Axial variable thermal conductivity coefficient of middle-deep buried pipe backfill structure and laying method

Abstract

This invention belongs to the field of buried pipe backfilling technology, and relates to a backfilling structure and layout method for medium-deep buried pipes with axial variable thermal conductivity. The backfilling structure consists of an upper insulation section, a middle transition section, and a lower high-efficiency heat exchange section arranged sequentially from the surface to the well bottom. The depth range of each section increases sequentially, and the average thermal conductivity of each section increases sequentially. The layout method includes: obtaining and modeling basic parameters, establishing a material library and parameterizing, constructing and solving an optimized model, interpreting the scheme, and designing segments. This invention can effectively suppress thermal short-circuiting at the wellhead, significantly enhance deep heat extraction, improve system performance, and extend the sustainability of geothermal fields. Therefore, this invention reduces thermal disturbance in shallow strata, is conducive to the thermal recovery of the rock and soil, and can slow down the performance degradation rate of the system during long-term operation.

Description

Technical Field

[0001] This invention belongs to the field of buried pipe backfilling technology, specifically relating to a medium-deep buried pipe backfilling structure and layout method with axially variable thermal conductivity. Background Technology

[0002] Currently, backfill materials for medium-deep coaxial casing buried pipes generally adopt a homogeneous design, meaning that the same proportion of backfill grout is used throughout the entire borehole depth, such as conventional bentonite-cement based grout. Its core functions are to fix the casing, seal the wellbore, and provide basic thermal conductivity channels. Related research largely focuses on finding a homogeneous material with the "optimal cost-performance ratio," achieving a balance between strength, cost, and overall thermal conductivity. Conventional solutions in existing technologies include: maintaining a constant thermal conductivity of the backfill material throughout the well section, typically 1.5–2.0 W / (m·K). During construction, a one-time or continuous injection method is used to ensure uniform material distribution within the annulus.

[0003] However, existing backfilling schemes for medium-deep coaxial casing buried pipes have the following drawbacks: Exacerbating the thermal short-circuit effect at the wellhead: When extracting heat from medium-deep buried pipes, the high-temperature fluid downhole (reaching 60-80℃) flows through the upper near-surface low-temperature section (0-500 meters, where the rock and soil temperature is close to the annual average air temperature, such as 5-10℃). The homogeneous backfill material in this section still maintains a high thermal conductivity, becoming a significant heat loss channel, resulting in a large amount of heat being lost to the shallow low-temperature rock layers, reducing the outlet water temperature and decreasing the system COP.

[0004] The heat extraction capacity of the high-temperature section downhole is not fully utilized: In the high-temperature section below 1500 meters in depth (where the temperature of the rock and soil can reach 50-80℃), we hope to enhance the heat exchange between the rock and soil and the pipe wall. However, the thermal conductivity of homogeneous backfill materials is limited by cost and strength, and is usually not optimal, which limits the extraction efficiency of deep geothermal resources.

[0005] The design concept is passive and fails to address the differences in axial thermal requirements: the existing design does not consider the fundamental changes in geothermal gradient and heat transfer tasks along well depth. The upper part needs "insulation" and the lower part needs "heat conduction," but homogeneous materials cannot simultaneously meet these two conflicting needs, making it a "compromise" rather than an "optimization" design.

[0006] Therefore, a backfilling method that can effectively suppress wellhead thermal short circuits and significantly enhance deep heat extraction is needed to solve the above-mentioned technical problems. Summary of the Invention

[0007] This invention aims to abandon the traditional approach of "homogeneous backfill" and propose a backfill structure with "axial functional gradient". Based on the objective law that the geothermal gradient increases with depth and the heat exchange task changes from "preventing heat loss" to "enhancing heat extraction", backfill materials with different thermal conductivity are actively designed and injected in sections along the well depth direction.

[0008] The present invention provides the following technical solutions: A backfill structure for medium-deep buried pipes with axially variable thermal conductivity. The backfill structure sequentially includes, from the ground surface to the bottom of the well: an upper insulation section and a lower high-efficiency heat exchange section. The depth range of the upper insulation section is H1, and the average thermal conductivity λ1 of the backfill material in the upper insulation section is 0.6 - 1.2 W / (m·K). The depth range of the lower high-efficiency heat exchange section is H3, and the average thermal conductivity λ3 of the backfill material in the lower high-efficiency heat exchange section is 2.2 - 3.5 W / (m·K). Among them, H1 < H3. The value range of H1 is 0 - 800 meters, and the value range of H3 is from 1500 meters underground to the bottom of the well.

[0009] Preferably, a middle transition section is further provided between the upper insulation section and the lower high-efficiency heat exchange section. The depth range of the middle transition section is H2, and the average thermal conductivity λ2 of the backfill material in the middle transition section is 1.5 - 2.0 W / (m·K). Among them, H1 < H2 < H3.

[0010] More preferably, the backfill material of the upper insulation section uses a composite material of bentonite doped with lightweight heat-insulating aggregates. The backfill material of the middle transition section uses a bentonite-cement-based backfill material. The backfill material of the lower high-efficiency heat exchange section uses a composite material of high-grade cement-based material doped with high-thermal-conductivity fillers.

[0011] The present invention also discloses a layout method for a backfill structure of medium-deep buried pipes with axially variable thermal conductivity. This layout method is used to layout the above-mentioned backfill structure, and the layout method includes the following steps: Step S1: Acquisition and modeling of basic parameters; collect the geothermal gradient curve of the site, the thermal physical properties of rock and soil layers, the designed well depth, the casing size, the expected operating flow rate and the inlet temperature; establish a three-dimensional transient heat-fluid coupling numerical model including detailed geological layers, coaxial casings and backfill layers.

[0012] Step S2: Establishment of a material library and parameterization; establish a database of backfill material properties, and the database includes the thermal conductivity λ, compressive strength, and cost of materials under different ratios; in the three-dimensional transient heat-fluid coupling numerical model, the backfill layer is discretized into multiple segments along the depth direction, and each segment is independently assigned the thermal conductivity λ(z) from the material library, where z is the depth coordinate.

[0013] Step S3: Optimization model construction and solution; establish an optimization function with the maximum total heat extraction during the entire heating season and the minimum wellhead heat loss power as multi-objectives; the design variable is the thermal conductivity λ(z) of the backfill material in each depth segment; the constraint conditions include: material strength requirements, cost budget, and the monotonic non-decreasing property of the thermal conductivity λ(z); use genetic algorithm or particle swarm algorithm coupled with numerical simulation for iterative optimization.

[0014] Step S4: Solution interpretation and segmented design; The optimal solution λ*(z) output by the optimization algorithm is a curve that varies with depth; Based on engineering feasibility, the curve is clustered into 2 to 4 steps, and the representative thermal conductivity λi and depth interval Hi of each step are determined, thus obtaining the final segmented design scheme.

[0015] Preferably, step S4 further includes: Step S5: Material Proportioning and Construction Guidance; Based on the thermal conductivity λi value, match the specific material proportions from the material library or determine them through experiments, and output the formula, injection volume, and injection sequence of the backfill grout for each section.

[0016] Preferably, in step S1, the three-dimensional transient thermal-fluid coupling numerical model adopts a three-dimensional, unsteady, thermal-water coupling modeling framework, based on the dual-continuous medium theory and the finite element method, and is implemented on an open-source multiphysics simulation platform.

[0017] More preferably, the governing equations of the three-dimensional transient thermal-fluid coupling numerical model include: Control equations for fluid flow in porous media:

[0018] in, Here, ρ_f is the Hamiltonian operator, K is the intrinsic permeability tensor of the rock and soil, μ_f is the dynamic viscosity of the fluid, p is the pore pressure, g is the gravitational acceleration, z is the vertical coordinate, Q_f is the source and sink term, S_s is the water storage coefficient, and t is the time.

[0019] The governing equation for heat transport in porous media:

[0020] Where T is the temperature, (ρ C_p)_{eff} is the effective volumetric heat capacity of the system, C_{p,f} is the specific heat capacity of the fluid at constant pressure, λ_{eff} is the effective thermal conductivity tensor of the system, and Q_T is the heat source sink term.

[0021] One-dimensional flow heat transfer control equations for circulating fluid within a coaxial sleeve:

[0022] Where T_f is the temperature of the circulating fluid, and A_f is the cross-sectional area of ​​the flow channel. q_w represents the mass flow rate, and q_w represents the heat exchanged with the pipe wall per unit length of pipe.

[0023] Thermal conductivity equations for backfill material and casing wall:

[0024] In this context, the subscript 'm' represents different materials.

[0025] More preferably, the settings of the three-dimensional transient thermal-fluid coupling numerical model include: Initial conditions: The initial temperature field of the entire computational domain is set according to the geothermal gradient and the annual average surface temperature; the initial pore pressure field is set according to the hydrostatic pressure distribution.

[0026] Boundary conditions include: flow boundary and thermal boundary; flow boundary is: constant head boundary or zero flux boundary on the side of the model; water-impermeable boundary at the bottom of the model; free water surface or constant head boundary at the top of the model; thermal boundary is: constant heat flux boundary or isothermal boundary at the bottom of the model; adiabatic boundary or isothermal boundary on the side of the model; and third-type boundary condition or isothermal boundary at the surface that exchanges temperature with the atmosphere.

[0027] More preferably, the coupling mechanism of the three-dimensional transient heat-fluid coupled numerical model includes: Coupling interface processing: Continuous boundary conditions are set between the outer wall of the buried pipe and the backfill material, and between the backfill material and the borehole wall soil. Energy exchange is achieved by spatially coupling the one-dimensional pipe flow model and the three-dimensional porous medium model through the residual flow method or the equivalent thermal resistance method.

[0028] Spatial discretization and non-uniformity: A highly unstructured three-dimensional mesh is established using mesh generation tools; the thermal properties of soil and rock are set independently for different geological layers; the backfill material area is subdivided along the axial direction, and different thermal conductivity λ_m values ​​are assigned to mesh cells in different depth ranges according to the optimized design scheme, thereby physically realizing the "axial variable thermal conductivity" structure.

[0029] Preferably, in step S3, the reliability verification method of the three-dimensional transient thermal-fluid coupling numerical model includes: Analytical solution verification: Under simple boundary conditions, the numerical solution of the model is compared with the analytical solution of the classic moving line heat source to ensure the correctness of the core algorithm.

[0030] Verification using measured engineering data: Long-term soil and rock temperature field data obtained from distributed fiber optic thermometry in the collaborative project, as well as inlet and outlet water temperature data during the operation of the buried pipe, were used to invert and correct the model, and key formation parameters were adjusted until the simulation results and measured data were within an acceptable error range.

[0031] The beneficial effects of this invention are: 1. This invention can effectively suppress wellhead thermal short circuits: the upper insulation section (λ1≈0.9 W / (m·K)) reduces heat loss of the annular fluid near the wellhead by 40%-60%. Simulations show that at a well depth of 500 meters, the annular fluid temperature of this invention is 3-5℃ higher than that of the homogeneous backfill scheme (λ=1.8 W / (m·K)).

[0032] 2. This invention can significantly enhance deep heat extraction: the lower high-efficiency heat exchange section (λ3≈2.5 W / (m·K)) reduces the thermal resistance between the borehole wall and the deep high-temperature rock and soil by 20%-30%. At a well depth of 2500 meters, the heat flux density increases by 15%-25%, which means that more heat is extracted from the deep rock and soil.

[0033] 3. This invention can comprehensively improve system performance: The combined effect of the above two points results in an increase of 1.5-3.5℃ in the outlet water temperature of the buried pipe under the same operating conditions. Taking the simulated data of a project in the Songliao Basin of Jilin Province as an example, after adopting this invention, the average outlet water temperature during the entire heating season increased from 38.9℃ to 41.8℃, and the system's coefficient of performance (COP) is expected to increase by 8%-12%.

[0034] 4. This invention can extend the sustainability of geothermal fields: by optimizing the heat flow distribution throughout the well section, it reduces the thermal disturbance of shallow strata, which is conducive to the thermal recovery of the rock and soil mass and can slow down the performance degradation rate of the system during long-term operation. Attached Figure Description

[0035] Figure 1 This is a schematic diagram of the backfill structure for a medium-deep buried pipe with axially variable thermal conductivity and its layout method, according to the present invention. Figure 2 This is a flowchart of the deployment method of the present invention. Detailed Implementation

[0036] 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.

[0037] like Figures 1-2 As shown in the figure, in a backfill structure and layout method for a medium-deep buried pipe with axially variable thermal conductivity according to this embodiment, the backfill structure is as follows: Figure 1 As shown, the backfill structure consists of at least two sections (preferably three sections) of material with increasing thermal conductivity from the surface to the bottom of the well.

[0038] Upper insulation section: Depth range H1 (e.g., 0–800 meters). The main function of this section is thermal insulation, reducing heat loss of the annular fluid upwards. Low thermal conductivity materials are selected, with an average thermal conductivity λ1 designed to be 0.6–1.2 W / (m·K). Composite materials made of bentonite mixed with lightweight insulating aggregates (such as vitrified microspheres, expanded perlite, and closed-cell ceramsite) can be used to significantly reduce thermal conductivity while meeting cementing strength requirements.

[0039] Lower high-efficiency heat exchange section: depth range H3 (e.g., 1500 meters to the bottom of the well). The main function of this section is to maximize heat extraction. High thermal conductivity materials are selected, with an average thermal conductivity λ3 designed to be 2.2–3.5 W / (m·K). High-grade cement-based materials mixed with high thermal conductivity fillers (such as graphite powder, quartz sand, silicon carbide, or appropriate amounts of metal oxide powder) can be used to significantly improve the heat transfer capacity of this section.

[0040] Intermediate transition section (optional): Depth range H2 (between H1 and H3). This section serves to connect the upper and lower sections, buffer stress, and match material properties. The average thermal conductivity λ2 is designed to be 1.5–2.0 W / (m·K). Conventional bentonite-cement based backfill material or a transitional mix of the two materials can be used.

[0041] The relationship among the three is: λ1 < λ2 < λ3. The depth ranges H1, H2, and H3 of each segment need to be optimized and determined based on the geothermal gradient, soil thermal properties, design well depth, and operating parameters of the specific site, using the following design method.

[0042] The method for laying out backfill structures for medium-deep underground pipes with axially variable thermal conductivity is used to determine the specific parameters of the aforementioned backfill structure with variable thermal conductivity, including segment depth and target thermal conductivity for each segment. The method flow is as follows: Figure 2 As shown, the specific steps are as follows: Step S1: Basic Parameter Acquisition and Modeling. Collect site geothermal gradient curves, geothermal properties (thermal conductivity, specific heat capacity), design well depth, casing size, expected operating flow rate, and inlet temperature. Establish a three-dimensional transient thermal-fluid coupled numerical model based on an open-source finite element platform (such as OpenGeoSys), including detailed geological stratification, coaxial casing, and backfill layers.

[0043] Step S2: Establish Material Library and Parameterization. Establish a database of backfill material properties, including thermal conductivity λ, compressive strength, and cost for materials with different proportions. In the model, the backfill layer is discretized into multiple segments along the depth direction. Each segment can be independently assigned a thermal conductivity λ(z) from the material library, where z is the depth coordinate.

[0044] Step S3: Model Construction and Solution. An optimization function is established with the multiple objectives of maximizing the total heat extraction (Qmax) and minimizing the wellhead heat loss (Ploss) for the entire heating season. The design variable is the thermal conductivity λ(z) of the backfill material at each depth. Constraints include: material strength requirements, cost budget, and the monotonicity of λ(z) (i.e., the thermal conductivity does not decrease with increasing depth). Iterative optimization is performed using a genetic algorithm (GA) or particle swarm optimization (PSO) coupled with numerical simulation.

[0045] Step S4: Solution Interpretation and Segmented Design. The optimal solution λ*(z) output by the optimization algorithm is a curve that varies with depth. Based on engineering feasibility, this curve is clustered into 2-4 steps, and the representative thermal conductivity λi and depth interval Hi of each step are determined, thus obtaining the final segmented design scheme.

[0046] Step S5: Material Proportioning and Construction Guidance. Based on the λi value, match the materials from the material library or determine the specific material proportions (e.g., the ratio of bentonite: vitrified microspheres: water; the ratio of cement: quartz sand: graphite powder: water). Output the formula, injection volume, and injection sequence of the backfill grout for each section.

[0047] The details of the three-dimensional transient thermal-fluid coupling numerical model described in step S1 are as follows: The model adopts a three-dimensional, unsteady (transient), heat-water (TH) coupled modeling framework, based on the dual continuous medium theory and the finite element method (FEM), and is implemented on the open-source multiphysics simulation platform OpenGeoSys (OGS).

[0048] I. Model governing equations: The core of the model consists of the following set of mutually coupled partial differential equations: (1) Control equations for fluid flow in porous media (groundwater seepage field): The modified Darcy's law is used to describe the transient flow of groundwater in porous soil and rock media:

[0049] in: ρ is the Hamiltonian operator; ρ_f is the fluid density (kg / m³); K is the intrinsic permeability tensor of the rock and soil mass (m²); μ_f is the fluid dynamic viscosity (Pa·s); p is the pore pressure (Pa); g is the gravitational acceleration (m / s²); z is the vertical coordinate (m); Q_f is the source and sink term (kg / (m³·s)); S_s is the water storage coefficient (1 / Pa); t is the time (s).

[0050] Solving this equation yields the pore pressure field p(x,y,z,t) and the Darcy velocity field v_d(x,y,z,t), providing the convection velocity for the heat transport equation.

[0051] (2) Controlling equations for heat transport in porous media (temperature field): Based on the obtained seepage field, an energy conservation equation considering heat conduction, heat convection, and heat dissipation is established:

[0052] Where: T is the temperature (K or °C); (ρ C_p)_{eff} is the effective volumetric heat capacity of the system (J / (m³·K)) [The calculation formula is:] φ represents porosity, ρ_s and C_{p,s} represent the density and specific heat capacity of the soil skeleton, respectively; C_{p,f} represents the specific heat capacity of the fluid at constant pressure (J / (kg·K)); λ_{eff} represents the effective thermal conductivity tensor of the system (W / (m·K)), considering the heat conduction of the solid and the heat diffusion effect of the fluid; Q_T represents the heat source sink term (W / m³), mainly used to characterize the heat exchange between the buried pipe and the soil.

[0053] This equation is key to heat-fluid coupling, where the convection term ρ_f C_{p,f}·(v_d· T) directly introduces the velocity field v_d from the flow equation, realizing the coupling of the two fields.

[0054] (3) One-dimensional flow heat transfer control equations for circulating fluid inside the coaxial sleeve: If the circulating fluid inside the buried pipe is considered as a one-dimensional flow along the well depth s, its energy equation simplifies to:

[0055] Where: T_f is the circulating fluid temperature (°C), which is a function of depth s and time t; A_f is the cross-sectional area of ​​the flow channel (m²), calculated separately for the inner tube and the annulus; q_w represents the mass flow rate (kg / s); q_w represents the heat exchanged with the pipe wall per unit length (W / m), serving as a bridge connecting the fluid inside the pipe with the soil and rock model.

[0056] For coaxial sleeves, the above equations need to be established for the fluid in the inner tube and the fluid in the annulus, and coupled through the heat conduction conditions of the tube wall.

[0057] (4) Thermal conductivity equations for backfill material and casing wall: For solid areas such as backfill material layers and the metal walls of casings, heat transfer is considered only by thermal conduction:

[0058] Here, the subscript m represents different materials (such as insulation layers, steel sleeves, and backfill material sections with different thermal conductivity). The axially variable thermal conductivity backfill structure is achieved by assigning different λ_m values ​​to backfill material units at different depth ranges in this equation.

[0059] II. Key Model Settings and Coupling Mechanisms: Initial conditions: The initial temperature field T(x,y,z, t=0) of the entire computational domain is set according to the geothermal gradient G (°C / 100m) and the annual average surface temperature T_surface as: T(z) = T_surface + G*z / 100. The initial pore pressure field is set according to the hydrostatic pressure distribution.

[0060] Boundary conditions: 1. Flow boundary: The sides of the model can be constant head boundary or zero flux boundary; the bottom is set as a watertight boundary; the top is a free water surface or constant head boundary.

[0061] 2. Thermal boundary: The bottom of the model is set as a constant heat flux boundary (reflecting the earth's heat flux density) or a isothermal boundary; the sides of the model are set as adiabatic boundaries or isothermal boundaries; the surface is set as a third type of boundary condition or an isothermal boundary that exchanges temperature with the atmosphere.

[0062] Coupling interface treatment: Continuous boundary conditions, i.e., continuous temperature and heat flux density, are set between the outer wall of the buried pipe and the backfill material, and between the backfill material and the borehole wall soil. Energy exchange is achieved by spatially coupling the one-dimensional pipe flow model with the three-dimensional porous medium model using the residual flow method or the equivalent thermal resistance method.

[0063] Spatial Discretization and Non-homogeneity: A highly unstructured 3D mesh is established using mesh generation tools such as Gmsh. The geothermal properties (K, λ_s, ρ_s, C_{p,s}, φ) are independently set for different geological layers (such as Quaternary loose layers, Neogene, and Cretaceous strata). The key innovation lies in subdividing the backfill material area along the axial direction (Z-direction). Based on the optimized design scheme, different thermal conductivity λ_m values ​​are assigned to mesh cells at different depth intervals, thus physically realizing an "axially variable thermal conductivity" structure.

[0064] III. Model Solving and Verification: The aforementioned governing equations were solved simultaneously in the OGS platform using a "thermal-water coupling (TH)" process. Spatial discretization was performed using the finite element method, and temporal discretization was performed using implicit time-progression methods (such as backward difference). Large linear equation systems were handled using parallel solvers such as PETSc or Eigen.

[0065] The reliability of the model was verified in the following ways: Analytical solution verification: Under simple boundary conditions (such as a constant linear heat source), the numerical solution of the model is compared with the analytical solution of a classic moving linear heat source to ensure the correctness of the core algorithm.

[0066] Verification using measured engineering data: Long-term soil and rock temperature field data obtained from distributed optical fiber temperature measurement (DTS) in cooperative projects (such as the Jilin Da'an medium-deep geothermal project), as well as inlet and outlet water temperature data during the operation of buried pipes, are used to invert and correct the model, and adjust key formation parameters until the simulation results and measured data are within an acceptable error range (such as mean absolute error MAE < 5%).

[0067] The construction method for backfill structures of medium-deep underground pipes with axially variable thermal conductivity is as follows: A bottom-up segmented grouting method is adopted to ensure clear interfaces between different grouts and prevent mixing.

[0068] 1. Lower the grouting pipe to a certain distance from the bottom of the well.

[0069] Second, first inject the backfill grout with the highest thermal conductivity (λ3), with the injection volume precisely corresponding to the annular volume of the lower high-efficiency heat exchange section (H3). After injection, you can wait for it to initially set, or temporarily seal it with biodegradable gel / physical partitions.

[0070] 3. Raise the grouting pipe and inject grout into the middle transition section (λ2).

[0071] IV. Finally, inject the slurry of the upper insulation section (λ1) to the wellhead.

[0072] 5. After all the slurry has solidified, a complete axially variable thermal conductivity backfill structure is formed.

[0073] Alternatives to this implementation include: a two-section structure: For projects with simple geological conditions or strict cost control, it can be simplified to a two-section structure consisting only of an "upper insulation section" and a "lower high-efficiency heat exchange section," omitting the middle transition section. The thermal conductivity of the two sections should satisfy λupper < λlower.

[0074] Material substitution: (1) Upper insulation section: In addition to the bentonite-vitrified microsphere system, foamed cement, hydrophobic expanded perlite slurry, or composite slurry mixed with polymer heat insulation microspheres can also be used.

[0075] (2) Lower high-efficiency heat exchange section: In addition to cement-graphite / quartz sand system, high thermal conductivity ceramsite concrete, thermally conductive cement-based composite materials with added carbon fiber or steel fiber can also be used.

[0076] Alternative construction methods: In addition to bottom-up segmented grouting, a double-layer concentric grouting pipe can be used to inject two different grouts simultaneously, forming radial stratification in the annulus, and then adjusting the flow rate to create an axial gradient. Alternatively, prefabricated solid backfill modules with different thermal conductivity can be used, lowered into the well in segments, and then grouted to fill the voids.

[0077] The key technical point of this invention is: The core idea of ​​"axial gradient design" is to propose and systematically apply the design principle of "increasing thermal conductivity" of backfill materials for deep buried pipes along the well depth direction for the first time, and to change from "homogeneous" thinking to "functional gradient" thinking in order to match the axially differentiated thermal environment and heat exchange requirements.

[0078] The "upper insulation and lower conduction" synergistic thermal management strategy: The function of the backfill structure is actively divided into "thermal insulation" in the upper part and "enhanced thermal conduction" in the lower part, forming an internally synergistic heat exchange system, which solves the two contradictory problems of "wellhead heat loss" and "insufficient heat extraction in the deep part" from the structural root.

[0079] A segmented design method based on numerical simulation and optimization algorithms provides a complete and quantitative design process. By establishing a refined heat-fluid coupling model and combining it with multi-objective optimization algorithms, it can "tailor-make" the optimal segmentation scheme (number of segments, depth, thermal conductivity value) for any specific geological conditions and engineering requirements, ensuring the scientific nature and optimality of the design.

[0080] The feasible composite slurry formulation system for the project: The specific material composition for achieving different thermal conductivity ranges has been clarified (such as the bentonite-lightweight aggregate system for insulation, and the cement-high thermal conductivity filler system for enhancing heat exchange). These materials are all commercially available or mature engineering materials, with reliable formulations and feasible construction.

[0081] The "bottom-up" segmented reverse-sequence grouting construction process proposes a key construction method to ensure the accurate positioning of grouts with different properties downhole and avoid mixing, thus ensuring the accurate realization of the design intent in the project.

[0082] This invention provides an innovative and practical backfill structure and layout method for medium-deep geothermal pipes with variable axial thermal conductivity. It exhibits significant advantages and unique value in several aspects. In terms of design concept, it breaks through the limitations of traditional "homogeneous" thinking, introducing a synergistic thermal management strategy of "axial gradient design" and "upper protection and lower conduction," precisely matching the thermal environment and heat exchange requirements at different axial positions of the medium-deep geothermal pipe. This effectively solves the problems of wellhead heat loss and insufficient deep heat extraction from the root, laying a solid theoretical foundation for the efficient utilization of medium-deep geothermal energy.

[0083] In terms of design methodology, a segmented design approach based on numerical simulation and optimization algorithms has been developed, establishing a complete and quantitative design process. By establishing a refined heat-fluid coupling model and employing multi-objective optimization algorithms, optimal segmentation schemes can be tailored to different geological conditions and engineering requirements, including the number of segments, depth, and thermal conductivity values, greatly improving the scientific rigor and optimization of the design.

[0084] Regarding material selection, a feasible composite grout formulation system for the project was clearly defined. All materials used are commercially available or mature engineering materials, such as the bentonite-lightweight aggregate system for thermal insulation and the cement-high thermal conductivity filler system for enhanced heat exchange. These materials are not only cost-controllable but also have reliable formulations and feasible construction, providing strong support for practical engineering applications.

[0085] In terms of construction technology, the "bottom-up" segmented reverse grouting construction process is a major innovation. This process can ensure the accurate positioning of grouts with different properties downhole, effectively avoid the problem of grout mixing, ensure the accurate realization of the design intent in the project, and improve construction quality and efficiency.

[0086] Furthermore, this invention provides various alternatives, such as two-section structures, continuous gradient materials, and substitutions for different material types and construction methods, demonstrating strong flexibility and adaptability to meet diverse geological conditions and project requirements. Through analytical verification and engineering measurement data validation, the reliability of the model has been fully guaranteed, further proving the feasibility and effectiveness of this invention in practical engineering applications.

[0087] In summary, the axially variable thermal conductivity backfill structure and layout method of the medium-deep geothermal pipe of the present invention has important application value in the field of medium-deep geothermal energy development and utilization, and is expected to bring positive impetus to the development of this field.

[0088] 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 backfill structure for medium-deep underground pipes with axially variable thermal conductivity, characterized in that, The backfill structure, from the surface to the bottom of the well, includes, in sequence: an upper insulation section and a lower high-efficiency heat exchange section. The depth range of the upper insulation section is H1, and the average thermal conductivity λ1 of the backfill material of the upper insulation section is 0.6 to 1.2 W / (m·K). The depth range of the lower high-efficiency heat exchange section is H3, and the average thermal conductivity λ3 of the backfill material of the lower high-efficiency heat exchange section is 2.2 to 3.5 W / (m·K). Where H1 < H3; the value of H1 ranges from 0 to 800 meters, and the value of H3 ranges from 1500 meters underground to the bottom of the well.

2. The backfill structure for medium-deep buried pipes with axially variable thermal conductivity according to claim 1, characterized in that, A middle transition section is provided between the upper insulation section and the lower high-efficiency heat exchange section; the depth range of the middle transition section is H2, and the average thermal conductivity λ2 of the backfill material of the middle transition section is 1.5~2.0W / (m·K); wherein, H1< H2< H3.

3. The backfill structure for medium-deep underground pipes with axially variable thermal conductivity according to claim 2, characterized in that, The upper insulation section backfill material is a composite material of bentonite mixed with lightweight heat-insulating aggregate; the middle transition section backfill material is a bentonite-cement-based backfill material; and the lower high-efficiency heat exchange section backfill material is a composite material of high-grade cement-based material mixed with high thermal conductivity filler.

4. A method for laying out a backfill structure for medium-deep underground pipes with axially variable thermal conductivity, characterized in that, The layout method is used to lay out the backfill structure according to any one of claims 1 to 3, and the layout method includes the following steps: Step S1: Acquisition and modeling of basic parameters; collect site geothermal gradient curves, geothermal properties of soil and rock layers, design well depth, casing size, expected operating flow rate and inlet temperature; establish a three-dimensional transient thermal-fluid coupling numerical model including detailed geological layers, coaxial casing and backfill layer; Step S2: Establish a material library and parameterize it; establish a backfill material performance database, which includes the thermal conductivity λ, compressive strength, and cost of materials with different proportions; in the three-dimensional transient thermal-fluid coupling numerical model, the backfill layer is discretized into multiple segments along the depth direction, and each segment is independently assigned a thermal conductivity λ(z) from the material library, where z is the depth coordinate; Step S3: Model construction and solution; establish an optimization function with the goal of maximizing total heat extraction and minimizing heat loss at the wellhead during the entire heating season; design variables are the thermal conductivity λ(z) of the backfill material at each depth; constraints include: material strength requirements, cost budget, and the monotonicity of the thermal conductivity λ(z); iterative optimization is performed using a genetic algorithm or particle swarm optimization coupled with numerical simulation. Step S4: Solution interpretation and segmented design; The optimal solution λ*(z) output by the optimization algorithm is a curve that varies with depth; Based on engineering feasibility, the curve is clustered into 2 to 4 steps, and the representative thermal conductivity λi and depth interval Hi of each step are determined, thus obtaining the final segmented design scheme.

5. The method for laying out a medium-deep buried pipe backfill structure with axially variable thermal conductivity according to claim 4, characterized in that, Step S4 is followed by: Step S5: Material Proportioning and Construction Guidance; Based on the thermal conductivity λi value, match the specific material proportions from the material library or determine them through experiments, and output the formula, injection volume, and injection sequence of the backfill grout for each section.

6. The method for laying out a medium-deep buried pipe backfill structure with axially variable thermal conductivity according to claim 4, characterized in that, In step S1, the three-dimensional transient thermal-fluid coupling numerical model adopts a three-dimensional, unsteady, thermal-water coupling modeling framework, based on the dual-continuous medium theory and the finite element method, and is implemented on an open-source multiphysics simulation platform.

7. The method for laying out a medium-deep buried pipe backfill structure with axially variable thermal conductivity according to claim 6, characterized in that, The governing equations of the three-dimensional transient thermal-fluid coupling numerical model include: Control equations for fluid flow in porous media: in, Here, ρ_f is the Hamiltonian operator, K is the intrinsic permeability tensor of the rock and soil mass, μ_f is the fluid dynamic viscosity, p is the pore pressure, g is the gravitational acceleration, z is the vertical coordinate, Q_f is the source and sink term, S_s is the water storage coefficient, and t is the time. The governing equation for heat transport in porous media: Where T is the temperature, (ρ C_p)_{eff} is the effective volumetric heat capacity of the system, C_{p,f} is the specific heat capacity of the fluid at constant pressure, λ_{eff} is the effective thermal conductivity tensor of the system, and Q_T is the heat source term; One-dimensional flow heat transfer control equations for circulating fluid within a coaxial sleeve: Where T_f is the temperature of the circulating fluid, and A_f is the cross-sectional area of ​​the flow channel. Here, q_w represents the mass flow rate, and q_w represents the heat exchanged with the pipe wall per unit length of pipe. Thermal conductivity equations for backfill material and casing wall: In this context, the subscript 'm' represents different materials.

8. The method for laying out a medium-deep buried pipe backfill structure with axially variable thermal conductivity according to claim 7, characterized in that, The setup of the three-dimensional transient thermal-fluid coupling numerical model includes: Initial conditions: The initial temperature field of the entire computational domain is set based on the geothermal gradient and the annual average surface temperature; the initial pore pressure field is set according to the hydrostatic pressure distribution. Boundary conditions include: flow boundary and thermal boundary; The flow boundaries are as follows: the sides of the model are constant head boundaries or zero flux boundaries; the bottom of the model is a watertight boundary; and the top of the model is a free water surface or a constant head boundary. The thermal boundaries are as follows: the bottom of the model is set as a constant heat flux boundary or a constant temperature boundary; the sides of the model are set as an adiabatic boundary or a constant temperature boundary; and the ground surface is set as a third type of boundary condition or a constant temperature boundary that exchanges temperature with the atmosphere.

9. The method for laying out a medium-deep buried pipe backfill structure with axially variable thermal conductivity according to claim 7, characterized in that, The coupling mechanism of the three-dimensional transient heat-fluid coupled numerical model includes: Coupling interface treatment: Continuous boundary conditions are set between the outer wall of the buried pipe and the backfill material, and between the backfill material and the borehole wall soil; energy exchange is achieved by spatially coupling the one-dimensional pipe flow model and the three-dimensional porous medium model through the residual flow method or the equivalent thermal resistance method. Spatial discretization and non-uniformity: A highly unstructured three-dimensional mesh is established using mesh generation tools; the thermal properties of soil and rock are set independently for different geological layers; the backfill material area is subdivided along the axial direction, and different thermal conductivity λ_m values ​​are assigned to mesh cells in different depth ranges according to the optimized design scheme, thereby physically realizing the "axial variable thermal conductivity" structure.

10. The method for laying out a medium-deep buried pipe backfill structure with axially variable thermal conductivity according to claim 5, characterized in that, In step S3, the reliability verification method of the three-dimensional transient thermal-fluid coupling numerical model includes: Analytical solution verification: Under simple boundary conditions, the numerical solution of the model is compared with the analytical solution of the classic moving line heat source to ensure the correctness of the core algorithm; Verification using measured engineering data: Long-term soil and rock temperature field data obtained from distributed fiber optic thermometry in the collaborative project, as well as inlet and outlet water temperature data during the operation of the buried pipe, were used to invert and correct the model, and key formation parameters were adjusted until the simulation results and measured data were within an acceptable error range.

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