A simulation system and experimental method for periodic heat storage in a fractured aquifer

Abstract

The application provides a simulation system and experimental method for fracture water-bearing zone with periodic heat storage, and relates to the technical field of geothermal energy development and underground energy storage, which comprises a model box, the model box is filled with a porous medium, at least one fracture zone structure is constructed in the porous medium, and the fracture zone structure is filled with a material with a permeability greater than that of the porous medium; an injection-production circulation system, the injection-production circulation system comprises an injection well and a production well, and is buried in the porous medium in the model box; the injection well and the production well are connected to a fluid circulation system through pipelines, the fluid circulation system has switchable first and second working modes, wherein, in the first working mode, high-temperature hot water is provided to the injection well, and in the second working mode, the hot water extracted from the production well is recirculated to the injection well after being cooled; and a monitoring unit, the monitoring unit at least comprises a plurality of sensors for collecting temperature and pressure data inside the model box. The application effectively improves the simulation accuracy of the fracture zone on the water and heat migration process.

Description

Technical Field

[0001] This invention relates to the fields of geothermal energy development and underground energy storage technology, and more specifically, to a periodic thermal energy storage simulation system and experimental method for fractured aquifers. Background Technology

[0002] Aquifer transseasonal thermal energy storage technology, by injecting excess thermal energy underground during the warm season and extracting it for utilization during the cold season, achieves spatiotemporal energy transfer and is a key technology for large-scale spatiotemporal energy transfer and scheduling. Currently, geological reservoirs generally exhibit complex heterogeneity, particularly the presence of high-permeability fault zones, which fundamentally alters the patterns of groundwater and heat transport. High-permeability fault zones in geological reservoirs can dominate hot water transport, forming dominant flows and causing premature heat loss, severely restricting thermal energy storage efficiency. Therefore, clarifying the control mechanism of fault zones on the thermal energy storage process is a core issue in optimizing ATES engineering design and assessing its long-term operational risks.

[0003] In related technologies, homogeneous or simple layered models are often used for simulation, making it difficult to accurately characterize the geometry of fault zones. Furthermore, experimental procedures typically do not include closed-loop operations simulating heat extraction and cooling water reinjection on the user side, which differs from the periodic operation mode in actual engineering. In addition, existing devices generally lack simulation of constant geothermal background in deep reservoirs, and the spatiotemporal resolution of monitoring methods is insufficient, making it impossible to fully capture the dynamic evolution details of multiple physical fields such as thermal fields and flow fields. Summary of the Invention

[0004] The problem addressed by this invention is to improve the simulation accuracy of hydrothermal transport processes in fracture zones.

[0005] To address the above problems, this invention provides a periodic thermal storage simulation system and experimental method for fractured aquifers.

[0006] In a first aspect, the periodic thermal storage simulation system for fractured aquifers of the present invention includes: A model box filled with a porous medium, wherein at least one fracture zone structure is constructed in the porous medium, and the fracture zone structure is filled with a material with a permeability greater than that of the porous medium; The injection-production circulation system includes an injection well and a production well, which are buried in a porous medium inside the model box. The injection well and the production well are connected to a fluid circulation system through pipelines. The fluid circulation system has a switchable first working mode and a second working mode. In the first working mode, high-temperature hot water is provided to the injection well. In the second working mode, the hot water extracted from the production well is cooled and then reinjected into the injection well. The monitoring unit includes at least a plurality of sensors for collecting temperature and pressure data inside the model box.

[0007] The periodic thermal storage simulation system for fractured aquifers of this invention constructs a fractured zone structure filled with a high-permeability material within a porous medium in a model chamber. This structure realistically reproduces the dominant control of fractured zones on groundwater and heat transport under actual geological conditions. It effectively solves the technical shortcomings of existing technologies that use homogeneous or simple layered models, which cannot accurately depict the geometry of fractured zones and reflect dominant flow effects. This provides an accurate physical model basis for revealing the control mechanism of fractured zones on thermal storage processes. Furthermore, by setting up a fluid circulation system with switchable first and second operating modes, it can not only simulate the injection process of high-temperature hot water into the reservoir but also cool and reinject the hot water extracted from the production well back into the injection well. This achieves a complete closed-loop operation including user-side heat extraction and cooling water reinjection, overcoming the limitations of existing technologies that can only simulate simple heat injection and extraction processes without considering the cooling and reinjection process. This ensures that the experimental conditions highly match the periodic operation mode of actual engineering projects, significantly enhancing the guiding value of experimental research conclusions for engineering practice. Furthermore, by setting up a monitoring unit that includes at least multiple temperature and pressure sensors, synchronous and high-precision acquisition of temperature and pressure data inside the model box is achieved, which can comprehensively capture the dynamic evolution details of the thermal front during its propulsion in a heterogeneous medium.

[0008] In summary, this invention effectively improves the simulation accuracy of hydrothermal migration processes in fracture zones by precisely constructing the fracture zone structure, integrating the closed-loop injection and extraction system, and setting up a multi-physics monitoring unit.

[0009] Secondly, the periodic thermal storage simulation experimental method of the fractured aquifer zone of the present invention is applied to a periodic thermal storage simulation system of the fractured aquifer zone, wherein the periodic thermal storage simulation experimental method of the fractured aquifer zone includes: S1: After vacuuming the model box filled with porous medium, inject fluid into the model box until the pores in the porous medium are filled with fluid. At the same time, start the heating unit set at the bottom of the model box so that the inside of the model box reaches the preset initial temperature and preset initial pressure. S2: Switch to the first working mode, inject high-temperature hot water into the porous medium in the model box through the injection well, and simultaneously extract fluid from the production well; S3: Stop the fluid circulation by closing the valves of the injection well and the production well; S4: Switch to the second working mode, extract fluid from the production well, and then reinject the extracted fluid into the injection well after cooling; S5: Repeat the process of stopping the fluid circulation and subsequent steps by closing the valves of the injection well (2) and the production well (3).

[0010] Optionally, the fractured structure is obtained by temporarily placing the forming component into the model box, filling the model box with a porous medium, filling the space defined by the forming component with fracture band material, and removing the forming component after filling. The forming component is at least one pair of rigid plates, which are temporarily placed into the model box at a preset angle and a preset spacing.

[0011] Optionally, the construction process of the fracture zone structure includes: The porous medium is filled into the model box up to the lower edge of the rigid plate; After filling the gap between the pair of rigid plates with fracture zone material, continue filling the model box with the porous medium until the fracture zone material and the rigid plates are completely covered; The rigid plate is extracted from the model box to obtain the fracture zone structure, wherein the permeability of the fracture zone material is greater than the permeability of the porous medium.

[0012] Optionally, the step of injecting fluid into the model box until the pores in the porous medium are filled with fluid, and simultaneously activating the heating unit located at the bottom of the model box, so that the interior of the model box reaches a preset initial temperature state and a preset initial pressure state, includes: After fluid is injected into the model box, the monitoring unit collects pore pressure data at different locations in the model box in real time until the change in pore pressure data at each location within a first preset time interval is less than a first preset threshold, and then determines that the interior of the model box has reached the preset initial pressure state. Fluid is injected into the model box, and the heating unit is activated and maintained. Temperature data at different heights inside the model box are collected in real time by the monitoring unit until the change in temperature readings at each height within a second preset time interval is less than a second preset threshold. At this point, it is determined that the interior of the model box has reached the preset initial temperature state.

[0013] Optionally, switching to the first working mode, injecting high-temperature hot water into the porous medium inside the model box via the injection well, and simultaneously extracting fluid from the production well, includes: High-temperature hot water is injected into the injection well at a constant flow rate, while fluid is extracted from the production well at the same constant flow rate for a preset duration, so that an initial thermal reservoir is formed in the model box.

[0014] Optionally, the switching to the second operating mode, extracting fluid from the production well, and then cooling and reinjecting the extracted fluid into the injection well, includes: Fluid is drawn from the well at a constant flow rate, cooled by a heat exchanger, and then enters a buffer storage tank. The fluid is then reinjected into the porous medium in the model box through the injection well at the same constant flow rate. This process continues until a preset termination condition is reached.

[0015] Optionally, S5 specifically includes: The steps of closing the valves of the injection well and the production well and stopping the fluid circulation are considered as a storage cycle; The steps of switching to the second working mode, extracting fluid from the production well, and then cooling and reinjecting the extracted fluid back into the injection well constitute a heat recovery and reinjection cycle. A complete thermal cycle is formed by sequentially executing a storage cycle and a heat recovery and reinjection cycle, and the thermal cycle is repeated at least once.

[0016] Optionally, after activating the heating unit located at the bottom of the model box, the static heating time is maintained for 24 to 48 hours.

[0017] Optionally, the temperature distribution of the preset initial temperature state at various locations along the height direction is such that the temperature in the bottom region is higher than the temperature in the top region.

[0018] The method for simulating periodic thermal storage in fractured aquifers of this invention achieves high-fidelity reproduction of the initial experimental state through a coordinated operation of vacuuming, fluid injection, and bottom heating. After filling the porous medium, vacuuming is performed to remove gas from the pores, followed by fluid injection until the pores are completely filled, ensuring the complete saturation of the porous medium and providing a realistic porous medium environment for subsequent hydrothermal migration simulation. Simultaneously, the bottom heating unit is activated and maintained to bring the model chamber to the preset initial temperature and pressure states, accurately simulating the constant geothermal background of deep aquifers. This initialization process overcomes the shortcomings of existing technologies that ignore geothermal gradients and distort initial conditions, providing an accurate and reliable starting benchmark for subsequent experiments. Dynamic balance control of the thermal injection process is achieved by simultaneously injecting high-temperature hot water and extracting fluid. While providing high-temperature hot water to the injection well, fluid is extracted from the production well, maintaining fluid volume balance within the model chamber. This avoids the risk of pressure surges due to unilateral injection that could damage the medium structure, and simulates the actual engineering process of injecting thermal energy into the reservoir, providing an initial thermal reservoir for subsequent storage and extraction stages. By closing valves and stopping fluid circulation, a realistic simulation of the natural diffusion process of heat in the reservoir was achieved. By simulating the storage stage of thermal energy in underground reservoirs in actual engineering projects, the heat was redistributed under natural heat conduction and convection, providing an experimental basis for studying the residence law of heat and the heat loss mechanism in fracture zone media. Through a closed-loop operation of fluid extraction-cooling-reinjection, a dual simulation of heat extraction and pressure maintenance on the user side was achieved in actual engineering practice. The extracted hot water was cooled and reinjected into the injection well, simulating the actual process of users extracting thermal energy from the reservoir for heating and other purposes, while maintaining reservoir pressure balance through reinjection, avoiding engineering problems such as system operation being affected by reservoir pressure drops due to fluid extraction.

[0019] By repeatedly performing the steps of closing the valves of the injection well and the production well to stop fluid circulation, and switching to the second working mode to extract fluid from the production well and then cooling and reinjecting the extracted fluid back into the injection well at least once, multi-cycle operation is achieved. This reveals the evolution law of thermal storage performance under long-term operating conditions, effectively simulating multiple complete operating cycles of cross-seasonal thermal storage in actual engineering. By comparing the temperature and pressure responses under different cycles, the variation law of thermal breakthrough time, the decay trend of heat recovery rate, and the impact of fault zones on multi-cycle operating performance can be analyzed, providing key data support for assessing long-term risks of the project and optimizing operating strategies.

[0020] In summary, this invention can realistically reproduce the hydrothermal migration process of fractured aquifers under periodic thermal storage conditions, effectively improving the simulation accuracy and providing a reliable technical means for the study of the control mechanism of fractured zones on thermal storage processes. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the structure of the periodic thermal storage simulation system in the fractured aquifer of the present invention. Figure 2 This is a schematic flowchart of the experimental method for simulating periodic thermal storage in fractured aquifers according to an embodiment of the present invention. Attached image description: 1-Model box; 2-Injection well; 3-Operation well; 4-Inlet pipe; 5-Insulation layer; 6-Flow meter; 7-Thermometer; 8-Heat exchanger; 9-Buffer storage tank; 10-Constant temperature hot water tank; 11-Valve; 12-Drain pipe; 13-Homogeneous fine sand; 14-Coarse sand; 15-Pump; 16-Heating element; 17-Sensor. Detailed Implementation

[0023] To make the above-mentioned objects, features, and advantages of the present invention more apparent and understandable, specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings. Although some embodiments of the present invention are shown in the drawings, it should be understood that the present invention can be implemented in various forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided to provide a more thorough and complete understanding of the present invention. It should be understood that the accompanying drawings and embodiments of the present invention are for illustrative purposes only and are not intended to limit the scope of protection of the present invention.

[0024] It should be understood that the various steps described in the method embodiments of the present invention may be performed in different orders and / or in parallel. Furthermore, the method embodiments may include additional steps and / or omit the steps shown. The scope of the present invention is not limited in this respect.

[0025] The term "comprising" and its variations as used herein are open-ended, meaning "including but not limited to"; the term "based on" means "at least partially based on"; the term "one embodiment" means "at least one embodiment"; the term "another embodiment" means "at least one additional embodiment"; the term "some embodiments" means "at least some embodiments"; and the term "optionally" means "optional embodiments". Definitions of other terms will be given in the following description. It should be noted that the concepts of "first," "second," etc., mentioned in this invention are used only to distinguish different devices, modules, or units, and are not intended to limit the order of functions performed by these devices, modules, or units or their interdependencies.

[0026] It should be noted that the terms "a" and "a plurality of" used in this invention are illustrative rather than restrictive. Those skilled in the art should understand that, unless otherwise expressly indicated in the context, they should be understood as "one or more".

[0027] The names of the messages or information exchanged between the multiple devices in the embodiments of the present invention are for illustrative purposes only and are not intended to limit the scope of these messages or information.

[0028] Combination Figure 1 As shown, the periodic thermal storage simulation system for fractured aquifers provided in this embodiment of the invention includes: Model box 1, the model box 1 is filled with a porous medium, the porous medium has at least one fracture zone structure, the fracture zone structure is filled with a material with a permeability greater than that of the porous medium; The injection-production circulation system includes an injection well 2 and a production well 3, which are buried in a porous medium within the model box 1. The injection well 2 and the production well 3 are connected to a fluid circulation system via pipelines. The fluid circulation system has a switchable first working mode and a second working mode. In the first working mode, high-temperature hot water is provided to the injection well 2. In the second working mode, the hot water extracted from the production well 3 is cooled and then reinjected into the injection well 2. The monitoring unit includes at least a plurality of sensors 17 for collecting temperature and pressure data inside the model box 1.

[0029] Specifically, model box 1 serves as the main load-bearing structure for the experiment (it can be a transparent sandbox), and its interior is filled with porous media to simulate the geological structure of a real aquifer. At least one fracture zone structure is constructed within the porous media, filled with a material having a permeability greater than that of the porous media, to simulate high-permeability channels under real geological conditions. The injection-production circulation system includes at least one injection well 2 and at least one production well 3, both buried within the porous media inside model box 1. Injection well 2 and production well 3 are connected to a fluid circulation system via pipelines. This fluid circulation system has a switchable first operating mode and a second operating mode. In the first operating mode (heat injection mode), the fluid circulation system provides high-temperature hot water to injection well 2 through a constant-temperature hot water tank 10. The hot water is injected into the porous media inside model box 1 via injection well 2, while simultaneously, ambient-temperature water is extracted at the same flow rate from production well 3 and discharged to an external storage tank. This stage is used to establish an initial thermal reservoir in the aquifer. In the second operating mode (heat extraction and reinjection mode), the fluid circulation system extracts hot water from the extraction well 3, which is then cooled by the heat exchanger 8 to simulate the user-side heat extraction process. The cooled water enters the buffer storage tank 9, and is then reinjected by pump 15 through the injection well 2 into the porous medium in the model box 1 at the same flow rate as the extracted flow rate, forming a complete closed loop of energy utilization and pressure maintenance. Through the switching control of valve 11, the system can realize the complete periodic operation process of heat injection, storage, heat extraction, and reinjection, which is highly consistent with the operation mode in actual engineering.

[0030] The monitoring unit includes at least multiple sensors 17 for collecting temperature and pressure data inside the model box 1. Specifically, high-precision, miniaturized sensors 17, such as temperature sensors and pore pressure sensors, are arranged in a matrix at equal intervals on key cross-sections inside the model box 1 to capture the spatiotemporal dynamic evolution of the temperature and pressure fields during the advance of the thermal front. In addition, high-precision flow meters 6 and thermometers 7 are installed on the inlet and outlet water pipes of the injection well 2 and the production well 3, respectively, for accurately calculating the system's energy balance and heat recovery efficiency. Signals from all sensors are connected to a multi-channel data acquisition unit and recorded and controlled uniformly by a computer.

[0031] In a preferred embodiment of the present invention, the main model unit includes a model box 1, which is the main load-bearing structure of the entire experimental device. It is made of a high-strength transparent material, such as a thickened acrylic sheet. Its typical dimensions are 100 cm × 50 cm × 40 cm. The transparent material design allows direct observation of the internal fluid transport trajectory and the propagation pattern of the thermal front during the experiment, facilitating cross-verification with sensor monitoring data. The model box 1 is filled with a porous medium to simulate the geological structure of a real aquifer. An insulation layer 5 surrounds the model box 1 (except for the reserved observation surface) and is made of high-performance insulation material. Its function is to minimize boundary heat loss, ensure controllable thermodynamic boundary conditions during the experiment, and avoid interference with experimental results due to ambient temperature fluctuations. Homogeneous fine sand 13 is filled inside the model box 1 as a porous medium to simulate low-permeability surrounding rock formations. The fine sand has a small particle size and low permeability, representing low-permeability areas in the reservoir during the experiment, creating a permeability difference with the fracture zone material. Coarse sand 14, acting as the fracture zone material, fills the space defined by the shaped component, simulating a highly permeable fracture zone. The coarse sand 14 has a larger particle size and significantly higher permeability than the homogeneous fine sand 13, representing a dominant channel for fluid and heat transport in the experiment. The permeability difference between coarse sand 14 and homogeneous fine sand 13 forms a permeability abrupt change interface, used to study the control mechanism of the fracture zone on the hydrothermal transport process. A heating element 16, a precisely temperature-controlled electric heating element, is placed at the bottom of the model chamber 1. Its function is to simulate the constant geothermal background of deep aquifers, providing a stable bottom heat flux for the system. Before the experiment begins, the heating element 16 is kept statically heated (e.g., for 24 to 48 hours) to establish a stable vertical temperature gradient inside the model chamber 1 (e.g., 45°C at the bottom, 40°C at the top), overcoming the deficiency of existing technologies that ignore geothermal gradients, leading to distorted heat loss assessments. Temperature and pressure sensors 17 are arranged in a matrix at equal intervals on key sections inside the model chamber 1, employing high-precision, miniaturized sensor elements. Temperature sensors are used to collect temperature data at different locations within model chamber 1 in real time, capturing the evolution of the temperature field during the advancement of the thermal front; pore pressure sensors are used to collect pore pressure data, monitoring the pressure response during the injection and extraction process. These sensors provide high-resolution data support for analyzing preferential heat transport paths, thermal breakthrough time, and pressure conduction patterns.

[0032] The injection-production circulation unit includes an injection well 2 embedded in a porous medium within a model box 1, used to inject high-temperature hot water into the reservoir. The injection well 2 is connected to a fluid circulation system via pipeline. In the first operating mode (heat injection mode), high-temperature hot water is injected into the model box 1 through the injection well 2, establishing an initial thermal reservoir in the reservoir. A production well 3 embedded in the porous medium within the model box 1 is used to extract hot water from the reservoir. The production well 3 is connected to the fluid circulation system via pipeline. In the first operating mode, it extracts and discharges ambient temperature water at the same flow rate as the injection. In the second operating mode (heat recovery and reinjection mode), the production well 3 extracts hot water for subsequent cooling and reinjection. An inlet pipe 4 is connected to a constant-temperature hot water tank 10, used to introduce high-temperature hot water into the fluid circulation system. A valve 11 is installed on the inlet pipe 4 to control the flow and flow rate of the fluid. A plate heat exchanger 8 is installed on the outlet pipe of the production well 3. Its function is to simulate the user-side heat extraction process and cool the hot water extracted from the production well 3. When hot water flows through heat exchanger 8, it exchanges heat with the cooling medium, carrying away heat and lowering the water temperature, simulating the process in actual engineering where users extract heat energy from the reservoir for heating or other purposes. A buffer storage tank 9 is connected between the outlet of heat exchanger 8 and the reinjection pipeline to temporarily store water cooled by heat exchanger 8. The function of the buffer storage tank 9 is to balance flow fluctuations, ensuring a stable and continuous reinjection process and providing a stable water source for subsequent reinjection operations. A constant-temperature hot water tank 10 serves as an external heat source to provide high-temperature hot water. The constant-temperature hot water tank 10 has precise temperature control, allowing the hot water temperature to be set according to experimental requirements (e.g., 60℃, 80℃, 90℃, etc.). In the first operating mode, the constant-temperature hot water tank 10 supplies high-temperature hot water to injection well 2 through pipelines. Valves 11 are installed on each section of the pipeline to control the flow and direction switching of the fluid. By combining the opening and closing of valves 11, the switching between the first and second operating modes and the independent control of each branch are achieved. Drain pipe 12 is connected to the outlet pipe of production well 3, used to discharge ambient temperature water extracted from production well 3 to an external storage tank in the first operating mode. A valve 11 is installed on drain pipe 12, which closes in the second operating mode to switch to the reinjection process. Pump 15 is installed on the pipeline of the fluid circulation system to drive fluid flow. Pump 15 is a precision metering pump, capable of independently controlling the injection and extraction flow rates to achieve constant flow injection and extraction. In both operating modes, pump 15 operates at a set flow rate to ensure injection-production balance.

[0033] In this embodiment, the injection-production circulation system refers to the injection well 2 and production well 3 buried in the porous medium within the model box 1, belonging to the downhole part, responsible for the injection and production of fluid in the reservoir; the fluid circulation system refers to the surface pipelines and equipment assemblies connected to the injection well 2 and production well 3, including pump 15, valve 11, constant temperature hot water tank 10, heat exchanger 8, and buffer storage tank 9, etc., responsible for providing high-temperature hot water, cooling the produced fluid, switching operating modes, and driving fluid circulation. The two are connected by pipelines to form a complete fluid loop: the fluid circulation system transports the processed high-temperature hot water to the injection-production circulation system, which is then injected into the reservoir by the injection well 2; the produced hot water returns to the fluid circulation system via the production well 3 for cooling and reinjection. The monitoring unit includes a flow meter 6 installed on the inlet and outlet pipelines of the injection well 2 and production well 3, using a high-precision flow meter to monitor the fluid flow data in real time. By monitoring the injection flow and production flow through the flow meter 6, the accuracy of flow control during the experiment is ensured, and basic data is provided for subsequent energy balance calculations. Thermometer 7 is installed on the inlet and outlet water pipelines of injection well 2 and production well 3. It employs a high-precision temperature sensor to monitor real-time temperature changes of the fluid in the pipelines. The data monitored by thermometer 7 includes the injected hot water temperature, the produced hot water temperature, and the inlet and outlet temperatures of heat exchanger 8. This data, combined with internal sensor data, can be used to accurately calculate the system's energy balance and heat recovery efficiency. Sensor 17 (temperature and pressure sensor) not only collects data from inside model box 1 but also, together with flow meter 6 and thermometer 7, forms a complete monitoring system. Signals from all sensors are connected to a multi-channel data acquisition instrument, which is then recorded and controlled by a computer, enabling automated data acquisition and real-time monitoring of the experimental process.

[0034] In summary, this embodiment, through the organic integration of the above three units, constructs an aquifer thermal storage simulation system that can accurately simulate the geometry of fault zones, realistically reproduce the deep geothermal environment, completely replicate the periodic engineering operation process, and possess high-precision synchronous monitoring capabilities of multiple physical fields. This provides a reliable experimental platform for the study of the control mechanism of fault zones on the hydrothermal migration process.

[0035] The periodic thermal storage simulation system for fractured aquifers of this invention constructs a fractured zone structure filled with a high-permeability material within a porous medium in a model chamber. This structure realistically reproduces the dominant control of fractured zones on groundwater and heat transport under actual geological conditions. It effectively solves the technical shortcomings of existing technologies that use homogeneous or simple layered models, which cannot accurately depict the geometry of fractured zones and reflect dominant flow effects. This provides an accurate physical model basis for revealing the control mechanism of fractured zones on thermal storage processes. Furthermore, by setting up a fluid circulation system with switchable first and second operating modes, it can not only simulate the injection process of high-temperature hot water into the reservoir but also cool and reinject the hot water extracted from the production well back into the injection well. This achieves a complete closed-loop operation including user-side heat extraction and cooling water reinjection, overcoming the limitations of existing technologies that can only simulate simple heat injection and extraction processes without considering the cooling and reinjection process. This ensures that the experimental conditions highly match the periodic operation mode of actual engineering projects, significantly enhancing the guiding value of experimental research conclusions for engineering practice. Furthermore, by setting up a monitoring unit that includes at least multiple temperature and pressure sensors, synchronous and high-precision acquisition of temperature and pressure data inside the model box is achieved, which can comprehensively capture the dynamic evolution details of the thermal front during its propulsion in a heterogeneous medium.

[0036] In summary, this invention effectively improves the simulation accuracy of hydrothermal migration processes in fracture zones by precisely constructing the fracture zone structure, integrating the closed-loop injection and extraction system, and setting up a multi-physics monitoring unit.

[0037] Combination Figure 2 As shown in the figure, this embodiment of the invention also provides a method for simulating periodic thermal storage in fractured aquifers, applied to a system for simulating periodic thermal storage in fractured aquifers. The method includes: S1: After vacuuming the model box 1 filled with porous medium, inject fluid into the model box 1 until the pores in the porous medium are filled with fluid. At the same time, start the heating unit set at the bottom of the model box 1 so that the interior of the model box 1 reaches the preset initial temperature state and preset initial pressure state. S2: Switch to the first working mode, inject high-temperature hot water into the porous medium in the model box 1 through injection well 2, and at the same time extract fluid from the production well 3; S3: Stop the fluid circulation by closing the valves of injection well 2 and production well 3; S4: Switch to the second working mode, extract fluid from the production well 3, and then reinject the extracted fluid into the injection well 2 after cooling; S5: Repeat the process of stopping the fluid circulation and subsequent steps by closing the valves of the injection well (2) and the production well (3).

[0038] Specifically, S1: Model initialization. After the porous medium filling is completed, the model box 1 is evacuated to remove gas from the pores. Then, fluid (such as water) is injected into the model box 1 until the pores are completely filled, achieving model saturation. Simultaneously, the heating unit (such as heating element 16) located at the bottom of the model box 1 is activated to bring the model box 1 to a preset initial temperature and pressure. After the porous medium filling is completed, the pores of the porous medium, including homogeneous fine sand 13 and coarse sand 14, are filled with air. If fluid is injected directly, residual air will lead to insufficient pore saturation, affecting the realism of the subsequent hydrothermal transport process. Therefore, the model box 1 is first evacuated using a vacuum pump to remove gas from the pores, creating conditions for subsequent fluid saturation. After the predetermined vacuum level is reached, fluid, typically deionized water or a solution simulating groundwater composition, is injected into the model box 1. The fluid enters the pores under negative pressure until all pores are completely filled. This operation ensures complete saturation of the porous medium, providing a realistic porous media environment for subsequent hydrothermal transport simulations. During saturation, the fluid injection rate can be monitored in real time via a monitoring unit to ensure complete saturation. Simultaneously or after fluid injection, the heating element 16 located at the bottom of the model chamber 1 is activated. The heating element 16 employs a precisely temperature-controlled electric heating element, its function being to simulate the constant geothermal background of deep aquifers. After activation, the heating element 16 is kept in a static heating state for a period of time (e.g., 24 to 48 hours) to allow heat to transfer upwards from the bottom.

[0039] Through continuous heating by the heating element 16 and heat conduction in the medium, a stable temperature distribution gradually forms inside the model box 1. Based on temperature data collected by the monitoring unit at different heights, a thermal equilibrium state is considered reached when the temperature readings at each height no longer change over time. In a preferred embodiment of the invention, the initial temperature state is preset to a temperature distribution along the height direction, with the temperature in the bottom region higher than the temperature in the top region (e.g., 45°C at the bottom and 40°C at the top), realistically simulating the geothermal characteristic that the deeper the strata, the hotter it gets. After saturation is complete and the temperature field stabilizes, the pore pressure inside the model box 1 should reach a uniform distribution state. Pore pressure data at different locations is collected by the monitoring unit, and a pressure equilibrium state is considered reached when the pressure readings at each location no longer change over time. The preset initial pressure state, a uniformly distributed pore pressure, provides stable initial pressure conditions for subsequent injection and production processes. Through the coordinated operation of vacuuming, fluid saturation, and bottom heating, a high-fidelity restoration of the experimental initial state is achieved. This step overcomes the shortcomings of existing technologies that ignore geothermal gradients and distort initial conditions, providing an accurate and reliable starting benchmark for subsequent experiments.

[0040] S2: During the heat injection stage, switch to the first working mode (heat injection mode). High-temperature hot water is injected into the porous medium in the model box 1 through injection well 2, while fluid is simultaneously extracted from the production well 3 at the same flow rate. The fluid circulation system is switched to the first working mode by controlling the opening and closing of valve 11. In the first working mode, the constant-temperature hot water tank 10 is connected to the pipeline of injection well 2, and the production well 3 is connected to the pipeline of drainage pipe 12, forming an injection-production passage. The constant-temperature hot water tank 10 stores high-temperature hot water at a predetermined temperature (e.g., 80℃). Pump 15 is started, and high-temperature hot water is injected into the porous medium in the model box 1 at a constant flow rate (e.g., 50 ml / min) through inlet pipe 4 and injection well 2. The constant injection flow rate is controlled by a precision metering pump to ensure the stability of the injection process. Simultaneously with the injection of high-temperature hot water, pump 15 connected to the production well 3 is started, extracting fluid from the production well 3 at a constant flow rate (e.g., 50 ml / min), the same as the injection flow rate. The extracted fluid is the original ambient temperature water in model tank 1, which is discharged to an external storage tank through drain pipe 12. High-temperature hot water is continuously injected, forming a high-temperature zone around injection well 2 and diffusing into the surrounding medium. Injection and extraction continue for a preset duration (e.g., 2 hours) to form an initial thermal reservoir with a certain scale and temperature distribution within model tank 1. During the injection process, data such as injection temperature, injection flow rate, extraction temperature, and extraction flow rate can be recorded in real time through a monitoring unit. Dynamic balance control of the heat injection process is achieved through the simultaneous injection of high-temperature hot water and extraction of fluid. Simultaneous extraction at the same flow rate maintains the fluid volume balance within model tank 1, avoiding the risk of pressure surges due to unilateral injection that could damage the medium structure. This step simulates the actual engineering scenario of injecting thermal energy into a reservoir, providing an initial thermal reservoir for subsequent storage and extraction stages.

[0041] S3: Storage Phase. By closing the valves of injection well 2 and production well 3, fluid circulation is stopped, allowing heat within model tank 1 to redistribute and diffuse naturally through heat conduction and convection. After the injection phase, valve 11 connected to injection well 2 and production well 3 is closed, cutting off the fluid circulation system. Simultaneously, all pumps 15 are stopped, bringing the fluid within model tank 1 to a standstill. The cessation of fluid circulation means there is no longer forced convection; heat within model tank 1 is redistributed solely through natural heat conduction and convection. This state simulates the storage phase of thermal energy in underground reservoirs in actual engineering. During the storage phase, the high-temperature thermal reservoir formed during the injection phase transfers heat to the surrounding low-temperature regions. Due to permeability differences in porous media, such as high permeability in coarse sand region 14 and low permeability in fine sand region 13, the heat transfer path and speed are controlled by the heterogeneous structure. Especially in fracture zone areas, heat may preferentially diffuse along dominant channels. The storage phase lasts for a preset duration (e.g., 4 hours) to allow for sufficient heat diffusion. This storage duration can be adjusted according to experimental needs to study the impact of different storage times on thermal breakthrough time and heat recovery rate. During storage, the monitoring unit continuously records temperature and pressure data at various locations within model chamber 1 to analyze the heat diffusion pattern. By closing the valves and stopping fluid circulation, a realistic simulation of the natural heat diffusion process in the reservoir is achieved. This step simulates the storage stage of thermal energy in underground reservoirs in actual engineering, allowing heat to redistribute under natural heat conduction and convection, providing an experimental basis for studying the residence law of heat and heat loss mechanism in fracture zone media.

[0042] S4: During the heat extraction and reinjection stage, switch to the second operating mode (heat extraction and reinjection mode). Fluid is extracted from the production well 3, cooled, and reinjected into the injection well 2. The fluid circulation system is switched to the second operating mode by controlling the opening and closing of valve 11. In the second operating mode, the pipeline between the production well 3 and heat exchanger 8 is connected, and the buffer water tank 9 is connected to the pipeline between the injection well 2, forming a cooling and reinjection pathway. Pump 15, connected to the production well 3, is started to extract hot water from the reservoir at a constant flow rate (e.g., 50 ml / min). The extraction flow rate is the same as the injection flow rate during the heat injection stage to maintain system flow balance. The extracted hot water flows through heat exchanger 8. Heat exchanger 8 is a plate heat exchanger with internal cooling medium channels. As the hot water flows through heat exchanger 8, it exchanges heat with the cooling medium, carrying away heat and lowering the water temperature. This process simulates the process in actual engineering where users extract heat energy from the reservoir for heating or other purposes. Thermometers 7 are installed at the inlet and outlet of heat exchanger 8 to monitor the water temperature changes before and after cooling in real time, providing data for calculating the heat recovery rate. The cooled water enters a buffer storage tank 9 for temporary storage. The function of the buffer storage tank 9 is to balance flow fluctuations, ensure a stable and continuous reinjection process, and avoid pressure fluctuations caused by unstable flow. Pump 15 injects the cooling water from the buffer storage tank 9 back into the porous medium in the model box 1 through injection well 2 at a constant flow rate (e.g., 50 ml / min), the same as the produced flow rate. Precise control of the reinjection flow rate is achieved by a precision metering pump to ensure a balance between the reinjection and produced flow rates. The heat extraction and reinjection process continues until a preset termination condition is reached. In a preferred embodiment of the invention, the preset termination condition is that the outlet temperature of the production well 3 drops to a predetermined threshold, such as close to the initial temperature. When the outlet temperature of the production well 3 is lower than this threshold, it indicates that the recoverable heat in the reservoir has been substantially extracted, and the heat extraction stage ends. By implementing a closed-loop operation of fluid extraction, cooling, and reinjection, the system achieves a dual simulation of heat extraction and pressure maintenance on the user side in actual engineering practice. The extracted hot water is cooled and then reinjected, which not only simulates the actual process of users extracting heat energy from the reservoir, but also maintains the reservoir pressure balance through equal flow reinjection, thus avoiding engineering problems such as the reservoir pressure drop caused by fluid extraction affecting system operation.

[0043] S5: Multi-cycle loop, repeating steps S3 and S4 at least once to simulate multiple complete thermal storage and extraction cycles. A complete thermal cycle is defined as the combination of steps S3 (storage phase) and S4 (extraction and reinjection phase). Step S3 simulates the storage process of thermal energy in the underground reservoir, and step S4 simulates the user's heat extraction and reinjection process; together, they constitute a complete operating cycle in a real-world project. After completing the heat extraction and reinjection of the first cycle, steps S3 (storage phase) and S4 (extraction and reinjection phase) are executed again to form the second thermal cycle. In a preferred embodiment of the invention, this is repeated 5 to 10 times to simulate multiple complete thermal storage and extraction cycles. The number of cycles can be adjusted according to experimental requirements to study the performance evolution under long-term operating conditions. Throughout the multi-cycle loop, the monitoring unit continuously records temperature and pressure data at various locations within model box 1, as well as flow and temperature data in the pipelines. This data provides a basis for subsequent analysis of performance changes during the cycle. Based on continuously acquired data, the thermal energy recovery rate (cycle efficiency) for each heating cycle and the cumulative thermal energy recovery rate throughout the entire experimental process can be calculated. By comparing performance indicators of different cycles, the variation pattern of thermal breakthrough time, the decay trend of thermal energy recovery rate, and the impact of fault zones on multi-cycle operation performance can be analyzed. By repeating S3 and S4 at least once in a multi-cycle cycle, the evolution law of thermal storage performance under long-term operating conditions was revealed. This step simulated multiple complete operating cycles of cross-seasonal thermal storage in actual engineering. By comparing the temperature and pressure responses under different cycles, the variation pattern of thermal breakthrough time, the decay trend of thermal energy recovery rate, and the impact of fault zones on multi-cycle operation performance can be analyzed, providing key data support for assessing long-term project risks and optimizing operation strategies.

[0044] In summary, an initial state with realistic geothermal background and pressure balance was established through coordinated operations of vacuuming, saturation, and bottom heating; an initial thermal reservoir was established in the reservoir through isostatic heat injection; the natural diffusion process of heat in the reservoir was simulated through static storage; the actual engineering process of heat extraction and pressure maintenance was fully reproduced through closed-loop operation of heat extraction, cooling, and reinjection; and the performance evolution law under long-term operating conditions was revealed through multi-cycle operation. Thus, the hydrothermal migration process of the fractured aquifer under periodic thermal storage conditions can be realistically reproduced, effectively improving the simulation accuracy.

[0045] The periodic thermal storage simulation method for fractured aquifers in this embodiment achieves high-fidelity reproduction of the initial experimental state through a coordinated operation of vacuuming, fluid injection, and bottom heating. After filling the porous medium, vacuuming is performed to remove gas from the pores, followed by fluid injection until the pores are completely filled, ensuring the complete saturation of the porous medium and providing a realistic porous medium environment for subsequent hydrothermal migration simulation. Simultaneously, the bottom heating unit is activated and maintained to bring the model box to the preset initial temperature and pressure states, accurately simulating the constant geothermal background of deep aquifers. This initialization process overcomes the shortcomings of existing technologies that ignore geothermal gradients and distort initial conditions, providing an accurate and reliable starting benchmark for subsequent experiments. Dynamic balance control of the thermal injection process is achieved by simultaneously injecting high-temperature hot water and extracting fluid. While providing high-temperature hot water to the injection well, fluid is extracted from the production well, maintaining fluid volume balance within the model box. This avoids the risk of pressure surges due to unilateral injection that could damage the medium structure, and simulates the actual engineering conditions of injecting thermal energy into the reservoir, providing an initial thermal reservoir for subsequent storage and extraction stages. By closing valves and stopping fluid circulation, a realistic simulation of the natural diffusion process of heat in the reservoir was achieved. By simulating the storage stage of thermal energy in underground reservoirs in actual engineering projects, the heat was redistributed under natural heat conduction and convection, providing an experimental basis for studying the residence law of heat and the heat loss mechanism in fracture zone media. Through a closed-loop operation of fluid extraction-cooling-reinjection, a dual simulation of heat extraction and pressure maintenance on the user side was achieved in actual engineering practice. The extracted hot water was cooled and reinjected into the injection well, simulating the actual process of users extracting thermal energy from the reservoir for heating and other purposes, while maintaining reservoir pressure balance through reinjection, avoiding engineering problems such as system operation being affected by reservoir pressure drops due to fluid extraction.

[0046] By repeatedly performing the steps of closing the valves of the injection well and the production well to stop fluid circulation, and switching to the second working mode to extract fluid from the production well and then cooling and reinjecting the extracted fluid back into the injection well at least once, multi-cycle operation is achieved. This reveals the evolution law of thermal storage performance under long-term operating conditions, effectively simulating multiple complete operating cycles of cross-seasonal thermal storage in actual engineering. By comparing the temperature and pressure responses under different cycles, the variation law of thermal breakthrough time, the decay trend of heat recovery rate, and the impact of fault zones on multi-cycle operating performance can be analyzed, providing key data support for assessing long-term risks of the project and optimizing operating strategies.

[0047] In summary, this embodiment can realistically reproduce the hydrothermal migration process of the fractured aquifer under periodic thermal storage conditions, effectively improving the simulation accuracy and providing a reliable technical means for the study of the control mechanism of the fractured zone on the thermal storage process.

[0048] Optionally, the molding component is temporarily placed into the model box 1, the porous medium is filled into the model box 1, and the fracture band material is filled into the space defined by the molding component. After filling, the molding component is removed to obtain the fracture band structure. The molding component is at least one pair of rigid plates, and the rigid plates are temporarily placed into the model box 1 at a preset tilt angle and a preset spacing.

[0049] Specifically, the forming component employs at least one pair of smooth-surfaced rigid plates. In a preferred embodiment, the rigid plates are made of stainless steel. Stainless steel has sufficient rigidity to maintain its shape during filling, ensuring the accuracy of the constructed fracture zone geometry. Simultaneously, the smooth surface of the stainless steel reduces friction with the filling medium, facilitating subsequent extraction operations.

[0050] In a preferred embodiment, the forming member is designed as a smooth-surfaced forming plate. When the forming plate is pulled out, the smooth surface reduces dragging and disturbance to the filled medium, avoiding damage to the integrity of the fracture zone boundary. The smooth surface makes the contact interface between the forming plate and the surrounding medium flat, and the boundary between the fracture zone formed after extraction and the surrounding rock is clear, which is beneficial for observing and monitoring the migration characteristics of the thermal front along the fracture zone in subsequent experiments.

[0051] The forming plate is temporarily placed into the model box 1 at a preset tilt angle and preset spacing. The preset tilt angle determines the inclination angle of the fracture zone relative to the horizontal plane, and the preset spacing determines the width of the fracture zone. These two geometric parameters are the core variables controlling the morphology of the fracture zone and can be precisely set according to research needs. In a preferred embodiment, the preset tilt angle can be selected from 30° to 60° (e.g., 30°, 45°, 60°), and the preset spacing can be selected from 2cm to 6cm (e.g., 2cm, 3cm, 4cm, 6cm). By systematically adjusting the tilt angle and spacing, parametric studies can be conducted to analyze the influence of fracture zones with different geometric shapes on the hydrothermal migration process.

[0052] In this embodiment, temporary placement means that the forming plate serves to define the space during model fabrication and is removed after filling is complete, not permanently remaining in the model. This design differs from existing technologies that use permanent partitions (such as nylon mesh or stainless steel wire mesh). Permanent partitions act as a heterogeneous interface, continuously interfering with the temperature and flow fields. In contrast, the forming plate of this invention is removed after defining the space, avoiding the introduction of additional artificial interfaces and ensuring the realism of the model.

[0053] After the forming plate is placed and fixed according to preset parameters, porous media (such as homogeneous fine sand 13) is first filled into the model box 1 up to the lower edge of the rigid plate to determine the bottom boundary of the fracture zone structure. This provides a reference for subsequent filling of the fracture zone material, ensuring the integrity of the contact between the bottom of the fracture zone and the surrounding rock, and avoiding cavities or poor contact at the bottom due to improper filling sequence. Fracture zone material (such as coarse sand 14) is then filled into the gap between a pair of rigid plates to define the space. The gap between the rigid plates is the preset fracture zone space, the width of which is determined by the spacing between the rigid plates, and the inclination angle by the tilt angle of the rigid plates. The physical constraint of the rigid plates ensures that the fracture zone material is filled within a precise geometric range. The fracture zone material uses a medium with a permeability greater than that of the porous media, such as coarse sand. The difference in particle size between coarse and fine sand makes the permeability of the coarse sand region significantly higher than that of the fine sand region, forming a permeability abrupt change interface. During the filling process, slight vibration can be used to assist in ensuring that the fracture zone material is uniformly and densely filled into the gaps, avoiding local voids or unevenness.

[0054] After filling the fracture zone material, continue filling the model box 1 with porous medium until the fracture zone material and rigid plate are completely covered, thereby determining the top boundary of the fracture zone structure, so that the fracture zone is completely wrapped in the surrounding rock, ensuring the integrity of the contact between the top of the fracture zone and the surrounding rock, and forming a closed fracture zone structure.

[0055] During the filling process, the permeability of the fracture zone material is consistently greater than that of the porous medium. This difference is the core of constructing the heterogeneous structure, determining that fluids and heat will preferentially migrate along the fracture zone region in subsequent experiments, forming a dominant flow effect. This embodiment achieves heterogeneous simulation through the permeability difference of the material itself, rather than an artificial interface.

[0056] After all filling is complete, the rigid plate is slowly extracted from model box 1. Micro-vibration can be used during extraction to reduce disturbance to the filled medium. The design of a smooth surface plays a crucial role at this stage, allowing the plate to be extracted smoothly without carrying away excessive medium. After extraction, the voids created by the plate are naturally collapsed or slightly filled by the surrounding porous medium. If voids remain, they will form unrealistic weak zones and uncontrollable thermal conductivity barriers in the model, leading to experimental distortion. Natural collapse or filling eliminates the artificial voids created by plate extraction. After filling, the fracture zone material (coarse sand 14) and the porous medium (fine sand 13) are in close contact at the interface. However, due to the difference in permeability between the materials, the fluid and heat transport speed in the coarse sand region is still much faster than in the fine sand region, forming a clear interface of abrupt permeability change. This close physical interface ensures that the subsequent hydrothermal coupling process takes place under stable and realistic material boundary conditions, avoiding uncontrollable heat conduction or fluid cross-flow caused by voids at the interface. After compaction, although the two particles are in close contact at the interface, the transport rate of fluids and heat in the coarse-grained region is still much faster than in the fine-grained region, forming a clear interface with abrupt changes in permeability. This operation also enhances the repeatable and observable heterogeneous effects controlled by the inherent properties of the material (such as particle size and porosity), making the experimental phenomena closer to geological reality.

[0057] Through the above steps, a fracture zone structure filled with fracture zone material is finally obtained. In this embodiment, the dip angle and width of the fracture zone are precisely controlled by the preset parameters of the forming plate, and it can be repeatedly constructed. The fracture zone is filled with a high-permeability material, and the surrounding rock is filled with a low-permeability material, forming a permeability abrupt change interface between the two. There are no artificial gaps between the fracture zone and the surrounding rock, forming a close physical interface, simulating the contact state between the fracture zone and the surrounding rock in real geology.

[0058] In this optional embodiment, the preset tilt angle and spacing of the forming plates enable precise control and repeatable construction of the fracture zone geometry (tilt angle, width), solving the technical problem that traditional homogeneous or layered models cannot realistically reproduce the dominant flow effect. Heterogeneous simulation is achieved through the permeability difference of the materials themselves (coarse sand and fine sand) rather than artificial interfaces, avoiding the interference of permanent barriers on the temperature and flow fields, making the experimental phenomena closer to geological reality. The natural collapse or refilling after extraction eliminates artificial cavities, creating a close physical interface between the fracture zone and the surrounding rock, ensuring that the hydrothermal coupling process occurs under real boundary conditions. By adjusting the tilt angle and spacing of the forming plates, the influence of fracture zones with different geometries on thermal storage performance can be systematically studied, providing an experimental basis for parameter sensitivity analysis and engineering optimization.

[0059] Optionally, the construction process of the fracture zone structure includes: The porous medium is filled into the model box 1 up to the lower edge of the rigid plate; After filling the gap between the pair of rigid plates with fracture zone material, continue to fill the model box 1 with porous medium until the fracture zone material and the rigid plates are completely covered. The rigid plate is extracted from the model box 1 to obtain the fracture zone structure, wherein the permeability of the fracture zone material is greater than the permeability of the porous medium.

[0060] Specifically, after at least one pair of smooth-surfaced rigid plates are temporarily placed into the model box 1 at a preset angle and preset spacing and then fixed, the filling of the porous medium begins. First, the matrix material is filled into the outer area. In this embodiment, the porous medium material is homogeneous fine sand 13. During the filling process, the operator or automatic filling device pours the fine sand 13 evenly into the model box 1 and controls the filling height. The end point of filling is set at the lower edge of the rigid plate, that is, the bottom of the rigid plate. Filling to the lower edge of the rigid plate clarifies the bottom starting position of the fracture zone structure. The lower edge of the rigid plate becomes the baseline for subsequent filling of the fracture zone material, ensuring that the interface position between the bottom of the fracture zone and the surrounding rock is accurately controllable; before filling the fracture zone material, a layer of surrounding rock material is laid first to provide a stable base for subsequent filling, avoiding direct contact between the fracture zone material and the bottom of the model box, which would lead to distortion of the boundary conditions; filling the outer material to the lower edge first allows the subsequently filled fracture zone material to form a complete contact interface with the surrounding rock at the bottom, avoiding cavities or poor contact at the bottom due to improper filling sequence. In existing technologies, simple layered sand filling methods often employ horizontal layering, which cannot construct heterogeneous structures with inclined angles. This embodiment, through the physical constraint of rigid plates and precise control of the filling height, lays the foundation for the subsequent construction of inclined fracture zones.

[0061] After filling the porous medium to the lower edge of the rigid plate, the fracture zone material is then filled into the gap defined by the pair of rigid plates. Preferably, coarse sand 14 is used as the fracture zone material; the particle size of coarse sand 14 is larger than that of fine sand 13, and its permeability is significantly higher. During the filling process, the coarse sand 14 is evenly filled into the gap between the rigid plates until the entire gap is filled; slight vibration can be used to ensure that the coarse sand 14 is uniformly compacted, avoiding local voids or unevenness. The gap between the rigid plates is defined by a preset spacing (determining the fracture zone width) and a preset inclination angle (determining the fracture zone tilt angle). By filling the space defined by the rigid plates with fracture zone material, the constructed fracture zone is ensured to have accurate geometric parameters. The rigid plate has a smooth surface, forming a flat contact interface with the filling medium. This interface, after the rigid plate is removed, will become the boundary between the fault zone and the surrounding rock, and its clarity directly affects the accuracy of thermal front observations during the experiment. The physical separation effect of the rigid plate prevents the fault zone material from mixing with the porous medium during the filling process, ensuring the abrupt change in permeability at the interface between the two materials. The high permeability of coarse sand 14 makes the transport speed of fluids and heat in this region much faster than in the fine sand 13 region, forming a clear abrupt change in permeability interface. This interface is controlled by the inherent properties of the materials, rather than relying on artificial barriers, and therefore more closely resembles real geological conditions.

[0062] After filling the fault zone with material, the porous medium is then filled. Fine sand 13 is poured into model box 1 until it completely covers the lower fault zone material and the upper part of the rigid plate. The material must be evenly distributed during filling to ensure a consistent thickness of the overburden layer. Completely covering the fault zone material defines the top boundary of the fault zone structure, ensuring the fault zone is completely encased in the surrounding rock. The encapsulation of the fault zone by the upper and lower layers of surrounding rock material forms a closed fault zone structure, simulating the state of a fault zone surrounded by surrounding rock in real geology. The overburden layer is the same as the lower surrounding rock material (both are fine sand 13), ensuring a close contact interface between the top of the fault zone and the surrounding rock. This embodiment, by filling the bottom surrounding rock first, then the middle fault zone, and finally the top surrounding rock, ensures that the fault zone is completely encased in the surrounding rock in the vertical direction, with both the upper and lower interfaces in contact with the surrounding rock, conforming to the spatial distribution characteristics of fault zones in real geology.

[0063] After all filling is complete, the rigid plate is slowly extracted from model box 1. Micro-vibration can be used during extraction to reduce disturbance to the filled medium. Due to the smooth surface of the rigid plate, it can be extracted smoothly without carrying away too much medium. After the rigid plate is removed, the space originally occupied by the rigid plate forms a void. However, due to the stability of the surrounding medium, this void will not remain open. The surrounding fine sand 13 collapses naturally under gravity, filling the void created by the removal of the rigid plate. If necessary, slight compaction can be performed to ensure the void is completely filled. Leaving voids will create unrealistic weak zones and uncontrollable thermal conductivity barriers in the model, leading to experimental distortion. Artificial voids created by plate removal can be eliminated through natural collapse or supplementary compaction. After compaction, the fracture zone material (coarse sand 14) and the porous medium (fine sand 13) are in close contact at the interface. After compaction, although the two are in close contact at the interface, the movement speed of fluid and heat in the coarse sand area is still much faster than that in the fine sand area, forming a clear interface with abrupt change in permeability.

[0064] Thus, a fracture zone structure filled with fracture zone material was finally obtained. The dip angle and width of the fracture zone were precisely controlled by the preset parameters of the rigid plate, and it could be repeatedly constructed. The fracture zone was filled with a high-permeability material (coarse sand 14), and the surrounding rock was filled with a low-permeability material (fine sand 13), forming a permeability abrupt change interface between the two. There were no artificial gaps between the fracture zone and the surrounding rock, forming a close physical interface, simulating the contact state between the fracture zone and the surrounding rock in real geology. By controlling the preset parameters of the rigid plate, fracture zones with the same geometric shape could be repeatedly constructed, providing a basis for comparative experiments and parameter studies.

[0065] In this optional embodiment, the preset tilt angle and spacing of the rigid plates enable precise control and repeatable construction of the fracture zone geometry, solving the technical problem that traditional homogeneous or layered models cannot realistically reproduce the dominant flow effect. Heterogeneous simulation is achieved through the permeability difference of the materials themselves (coarse sand and fine sand) rather than an artificial interface, avoiding the interference of permanent barriers on the temperature and flow fields, making the experimental phenomena closer to geological reality. The natural collapse or refilling after extraction eliminates artificial cavities, creating a close physical interface between the fracture zone and the surrounding rock, ensuring that the hydrothermal coupling process occurs under real boundary conditions. After refilling, the fracture zone material and the surrounding rock material are in close contact at the interface, but due to the difference in the inherent properties of the materials, a clear abrupt change in permeability is maintained, providing an ideal physical model for studying the dominant flow effect.

[0066] Optionally, the step of injecting fluid into the model box 1 until the pores in the porous medium are filled with fluid, and simultaneously activating the heating unit located at the bottom of the model box 1, so that the interior of the model box 1 reaches a preset initial temperature state and a preset initial pressure state, includes: After fluid is injected into the model box 1, the pore pressure data at different locations in the model box 1 is collected in real time by the monitoring unit until the change in pore pressure data at each location within a first preset time interval is less than a first preset threshold, and it is determined that the interior of the model box 1 has reached the preset initial pressure state. Fluid is injected into the model box 1, and the heating unit is activated and maintained. Temperature data at different height positions inside the model box 1 are collected in real time by the monitoring unit until the change in temperature readings at each height position within a second preset time interval is less than a second preset threshold. At this point, it is determined that the interior of the model box 1 has reached the preset initial temperature state.

[0067] Specifically, after filling the porous medium and evacuating the model box 1, fluid, such as deionized water or a solution simulating groundwater composition, is injected into the model box 1. Under negative pressure, the fluid enters the pores of the porous medium, gradually displacing and expelling the air within the pores. As the fluid continues to be injected, the pores are gradually filled, and the pore pressure inside the model box 1 begins to build up. The monitoring unit includes pore pressure sensors 17 positioned at different locations inside the model box 1. These sensors are arranged in a matrix at equal intervals on the key profile, enabling them to collect pore pressure data at different spatial points within the porous medium. During the fluid injection process, the monitoring unit collects and records the readings of each pressure sensor in real time at a set sampling frequency, such as once per second or once per minute.

[0068] The criterion for determining pressure stability is that the change in pore pressure data at each location within a first preset time interval is less than a first preset threshold. The first preset time interval refers to the time window used to determine pressure stability. In a preferred embodiment, the first preset time interval can be set to 10 minutes, 30 minutes, or 1 hour, and can be adjusted according to experimental accuracy requirements and media permeability characteristics. The time interval should be long enough to eliminate the influence of short-term fluctuations, but not so long as to lead to low experimental efficiency. The first preset threshold refers to the allowable pressure fluctuation range. In a preferred embodiment, the first preset threshold can be set as a multiple of the sensor's accuracy range (e.g., if the sensor accuracy is 0.1 kPa, the threshold can be set to 0.2 kPa or 0.5 kPa), or as a percentage of the absolute pressure value (e.g., 0.5%). The threshold selection should reflect that the pressure has reached a stable state, but should not be too stringent, resulting in excessively long waiting times. The monitoring unit continuously collects readings from each pressure sensor and calculates in real time the change in each sensor within the current time window (e.g., the difference between the maximum and minimum values, or the standard deviation). When the changes in all sensor readings are less than a first preset threshold, the system determines that the model chamber 1 has reached a stable state with uniform pore pressure distribution, i.e., the preset initial pressure state. By using preset thresholds and time intervals, the system avoids the arbitrariness of operators' subjective judgments regarding whether stability has been achieved, ensuring good consistency of initial conditions for each experiment. Regardless of which operator performs the experiment, as long as the same preset parameters are used, the same stable state can be determined, improving the repeatability of the experiment. The determination based on real-time monitoring data accurately reflects the pressure evolution process inside the porous medium, ensuring the accuracy of the initial pressure state.

[0069] Simultaneously or subsequently, a heating unit (such as heating element 16) located at the bottom of model box 1 is activated. Heating element 16 begins heating in a constant power or constant temperature mode, with heat transferred upwards from the bottom, forming a vertical temperature gradient within the porous medium. Due to differences in permeability within the porous medium (such as between coarse sand 14 and fine sand 13), the rate of heat transfer may vary in different areas. Therefore, heating needs to be continued for a sufficiently long time to allow the temperature field inside the entire model box 1 to reach thermal equilibrium. The monitoring unit includes temperature sensors 17 positioned at different heights inside model box 1. These sensors are arranged in a matrix at equal intervals along the vertical direction, enabling the collection of temperature data at different depths. During heating, the monitoring unit collects and records the readings of each temperature sensor in real time at a set sampling frequency. The criterion for determining temperature stability is that the change in temperature readings at each height position within a second preset time interval is less than a second preset threshold. The second preset time interval refers to the time window used to determine temperature stability. Since heat conduction is typically slower than pressure equilibrium, the second preset time interval can be set longer than the first preset time interval. In a preferred embodiment, the second preset time interval can be set to 30 minutes, 1 hour, or 2 hours. The second preset threshold refers to the allowable temperature fluctuation range. In a preferred embodiment, the second preset threshold can be set as a multiple of the sensor's accuracy range (e.g., if the sensor accuracy is 0.1℃, the threshold can be set to 0.2℃ or 0.5℃), or as a percentage of the absolute temperature value (e.g., 0.5%). The selection of the threshold should consider the overall temperature difference of the ground temperature gradient (e.g., 45℃ at the bottom, 40℃ at the top, a total temperature difference of 5℃) to ensure that the stabilized temperature field accurately reflects the preset gradient characteristics. The monitoring unit continuously collects readings from temperature sensors at various altitudes and calculates the change in each sensor within the current time window in real time. When the changes in all sensors are less than the second preset threshold, the system determines that the model box 1 has reached a thermal equilibrium state, i.e., the preset initial temperature state. When the preset initial temperature state is reached, a stable temperature distribution along the height direction is formed inside the model box 1. In a preferred embodiment, the temperature distribution is characterized by a higher temperature in the bottom region than in the top region (e.g., 45°C at the bottom and 40°C at the top), realistically simulating the geothermal characteristic of "the deeper the strata, the hotter it gets".

[0070] In this optional embodiment, the injection of fluid into model chamber 1 is simultaneous with the activation of the heating unit, meaning that the establishment of pressure and temperature states occurs synchronously. During fluid injection, pore pressure gradually builds up; simultaneously, bottom heating causes the temperature field to evolve gradually. These two processes may have different time scales: pressure equilibrium is usually faster (stabilizing shortly after fluid injection), while thermal equilibrium requires more time (heat conduction is slower). The monitoring unit simultaneously collects pressure and temperature data, with two sets of judgment logic operating in parallel. When the pressure data reaches the stabilization standard first, the system records that the pressure has reached the standard but continues to wait for temperature stabilization; the same applies when the temperature data reaches the stabilization standard first. Only when both reach the stabilization standard is the preset initial temperature and pressure state considered to have been achieved inside model chamber 1. Based on the real-time judgment results of the monitoring unit, the experimental system can achieve automatic process control. When the preset initial state is determined to be reached, the system can automatically issue a command to allow entry into the next stage (such as the heating stage). This automated control improves experimental efficiency and avoids the uncertainty of manual judgment. This embodiment uses a change less than a preset threshold as the judgment criterion, transforming the abstract steady state into a measurable and quantifiable indicator, thus avoiding the arbitrariness of subjective judgment. Using the same preset time interval and preset threshold, different batches of experiments can obtain consistent initial conditions, providing a reliable basis for comparative experiments and parameter studies. Through bottom heating and thermal equilibrium determination, the geothermal characteristics of deep aquifers are realistically reproduced, overcoming the deficiency of existing technologies that ignore geothermal gradients. Automatic judgment based on real-time monitoring reduces human intervention and improves experimental efficiency and data objectivity. Simultaneous monitoring of the evolution of the pressure and temperature fields ensures that both reach a steady state before subsequent experiments begin, providing accurate initial conditions for multiphysics coupling studies.

[0071] Optionally, switching to the first working mode, injecting high-temperature hot water into the porous medium inside the model box 1 via injection well 2, and simultaneously extracting fluid from the production well 3, includes: High-temperature hot water is injected into injection well 2 at a constant flow rate, while fluid is extracted from production well 3 at the same constant flow rate for a preset duration, so that an initial thermal reservoir is formed in model box 1.

[0072] Specifically, after completing model initialization (S1) and confirming that the model chamber 1 has reached the preset initial temperature and pressure, the experimental system enters the heat injection stage. The operator or the automatic control system can first switch valve 11 to set the fluid circulation system to the first operating mode. In the first operating mode, the constant temperature hot water tank 10 is connected to the injection well 2, and the extraction well 3 is connected to the drainage pipe 12, forming an injection-extraction pathway. Next, the operating parameters for the heat injection stage are set. According to the experimental plan, the hot water temperature of the constant temperature hot water tank 10 is set to a predetermined value (e.g., 80℃), the injection flow rate is set to a constant value (e.g., 50 ml / min), and the heat injection duration is set to a preset value (e.g., 2 hours). These parameters are input through the control panel and executed automatically by the system. The pump 15 connected to the injection well 2 is started, and high-temperature hot water is injected into the porous medium inside the model chamber 1 at a set constant flow rate through the inlet pipe 4 and the injection well 2. Simultaneously, pump 15, connected to production well 3, is activated to extract fluid from production well 3 at a constant flow rate (50 ml / min), identical to the injection flow rate. The extracted fluid is the ambient temperature water originally present in model tank 1, discharged to an external storage tank via drain pipe 12. Throughout the entire heat injection process, pump 15 operates continuously, and the flow control system monitors and adjusts the pump speed in real time to ensure that the injection and extraction flow rates remain at the set values, with fluctuations controlled within the range allowed by the sensor accuracy. The monitoring unit operates synchronously: flow meter 6 records the injection and extraction flow rate data in real time, thermometer 7 records the temperature of the injected hot water and the extracted fluid, and temperature and pressure sensors 17 inside model tank 1 continuously collect temperature and pressure data at various points. The heat injection process continues until the preset duration (e.g., 2 hours) is reached. During this period, high-temperature hot water continuously enters the porous medium, carrying heat away from the injection well through convection, while simultaneously exchanging heat with the medium skeleton, causing the medium temperature to rise. Over time, a high-temperature zone gradually forms around the injection well, i.e., the initial thermal reservoir.

[0073] In this optional embodiment, a precision metering pump is used to maintain constant injection and extraction flow rates, eliminating the interference of flow fluctuations on the thermal front propagation law. This ensures that the injection conditions are completely consistent for each experiment, providing a reliable foundation for subsequent comparative experiments and parameter studies. Simultaneously, the constant flow rate allows for a clear mathematical description of the injection process, facilitating the setting of boundary conditions and model verification in subsequent numerical simulations. Equal injection and extraction flow rates prevent pressure buildup due to unilateral injection or pressure drop due to unilateral extraction, keeping the system in a near-steady-state flow state. This design not only protects the integrity of the porous media structure but also simplifies the analysis of convection and conduction relationships in subsequent heat transport analysis. By setting an appropriate injection duration, the total amount of injected hot water is controlled, ensuring that the size and temperature distribution of the thermal reservoir meet the experimental requirements of subsequent storage and heat harvesting stages. In homogeneous media, the thermal reservoir exhibits a symmetrical distribution; in heterogeneous media containing fracture zones, the thermal reservoir preferentially expands along high-permeability channels, providing an intuitive physical phenomenon for studying the control effect of fracture zones on heat transport.

[0074] Optionally, the switching to the second operating mode, extracting fluid from the production well 3, and then cooling the extracted fluid before reinjecting it into the injection well 2 includes: Fluid is drawn from the extraction well 3 at a constant flow rate, cooled by a heat exchanger, and then enters a buffer storage tank. The fluid is then reinjected into the porous medium in the model box 1 through the injection well 2 at the same constant flow rate. This process continues until a preset termination condition is reached.

[0075] Specifically, after the storage phase (S3) ends, the experimental system enters the heat recovery and reinjection phase. The operator or automatic control system can first switch valve 11 to set the fluid circulation system to the second operating mode. In the second operating mode, the pipeline between the production well 3 and the heat exchanger 8 is connected, the pipeline between the buffer storage tank 9 and the injection well 2 is connected, and the valve of the drain pipe 12 is closed, forming a closed-loop circulation path of production-cooling-reinjection. Next, the operating parameters for the heat recovery and reinjection phase are set. According to the experimental plan, the production flow rate is set to a constant value (e.g., 50 ml / min), which is the same as the injection flow rate in the heat injection phase, to maintain the system's flow balance. The cooling medium parameters of the heat exchanger 8 are set to ensure that the cooling capacity meets the experimental requirements. A preset termination condition is set; preferably, the preset termination condition is that the outlet temperature of the production well 3 drops to a predetermined threshold (e.g., close to the initial temperature or below a certain set value). The pump 15 connected to the production well 3 is started to extract hot water from the reservoir from the production well 3 at a set constant flow rate (50 ml / min). Hot water flows out through pipelines and first enters heat exchanger 8. Heat exchanger 8 is a plate heat exchanger with internal cooling medium channels. As the hot water flows through heat exchanger 8, it exchanges heat with the cooling medium, carrying away heat and lowering the water temperature. This process simulates the process in actual engineering where users extract heat energy from the reservoir for heating and other purposes. Thermometers 7 are installed at the inlet and outlet of heat exchanger 8 to monitor the water temperature changes before and after cooling in real time. At the same time, flow meter 6 monitors the flow rate through heat exchanger 8 in real time to ensure that the flow rate is stable at the set value. The cooled water leaves heat exchanger 8 and enters buffer storage tank 9 for temporary storage. The function of buffer storage tank 9 is to balance possible flow fluctuations, ensure a stable and continuous reinjection process, and avoid mutual interference caused by direct pipeline connection. Subsequently, pump 15, connected to injection well 2, draws cooled water from buffer storage tank 9 and reinjects it into the porous medium in model box 1 through injection well 2 at the same constant flow rate (50 ml / min) as the extracted water. During reinjection, thermometer 7 monitors the reinjection water temperature, and flow meter 6 monitors the reinjection flow rate to ensure a balance between the reinjection and extraction flow rates. The heat extraction and reinjection process continues uninterrupted. The monitoring unit records the outlet temperature of the extraction well, the inlet and outlet temperatures of the heat exchanger, the reinjection temperature, the flow rates at each point, and the temperature and pressure data inside model box 1 throughout the process. The system continuously monitors the outlet temperature of extraction well 3 and compares it in real time with a preset termination condition threshold.

[0076] In this optional embodiment, the fluid volume in the model tank 1 remains stable throughout multiple cycles by reinjecting at the same constant flow rate as the extracted fluid. This prevents the system from operating normally due to a drop in reservoir pressure caused by fluid extraction. This design provides pressure condition assurance for the repeated execution of storage and heat extraction steps, making multi-cycle experiments possible. The extracted hot water is cooled by heat exchanger 8, simulating the process in actual engineering where users extract heat energy from the reservoir for heating or other purposes. The temperature and flow rate data at the inlet and outlet of the heat exchanger are accurately recorded and can be used to calculate the heat extracted in a single heat extraction cycle (i.e., the heat energy obtained by the user), providing accurate data support for calculating the heat recovery rate. The buffer water tank 9 temporarily stores the cooled water, balancing possible flow fluctuations and avoiding mutual interference caused by direct pipeline connections, ensuring a continuous and stable reinjection process. At the same time, the buffer water tank also serves as a gas-liquid separator, accommodating any gas that may precipitate and preventing gas from entering the reinjection pipeline and affecting the saturation state of the porous medium. When the outlet temperature of well 3 drops to a predetermined threshold, it indicates that the recoverable heat in the reservoir has been largely extracted, and the thermal front has advanced to the well or the overall temperature of the reservoir has decreased. Using this physical phenomenon as the termination condition avoids the subjectivity of relying solely on time for judgment.

[0077] Optionally, S5 specifically includes: The steps of closing the valves of injection well 2 and production well 3 and stopping fluid circulation are considered as a storage cycle; The steps of switching to the second working mode, extracting fluid from the production well 3, and then cooling and reinjecting the extracted fluid back into the injection well 2 constitute a heat recovery and reinjection cycle. A complete thermal cycle is formed by sequentially executing a storage cycle and a heat recovery and reinjection cycle, and the thermal cycle is repeated at least once.

[0078] Specifically, after completing the first heat recovery and reinjection phase (S4), the experimental system enters a multi-cycle phase. First, the storage cycle is defined as the step of closing the valves of injection well 2 and production well 3 and stopping fluid circulation. The core of this cycle is valve closure, pump stoppage, and fluid stillness; the heat within model chamber 1 is redistributed and diffused solely through natural heat conduction and convection. The duration of the storage cycle can be set according to the experimental plan (e.g., 4 hours). During this time period, the monitoring unit continuously records temperature and pressure data at various locations within model chamber 1 to analyze the heat diffusion patterns.

[0079] The definition of a heat recovery and reinjection cycle: The process of switching to the second operating mode, extracting fluid from the production well 3, and then reinjecting the cooled fluid back into the injection well 2 is defined as a heat recovery and reinjection cycle. The core of this cycle is: hot water is extracted from the production well 3 at a constant flow rate, cooled by the heat exchanger 8, and then enters the buffer storage tank 9. It is then reinjected into the model box 1 through the injection well 2 at the same flow rate. The duration of the heat recovery and reinjection cycle is controlled by preset termination conditions, such as the temperature at the production well outlet falling back to a predetermined threshold. During this process, the monitoring unit records the inlet and outlet temperatures of the heat exchanger, the flow rate, and the temperature and pressure data at various points within the model box, which are used to calculate the heat recovery rate.

[0080] The thermal cycle consists of a storage cycle and a heat recovery and reinjection cycle executed sequentially, forming a complete thermal cycle. The storage cycle simulates the residence and diffusion of heat in the underground reservoir, while the heat recovery and reinjection cycle simulates the process of the user extracting heat energy from the reservoir and reinjecting it back. Together, they correspond to a complete operating cycle in a real-world engineering project.

[0081] After completing the division of the cycle units, the experimental system begins to execute a multi-cycle sequence. Taking the first thermal cycle as an example, the execution process is as follows: Execution of the first storage cycle: After the first heat recovery and reinjection stage ends, the system automatically or manually closes valve 11 connected to injection well 2 and production well 3, stops the operation of all pumps 15, and keeps the fluid in model box 1 in a static state. At this time, model box 1 still retains the heat that was not fully extracted in the previous cycle. The system remains static for a preset duration (e.g., 4 hours) to allow the heat to further diffuse and redistribute under natural heat conduction and convection. During this period, the monitoring unit continuously collects temperature and pressure data at each point. Execution of the first heat recovery and reinjection cycle: After the storage cycle ends, the system switches to the second working mode, starts pump 15, and extracts fluid from production well 3 at a constant flow rate. The extracted fluid flows through heat exchanger 8 for cooling and then enters buffer storage tank 9, and is reinjected into model box 1 through injection well 2 at the same flow rate. The heat recovery and reinjection process continues until the outlet temperature of production well 3 drops back to a predetermined threshold (e.g., close to the initial temperature). The monitoring unit records all relevant data throughout the process.

[0082] Execution of the second and subsequent cycles: After the first thermal cycle ends, the system automatically enters the second thermal cycle, repeating the aforementioned storage cycle and heat recovery cycle. In a preferred embodiment, this is repeated 5 to 10 times to simulate multiple complete heat storage-heat extraction cycles. During cycle switching, the system achieves seamless connection through automatic control of valve 11 and start / stop of pump 15, ensuring the continuity and independence between each cycle. During the execution of the cycle sequence, all operating parameters (such as storage duration, heat extraction flow rate, and termination temperature threshold) remain consistent with the first cycle to ensure comparability between cycles. To study the impact of parameter changes on long-term performance, parameter settings can be adjusted in different experimental batches.

[0083] In a preferred embodiment of the present invention, the monitoring unit operates continuously throughout the multi-cycle process, providing complete data support for subsequent analysis. Temperature and pressure sensors 17, flow meters 6, thermometers 7, and other monitoring devices operate continuously throughout the multi-cycle process, recording data from all measuring points at a set sampling frequency (e.g., once per second). This data forms a time series covering multiple complete cycles, providing a basis for analyzing performance changes during the cycles. For each heat recovery and reinjection cycle, the heat extracted (heat energy obtained by the user side) is calculated based on the temperature difference and flow rate data at the inlet and outlet of the heat exchanger 8. The extracted heat is compared with the recoverable heat in the reservoir before the start of the cycle (or with the heat injected during the injection phase) to obtain the heat recovery rate (cycle efficiency) for that cycle. The cycle efficiencies of multiple cycles are summed to obtain the cumulative heat recovery rate for the entire experimental process. Furthermore, analyzing the performance evolution patterns during cycles and comparing performance indicators across different cycles reveals the evolution trend under long-term operating conditions: as the number of cycles increases, the thermal breakthrough time (the time required from the start of thermal extraction to a significant increase in the temperature of the production well) may show a shortening trend, indicating a decrease in the effective heat retention capacity in the reservoir. Cycle efficiency may gradually decrease with increasing cycle number, reflecting the cumulative effect of heat loss during long-term operation. In models containing fracture zones, comparing with homogeneous models allows analysis of the control effect of fracture zones on multi-cycle performance evolution, such as whether they exacerbate efficiency decay or accelerate thermal breakthrough. Simultaneously, based on the experimental results of multi-cycle cycles, key parameters affecting long-term thermal storage performance (such as fracture zone geometry, injection-production flow rate, injection temperature, etc.) can be identified, providing direct basis for system design and operational strategy optimization in practical engineering. For example, if experiments show that the width of the fracture zone significantly affects the efficiency decay rate, then fracture zone characteristics should be given special attention in practical engineering site selection and design.

[0084] Optionally, after activating the heating unit located at the bottom of the model box 1, the static heating time is maintained for 24 to 48 hours.

[0085] In this embodiment, after the porous medium filling and fluid saturation of the model box 1 are completed, the heating element 16 located at the bottom of the model box 1 is activated. The heating element 16 uses a precisely temperature-controlled electric heating element and starts heating in a constant power or constant temperature mode. Heat is transferred from the bottom upwards, forming a vertical temperature gradient in the porous medium. After heating is started, the system enters a static heating state. In this state, no injection or sampling operations are performed, all valves 11 are closed, pump 15 stops running, and the fluid is in a static state. The heating element 16 continues to work, and heat is slowly transferred upwards through the thermal conduction of the porous medium skeleton and the pore fluid. The static heating time can be set to 24 to 48 hours. For example, for a model box with a height of 40 cm, it takes sufficient time for heat to be transferred from the bottom to the top, which can be 24 hours or longer to transfer heat to the top and initially establish a temperature gradient. At the same time, parameters such as the thermal conductivity and specific heat capacity of the porous medium affect the heat transfer rate. The difference in thermal properties between fine sand 13 and coarse sand 14 may lead to different local heat conduction rates, requiring sufficient time to homogenize the overall temperature field. To establish a stable temperature gradient of 45°C at the bottom and 40°C at the top, it is necessary to ensure that heat transfer reaches a dynamic equilibrium, i.e., a balance between continuous heating at the bottom and heat dissipation at the top. This requires a longer period, such as 48 hours, for this equilibrium to stabilize. During the static heating process, the monitoring unit collects temperature data at different heights within model box 1 in real time. By observing the changing trends of temperature readings at each height, it can be determined whether the temperature field has reached a stable state. When the change in temperature readings at all heights within a preset time interval (e.g., 1 hour) is less than a preset threshold (e.g., 0.2°C), it indicates that the temperature field has stabilized, and static heating can be terminated. The time range of 24 to 48 hours provides an empirical time window for this, but the final criterion is temperature stability. In this embodiment, through continuous heating for 24-48 hours, heat is transferred from the bottom upwards, ultimately forming a stable temperature distribution with a high bottom and a low top, realistically simulating the deep geothermal environment. Through sufficient static heating, the temperature distribution inside the model is made consistent with the actual geological environment. Only after the temperature field reaches stability can subsequent heat injection, storage, and heat extraction experiments be repeatable and comparable.

[0086] Optionally, the temperature distribution of the preset initial temperature state at various locations along the height direction is such that the temperature in the bottom region is higher than the temperature in the top region.

[0087] In this embodiment, the heating element 16 located at the bottom of the model box 1 becomes the heat source for the entire system after activation, and the bottom area is heated first, causing its temperature to rise. Since heat is transferred upwards from the bottom, the bottom area is always the hottest region. Heat is transferred upwards through the thermal conduction of the porous medium. During this transfer, some heat is absorbed by the medium, some continues to be transferred upwards, and some heat may be lost slightly through the insulation layer 5 on the side wall of the model box. Therefore, the temperature gradually decreases with increasing height, resulting in a temperature distribution that is high at the bottom and low at the top. After static heating for 24-48 hours, the temperature field reaches a stable state. At this time, the temperature measured at various heights along the vertical direction exhibits the following characteristics: the area immediately adjacent to the heating element 16 reaches the set temperature (e.g., 45°C). At the middle height of the model box, the temperature is between the bottom and the top. The top area, far from the heat source, has the lowest temperature (e.g., 40°C).

[0088] Under actual geological conditions, due to the presence of heat flow within the Earth, the temperature gradually increases with depth. Deep aquifers are typically situated within this geothermal gradient environment. This embodiment uses bottom heating to create a temperature distribution within model box 1 where the bottom temperature is higher than the top temperature, accurately replicating this geological characteristic. In the model with a realistic geothermal gradient, the heat loss during upward transfer is consistent with actual conditions, making the heat loss assessment results more valuable. This embodiment provides initial conditions with a realistic geothermal gradient, making the heat transport process after injection more closely resemble actual engineering scenarios.

[0089] By allowing the heating process to stand still (e.g., 24-48 hours), the time required for the temperature field to stabilize from startup is ensured, guaranteeing sufficient heat transfer. The higher bottom temperature compared to the top ensures that the stabilized temperature field exhibits spatial distribution characteristics consistent with real geology. This ensures that the initial temperature state inside model box 1 conforms to the physical laws of geothermal gradients and possesses repeatable and quantifiable experimental operability, providing a reliable starting point for subsequent thermal storage simulation studies.

[0090] While the present invention has been disclosed above, its scope of protection is not limited thereto. Those skilled in the art can make various changes and modifications without departing from the spirit and scope of the present invention, and all such changes and modifications will fall within the scope of protection of the present invention.

Claims

1. A periodic thermal storage simulation system for fractured aquifers, characterized in that, include: Model box (1), the model box (1) is filled with a porous medium, the porous medium has at least one fracture zone structure, the fracture zone structure is filled with a material with a permeability greater than that of the porous medium; The injection-production circulation system includes an injection well (2) and a production well (3), which are buried in a porous medium inside the model box (1). The injection well (2) and the production well (3) are connected to a fluid circulation system through pipelines. The fluid circulation system has a switchable first working mode and a second working mode. In the first working mode, high-temperature hot water is provided to the injection well (2), and in the second working mode, the hot water extracted from the production well (3) is cooled and then reinjected into the injection well (2). The monitoring unit includes at least a plurality of sensors (17) for collecting temperature and pressure data inside the model box (1).

2. A method for simulating periodic thermal storage in fractured aquifers, characterized in that, The method for simulating periodic thermal storage in fractured aquifers, as described in claim 1, comprises: S1: After vacuuming the model box (1) filled with porous medium, inject fluid into the model box (1) until the pores in the porous medium are filled with fluid. At the same time, start the heating unit set at the bottom of the model box (1) so that the interior of the model box (1) reaches the preset initial temperature state and the preset initial pressure state. S2: Switch to the first working mode, inject high-temperature hot water into the porous medium in the model box (1) through the injection well (2), and extract fluid from the mining well (3); S3: Stop the fluid circulation by closing the valves of the injection well (2) and the production well (3); S4: Switch to the second working mode, extract fluid from the extraction well (3), and cool the extracted fluid before reinjecting it into the injection well (2). S5: Repeat the process of stopping the fluid circulation and subsequent steps by closing the valves of the injection well (2) and the production well (3).

3. The experimental method for simulating periodic thermal storage in fractured aquifers according to claim 2, characterized in that, By temporarily placing the forming component into the model box (1), filling the model box (1) with a porous medium, and filling the space defined by the forming component with fracture band material, and removing the forming component after filling, a fracture band structure is obtained. The forming component is at least one pair of rigid plates, and the rigid plates are temporarily placed into the model box (1) at a preset tilt angle and a preset spacing.

4. The experimental method for simulating periodic thermal storage in fractured aquifers according to claim 3, characterized in that, The construction process of the fracture zone structure includes: The porous medium is filled into the model box (1) up to the lower edge of the rigid plate; After filling the gap between the pair of rigid plates with fracture zone material, continue to fill the model box (1) with porous medium until the fracture zone material and the rigid plates are completely covered; The rigid plate is extracted from the model box (1) to obtain the fracture zone structure, wherein the permeability of the fracture zone material is greater than the permeability of the porous medium.

5. The experimental method for simulating periodic thermal storage in fractured aquifers according to claim 4, characterized in that, The process of injecting fluid into the model box (1) until the pores in the porous medium are filled with fluid, and simultaneously activating the heating unit located at the bottom of the model box (1), so that the interior of the model box (1) reaches a preset initial temperature state and a preset initial pressure state, includes: After fluid is injected into the model box (1), the pore pressure data at different locations in the model box (1) are collected in real time by the monitoring unit until the change in pore pressure data at each location within the first preset time interval is less than the first preset threshold, and the interior of the model box (1) is determined to have reached the preset initial pressure state. Fluid is injected into the model box (1) and the heating unit is started and maintained. Temperature data at different height positions inside the model box (1) are collected in real time by the monitoring unit until the change in temperature readings at each height position within a second preset time interval is less than a second preset threshold. Then, it is determined that the interior of the model box (1) has reached the preset initial temperature state.

6. The experimental method for simulating periodic thermal storage in fractured aquifers according to claim 5, characterized in that, The switching to the first working mode involves injecting high-temperature hot water into the porous medium inside the model box (1) via the injection well (2), while simultaneously extracting fluid from the extraction well (3), including: High-temperature hot water is injected into the injection well (2) at a constant flow rate, and fluid is extracted from the production well (3) at the same constant flow rate for a preset duration, so that an initial thermal reservoir is formed in the model box (1).

7. The experimental method for simulating periodic thermal storage in fractured aquifers according to claim 6, characterized in that, The switching to the second working mode, extracting fluid from the production well (3), and then cooling the extracted fluid before reinjecting it into the injection well (2) includes: Fluid is drawn from the mining well (3) at a constant flow rate, cooled by the heat exchanger and then enters the buffer storage tank. The fluid is then injected back into the porous medium in the model box (1) through the injection well (2) at the same constant flow rate. This process continues until the preset termination condition is met.

8. The experimental method for simulating periodic thermal storage in fractured aquifers according to claim 7, characterized in that, S5 specifically includes: The steps of closing the valves of the injection well (2) and the production well (3) and stopping the fluid circulation are considered as a storage cycle; The steps of switching to the second working mode, extracting fluid from the production well (3), and then cooling the extracted fluid and reinjecting it back into the injection well (2) constitute a heat recovery and reinjection cycle; A complete thermal cycle is formed by sequentially executing a storage cycle and a heat recovery and reinjection cycle, and the thermal cycle is repeated at least once.

9. The experimental method for simulating periodic thermal storage in fractured aquifers according to claim 5, characterized in that, After activating the heating unit located at the bottom of the model box (1), the static heating time is maintained for 24 to 48 hours.

10. The experimental method for simulating periodic thermal storage in fractured aquifers according to claim 9, characterized in that, The temperature distribution of the preset initial temperature state at various locations along the height direction is such that the temperature in the bottom region is higher than the temperature in the top region.

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