore 102 to the first main wellbore 101 and the working fluid flowing from the fourth main wellbore 104 to the third main wellbore 103 can be heat exchanged through the surface heat exchange subsystem 400; the lower end of the first main wellbore 101, the lower end of the second main wellbore 102, the lower end of the third main wellbore 103 and the lower end of the fourth main wellbore 104 all extend into the geothermal layer 200 below the ground in a vertical direction.

[0029] See also Figures 1 to 3 As shown, the branch wellbore subsystem 300 is located inside the geothermal layer 200; the branch wellbore subsystem 300 may include a plurality of branch wellbores arranged horizontally; the plurality of branch wellbores may include at least one first branch wellbore 3011 and at least one second branch wellbore 3021, the left end of each first branch wellbore 3011 being connected to the lower end of the first main wellbore 101, and the right end of each first branch wellbore 3011 being connected to the lower end of the fourth main wellbore 104; the left end of each second branch wellbore 3021 being connected to the lower end of the second main wellbore 102, and the right end of each second branch wellbore 3021 being connected to the lower end of the third main wellbore 103.

[0030] The embodiment of the present invention provides a mid-deep geothermal heat exchange system based on a multi-branch wellbore. By introducing multiple branch wellbores, the total contact area between the wellbore and the underground heat source rock is significantly increased compared with the traditional geothermal system, thereby significantly improving the extraction efficiency of heat energy. Since the working fluid is circulated in a closed wellbore, the permeation loss and environmental pollution risk caused by the mixing of the working fluid and the formation fluid in the traditional system are effectively avoided. Moreover, the working fluid is heated up and returned to the ground after heat exchange with the underground hot rock in the closed wellbore, so the heat loss is low, and the temperature and chemical properties of the working fluid can be maintained. Stability, thereby improving the extraction efficiency of heat energy and the long-term stability of the system; by utilizing the thermal siphon of the working fluid to drive the circulation of the working fluid, the dependence on external pumping equipment is eliminated, and the thermal siphon effect can be established within a few minutes after the system is started and can continuously maintain the stable operation of the system. The system can quickly resume operation even after a long shutdown; since the temperature and density of the working fluid inside the system are dynamically changing, the thermal siphon effect can automatically adjust the circulation rate of the working fluid according to the real-time temperature change, thereby realizing efficient utilization of geothermal energy.

[0031] As a possible implementation, see Figure 1 and Figure 2As shown, the branch wellbore subsystem 300 may also include a first auxiliary wellbore 301 arranged horizontally; the left end of the first auxiliary wellbore 301 is connected to the lower end of the first main wellbore 101, and the right end of the first auxiliary wellbore 301 is connected to the lower end of the fourth main wellbore 104; the left end and the right end of each first branch wellbore 3011 are respectively connected to different positions on the side wall of the first auxiliary wellbore 301. With this design, the total contact area between the wellbore and the underground heat source rock can be further increased, and the time for the working fluid to heat the geothermal layer can be further extended, thereby further improving the efficiency of heat energy extraction.

[0032] As a possible implementation, see Figure 1 and Figure 2 As shown, the branch wellbore subsystem 300 may also include a second auxiliary wellbore 302 arranged horizontally; the left end of the second auxiliary wellbore 302 is connected to the second main wellbore 102, and the right end of the second auxiliary wellbore 302 is connected to the lower end of the third main wellbore 103; the left end and the right end of each second branch wellbore 3021 are respectively connected to different positions on the side wall of the second auxiliary wellbore 302. With this design, the total contact area between the wellbore and the underground heat source rock can be further increased, and the time for the working fluid to heat the geothermal layer can be further extended, thereby further improving the extraction efficiency of heat energy.

[0033] In actual application, see Figure 2 As shown, in order to ensure the utilization effect of geothermal energy, the vertical distance between the lower end of the first main shaft 101, the lower end of the second main shaft 102, the lower end of the third main shaft 103 and the lower end of the fourth main shaft 104 and the ground can be no less than 2000m.

[0034] In actual application, see Figure 2 As shown, in order to control the cost of wellbore layout, the vertical distance between the lower end of the first main wellbore 101, the lower end of the second main wellbore 102, the lower end of the third main wellbore 103 and the lower end of the fourth main wellbore 104 and the ground may not be greater than 5000m.

[0035] As a possible implementation, see Figures 1 to 3 As shown, the surface heat exchange subsystem 400 may include a first heat exchanger 401 and a second heat exchanger 402; the upper end of the first main wellbore 101 is connected to the upper end of the second main wellbore 102 through the first heat exchanger 401, and the working fluid flowing from the second main wellbore 102 to the first main wellbore 101 can be heat exchanged through the first heat exchanger 401, so that the heat energy extracted by the first heat exchanger 401 can be used by the user; the upper end of the third main wellbore 103 is connected to the upper end of the fourth main wellbore 104 through the second heat exchanger 402, and the working fluid flowing from the fourth main wellbore 104 to the third main wellbore 103 is heat exchanged through the second heat exchanger 402, so that the heat energy extracted by the second heat exchanger 402 can be used by the user.

[0036] Based on the above Figure 1 The mid-deep geothermal heat exchange system shown in the figure, the embodiment of the present invention also provides another mid-deep geothermal heat exchange system based on a multi-branch wellbore, see Figures 1 to 4 As shown, the mid-deep geothermal heat exchange system may also include a data acquisition subsystem 600 and a regulating subsystem 700; the data acquisition subsystem 600 may be used to collect the operating data of the main wellbore subsystem 100 and the branch wellbore subsystem 300; the regulating subsystem 700 may be used to regulate the circulation process of the working fluid 500 in the main wellbore subsystem 100 and the branch wellbore subsystem 300. Relevant personnel may control the regulating subsystem 700 according to the operating data collected by the data acquisition subsystem 600 to regulate the self-circulation process of the working fluid 500 using thermal siphon, thereby ensuring that the mid-deep geothermal heat exchange system has sufficient driving force to drive the normal circulation of the working fluid 500.

[0037] As a possible implementation, see Figures 1 to 4 As shown, the mid-deep geothermal heat exchange system can also include a control subsystem 800, which is connected to the data acquisition subsystem 600 and the regulating subsystem 700 respectively, and can be used to control the regulating subsystem 700 to regulate the circulation process of the working fluid 500 in the main wellbore subsystem 100 and the branch wellbore subsystem 300 based on the above-mentioned operating condition data.

[0038] In actual application, the above working condition data may include temperature data, pressure data, flow data, etc., which are not limited. Figure 4 As shown, the data acquisition subsystem 600 may include a temperature sensor, a pressure sensor and a flow sensor; the temperature sensor may be used to collect the above-mentioned temperature data; the pressure sensor may be used to collect the above-mentioned pressure data; the flow sensor may be used to collect the above-mentioned flow data; the control subsystem 800 may also be used to determine the pressure loss of the medium-deep geothermal heat exchange system based on the pipeline parameters of the medium-deep geothermal heat exchange system and the flow parameters of the working fluid 500.

[0039] The pipeline parameters may include the length and diameter of the pipeline and the wellbore, respectively, and the flow parameters may include the friction resistance of the working fluid 500 in the pipeline and the wellbore, and the average density and average flow rate of the working fluid 500.

[0040] As a possible implementation, see Figures 1 to 4As shown, the regulating subsystem 700 may include a variable speed pump and a flow control valve; the control subsystem 800 may also be used to control the respective working states of the variable speed pump and the flow control valve based on temperature data, pressure data, flow data and pressure loss, so as to regulate the circulation process of the working fluid 500 in the main wellbore subsystem 100 and the branch wellbore subsystem 300 through the variable speed pump and the flow control valve.

[0041] Exemplarily, after obtaining the temperature data, pressure data, flow data and pressure loss, the control subsystem 800 can determine the pressure difference used to drive the circulation of the working fluid 500 based on the temperature data, pressure data and flow data, and compare the pressure difference with the pressure loss to determine whether the pressure difference is greater than the pressure loss; if not (that is, the pressure difference is not greater than the pressure loss), the control subsystem 800 controls the working states of the variable speed pump and the flow control valve to change in the direction of increasing the pressure difference until the pressure difference is greater than the pressure loss and then stops the control; if yes (that is, the pressure difference is greater than the pressure loss), the control subsystem 800 does not need to control the working states of the variable speed pump and the flow control valve, and can achieve normal circulation of the working fluid by only using the thermal siphon of the working fluid.

[0042] For ease of understanding, the implementation method of the above-mentioned mid-deep geothermal heat exchange system is exemplarily described below by taking a specific application as an example.

[0043] See also Figures 1 to 4 As shown, the mid-deep geothermal heat exchange system may include a main wellbore subsystem 100, a branch wellbore subsystem 300, a data acquisition subsystem 600, a regulating subsystem 700, a control subsystem 800, and a surface heat exchange subsystem 400 located above the ground; the branch wellbore subsystem 300 is located inside the geothermal layer 200 below the ground; the control subsystem 800 is connected to the data acquisition subsystem 600 and the regulating subsystem 700 respectively;

[0044] The main wellbore subsystem 100 includes a first main wellbore 101, a second main wellbore 102, a third main wellbore 103 and a fourth main wellbore 104 which are arranged vertically; the surface heat exchange subsystem 400 includes a first heat exchanger 401 and a second heat exchanger 402; the upper end of the first main wellbore 101 is connected to the upper end of the second main wellbore 102 through the first heat exchanger 401, and the upper end of the third main wellbore 103 is connected to the upper end of the fourth main wellbore 104 through the second heat exchanger 402. 02; the lower ends of the first main shaft 101, the second main shaft 102, the third main shaft 103 and the fourth main shaft 104 are all vertically extended into the geothermal layer 200 below the ground; the vertical distance between the lower ends of the first main shaft 101, the second main shaft 102, the third main shaft 103 and the fourth main shaft 104 and the ground is not less than 2000m and not more than 5000m;

[0045] The branch wellbore subsystem 300 includes a first auxiliary wellbore 301, a second auxiliary wellbore 302, at least two first branch wellbores 3011 and at least two second branch wellbores 3021 which are arranged horizontally; the left end of the first auxiliary wellbore 301 is connected to the lower end of the first main wellbore 101, and the right end of the first auxiliary wellbore 301 is connected to the lower end of the fourth main wellbore 104; the left end and the right end of each first branch wellbore 3011 are respectively connected to different positions on the side wall of the first auxiliary wellbore 301; the left end of the second auxiliary wellbore 302 is connected to the second main wellbore 102, and the right end of the second auxiliary wellbore 302 is connected to the lower end of the third main wellbore 103; the left end and the right end of each second branch wellbore 3021 are respectively connected to different positions on the side wall of the second auxiliary wellbore 302;

[0046] The data acquisition subsystem 600 includes a temperature sensor for collecting temperature data, a pressure sensor for collecting pressure data, and a flow sensor for collecting flow data; the regulation subsystem 700 includes a variable speed pump, a flow control valve, etc.

[0047] During the operation of the mid-deep geothermal heat exchange system, the thermal siphon of the working fluid 500 can be used to drive the self-circulation of the working fluid 500 in the main wellbore, the auxiliary wellbore and the branch wellbore; the working fluid flowing from the second main wellbore 102 to the first main wellbore 101 can be heat exchanged through the first heat exchanger 401, and the working fluid flowing from the fourth main wellbore 104 to the third main wellbore 103 can be heat exchanged through the second heat exchanger 402, so that the heat energy extracted by the first heat exchanger 401 and the second heat exchanger 402 can be used by users.

[0048] The thermosiphon effect is a natural circulation phenomenon caused by the difference in fluid density due to temperature. When the working fluid flows from the surface to deep underground in the wellbore, the temperature gradually increases, causing the fluid density to decrease. The lower density hot fluid rises to the surface in the production wells (such as the second main wellbore 102 and the fourth main wellbore 104), while the higher density cold fluid re-enters the system through the injection wells (such as the first main wellbore 101 and the third main wellbore 103), forming a self-driven fluid circulation mechanism. This natural circulation formed by the density difference greatly reduces the demand for external energy input, making the system more economical and efficient.

[0049] According to the thermosiphon effect, there is an inverse relationship between the density and temperature of the fluid. When the temperature of the fluid increases, the molecular kinetic energy of the fluid increases, resulting in an increase in the distance between the molecules of the fluid and a decrease in the density of the fluid. On the contrary, when the temperature of the fluid decreases, the distance between the molecules of the fluid decreases and the density of the fluid increases. In the mid-deep geothermal heat exchange system, the fluid has different temperature distributions underground. The fluid absorbs heat in the heat source area (i.e., the underground hot area where the geothermal layer 200 is located in the deeper part of the underground), the temperature of the fluid increases, and the density of the fluid decreases, which makes the fluid "light" (i.e., the fluid has a smaller weight) and tends to rise. At the same time, the temperature of the fluid in the cold source area (i.e., the underground cold area where the geothermal layer 200 is located in the shallower part of the underground, the underground cold area where the geothermal layer 200 is located in the underground farther from the heat source area, and the cold area on the ground) is lower, and the density of the fluid is higher. This part of the fluid in the cold source area sinks because it is "heavy" (i.e., the fluid has a larger weight). The temperature increase in the heat source area causes the working fluid temperature to increase, which in turn causes the fluid density to decrease. Due to its low density, the hot fluid will naturally flow upward into the cold source area (i.e., the surface heat exchange device); after the fluid undergoes heat exchange, its density increases due to a drop in temperature, which causes the cooled fluid to sink near the heat source area, thereby pushing more hot fluid to flow upward, forming a cycle.

[0050] When the temperature of the heat source area rises, the hot fluid absorbs more heat in the area, the fluid temperature further rises, and the fluid density further decreases. Due to the increase in the fluid density difference, the fluid's rising velocity increases, pushing more cold fluid into the heat source area, thereby accelerating the fluid's circulation in the mid-deep geothermal heat exchange system. Conversely, when the temperature of the heat source area drops, the fluid absorbs less heat, the fluid temperature also decreases, the fluid density increases, and the fluid's rising rate slows down. At this time, the fluid's circulation rate in the mid-deep geothermal heat exchange system will automatically decrease to maintain a stable energy exchange efficiency.

[0051] The self-regulating ability of the medium-deep geothermal heat exchange system mainly comes from the natural flow of the fluid. When the temperature of the heat source area increases, the fluid flow speeds up; when the temperature of the heat source area drops, the fluid flow slows down. This mechanism enables the medium-deep geothermal heat exchange system to automatically adjust the circulation rate of the fluid under different temperature environments without relying on external energy or human intervention. Through the thermal siphon effect, the medium-deep geothermal heat exchange system can maintain efficient heat exchange under different temperature conditions. For example, when the temperature of the heat source area is high, the medium-deep geothermal heat exchange system can increase the flow rate of the fluid, increase the absorption and exchange of heat; when the temperature of the heat source area is low, the medium-deep geothermal heat exchange system can reduce the flow rate of the fluid, thereby avoiding excessive energy loss.

[0052] The thermosiphon effect actually forms a feedback loop mechanism. If the temperature of the heat source area rises, the fluid flow rate increases, and more cold fluid enters the heat source area; if the temperature of the heat source area drops, the fluid flow rate slows down to avoid excessive heat loss caused by rapid fluid flow. This feedback loop mechanism enables the medium-deep geothermal heat exchange system to automatically adjust according to the temperature changes of the underground environment, thereby maintaining stable performance under different climatic conditions and underground temperature distributions. Therefore, this self-regulating mechanism generated by the thermosiphon effect enables the medium-deep geothermal heat exchange system to automatically adapt and optimize operating parameters when facing different geological conditions and operational requirements, further improving the working efficiency and stability of the medium-deep geothermal heat exchange system.

[0053] In traditional geothermal systems, surface facilities usually include large pumping stations, cooling towers, and complex pipeline networks. These facilities not only occupy a large area, but also require a lot of maintenance and management. In addition, traditional geothermal systems often need to deal with a large amount of wastewater and waste heat, which increases the environmental burden and operating costs. The compactness and environmental friendliness of surface facilities are fully considered in the design process of the medium-deep geothermal heat exchange system. The surface facilities of the medium-deep geothermal heat exchange system mainly include storage devices for working fluids, solid removal devices, heat exchangers (i.e., heat exchangers), etc. The design of these facilities can be optimized to reduce the floor space and maintenance requirements of surface facilities, and all fluid circulation is completed in a closed-loop system, avoiding the discharge of wastewater and the use of groundwater.

[0054] Figure 3 The circulation process of the working fluid in the medium-deep geothermal heat exchange system is schematically depicted. Through this circulation method, thermal changes or insufficient production in the medium-deep geothermal heat exchange system can be avoided. Therefore, the working fluid can be rerouted and distributed within the configured wellbore, and the energy recovery from the geothermal gradient by the working fluid can be maximized by measures such as completely changing the working fluid composition, modifying the working fluid composition with additives, changing the working fluid flow rate, and changing the working fluid direction. In addition, the working fluid can be completely stopped at a predetermined location in the medium-deep geothermal heat exchange system based on performance and / or thermal issues.

[0055] After the working fluid returns to the ground from the wellbore, it first enters the solid removal device to remove solid particles in the working fluid to prevent the solid particles in the working fluid from causing wear to the subsequent components of the system. Then, the fluid after solid removal enters the heat exchanger and releases heat to the ground facilities through heat exchange with the cooling medium for power generation or heating. The cooled fluid is re-injected into the wellbore through the thermal siphon effect and continues to participate in the cycle.

[0056] The composition of the working fluid in the deep geothermal heat exchange system and the optimal flow rate of the working fluid over time can be determined according to the actual application requirements. When selecting the composition of the working fluid, it is necessary to achieve the optimal thermodynamic properties of the working fluid and ensure the integrity of the wellbore structure; based on this, the working fluid may include water, supercritical fluid, hydrocarbon, refrigerant or other types of fluids; the working fluid may also include wellbore integrity additives, which are composed of sealants, reactants, solid particles, bridging agents, plugging materials, densifiers, etc., in order to maintain sufficient compressive strength of the wellbore; drag reducers may also be added to the working fluid to achieve a wider wellbore network (i.e., a network formed by the main wellbore, auxiliary wellbore, and branch wellbore) configuration while avoiding reaching the hydraulic limit or compromising the overall thermodynamic efficiency. The circulation of the working fluid in the wellbore network is achieved by connecting the wellbore network in series, and the total flow rate of the working fluid is usually set at 40L / s to 200L / s of water equivalent. If the wellbore network adopts a parallel well loop arrangement or a combination of series and parallel well loops, the total flow rate of the working fluid will be adjusted accordingly.

[0057] During the operation of the medium-deep geothermal heat exchange system, the pressure loss of the medium-deep geothermal heat exchange system can also be determined by the control subsystem 800 based on the pipeline parameters of the medium-deep geothermal heat exchange system and the flow parameters of the working fluid 500, and the working states of the variable speed pump and the flow control valve can be controlled by the control subsystem 800 based on the temperature data, pressure data, flow data and pressure loss, so as to adjust the circulation process of the working fluid 500 in the main wellbore subsystem and the branch wellbore subsystem through the variable speed pump and the flow control valve. For example, in order to further improve the automation level and operation efficiency of the medium-deep geothermal heat exchange system, temperature sensors, pressure sensors and flow sensors can be arranged at different depths of the wellbore and in the ground pipeline respectively, so as to collect the working condition data in real time through the arranged sensors to form a data acquisition subsystem 600; the collected working condition data is transmitted to the central control unit of the control subsystem 800 through the data acquisition subsystem 600 for analysis and decision-making.

[0058] When performing analysis and decision-making, the control subsystem 800 can calculate the pipeline pressure loss and the actual driving force of the thermal siphon (i.e., the pressure difference) according to the Darcy-Weisbach equation, and automatically adjust the operation of the flow control valve (if any) and the variable speed pump (if any) to maintain the circulation of the working fluid in the optimal state.

[0059] Thermosyphon is a natural circulation based on the density difference between hot and cold fluids. For a fluid circulation system with a height difference of H, if the resistance along the way is ignored, the pressure difference between the hot and cold fluids can be approximately expressed as:

[0060] ΔP≈g·(ρ cold -ρ hot )·H

[0061] Where ΔP is the pressure difference, g is the acceleration due to gravity, and ρ cold is the cold fluid density (calculated from the temperature data combined with the relationship between temperature and fluid density), ρ hot is the thermal fluid density (calculated from the temperature data combined with the relationship between temperature and fluid density).

[0062] When considering the frictional resistance of the fluid in the pipe and wellbore, the flow of the working fluid can be analyzed using the following equation:

[0063]

[0064] Among them, f is the friction resistance coefficient, L is the length of the pipe section, D is the pipe diameter, ρ is the fluid density, and v is the average flow velocity of the fluid (calculated based on the flow data and the pipe diameter).

[0065] When the ΔP generated by the thermal siphon is greater than the flow loss (i.e., pressure loss) in the pipeline, the natural circulation of the working fluid can be achieved.

[0066] In addition, the multi-branch wellbore structure can increase the heat transfer area and improve the heat exchange efficiency under the condition of a certain overall pressure loss, thereby ensuring the effectiveness of the natural circulation of the working fluid in a larger range. The heat exchange efficiency can also be calculated in real time by detecting the inlet and outlet temperatures of the heat exchanger. If the heat exchange capacity of the system decreases or the temperature difference is abnormal, the control subsystem 800 can quickly adjust the fluid circulation rate (by adjusting the working state of the variable speed pump) or perform bypass adjustment (by adjusting the working state of the flow control valve) under the premise of ensuring the safety of fluid circulation; when the data acquisition subsystem 600 detects that the pressure or temperature of the system exceeds the corresponding threshold, the control subsystem 800 can quickly adjust the fluid circulation rate or perform bypass adjustment to adjust the pressure or temperature of the system to a normal value.

[0067] In actual application, in order to improve the modularity and scalability of deep-seated geothermal heat exchange systems, more flexible system designs can be developed to achieve rapid deployment and adjustment under different scales and different needs. For example, pluggable heat exchange modules and adaptive fluid management modules can be designed to quickly adapt and optimize operating effects under different geographical and environmental conditions. This modular strategy will also simplify the system maintenance and upgrade process and reduce the long-term operating costs of the system.

[0068] The beneficial effects of the above-mentioned mid-deep geothermal heat exchange system are mainly reflected in the following aspects:

[0069] (1) The multi-branch wellbore design significantly increases the contact area between the wellbore and the underground heat source rock, solving the problem of low energy extraction efficiency in traditional geothermal systems by optimizing heat exchange efficiency and energy output. This design allows efficient heat energy extraction within a smaller surface area.

[0070] (2) The thermosiphon effect is used to drive the circulation of the working fluid. This naturally driven circulation mechanism reduces the demand for external energy, reduces the operating cost of the system, and improves the energy efficiency of the system.

[0071] In summary, the above-mentioned medium-deep geothermal heat exchange system adopts a fully closed circulation system design, which forms an underground annular heat exchange network by connecting two vertical wellbores with multiple horizontal wellbores. The key advantage of this design is that it does not rely on the existence of specific geothermal conditions, greatly expanding the scope of geothermal energy. This design allows the working fluid (i.e., a specially designed liquid or gas) to circulate in a closed environment, be heated by contacting hot dry rocks deep in the earth's crust, and then bring the heat back to the ground. Compared with traditional open water source geothermal systems, this technology does not rely on groundwater, avoiding environmental pollution and geological risks. In the above-mentioned medium-deep geothermal heat exchange system, the working fluid relies on the thermal siphon effect to circulate in a closed loop (i.e., the natural flow of hot fluid rising and cold fluid falling), and is heated by the natural thermal gradient deep in the earth's crust. Since the circulation of the working fluid is less dependent on the pumping system, the energy efficiency of the system is significantly improved compared to traditional geothermal systems and the operating cost of the system is reduced.

[0072] The multi-branch wellbore design can cover different geothermal areas more flexibly, making the wellbore network more fully developed for geothermal reservoirs. The multi-branch wellbore design disperses the area of ​​heat extraction. Heat collection and recovery through multiple branch wellbores can help alleviate the problem of temperature drop in local areas, extend the service life of geothermal reservoirs, and improve the sustainability of geothermal resources. The multi-branch wellbore design enables the working fluid to exchange heat simultaneously in a wider reservoir area, and has a higher heat collection efficiency than traditional geothermal systems. The multi-branch wellbore technology can more efficiently capture the thermal energy of geothermal fluids and improve the heat recovery rate of the entire system by adjusting the layout direction and layout depth of each branch wellbore. At the same time, the flexibility brought by the use of multi-branch wellbore technology allows the system to operate optimally under different reservoir temperature and pressure conditions.

[0073] Since the above-mentioned medium-deep geothermal heat exchange system adopts a closed design, it does not produce greenhouse gas emissions during operation, does not involve the exploitation and use of groundwater resources, and does not have the risk of causing earthquakes or other geological problems. As an environmentally friendly energy solution, the above-mentioned medium-deep geothermal heat exchange system is suitable for use in ecologically sensitive areas.

[0074] This multi-branch wellbore design allows multiple horizontal and vertical wellbores to be arranged in a single well site, so that the above-mentioned medium-deep geothermal heat exchange system can exchange heat with a large amount of underground rocks within a smaller surface area. The effective well length of the multi-branch wellbore can reach more than 50,000 meters. Compared with the traditional closed geothermal system, the contact area between the wellbore and the rock has increased by nearly 100 times, thereby significantly improving the heat extraction rate. Since each branch wellbore is in direct contact with the underground heat source rock, the entire system can extract heat from more strata, and its energy utilization rate far exceeds that of the traditional single-wellbore geothermal system. In addition, the multi-branch wellbore design also helps to reduce the overall construction and operation costs of the system. By drilling and arranging multiple wellbores at the same time in a well site, the system can achieve large-scale construction and reduce the drilling cost per meter of wellbore. At the same time, multiple wellbores share infrastructure and surface facilities, further improving the economy of the system.

[0075] Unless otherwise specifically stated, the relative steps, numerical expressions and values ​​of the components and steps set forth in these embodiments do not limit the scope of the present invention.

[0076] If the functions are implemented in the form of software functional units and sold or used as independent products, they can be stored in a non-volatile computer-readable storage medium that is executable by a processor. Based on this understanding, the technical solution of the present invention, or the part that contributes to the prior art, or the part of the technical solution, can be embodied in the form of a software product. The computer software product is stored in a storage medium, including several instructions for enabling a computer device (which can be a personal computer, a server, or a network device, etc.) to perform all or part of the steps of the methods described in each embodiment of the present invention. The aforementioned storage medium includes: various media that can store program codes, such as a USB flash drive, a mobile hard disk, a read-only memory (ROM), a random access memory (RAM), a magnetic disk or an optical disk.

[0077] In the description of the present invention, it should be noted that the terms "center", "upper", "lower", "left", "right", "vertical", "horizontal", "inner", "outer", etc., indicating the orientation or positional relationship, are based on the orientation or positional relationship shown in the drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as limiting the present invention. In addition, the terms "first", "second", and "third" are used for descriptive purposes only, and cannot be understood as indicating or implying relative importance.

[0078] Finally, it should be noted that the above-described embodiments are only specific implementations of the present invention, which are used to illustrate the technical solutions of the present invention, rather than to limit them. The protection scope of the present invention is not limited thereto. Although the present invention is described in detail with reference to the above-described embodiments, ordinary technicians in the field should understand that any technician familiar with the technical field can still modify the technical solutions recorded in the above-described embodiments within the technical scope disclosed by the present invention, or can easily think of changes, or make equivalent replacements for some of the technical features therein; and these modifications, changes or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present invention, and should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the protection scope of the claims.

Claims

1. A mid-deep geothermal heat exchange system based on a multi-branch wellbore, characterized in that: It includes a main wellbore subsystem, a branch wellbore subsystem and a surface heat exchange subsystem located above the ground; the working fluid is driven to circulate in the main wellbore subsystem and the branch wellbore subsystem by utilizing the thermal siphon of the working fluid; The main wellbore subsystem comprises a first main wellbore, a second main wellbore, a third main wellbore and a fourth main wellbore which are arranged vertically; the upper end of the first main wellbore is communicated with the upper end of the second main wellbore through the surface heat exchange subsystem, and the upper end of the third main wellbore is communicated with the upper end of the fourth main wellbore through the surface heat exchange subsystem, and the working fluid flowing from the second main wellbore to the first main wellbore and the working fluid flowing from the fourth main wellbore to the third main wellbore are heat exchanged through the surface heat exchange subsystem; the lower end of the first main wellbore, the lower end of the second main wellbore, the lower end of the third main wellbore and the lower end of the fourth main wellbore all extend into the geothermal layer below the ground in the vertical direction; The branch wellbore subsystem is located inside the geothermal layer; the branch wellbore subsystem includes a plurality of branch wellbores arranged horizontally; the plurality of branch wellbores include at least one first branch wellbore and at least one second branch wellbore, the left end of each of the first branch wellbore is connected to the lower end of the first main wellbore, and the right end of each of the first branch wellbore is connected to the lower end of the fourth main wellbore; the left end of each of the second branch wellbore is connected to the lower end of the second main wellbore, and the right end of each of the second branch wellbore is connected to the lower end of the third main wellbore.

2. The mid-deep geothermal heat exchange system according to claim 1, characterized in that: The branch wellbore subsystem also includes a horizontally arranged first auxiliary wellbore; the left end of the first auxiliary wellbore is connected to the lower end of the first main wellbore, and the right end of the first auxiliary wellbore is connected to the lower end of the fourth main wellbore; the left end and the right end of each of the first branch wellbores are respectively connected to different positions on the side wall of the first auxiliary wellbore.

3. The mid-deep geothermal heat exchange system according to claim 1, characterized in that: The branch wellbore subsystem also includes a horizontally arranged second auxiliary wellbore; the left end of the second auxiliary wellbore is connected to the second main wellbore, and the right end of the second auxiliary wellbore is connected to the lower end of the third main wellbore; the left end and the right end of each second branch wellbore are respectively connected to different positions on the side wall of the second auxiliary wellbore.

4. The mid-deep geothermal heat exchange system according to claim 1, characterized in that: The vertical distance between the lower end of the first main shaft, the lower end of the second main shaft, the lower end of the third main shaft and the lower end of the fourth main shaft and the ground is not less than 2000m.

5. The mid-deep geothermal heat exchange system according to claim 4, characterized in that: The vertical distance between the lower end of the first main shaft, the lower end of the second main shaft, the lower end of the third main shaft and the lower end of the fourth main shaft and the ground is not more than 5000m.

6. The mid-deep geothermal heat exchange system according to claim 1, characterized in that: The surface heat exchange subsystem includes a first heat exchanger and a second heat exchanger; the upper end of the first main wellbore is connected to the upper end of the second main wellbore through the first heat exchanger, and the working fluid flowing from the second main wellbore to the first main wellbore is heat exchanged through the first heat exchanger; the upper end of the third main wellbore is connected to the upper end of the fourth main wellbore through the second heat exchanger, and the working fluid flowing from the fourth main wellbore to the third main wellbore is heat exchanged through the second heat exchanger.

7. The mid-deep geothermal heat exchange system according to claim 1, characterized in that: It also includes a data acquisition subsystem and a regulation subsystem; the data acquisition subsystem is used to collect the respective operating data of the main wellbore subsystem and the branch wellbore subsystem; the regulation subsystem is used to regulate the circulation process of the working fluid in the main wellbore subsystem and the branch wellbore subsystem.

8. The mid-deep geothermal heat exchange system according to claim 7, characterized in that: It also includes a control subsystem, which is connected to the data acquisition subsystem and the regulating subsystem respectively, and is used to control the regulating subsystem to regulate the circulation process of the working fluid in the main wellbore subsystem and the branch wellbore subsystem based on the working condition data.

9. The mid-deep geothermal heat exchange system according to claim 8, characterized in that: The operating condition data includes temperature data, pressure data and flow data; the data acquisition subsystem includes a temperature sensor, a pressure sensor and a flow sensor; the temperature sensor is used to collect the temperature data; the pressure sensor is used to collect the pressure data; the flow sensor is used to collect the flow data; the control subsystem is also used to determine the pressure loss of the medium-deep geothermal heat exchange system based on the pipeline parameters of the medium-deep geothermal heat exchange system and the flow parameters of the working fluid.

10. The mid-deep geothermal heat exchange system according to claim 9, characterized in that: The regulating subsystem includes a variable speed pump and a flow control valve; the control subsystem is also used to control the respective working states of the variable speed pump and the flow control valve based on the temperature data, the pressure data, the flow data and the pressure loss, so as to regulate the circulation process of the working fluid in the main wellbore subsystem and the branch wellbore subsystem through the variable speed pump and the flow control valve.

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