Arrangement method and design method of medium-deep layer variable-depth buried pipe group

Abstract

The invention belongs to the technical field of buried pipe geothermal heating, and relates to an arrangement method and a design method for a middle-deep layer variable-depth buried pipe group, the buried pipe group comprises not less than three buried pipes, the depth range of the buried pipes is 2000-4000 m, and the buried pipes are arranged according to different depths in the dominant underground water flow direction or the thermal load distribution direction; the depth arrangement mode of the buried pipes comprises shallow, medium, deep, medium and shallow, and deep, medium, shallow, medium and deep; the medium-deep layer buried pipes are designed into a plurality of three-dimensional heat exchange structures which are different in depth and are arranged in a staggered mode in the same area, so that heat interference is dispersed, the heat recovery effect is improved, temperature field distribution can be remarkably improved, cold accumulation can be relieved, and the sustainability and stability of long-term operation of a buried pipe group are effectively improved.

Description

Technical Field

[0001] This invention belongs to the field of geothermal heating technology, specifically relating to a method and design for arranging medium-deep variable-depth buried pipe groups. Background Technology

[0002] Currently, medium-deep geothermal systems generally adopt a design of buried pipe groups at equal depths, meaning that all buried pipes are drilled to the same depth, typically 2500 meters or 3000 meters. Existing research and optimization mainly focus on horizontal layout parameters, such as pipe spacing and arrangement (e.g., linear, rectangular, quincunx), or on adjusting operational strategies, such as intermittent operation and variable flow control.

[0003] However, vertically, all buried pipes remain at similar or the same depth, resulting in a continuous and concentrated extraction of heat from the soil and rock mass during long-term continuous heat extraction. This creates a significant thermal interference zone, especially at the same depth level, making it difficult for the soil and rock mass to recover its heat and leading to a "cold accumulation" in the central area. Specifically, this manifests as follows: 1. Severe and difficult-to-recover thermal interference: Since all buried pipe heat exchange sections are at the same depth, the surrounding soil and rock mass forms a continuous low-temperature zone (commonly known as "cold accumulation") under long-term continuous heat extraction. For example, after 5 to 10 years of operation, the temperature of the soil and rock mass in this area may drop by 8 to 12°C, and the natural recovery is extremely slow, which seriously affects the subsequent heat extraction efficiency.

[0004] 2. System performance declines year by year: As the operating time increases, the outlet water temperature shows a significant downward trend. Data from a real project shows that, under the same depth layout, the average winter outlet water temperature in the 10th year of system operation is about 4-6°C lower than that in the first year, resulting in a decrease in the coefficient of performance (COP) of the heat pump unit and a decline in the heating supply guarantee capacity.

[0005] 3. Uneven utilization of underground heat resources: Existing designs fail to fully utilize the vertical geothermal gradient. There are differences in thermal properties and thermal recovery rates between shallow strata (e.g., 2000–2500 meters) and deep strata (e.g., 2500–4000 meters), but equal-depth designs cannot achieve differentiated heat extraction and stratified recovery, resulting in a situation where some strata are "over-exploited" while others are "idle."

[0006] Therefore, there is an urgent need for a medium-deep underground pipeline system that offers more balanced utilization of thermal resources and better sustainability and stability, in order to solve the aforementioned technical problems. Summary of the Invention

[0007] This invention designs medium-deep underground pipes (2000–4000m) at multiple different depths within the same area, which are staggered to form a three-dimensional heat exchange structure, thereby dispersing thermal interference and improving the heat recovery effect.

[0008] The present invention provides the following technical solution: a method for arranging a medium-deep variable-depth underground pipe group, wherein the underground pipe group includes no less than 3 underground pipes, the depth range of the underground pipes is 2000 meters to 4000 meters, and the underground pipes are arranged at different depths along the dominant groundwater flow direction or heat load distribution direction; the depth arrangement of the underground pipes includes: shallow-medium-deep-medium-shallow, deep-medium-shallow-medium-deep.

[0009] Preferably, the buried pipe is a coaxial sleeve, and the backfill material for the buried pipe group is reinforced thermally conductive cement.

[0010] This invention also discloses a design method for the layout of medium-deep variable-depth buried pipe groups. This design method is used to design the aforementioned medium-deep variable-depth buried pipe group layout, and the design method includes the following steps: Step S1: Determine basic parameters, including the geological structure, thermal conductivity of soil and rock, geothermal gradient, static water level and flow velocity of groundwater; and preliminarily determine the total number of buried pipes based on the building heat load and the heat extraction capacity of a single pipe.

[0011] Step S2: Three-dimensional numerical model construction. Establish a three-dimensional unsteady heat-fluid coupling model covering the entire buried pipe group and the surrounding soil and rock, and set the initial geothermal field and boundary conditions.

[0012] Step S3: Variable depth scheme design and simulation. Define multiple depth combination schemes in the three-dimensional unsteady thermal-fluid coupling model, and set the simulation operation cycle, circulation flow rate, and inlet water temperature.

[0013] Step S4, Performance Evaluation and Scheme Optimization: Extract the outlet water temperature time series data, soil temperature field distribution cloud map, and cumulative heat extraction of each scheme throughout the entire simulation period. Use a multi-objective evaluation method to conduct a comprehensive comparison and select the optimal depth combination.

[0014] Preferably, in step S2, the model is established using the OpenGeoSys platform; the initial geothermal field is calculated based on the geothermal gradient.

[0015] More preferably, the governing equations of the coupled model include: Fluid flow governing equations:

[0016] in, This is the Darcy velocity vector; For soil and rock mass permeability tensor; For fluid dynamic viscosity; Pore ​​pressure; This is the water storage coefficient; For fluid density; For source and sink items; Heat transfer control equation:

[0017] in, For temperature; For time; For the system's effective volumetric heat capacity, Porosity These are the density and specific heat capacity of the soil and rock skeleton, respectively. The effective thermal conductivity of the system; Specific heat capacity of the fluid; It is a heat source sink.

[0018] More preferably, the boundary conditions of the coupled model include: flow boundary and thermal boundary; the flow boundary includes: constant head boundary or zero flux boundary on the side of the model, watertight boundary at the bottom of the model, constant head boundary at the top of the model or considering recharge; the thermal boundary includes: constant heat flux boundary or isothermal boundary at the bottom of the model, adiabatic boundary or isothermal boundary on the side of the model, isothermal boundary or heat flux boundary at the surface.

[0019] Preferably, in step S4, the multi-objective evaluation method uses the highest average outlet water temperature and the peak temperature difference of each rock and soil body in the profile as the main indicators, and the system energy efficiency ratio decay rate as an auxiliary indicator for comprehensive comparison.

[0020] Preferably, step S4 further includes: Step S5, Construction Drawing Design and Implementation: The selected depth combination, plane coordinates, and drilling deviation control requirements are used to form construction drawings. During drilling, temperature measurement while drilling technology is used to check the temperature of key formations to ensure that the actual depth of each well deviates from the design value by less than ±50 meters. When installing buried pipes, appropriate casing running and cementing processes are used for wells of different depths to ensure wellbore integrity and heat exchange efficiency.

[0021] The beneficial effects of this invention are: 1. This invention can significantly improve the temperature field distribution and alleviate cold accumulation: Numerical simulation results show that after 10 years of operation, the temperature difference between the lowest and highest points of the soil and rock mass at a horizontal profile at a depth of 2000 meters reaches 1.35°C, while the temperature difference of the constant depth arrangement (Group A) is only 0.18°C. This indicates that the variable depth design successfully "disperses" the concentrated low-temperature zone, forming multiple independent and smaller temperature disturbance zones, which is beneficial for the soil and rock mass to recover thermally from the surrounding environment during the cooling period.

[0022] 2. This invention effectively improves long-term operational performance: After 10 years of simulated operation, the outlet water temperature of the variable depth arrangement (Group B) can still be maintained above 28.5°C at the end of the tenth heating season, which is about 8.2°C higher than the outlet water temperature of approximately 20.3°C for the same period of the constant depth arrangement (Group A). ​​Converted to the total heat output for the entire heating season, the variable depth arrangement increases the output by approximately 18% to 22% compared to the constant depth arrangement.

[0023] 3. This invention enhances the sustainability and stability of medium-deep buried pipe systems: Due to reduced thermal interference, the annual performance degradation rate of the system is significantly lowered. The annual water temperature degradation rate for a constant-depth layout is approximately 0.6°C / year, while for a variable-depth layout, it can be controlled to within 0.3°C / year, greatly extending the effective service life of geothermal fields.

[0024] 4. This invention has strong engineering feasibility and high cost-effectiveness: This invention does not require the development of new pipe materials or complex control systems; it can be achieved simply by optimizing the drilling depth design. Although the cost per meter of deep wells (such as 2900 meters) is slightly higher, by combining it with some medium and shallow wells, the increase in total system investment is controllable (approximately 5% to 10%), while the resulting long-term performance improvement and reduced degradation can significantly improve the economic efficiency of the project throughout its entire life cycle. Attached Figure Description

[0025] Figure 1 This is a schematic diagram illustrating the design steps of a method for arranging and designing a medium-deep variable-depth buried pipe group according to the present invention. Figure 2 This is a schematic diagram of the three-dimensional heat exchange model of the five-port variable depth buried pipe group of the present invention; Figure 3 This is a temperature field distribution diagram of the soil and rock mass at a horizontal profile at a depth of 2000 meters, obtained after 10 years of operation of the variable depth arrangement (Group B) and constant depth arrangement (Group A) of the present invention. Figure 4 This is a comparison of the inlet and outlet water temperatures of the five wells in the pipe group under the two arrangements of Group A and Group B of this invention, as a function of operating time (10 years). Detailed Implementation

[0026] The relevant technologies of this invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments. Obviously, the described embodiments are only some embodiments of this invention, and not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0027] The arrangement method for medium-deep variable-depth buried pipe groups in this embodiment is as follows: The underground pipe network comprises at least three underground pipes, each with a depth intentionally designed to vary between 2000 and 4000 meters. In preferred configurations, an odd number of underground pipes (e.g., five) are used, arranged along the dominant groundwater flow direction or heat load distribution direction, with depths arranged symmetrically or asymmetrically in a "shallow-medium-deep-medium-shallow" or "deep-medium-shallow-medium-deep" pattern. For example, in a rectangular site of 300m × 500m, the depths of the five underground pipes from west to east could be designed as 2300m, 2500m, 2900m, 2500m, and 2300m respectively. Each underground pipe can employ a conventional coaxial sleeve structure, with an outer pipe diameter of 177.8 mm and an inner pipe diameter of 110 mm, and the backfill material is reinforced thermally conductive cement.

[0028] The specific steps of the design method for medium-deep variable-depth buried pipe groups in this embodiment are as follows: Figure 1 As shown: Step S1: Determining Basic Parameters. Collect the site geological survey report to obtain key parameters such as stratum structure, thermal conductivity of soil and rock (e.g., 2.0–2.5 W / (m·K)), geothermal gradient (e.g., 2.8–3.2 °C / 100m), groundwater static level and flow velocity. Based on the building heat load (e.g., 5 MW) and the heat extraction capacity of a single pipe (e.g., 120–150 W / m), preliminarily determine the total number of buried pipes (e.g., 5–7).

[0029] Step S2: Construction of the 3D Numerical Model. Using the OpenGeoSys (OGS) platform, a 3D unsteady-state heat-fluid coupling model covering the entire buried pipe network and surrounding soil and rock mass is established. The model needs to accurately characterize the drilling structure, pipe materials, backfill materials, and thermal properties of each stratum, and set reasonable initial geothermal fields (calculated based on geothermal gradients) and boundary conditions (constant heat flux boundary at the bottom, and adiabatic or isothermal boundaries on the sides). The specific coupling model scheme is as follows: A three-dimensional unsteady thermal-fluid coupled model covering the entire underground pipe network and surrounding soil and rock mass was established using the OpenGeoSys (OGS) platform. This model aims to accurately simulate the interaction between groundwater seepage and heat transfer within the soil and rock mass. Its core consists of the following governing equations and key settings: 1. Fluid flow governing equations (Darcy's law and continuity equation): To simulate the effect of groundwater seepage on heat transfer, the following equation is used to describe the flow of fluid in porous media:

[0030] in, The Darcy velocity vector (m / s); is the permeability tensor of the rock and soil mass (m²). The fluid dynamic viscosity is expressed in Pa·s. Pore ​​pressure (Pa); The water storage coefficient (1 / Pa); The fluid density is (kg / m³). The source and sink terms (kg / (m³·s)) are used to simulate pumping or reinjection.

[0031] 2. The governing equation for heat transfer (energy conservation equation): To simulate the unsteady heat transfer process in rock and soil containing groundwater seepage, the following energy equation is used:

[0032] in, Temperature (K or °C); Time (s); The effective volumetric heat capacity of the system is (J / (m³·K)). Porosity These are the density and specific heat capacity of the soil and rock skeleton, respectively. The effective thermal conductivity of the system (W / (m·K)) is usually the volume-weighted average of the thermal conductivity of the fluid and the skeleton. Specific heat capacity of the fluid (J / (kg·K); The heat source concentration (W / m³) is used to simulate the heat extraction process of buried pipes.

[0033] 3. Key Model Settings: Coupling Mechanism: The above equations are solved simultaneously in the OGS platform through a "thermal-water coupling (TH)" process, i.e., the Darcy velocity obtained from the fluid equations is obtained. Substituting directly into the convection term of the energy equation, the temperature field obtained from the solution of the energy equation affects the fluid viscosity. and density Feedback is fed back to the flow equation.

[0034] Initial conditions: Initial temperature field of the entire computational domain The geothermal gradient (e.g., 2.8-3.2°C / 100m) and the annual average surface temperature are set as linear distributions.

[0035] Boundary conditions: (1) Flow boundary: The side of the model can be a constant head boundary or a zero flux boundary; the bottom is usually set as a water-impermeable boundary; the top is the ground surface, which is connected to the atmosphere, and can be set as a constant head boundary or consider recharge.

[0036] (2) Thermal boundary: The bottom of the model is set as a constant heat flow boundary (reflecting the heat flow of the earth) or a constant temperature boundary; the sides of the model are set as an adiabatic boundary or a constant temperature boundary; the surface is set as a constant temperature boundary (equal to the local annual average temperature) or a heat flux boundary.

[0037] Buried Pipe Model: The buried pipe heat exchanger (DBHE) can be integrated into the aforementioned continuous medium model using either a "line heat source" or a "discrete pipe group" model. The preferred approach is to use a discrete pipe group model, treating the inner pipe, outer pipe, and backfill material as independent entities for mesh generation. Convective heat transfer boundary conditions are applied between the inner wall surface and the circulating fluid to achieve more accurate pipe-soil heat transfer simulation. Step S3: Variable Depth Scheme Design and Simulation. Define multiple depth combination schemes in the model. For example, in addition to the control group (Group A, all 2500 meters), set up Group B (2300, 2500, 2900, 2500, 2300 meters), Group C (2300, 2700, 2500, 2700, 2300 meters), etc. The simulation is set with an operating cycle of 10 years (including 4 months of continuous heating season and 8 months of natural recovery in the non-heating season), a circulation flow rate of 25 m³ / h, and the inlet water temperature dynamically set according to the outdoor air temperature.

[0038] Step S4: Performance Evaluation and Scheme Optimization. Extract the time-series data of outlet water temperature, the temperature field distribution cloud map of the soil and rock mass, and the cumulative heat extraction for each scheme throughout the entire simulation period. Employ a multi-objective evaluation method, using "highest average outlet water temperature in the 10th heating season" and "peak temperature difference of soil and rock masses at a depth of 2000 meters" as the main indicators, supplemented by "system energy efficiency ratio attenuation rate," for comprehensive comparison, and select the optimal depth combination.

[0039] Step S5: Construction Drawing Design and Implementation. The optimized depth combinations, plane coordinates, and drilling deviation control requirements will be compiled into construction drawings. During drilling, temperature monitoring while drilling (TMD) technology must be used to verify the temperature of key formations, ensuring that the actual depth of each well deviates from the design value by less than ±50 meters. When installing underground casing, appropriate casing running and cementing processes must be used for wells of different depths to ensure wellbore integrity and heat exchange efficiency.

[0040] The model in this implementation is built on the OpenGeoSys (OGS) platform and consists of two coupled sub-models. In addition to the macroscopic geothermal-fluid coupled model for predicting the evolution of the underground thermal environment, it also includes a buried pipe well heat transfer model as a bridge connecting the design operating parameters and the underground thermal field. This model consists of a set of equations describing the heat transfer process between the fluid inside the coaxial casing, the pipe wall, and the backfill material (i.e., the core control equations (1) to (10)). Its core function is to transform the operating parameters (such as flow rate and inlet temperature) determined by S1 and the variable depth scheme defined by S3 into heat source terms in the macroscopic model. This allows for a quantitative reflection of the actual heat extraction power from the soil and rock by pipes buried at different depths.

[0041] The OpenGeoSys (OGS) platform is used, and the core control equations are as follows: Based on the symmetry characteristics of the coaxial casing buried pipe heat exchanger and the above assumptions about the model, the heat exchange inside the well can be simplified into a one-dimensional heat transfer process, and the heat conduction of the rock and soil outside the well can be simplified into a two-dimensional heat transfer process.

[0042] The fluid in the inner tube exchanges heat with the fluid in the annular cavity, and its energy equation can be expressed by the following formula: (1) In the formula: Temperature of the fluid in the inner tube (°C); The fluid velocity in the inner tube is given in m·s. -1 ; The heat transfer coefficient between the fluid in the inner tube and the fluid in the annular cavity is given by W·m. -1 ·K 1 ; Temperature of the fluid in the annular cavity / °C; The cross-sectional area of ​​the inner tube / m 2 ; For time / s, The coordinates are in meters along the well depth direction (with the wellhead as the origin and downward as positive). Fluid density / kg·m -3 , Specific heat capacity at constant pressure of fluid / J·kg -1 ·K -1 .

[0043] (2) In the formula: The convective heat transfer coefficient between the fluid in the inner tube and the tube wall is given by W·m. -2 ·K -1 ; Inner diameter of the tube / m; Thermal conductivity of the inner tube / W·m -1 ·K -1 ; The inner diameter is the outer diameter of the tube in meters. The convective heat transfer coefficient between the fluid in the annular cavity and the pipe wall is given by W·m. -2 ·K -1 .

[0044] During the flow process, the fluid in the annular cavity undergoes heat exchange simultaneously with the fluid in the inner tube and the backfill material. Its energy equation can be expressed by the following formula: (3) In the formula: The fluid velocity in the annular cavity is given in m·s. -1 ; The heat transfer coefficient between the fluid in the annular cavity and the backfill material is given by W·m. -1 ·K-1 ; Temperature of backfill material (°C); Cross-sectional area of ​​the annular cavity / m 2 .

[0045] The heat transfer coefficient between the fluid and the backfill material in the annular cavity can be expressed as: (4) In the formula: The inner diameter of the outer tube is in meters (m). Thermal conductivity of the outer tube / W·m -1 ·K -1 ; Outer diameter of the outer tube / m; Thermal conductivity of backfill material / W·m -1 ·K -1 ; The value is the borehole radius in meters. The convective heat transfer coefficient between the fluid in the annular cavity and the inner wall of the outer tube is given by W·m. -2 ·K -1 .

[0046] The convective heat transfer coefficient is calculated based on the Petukhov equation: (5) In the formula: The convective heat transfer coefficient is given by W·m. -2 ·K -1 ; For Nusselt numbers; Thermal conductivity of the fluid / W·m -1 ·K -1 ; The hydraulic diameter is given in meters (m), which can be determined by... Calculations show that Cross-sectional area in the direction of fluid flow / m 2 , The wetted perimeter is in meters.

[0047] Nusselt numbers can be calculated using the following formula: (6) In the formula: It is the Reynolds number; Based on hydraulic diameter Darcy friction factor. For Prandtl numbers, is the Darcy friction coefficient.

[0048] Simultaneously appearing in equation (6) and Both are essentially Darcy's coefficients of friction. In practical calculations, if the flow channel cross-section is uniform (hydraulic diameter constant), it is usually taken as... That is, using the same value.

[0049] Darcy's coefficient of friction is calculated by the following formula: (7) The backfill material undergoes heat exchange simultaneously with the fluid in the annular cavity and the drill wall; its energy equation is: (8) In the formula: Density of backfill material (kg·m³) -3 ; Cross-sectional area of ​​backfill material / m 2 ; Specific heat capacity of backfill material / J·kg -1 ·K -1 ; The heat transfer coefficient between the backfill material and the borehole wall is given by W·m. -1 ·K -1 ; The borehole wall temperature is expressed in °C.

[0050] The heat transfer coefficient between the backfill material and the wellbore can be expressed as: (9) The heat conduction equation for soil and rock is as follows: (10) In the formula: Density of soil and rock mass / kg·m -3 ; Specific heat capacity of rock and soil / J·kg -1 ·K -1 ; Rock and soil temperature / °C; Thermal conductivity of soil / W·m -1 ·K -1 ; The value is a radial coordinate in meters, representing the radial distance from the calculation point to the drilling center axis.

[0051] The initial geothermal field of the model is set linearly according to the geothermal gradient. The boundary conditions are set as follows: the bottom is a constant heat flux boundary, the sides are adiabatic or isothermal boundaries, and the surface is an isothermal boundary. The model transforms the radial heat transfer process of the fluid, pipe wall, and backfill material into a thermal resistance form, effectively avoiding the need for radial meshing of these parts. This overcomes the problem of modeling difficulties caused by an excessively large ratio of longitudinal to radial step size in the modeling of medium-deep buried pipe heat exchangers.

[0052] Figure 2 This is a schematic diagram of a three-dimensional heat exchange model of a group of five underground pipes with varying depths. The figure shows the control group (Group A, all 2500 meters).

[0053] like Figure 3 As shown in the diagram, after 10 years of operation, the temperature field distribution of the soil and rock mass at a horizontal profile (central profile) at a depth of 2000 meters for the variable-depth layout (Group B) and the constant-depth layout (Group A) clearly shows that the low-temperature zone is more dispersed under the variable-depth layout. This indicates that the heat absorption capacity at the same depth is different, with less mutual influence and a more reasonable thermal influence range. Through reasonable depth configuration, the variable-depth layout effectively reduces thermal interference and cold accumulation, thereby improving thermal recovery capacity and system stability.

[0054] like Figure 4 As shown, at the end of the 10th heating season, the highest outlet water temperature of the variable depth arrangement reached over 28 ℃, which is about 8.25 ℃ higher than that of the constant depth arrangement. This indicates that the variable depth arrangement can more effectively maintain stable heat extraction performance, especially during long-term operation, where its heat recovery capacity and stability are significantly improved.

[0055] The key point of this invention is the "vertical depth differentiation" spatial layout structure of the buried pipe group, that is, the depth of each well in the buried pipe group is intentionally designed to be different, forming a three-dimensional heat exchange network, which fundamentally changes the traditional planar heat extraction mode.

[0056] The "simulation-driven, performance-oriented" variable-depth optimization design process emphasizes establishing a refined three-dimensional heat-fluid coupling numerical model to perform long-term performance simulation and prediction for various candidate depth combinations. Based on clear performance indicators (water temperature, temperature drop, and heat extraction), the process compares and optimizes different schemes to ensure the reliability and superiority of the design. This process closely integrates theoretical design with engineering practice.

[0057] The thermal management mechanism of "dispersed interference and layered recovery" is as follows: by alternating deep and shallow wells, the heat extraction load is distributed in layers in the vertical direction to avoid excessive heat extraction at the same depth level; at the same time, by utilizing the difference in heat recovery rate of strata at different depths, the heat of the rock and soil is naturally replenished in a temporal and layered manner, thereby improving the overall sustainability of the system.

[0058] In summary, this invention is not only theoretically innovative and advanced, but also offers significant economic and environmental benefits in practical applications. Economically, by improving heat extraction efficiency and reducing thermal interference and cold accumulation, it lowers system operating costs, extends the effective service life of geothermal fields, and enhances the project's overall economic viability throughout its lifecycle. Environmentally, the medium-deep buried pipe system, as a clean energy utilization method, allows for more effective utilization of underground thermal energy through variable-depth layout, reducing dependence on traditional fossil fuels, lowering carbon emissions, and aligning with national energy conservation, emission reduction, and sustainable development strategies. In the future, this invention has room for further expansion and optimization. It can be combined with more practical engineering cases to continuously improve the variable-depth layout and design methods, making it applicable to a wider range of geological conditions and heat load demands. Furthermore, it can be explored for combined applications with other energy systems, such as solar and wind power, to build a more stable and efficient energy supply system. Simultaneously, with continuous technological advancements, more advanced monitoring and control systems can be introduced to monitor the operational status of the buried pipe network and changes in soil and rock temperature in real time, further improving the system's intelligence and operational performance. The method for arranging and designing medium-deep variable-depth buried pipe groups in this invention provides a practical and efficient solution for the development and utilization of medium-deep geothermal energy. It is expected to play an important role in the future energy field and promote the development and innovation of geothermal energy utilization technology in my country and even globally.

[0059] It should be emphasized that the above are merely preferred embodiments of the present invention and are not intended to limit the present invention in any way. Any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention shall still fall within the scope of the technical solution of the present invention.

Claims

1. A method for arranging medium-deep variable-depth buried pipe groups, characterized in that, The underground pipe group includes no less than 3 underground pipes, the depth of which ranges from 2,000 meters to 4,000 meters, and the underground pipes are arranged at different depths along the dominant groundwater flow direction or heat load distribution direction. The depth arrangement of the buried pipes includes: shallow-medium-deep-medium-shallow and deep-medium-shallow-medium-deep.

2. The method for arranging medium-deep variable-depth buried pipe groups according to claim 1, characterized in that, The buried pipe is a coaxial sleeve, and the backfill material for the buried pipe group is reinforced thermally conductive cement.

3. A design method for the layout of medium-deep variable-depth buried pipe groups, characterized in that, The design method is used to design the layout of the medium-deep variable-depth buried pipe group as described in claim 1 or 2, and the design method includes the following steps: Step S1: Determine basic parameters, including the geological structure, thermal conductivity of soil and rock, geothermal gradient, static water level and flow velocity of groundwater; preliminarily determine the total number of buried pipes based on the building heat load and the heat extraction capacity of a single pipe. Step S2: Three-dimensional numerical model construction. Establish a three-dimensional unsteady heat-fluid coupling model covering the entire buried pipe group and the surrounding soil and rock, and set the initial geothermal field and boundary conditions. Step S3: Variable depth scheme design and simulation. Define multiple depth combination schemes in the three-dimensional unsteady heat-fluid coupling model, and set the simulation operation cycle, circulation flow rate, and inlet water temperature. Step S4, Performance Evaluation and Scheme Optimization: Extract the outlet water temperature time series data, soil temperature field distribution cloud map, and cumulative heat extraction of each scheme throughout the entire simulation period. Use a multi-objective evaluation method to conduct a comprehensive comparison and select the optimal depth combination.

4. The design method for arranging medium-deep variable-depth buried pipe groups according to claim 3, characterized in that, In step S2, a coupled model is established using the OpenGeoSys platform; the initial geothermal field is calculated based on the geothermal gradient.

5. The design method for arranging medium-deep variable-depth buried pipe groups according to claim 4, characterized in that, The governing equations of the coupled model include: Fluid flow governing equations: in, This is the Darcy velocity vector; For soil and rock mass permeability tensor; For fluid dynamic viscosity; Pore ​​pressure; This is the water storage coefficient; For fluid density; For source and sink items; Heat transfer control equation: in, For temperature; For time; For the system's effective volumetric heat capacity, Porosity These are the density and specific heat capacity of the soil and rock skeleton, respectively. The effective thermal conductivity of the system; Specific heat capacity of the fluid; It is a heat source sink.

6. The design method for arranging medium-deep variable-depth buried pipe groups according to claim 5, characterized in that, The boundary conditions of the coupled model include: flow boundary and thermal boundary; The flow boundaries include: constant head boundaries or zero flux boundaries on the sides of the model, impermeable boundaries at the bottom of the model, and constant head boundaries or boundaries considering recharge at the top of the model. The thermal boundaries include: the constant heat flux boundary or isothermal boundary at the bottom of the model, the adiabatic boundary or isothermal boundary on the sides of the model, and the isothermal boundary or heat flux boundary on the ground surface.

7. The design method for arranging medium-deep variable-depth buried pipe groups according to claim 3, characterized in that, In step S4, the multi-objective evaluation method uses the highest average outlet water temperature and the peak temperature difference of each rock and soil body in the profile as the main indicators, and the system energy efficiency ratio decay rate as an auxiliary indicator for comprehensive comparison.

8. The design method for arranging medium-deep variable-depth buried pipe groups according to claim 3, characterized in that, Step S4 is followed by: Step S5, Construction Drawing Design and Implementation: The selected depth combination, plane coordinates, and drilling deviation control requirements are used to form construction drawings. During drilling, temperature measurement while drilling technology is used to check the temperature of key formations to ensure that the actual depth of each well deviates from the design value by less than ±50 meters. When installing buried pipes, appropriate casing running and cementing processes are used for wells of different depths to ensure wellbore integrity and heat exchange efficiency.

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