A medium-deep geothermal well exploitation and recharge system

Abstract

This invention belongs to the field of geothermal energy development technology, and particularly relates to a medium-deep geothermal well production and reinjection system. It includes a production well, a reinjection well, a submersible pump located in the production well, a heat exchanger connected to the submersible pump outlet, and a reinjection pipeline that sends the heat exchanger tailwater back to the reinjection well. The reinjection screen pipe of the heat storage section of the reinjection well is vertically divided into upper, middle, and lower sections. Along the tailwater flow direction, the reinjection pipeline is sequentially equipped with a cyclone microbubble deaerator, a seed-induced scale-inhibiting reactor, a precision filter, and a reinjection pressurization pump. The production wellhead and the reinjection wellhead are connected via a feedforward pipeline. The system also includes a central controller, which performs thermal breakthrough prediction based on the temperature difference between the production well and the reinjection well and the cumulative production and injection volume. When the predicted thermal breakthrough time is less than a set value, the central controller controls the submersible pump and the reinjection pressurization pump to stop, start, and reverse, thus switching the production and injection roles of the production well and the reinjection well. This invention can solve the problems of easy clogging during reinjection, premature thermal breakthrough, and high energy consumption.

Description

Technical Field

[0001] This invention belongs to the field of geothermal energy development technology, and particularly relates to a medium-deep geothermal well extraction and reinjection system. Background Technology

[0002] Currently, medium-deep geothermal heating generally adopts a "one-extraction-one-injection" well-to-well model, where the tailwater is extracted for heat exchange and then reinjected to maintain reservoir pressure. However, existing technologies have the following significant bottlenecks: First, reinjection wells often use a single-layer, indiscriminate injection method, concentrating the reinjection water into a specific layer of the reservoir. This easily leads to scaling and blockage of the screen pipes and near-wellbore areas, causing a continuous increase in reinjection pressure and a gradual decline in the reinjection rate. Second, dissolved CO2 in the tailwater precipitates under pressure and temperature changes, triggering CaCO3 crystallization. Existing surface treatment processes mostly rely on aeration heating for degassing, which is energy-intensive and prone to introducing secondary pollution. Third, the roles of the extraction and injection wells are fixed. As operating time progresses, the low-temperature reinjection water migrates to the production well, creating a "thermal breakthrough," resulting in a decrease in the outlet water temperature and a shortened operating life. Finally, there is a lack of effective pressure differential utilization mechanisms at the wellhead, leading to high energy consumption for reinjection pressurization.

[0003] Therefore, it is necessary to design a medium-deep geothermal well extraction and reinjection system to solve the above problems. Summary of the Invention

[0004] The purpose of this invention is to provide a medium-deep geothermal well extraction and reinjection system to solve the problems of easy blockage during reinjection, premature thermal breakthrough, and high energy consumption.

[0005] To achieve the above objectives, the present invention provides the following solution: a medium-deep geothermal well production and reinjection system, comprising a production well, a reinjection well, a submersible pump installed in the production well, a heat exchanger connected to the outlet of the submersible pump, and a reinjection pipeline for sending the tailwater of the heat exchanger back to the reinjection well. The reinjection screen pipe of the hot storage section of the reinjection well is vertically divided into three sections: upper, middle and lower. Each section of the reinjection screen pipe is connected to the reinjection pipeline through an independent branch pipe with an electrically controlled water valve. The reinjection pipeline is sequentially equipped with a cyclone microbubble deaerator, a seed-induced scale-inhibiting reactor, a precision filter, and a reinjection pressurization pump along the tailwater flow direction. The seed-induced scale-inhibiting reactor contains a quartz sand packing frame with a surface pre-coated with CaCO3 seeds. The production wellhead and the reinjection wellhead are connected by a feeder pipeline, and the feeder pipeline is equipped with a bidirectional check valve and a nitrogen isolation buffer tank. It also includes a central controller, which performs thermal breakthrough prediction based on the temperature difference between the production well and the reinjection well and the cumulative production and injection volume, and controls the submersible pump and the reinjection pressurization pump to reverse when the predicted thermal breakthrough time is less than a set value.

[0006] Preferably, the lower reinjection screen is located at 0–1 / 3 of the thickness above the bottom boundary of the thermal reservoir, the middle reinjection screen is located at 1 / 3–2 / 3 of the thickness of the thermal reservoir, and the upper reinjection screen is located at 2 / 3 of the thickness of the thermal reservoir to the top boundary; the reinjection screen is covered with a quartz sand filter layer, and the lower quartz sand filter layer has a mesh size of 20–40 mesh, the middle quartz sand filter layer has a mesh size of 40–60 mesh, and the upper quartz sand filter layer has a mesh size of 60–100 mesh.

[0007] Preferably, the central controller is configured to allocate the reinjection flow according to the logic of "lower section priority, upper section pressure maintenance": in the initial stage of reinjection, the opening of the lower electrically controlled water valve is controlled to make the lower section flow account for 60%-80%, the opening of the middle electrically controlled water valve is controlled to make the middle section flow account for 20%-40%, and the upper electrically controlled water valve is controlled to be closed or kept open at less than 10%.

[0008] Preferably, the central controller monitors the rate of increase of the reinjection pressure in real time. When the rate of increase of the pressure exceeds 0.5 kPa / d, it gradually closes the lower electrically controlled water valve and simultaneously opens the upper electrically controlled water valve, so that the scale layer is distributed vertically and the blockage of a single layer is delayed.

[0009] Preferably, the inlet of the cyclone microbubble deaerator is tangentially connected to the cylinder, and an exhaust core tube is provided at the center of the top of the cylinder. The exhaust core tube is connected to a micro vacuum pump with a vacuum degree of -5 to -20 kPa. The dissolved oxygen content of the tailwater after treatment by the cyclone microbubble deaerator is less than 1 mg / L, and the free CO2 removal rate is not less than 85%.

[0010] Preferably, the seed-induced scale-inhibiting reactor is a horizontal tank with a suspended stainless steel packing rack inside. The packing rack is loaded with quartz sand with a particle size of 2–3 mm. The surface of the quartz sand is pre-coated with a CaCO3 seed layer, and the coating amount of the seed layer is 0.8–1.2 g / kg of quartz sand. The residence time of the tailwater in the seed-induced scale-inhibiting reactor is 8–15 min, and the temperature inside the seed-induced scale-inhibiting reactor is controlled within the range of ±2℃ of the original temperature of the reinjected tailwater.

[0011] Preferably, the bidirectional check valve is configured to allow gravity flow from the production well to the reinjection well, and also to allow water in the reinjection well to flow back into the production well via the feeder pipeline for replenishment when the static water level in the reinjection well is more than 2 m higher than that in the production well and the reinjection booster pump is stopped; the nitrogen isolation buffer tank has a volume of 2–5 m³. 3 It has a nitrogen capsule inside to prevent air from contacting the wastewater.

[0012] Preferably, the central controller has a built-in thermal breakthrough prediction model, based on the formula... in, For thermal breakthrough time, For the viscosity of the thermal storage fluid, For thermal storage porosity, This refers to the distance between production wells and injection wells. For thermal storage permeability, The allowable temperature drop threshold; When the temperature of the water discharged from the production well decreases by a certain value compared to the initial temperature Or prediction When the heating season is less than one heating season, the central controller stops the submersible pump of the original production well and switches it to a reinjection well. The reinjection booster pump of the original reinjection well stops running and starts the submersible pump in the well to switch it to a production well. At the same time, the opening logic of the electrically controlled water valve is synchronously mirrored and reset.

[0013] Preferably, the outer sides of the upper, middle, and lower reinjection screen pipes are all wrapped with double layers of 60-mesh stainless steel mesh, and the spaces between the double layers of stainless steel mesh are filled with a slow-release seed ring layer with a thickness of 30-50 mm; the slow-release seed ring layer is composed of a mixture of quartz sand and wt% pre-calcified ceramic particles, and the surface of the pre-calcified ceramic particles is loaded with Zn. 2+ Polyaspartic acid scale inhibitor membrane is used to crystallize supersaturated ions in tailwater at a depth of 0–5 cm outside the well wall.

[0014] Preferably, the outlet end of the seed-induced scale-inhibiting reactor is equipped with an online turbidity meter. When the online turbidity meter reading suddenly increases by more than 5 NTU compared to the reference value, the central controller controls the start of the backwashing pipeline inside the seed-induced scale-inhibiting reactor. The backwashing water flow direction is opposite to the tailwater flow direction, the backwashing time is 3-5 minutes, and the backwashing waste liquid is discharged from the system through a dedicated sewage pipe.

[0015] Compared with the prior art, the present invention has the following advantages and technical effects: This invention utilizes a vertical three-section layered water distribution system and a downhole slow-release seed ring to forcibly disperse scale from a single well section to the entire geothermal reservoir profile, significantly delaying reinjection well blockage and extending reinjection lifespan. By replacing traditional aeration heating processes with swirl-flow micro-vacuum degassing and seed-induced scale prevention, it achieves efficient scale removal of tailwater while significantly reducing surface treatment energy consumption. The invention also leverages the wellhead kinetic energy feedback pipeline to recover the production-injection pressure difference, effectively reducing the frequency of reinjection pump start-ups and shutdowns and power consumption. More importantly, by dynamically executing the production-injection role switching through a built-in thermal breakthrough prediction model, it breaks the limitations of the traditional fixed-well mode, successfully postponing the thermal breakthrough time by more than one heating season, ensuring long-term stable heat extraction efficiency and reinjection rate of the geothermal system, and achieving a high level of development of geothermal resources characterized by "heat extraction without water consumption, pressureless injection within the same layer, and economic sustainability." Attached Figure Description

[0016] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0017] Figure 1 This is a schematic diagram of the overall system of the present invention; Figure 2 This is a schematic diagram of the reinjection screen pipe of the present invention.

[0018] The components include: 1. Production well; 2. Recharge well; 3. Heat exchanger; 4. Production pipeline; 5. Recharge pipeline; 6. Cyclone microbubble deaerator; 7. Seed-induced scale-inhibiting reactor; 8. Precision filter; 9. Recharge pressurization pump; 10. Production screen pipe; 11. Feedback pipeline; 12. Two-way check valve; 13. Nitrogen isolation buffer tank; 14. Central controller; 15. Recharge screen pipe; and 16. Electrically controlled water valve. Detailed Implementation

[0019] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0020] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0021] Reference Figures 1 to 2 As shown, the present invention provides a medium-deep geothermal well production and reinjection system, including a production well 1, a reinjection well 2, a submersible pump installed in the production well 1, a heat exchanger 3 connected to the outlet of the submersible pump, and a reinjection pipeline 5 for sending the tailwater of the heat exchanger 3 back to the reinjection well 2. The reinjection screen pipe 15 of the hot storage section of the reinjection well 2 is divided into three sections vertically: upper, middle and lower. Each section of the reinjection screen pipe 15 is connected to the reinjection pipeline 5 through an independent branch pipe with an electrically controlled water valve 16. Along the tailwater flow direction, the reinjection pipeline 5 is equipped with a cyclone microbubble deaerator 6, a seed-induced scale inhibitor 7, a precision filter 8, and a reinjection pressurization pump 9. The seed-induced scale inhibitor 7 has a quartz sand packing frame with CaCO3 seed crystals pre-coated on its surface suspended inside. The wellhead of production well 1 and the wellhead of reinjection well 2 are connected by a feeder pipeline 11. The feeder pipeline 11 is equipped with a two-way check valve 12 and a nitrogen isolation buffer tank 13. It also includes a central controller 14, which performs thermal breakthrough prediction based on the temperature difference between production well 1 and reinjection well 2 and the cumulative production and injection volume. When the predicted thermal breakthrough time is less than the set value, it controls the submersible pump and reinjection pressurization pump 9 to reverse, so that the production well 1 and reinjection well 2 switch production and injection roles.

[0022] The scheme is further optimized as follows: the lower reinjection screen 15 is located at 0–1 / 3 of the thickness above the bottom boundary of the thermal reservoir, the middle reinjection screen 15 is located at 1 / 3–2 / 3 of the thickness of the thermal reservoir, and the upper reinjection screen 15 is located at 2 / 3 of the thickness of the thermal reservoir to the top boundary; the reinjection screen 15 is covered with a quartz sand filter layer on the outside, and the mesh size of the lower quartz sand filter layer is 20–40 mesh, the mesh size of the middle quartz sand filter layer is 40–60 mesh, and the mesh size of the upper quartz sand filter layer is 60–100 mesh.

[0023] The scheme is further optimized. The central controller 14 is configured to allocate the reinjection flow according to the logic of "lower section priority, upper section pressure maintenance": in the early stage of reinjection, the opening of the lower electrically controlled water valve 16 is controlled to make the lower section flow account for 60%-80%, the opening of the middle electrically controlled water valve 16 is controlled to make the middle section flow account for 20%-40%, and the upper electrically controlled water valve 16 is controlled to be closed or kept open at less than 10%.

[0024] To further optimize the scheme, the central controller 14 monitors the rate of increase of the reinjection pressure in real time. When the rate of increase of the pressure exceeds 0.5 kPa / d, the lower electrically controlled water valve 16 is gradually closed, while the upper electrically controlled water valve 16 is opened simultaneously, so that the scale layer is distributed vertically and the blockage of a single layer is delayed.

[0025] The scheme is further optimized by tangentially connecting the inlet of the cyclone microbubble deaerator 6 to the cylinder, and the exhaust core pipe is provided at the center of the top of the cylinder. The exhaust core pipe is connected to a micro vacuum pump with a vacuum degree of -5 to -20 kPa. The dissolved oxygen content of the tailwater after treatment by the cyclone microbubble deaerator 6 is less than 1 mg / L, and the free CO2 removal rate is not less than 85%.

[0026] Further optimization of the scheme: the seed-induced scale prevention reactor 7 is a horizontal tank with a suspended stainless steel packing rack inside. The packing rack is loaded with quartz sand with a particle size of 2–3 mm. The surface of the quartz sand is pre-coated with a CaCO3 seed layer, and the coating amount of the seed layer is 0.8–1.2 g / kg of quartz sand. The residence time of the tailwater in the seed-induced scale prevention reactor 7 is 8–15 min, and the temperature inside the seed-induced scale prevention reactor 7 is controlled within the range of ±2℃ of the original temperature of the reinjection tailwater.

[0027] The scheme was further optimized, and the two-way check valve 12 was configured to allow gravity flow from production well 1 to reinjection well 2, and also to allow water in reinjection well 2 to flow back into production well 1 via the feeder pipeline 11 for replenishment when the static water level in reinjection well 2 is more than 12 m higher than that in production well 12 and the reinjection booster pump 9 is stopped; the nitrogen isolation buffer tank 13 has a volume of 2–5 m³.3 It has a nitrogen capsule inside to prevent air from contacting the wastewater.

[0028] Further optimization of the scheme: the central controller 14 incorporates a thermal breakthrough prediction model, based on the formula... in, For thermal breakthrough time, For the viscosity of the thermal storage fluid, For thermal storage porosity, The distance between production well 1 and injection well 2. For thermal storage permeability, The allowable temperature drop threshold; When the water temperature from production well 1 decreases by a certain value compared to the initial temperature... Or prediction When the heating season is less than one heating season, the central controller 14 controls the submersible pump of the original production well 1 to stop running and switch to the reinjection well 2. The reinjection booster pump 9 of the original reinjection well 2 stops running and starts the submersible pump in the well to switch to the production well 1. At the same time, the opening logic of the electrically controlled water valve 16 is synchronously mirrored and reset.

[0029] The design was further optimized by wrapping double layers of 60-mesh stainless steel mesh around the outer sides of the upper, middle, and lower reinjection screens 15. Between the double layers of stainless steel mesh is a 30-50mm thick slow-release seed ring layer. The slow-release seed ring layer is composed of a mixture of quartz sand and wt% pre-calcified ceramic particles, with Zn loaded on the surface of the pre-calcified ceramic particles. 2+ Polyaspartic acid scale inhibitor membrane is used to crystallize supersaturated ions in tailwater at a depth of 0–5 cm outside the well wall.

[0030] The scheme is further optimized by installing an online turbidity meter at the outlet of the seed-induced scale-inhibiting reactor 7. When the online turbidity meter reading suddenly increases by more than 5 NTU compared to the reference value, the central controller 14 controls the start of the backwashing pipeline inside the seed-induced scale-inhibiting reactor 7. The backwashing water flow direction is opposite to the tailwater flow direction, and the backwashing time is 3–5 minutes. The backwashing waste liquid is discharged from the system through a dedicated sewage pipe.

[0031] Example: This embodiment provides a geothermal well production and reinjection system suitable for the Guantao Formation bedrock geothermal reservoir (well depth approximately 1200–1500 m). The system includes one production well 1 and one reinjection well 2, with a well spacing of 450 m. The production well 1 is equipped with a variable frequency submersible pump, which extracts geothermal water through a production pipeline 4 and transports it to a plate heat exchanger 3 on the surface. A production screen 10 is installed on the production pipeline 4, and the tailwater (temperature reduced from approximately 55°C to 25°C) after heat exchange enters the reinjection pipeline.

[0032] The screen pipe of the reservoir section in reinjection well 2 is vertically divided into three sections. The lower reinjection screen pipe 15 is located at 0–1 / 3 of the reservoir thickness above the bottom boundary; the middle reinjection screen pipe 15 is located at 1 / 3–2 / 3 of the reservoir thickness; and the upper reinjection screen pipe 15 is located at 2 / 3 of the reservoir thickness to the top boundary. To accommodate the different particle sizes of the reservoir pores, each section of the reinjection screen pipe 15 is coated with a quartz sand filter layer of different mesh sizes: the lower section uses 20–40 mesh coarse sand, the middle section uses 40–60 mesh medium sand, and the upper section uses 60–100 mesh fine sand. This gradient filter layer design aims to achieve uniform vertical diffusion of tailwater.

[0033] During the initial system commissioning phase, the central controller 14 implements a "lower section priority, upper section pressure maintenance" strategy. Specifically, it controls the opening of the electrically controlled water valve 16 of the lower section reinjection screen pipe 15 to 70%, the opening of the electrically controlled water valve 16 of the middle section reinjection screen pipe 15 to 30%, and keeps the electrically controlled water valve 16 of the upper section reinjection screen pipe 15 closed. This approach utilizes the high permeability of the lower section to quickly establish a reinjection channel, while simultaneously maintaining the static pressure inside the well and preventing gas leakage by sealing off the upper section.

[0034] After 30 days of operation, the central controller 14 detected a reinjection pressure rise rate of 0.6 kPa / d (exceeding the set threshold of 0.5 kPa / d). At this point, the controller automatically adjusted the valve opening: closing the lower section electrically controlled water valve 16 to 40%, maintaining the middle section electrically controlled water valve 16 at 30%, and simultaneously opening the upper section electrically controlled water valve 16 to 30%. This dynamic adjustment forced a change in the tailwater flow direction, causing the scale layer to be distributed vertically, effectively preventing a single section from failing due to excessively rapid scaling.

[0035] The wastewater first enters the cyclone microbubble deaerator 6. The water enters the cylinder tangentially, generating a strong vortex. A miniature vacuum pump is connected to the central exhaust core pipe, maintaining a vacuum of -12 kPa inside the cylinder. Under this negative pressure environment, the removal rate of free CO2 in the water reaches 90%, and the dissolved oxygen content drops to 0.8 mg / L. This effectively inhibits the crystallization tendency of CaCO3 in subsequent pipelines and prevents oxygen corrosion of the pipelines.

[0036] The degassed effluent enters a horizontal seed-induced scale-inhibiting reactor 7. Inside the reactor, a suspended stainless steel packing rack holds 2.5mm of silica sand, pre-coated with a 1.0g / kg CaCO3 seed layer. The effluent flows through the reactor with a residence time of 12 minutes, and the supersaturated CaCO3... 2+ and CO3 2- Ions preferentially grow on the surface of the seed crystals, rather than forming scale on the pipes or well walls. The seed-induced scale-inhibiting reactor 7 strictly controls the water temperature at 25±2℃ through jacket heat exchange to avoid new supersaturation precipitation caused by temperature changes.

[0037] The production well 1 and the reinjection well 2 are connected by a feeder pipeline 11. A bidirectional check valve 12 and a 3m³ / h valve are connected in series on the feeder pipeline 11. 3 The nitrogen-isolated buffer tank 13 is used. At the beginning of the heating season, the static water level in the reinjection well 2 is relatively high. When the pressure difference reaches 2.5m, the tailwater flows into the reinjection well 2 by gravity through the feeder pipeline 11 without the need to start the reinjection booster pump 9, thus realizing pressure energy recovery. The nitrogen capsule inside the nitrogen-isolated buffer tank 13 effectively isolates the air from the tailwater, preventing secondary oxidation and scaling.

[0038] The central controller 14 has a built-in thermal breakthrough prediction model. After one heating season, the outlet water temperature of production well 1 dropped from 55℃ to 52℃, reaching the set temperature drop target. (3℃). The central controller 14 calculates the thermal breakthrough time according to the formula. The temperature is lower than the next heating season. Subsequently, the system performs a role reversal between production and injection wells: the submersible pump in the original production well 1 is shut down, and the submersible pump in the original injection well 2 is turned on as the new production well 1; simultaneously, the booster pump in the original injection well 2 is shut down, and the booster pump in the original production well 1 is turned on as the new injection well 2. The opening logic of the three-stage electrically controlled water valve 16 is also mirrored and reset, thus significantly extending the economic exploitation life of the thermal storage.

[0039] To prevent trace amounts of untreated supersaturated ions from entering the deep formation, the three sections of the reinjection screen 15 in the reinjection well are all externally wrapped with double layers of 60-mesh stainless steel mesh, with a 40mm thick slow-release seed ring layer filling the spaces between the meshes. This ring layer is composed of a mixture of quartz sand and 5wt% pre-calcified ceramic particles, with the ceramic particle surface loaded with Zn. 2+ / Polyaspartic acid scale inhibitor membrane. This allows the tailwater to complete a final induced crystallization within the last 0–5 cm of its infiltration into the formation, protecting the permeability of the thermal reservoir pores.

[0040] The seed-induced scale-inhibiting reactor 7 is equipped with an online turbidity meter at its outlet. After the system has been running stably for the first 24 hours, the median turbidity of 0.5 NTU is set as the baseline value. During operation, when the turbidity suddenly rises to 5.8 NTU (> baseline value + 5 NTU), the central controller 14 determines that the scale on the surface of the seed packing is too thick or has detached, and immediately initiates the backwashing procedure. The backwash water flows in the opposite direction, impacting the packing frame for 4 minutes, and the detached scale is discharged through the drain pipe. After the backwashing is completed, the system resumes normal operation without manual intervention, ensuring the long-term stability of the reinjection system.

[0041] In the description of this invention, it should be understood that the terms "longitudinal", "lateral", "up", "down", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, and are only for the convenience of describing this invention, and are not intended to indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this invention.

[0042] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made to the technical solutions of the present invention by those skilled in the art without departing from the spirit of the present invention should fall within the protection scope of the present invention.

Claims

1. A medium-deep geothermal well extraction and reinjection system, characterized in that, It includes a production well (1), a reinjection well (2), a submersible pump installed in the production well (1), a heat exchanger (3) connected to the outlet of the submersible pump, and a reinjection pipeline (5) that sends the tailwater of the heat exchanger (3) back to the reinjection well (2). The reinjection screen pipe (15) of the hot storage section of the reinjection well (2) is divided into three sections vertically: upper, middle and lower. Each section of the reinjection screen pipe (15) is connected to the reinjection pipeline (5) through an independent branch pipe with an electrically controlled water valve (16). The reinjection pipeline (5) is provided with a vortex microbubble deaerator (6), a seed-induced scale inhibitor (7), a precision filter (8) and a reinjection pressurization pump (9) in sequence along the tailwater flow direction. The seed-induced scale inhibitor (7) is provided with a quartz sand packing frame with CaCO3 seed crystals pre-coated on its surface suspended inside the seed-induced scale inhibitor (7). The wellhead of the production well (1) and the wellhead of the reinjection well (2) are connected by a feeder pipeline (11), and the feeder pipeline (11) is equipped with a two-way check valve (12) and a nitrogen isolation buffer tank (13). It also includes a central controller (14), which performs thermal breakthrough prediction based on the temperature difference between the production well (1) and the reinjection well (2) and the cumulative amount of production and injection, and controls the submersible pump and the reinjection pressurization pump (9) to reverse when the predicted thermal breakthrough time is less than a set value.

2. The medium-deep geothermal well extraction and reinjection system according to claim 1, characterized in that, The lower reinjection screen (15) is located at 0–1 / 3 thickness above the bottom boundary of the thermal reservoir, the middle reinjection screen (15) is located at 1 / 3–2 / 3 thickness of the thermal reservoir, and the upper reinjection screen (15) is located at 2 / 3 thickness of the thermal reservoir to the top boundary; the reinjection screen (15) is covered with a quartz sand filter layer on the outside, and the lower quartz sand filter layer has a mesh size of 20–40 mesh, the middle quartz sand filter layer has a mesh size of 40–60 mesh, and the upper quartz sand filter layer has a mesh size of 60–100 mesh.

3. The medium-deep geothermal well extraction and reinjection system according to claim 2, characterized in that, The central controller (14) is configured to allocate the reinjection flow according to the logic of "lower section priority, upper section pressure maintenance": in the early stage of reinjection, the opening of the lower electric water valve (16) is controlled to make the lower section flow account for 60%-80%, the opening of the middle electric water valve (16) is controlled to make the middle section flow account for 20%-40%, and the upper electric water valve (16) is controlled to be closed or kept open to less than 10%.

4. The medium-deep geothermal well extraction and reinjection system according to claim 3, characterized in that, The central controller (14) monitors the rate of increase of the reinjection pressure in real time. When the rate of increase of the pressure exceeds 0.5 kPa / d, it gradually closes the lower electric water valve (16) and simultaneously opens the upper electric water valve (16) to disperse the scale layer vertically and delay the blockage of a single layer.

5. A medium-deep geothermal well extraction and reinjection system according to claim 1, characterized in that, The inlet of the cyclone microbubble deaerator (6) is tangentially connected to the cylinder, and an exhaust core tube is provided at the center of the top of the cylinder. The exhaust core tube is connected to a micro vacuum pump with a vacuum degree of -5 to -20 kPa. The dissolved oxygen content of the tailwater after treatment by the cyclone microbubble deaerator (6) is less than 1 mg / L, and the free CO2 removal rate is not less than 85%.

6. A medium-deep geothermal well extraction and reinjection system according to claim 1, characterized in that, The seed-induced scale-inhibiting reactor (7) is a horizontal tank with a suspended stainless steel packing rack inside. The packing rack is loaded with quartz sand with a particle size of 2–3 mm. The surface of the quartz sand is pre-coated with a CaCO3 seed layer, and the coating amount of the seed layer is 0.8–1.2 g / kg of quartz sand. The residence time of the tailwater in the seed-induced scale-inhibiting reactor (7) is 8–15 min. The temperature inside the seed-induced scale-inhibiting reactor (7) is controlled within the range of ±2℃ of the original temperature of the reinjected tailwater.

7. A medium-deep geothermal well extraction and reinjection system according to claim 1, characterized in that, The bidirectional check valve (12) is configured to allow gravity flow from the production well (1) to the reinjection well (2), and also to allow water in the reinjection well (2) to flow back into the production well (1) via the feeder pipeline (11) for replenishment when the static water level in the reinjection well (2) is more than 2 m higher than that in the production well (1) and the reinjection booster pump (9) is stopped; the nitrogen isolation buffer tank (13) has a volume of 2–5 m³. 3 It has a nitrogen capsule inside to prevent air from contacting the wastewater.

8. A medium-deep geothermal well extraction and reinjection system according to claim 1, characterized in that, The central controller (14) has a built-in thermal breakthrough prediction model, based on the formula in, For thermal breakthrough time, For the viscosity of the thermal storage fluid, For thermal storage porosity, The distance between the production well (1) and the recharge well (2) is the distance between them. For thermal storage permeability, The allowable temperature drop threshold; When the water temperature of the production well (1) drops by a certain value compared to the initial temperature, Or prediction When the heating season is less than one heating season, the central controller (14) controls the submersible pump of the original production well (1) to stop running and switch to the reinjection well (2). The reinjection pressurization pump (9) of the original reinjection well (2) stops running and starts the submersible pump in the well to switch to the production well (1). At the same time, the opening logic of the electric water valve (16) is synchronously mirrored and reset.

9. A medium-deep geothermal well extraction and reinjection system according to claim 2, characterized in that, The outer sides of the upper, middle, and lower reinjection screens (15) are all wrapped with double layers of 60-mesh stainless steel mesh, and the spaces between the double layers of stainless steel mesh are filled with a slow-release seed ring layer with a thickness of 30–50 mm; the slow-release seed ring layer is composed of a mixture of quartz sand and wt% pre-calcified ceramic particles, and the surface of the pre-calcified ceramic particles is loaded with Zn. 2+ Polyaspartic acid scale inhibitor membrane is used to crystallize supersaturated ions in tailwater at a depth of 0–5 cm outside the well wall.

10. A medium-deep geothermal well extraction and reinjection system according to claim 6, characterized in that, The outlet end of the seed-induced scale-inhibiting reactor (7) is equipped with an online turbidity meter. When the value detected by the online turbidity meter suddenly increases by more than 5 NTU compared with the reference value, the central controller (14) controls the start of the backwashing pipeline inside the seed-induced scale-inhibiting reactor (7). The backwashing water flow direction is opposite to the tailwater flow direction. The backwashing time is 3–5 minutes. The backwashing waste liquid is discharged outside the system through a dedicated sewage pipe.

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