A method and system for coordinating carbon dioxide sequestration and thermal recovery in deep artificial reservoirs

Abstract

This invention discloses a method and system for coordinating carbon dioxide sequestration and thermal recovery in deep artificial reservoirs, belonging to the fields of CCUS and green coal mining. The method includes: constructing a heterogeneous artificial reservoir from the injection end to the extraction end using a three-stage graded coal gangue filling system (coarse-medium-fine); preparing a low-temperature carbon-rich fluid by dissolving CO2 in deep mine water cooled by heat exchange; injecting the fluid into the artificial reservoir from the injection end using gravity self-driving and pulse oscillation methods; utilizing deep geothermal stimulation to generate carbonates through in-situ mineralization of the fluid and coal gangue, achieving permanent CO2 sequestration while simultaneously heating the fluid; collecting decarbonized high-temperature mine water at the extraction end, extracting thermal energy, and recycling it; and intelligently switching the filling area based on a depletion threshold. This invention has the advantages of zero external freshwater consumption, ultra-low injection energy consumption, high-value utilization of coal-based solid waste, and a significant reduction in the carbon footprint throughout its entire life cycle.

Description

Technical Field

[0001] This invention belongs to the fields of carbon capture, utilization and storage (CCUS), geothermal energy development and green coal mining technology, specifically involving a method and system for coordinating carbon dioxide storage and heat extraction in deep artificial reservoirs. Background Technology

[0002] Geological carbon dioxide (CO2) sequestration is a key technological approach to reducing greenhouse gas emissions. Traditional deep saline aquifer CO2 sequestration technology has two significant bottlenecks: first, it requires the consumption of a large amount of precious external water resources (usually tens of times the mass of CO2), which is extremely unfriendly to water-scarce mining areas; second, it requires extremely high surface injection pressure to overcome deep formation pressure, resulting in huge engineering energy consumption and the risk of gas buoyancy escape, making it difficult to guarantee the safety of sequestration.

[0003] At the same time, coal mining faces multiple challenges as it extends into deeper areas: First, mining generates a large amount of coal gangue, fly ash and other coal-based solid waste, which occupies land and pollutes the environment when stored on the surface, and urgently needs to be disposed of on a large scale in a harmless manner; Second, deep mines have large water inflows and high water temperatures (usually reaching 40~50℃), which pose both a threat of heat hazards and contain abundant geothermal resources; Third, the collapse zones in goaf areas provide potential space for waste disposal and CO2 sequestration.

[0004] Currently, underground grouting and backfilling technologies in coal mines mainly focus on solid waste disposal and water hazard control. For example, Chinese patent application CN121345622A discloses a method for grouting and backfilling gangue in caving areas of coal mines by recycling water resources. This method achieves the utilization of mine water resources and underground disposal of gangue by recycling accumulated water from old goaf areas to prepare gangue slurry for backfilling. However, this scheme completely fails to address CO2 sequestration and geothermal synergy, resulting in a single backfilling function that cannot achieve the dual benefits of carbon emission reduction and heat recovery.

[0005] In contrast, Chinese patent application CN118223940A discloses an integrated disaster prevention method and system for comprehensive utilization of CO2 in coal mines, which is the first attempt to combine CO2 mineralization and storage for fire prevention and extinguishing with liquid CO2 for geothermal mining and cooling. However, analysis revealed the following fundamental deficiencies in the scheme: (1) In its "liquid CO2 mine geothermal extraction and cooling" technology, the liquid CO2 absorbs heat and transforms into gaseous state before being pumped back to the storage tank for recycling. The CO2 is not permanently sealed and is only used as a heat transfer medium, which cannot achieve carbon emission reduction; (2) Its "CO2 mineralization and sealing fire prevention" adopts the method of preparing carbonate slurry by pre-reaction of coal-based solid waste and CO2 and then injecting it. This is an external preparation of slurry for filling, rather than in-situ mineralization inside the already formed gangue filling body. The pore utilization rate and sealing efficiency are low; (3) The scheme relies on the mine water storage tank for water replenishment and does not solve the problem of external fresh water consumption; (4) The pore structure of the filling body is not designed for fluid injection and mineralization reaction, and the engineering problem of mineralization products blocking the pores and shortening the sealing life cycle is not considered; (5) The gravity of the deep well liquid column is not used to achieve low-energy injection, and there is also a lack of intelligent monitoring and exit mechanism for the system life cycle.

[0006] In summary, existing technologies generally suffer from problems such as the disconnect between CO2 sequestration and geothermal extraction, high water consumption, high energy consumption during injection, easy clogging of the filling material pores, and poor system synergy. There is an urgent need to develop a comprehensive closed-loop process that can achieve environmentally friendly solid waste disposal, permanent CO2 sequestration with zero external freshwater source and low energy consumption, and simultaneous extraction of deep geothermal energy. Summary of the Invention

[0007] To address the shortcomings of existing technologies, this invention provides a method and system for coordinating carbon dioxide sequestration and geothermal extraction in deep artificial reservoirs. By constructing a closed-loop system of "cooling-carbon dissolution-injection-mineralization-heat extraction" for mine water circulation, and combining a three-stage heterogeneous artificial reservoir with gravity-driven pulse injection technology, it achieves in-situ permanent mineralization and sequestration of CO2 with geothermal synergistic extraction, achieving comprehensive benefits such as zero external freshwater consumption, ultra-low injection energy consumption, high-value utilization of coal-based solid waste, and a significant reduction in carbon footprint throughout the entire life cycle.

[0008] This invention is achieved through the following technical solution:

[0009] A method for coordinating carbon dioxide sequestration and heat recovery in deep artificial reservoirs includes the following steps:

[0010] Step 1: In the target goaf, a three-stage graded coal gangue backfilling process of "coarse-medium-fine" is adopted to construct a high-permeability injection zone, a medium-permeability reaction zone, and a low-permeability collection zone in sequence along the direction from the injection end to the extraction end, forming an artificial reservoir with a heterogeneous pore flow field; wherein, the injection end is the end closer to the injection well, and the extraction end is the end closer to the extraction well;

[0011] Step 2: The high-temperature mine water extracted from the deep mine is cooled to 15~20℃ after heat energy is extracted by the ground heat exchange system to obtain low-temperature mine water; then the low-temperature mine water is mixed with the captured CO2 in the form of micro-nano bubbles in a gas-liquid mixing device to prepare low-temperature carbon-rich mine water that is weakly acidic.

[0012] Step 3: The low-temperature carbon-rich mine water obtained in Step 2 is injected into the artificial reservoir constructed in Step 1 through the injection end pipeline network. The injection process adopts a low-frequency high-pressure pulse oscillation method, and gravity self-driven injection is achieved by utilizing the gravity of the deep well fluid column. The bottom hole pressure is dynamically controlled to be 60%~80% of the fracturing pressure of the filling body and the surrounding rock of the top and bottom plates.

[0013] Step 4: The injected low-temperature carbon-rich mine water seeps into the intermediate infiltration reaction zone of the artificial reservoir and is heated by deep geothermal energy. The thermal activation accelerates the dissolution of calcium and magnesium alkaline ions in the coal gangue, which react with CO2 to form carbonate minerals, thus achieving in-situ mineralization and sequestration of CO2. At the same time, the exothermic reaction of mineralization and the geothermal temperature work together to raise the temperature of the mine water.

[0014] Step 5: Collect the decarbonized mine water whose temperature has increased after the mineralization reaction in Step 4 at the extraction end, and pump it to the ground heat exchange station to extract heat energy. After cooling, the mine water is returned to the gas-liquid mixing device in Step 2 for recycling after solid-liquid separation.

[0015] Step 6: Monitor the bottom pressure of the injection well, the injection flow rate, and the dissolved inorganic carbon concentration and temperature of the return water from the extraction well in real time. When at least one of the thresholds for hydraulic depletion, chemical depletion, or thermodynamic depletion is met, stop water injection in the current area and switch to the next artificial reservoir unit.

[0016] Preferably, in step one, in the total length of the artificial reservoir along the direction from the injection end to the extraction end, the extension length of the high-permeability injection zone accounts for 15% to 20% of the total length, and is filled with large-particle coal gangue with a particle size of 50 to 100 mm; the extension length of the medium-permeability reaction zone accounts for 65% to 70% of the total length, and is filled with mixed-particle coal gangue with a particle size of 20 to 50 mm; the extension length of the low-permeability collection zone accounts for 10% to 15% of the total length, and is filled with fine-particle coal gangue with a particle size of less than 20 mm.

[0017] Preferably, the mixing pressure in the gas-liquid mixing device in step two is controlled at 1.2~2.0 MPa; the pH value of the low-temperature carbon-rich mine water is 3.0~5.0, and its density is greater than that of formation water.

[0018] Preferably, the low-frequency high-pressure pulse oscillation injection method in step three is as follows: a pressure pulse lasting 5 to 15 minutes is applied every 2 to 6 hours, and the pulse pressure amplitude is 1.2 to 1.5 times the stable injection pressure.

[0019] Preferably, the temperature of the decarbonized mine water collected at the extraction end in step five is 20-30°C higher than the temperature of the injected water at the injection end in step three, and its dissolved inorganic carbon concentration is 30%-50% lower than that of the injected water at the injection end.

[0020] Preferably, the threshold for hydraulic depletion in step six is ​​set as follows: while maintaining the upper limit of injection pressure, the injection flow rate decreases to less than 10% of the initial economic flow rate; the threshold for chemical depletion is set as follows: the dissolved inorganic carbon concentration in the return water of the extraction well tends to be consistent with that at the injection end and the pH value of the return water remains acidic; the threshold for thermodynamic depletion is set as follows: the temperature of the return water in the extraction well continues to decrease and falls below 30°C.

[0021] A deep artificial reservoir carbon dioxide sequestration and heat recovery synergy system for the above method includes:

[0022] The heterogeneous artificial reservoir unit is formed by filling the target goaf with three-stage graded coal gangue in a “coarse-medium-fine” manner. From the injection end to the extraction end, it includes a high-permeability injection zone, a medium-permeability reaction zone, and a low-permeability collection zone.

[0023] A gas-liquid mixing unit includes a gas-liquid mixing device, the inlet of which is connected to a mine water circulation pipeline and a CO2 gas source, respectively, for mixing low-temperature mine water with CO2 to form low-temperature carbon-rich mine water.

[0024] The injection unit includes an injection pipeline network, a pulse generator, and an injection well located at the injection end; the injection pipeline network is connected to the outlet of the gas-liquid mixing unit; the pulse generator is located on the injection pipeline network and is used to apply low-frequency high-pressure pulse oscillations to the injected fluid.

[0025] The extraction unit includes a liquid collection system, an extraction pump, and an extraction well located at the extraction end, used to collect decarbonized high-temperature mine water after mineralization reaction;

[0026] The heat exchange unit is connected to the outlet of the extraction unit and is used to extract the heat energy from the decarbonized high-temperature mine water and send the cooled mine water back to the gas-liquid mixing unit.

[0027] The intelligent monitoring unit includes pressure sensors, flow meters, pH meters, dissolved inorganic carbon analyzers, and temperature sensors respectively installed in the injection well and the extraction well, as well as a controller that receives relevant sensor data and determines the system exit threshold.

[0028] Preferably, the pulse generating device is a pulse water pump or a hydraulic pulse generator, with an output pulse frequency of 0.01~0.1 Hz and a pulse pressure amplitude of 2~8 MPa.

[0029] Preferably, the gas-liquid mixing device is equipped with a micro-nano bubble generator connected to a CO2 gas source at the inlet, and the generated CO2 bubbles have a diameter of 10 nm to 50 μm.

[0030] Preferably, the outlet of the heat exchange unit is provided with a cyclone separator and a sedimentation tank for removing suspended particulate matter generated by the mineralization reaction.

[0031] The beneficial effects of this invention are as follows:

[0032] (1) This invention uses deep mine water as a circulating carrier for CO2 sequestration and thermal energy extraction. Through a closed-loop process of "cooling-carbon dissolution-injection-mineralization-heating-extraction-heat exchange-reuse", it completely eliminates the dependence of traditional CO2 geological sequestration on external freshwater. The mine water is recycled within the system, which not only solves the environmental burden caused by the discharge of deep mine water, but also avoids the water resource bottleneck for CO2 sequestration in water-scarce mining areas, achieving zero water discharge and efficient recycling.

[0033] (2) This invention utilizes the hydrostatic pressure of the liquid column in the deep mine (approximately 8 MPa at a depth of 800 m) to achieve gravity-driven injection of carbon-rich fluid, significantly reducing the operating energy consumption of the surface high-pressure water injection pump. At the same time, CO2 is injected in a dissolved state in the weakly acidic mine water, rather than in a pure gas phase or supercritical state, fundamentally eliminating the risk of gas buoyancy escape and significantly improving the safety of storage.

[0034] (3) This invention organically combines in-situ CO2 mineralization and sequestration with geothermal energy extraction to form a positive feedback mechanism: after the low-temperature carbon-rich fluid is injected, it is heated by deep geothermal energy to stimulate the mineralization reaction, accelerating the dissolution of calcium and magnesium ions in coal gangue and combining with CO2 to form carbonate minerals, thus achieving permanent CO2 solidification; the exothermic mineralization reaction further heats the fluid, raising the temperature of the extracted mine water by 20-30°C compared to the injection end, and then recycles it after extracting high-quality thermal energy through a heat exchange system. This "cold fluid injection, hot fluid output" model achieves the dual benefits of carbon sequestration and clean energy production.

[0035] (4) This invention features a unique three-stage graded coal gangue backfilling process: coarse-medium-fine. This process actively constructs a high-permeability injection zone, a medium-permeability reaction zone, and a low-permeability collection zone along the injection end to the extraction end, respectively meeting the functional requirements of rapid fluid diffusion, sufficient mineralization reaction, and interception of suspended particles. Simultaneously, a low-frequency, high-pressure pulse oscillation injection method is employed. The cyclical disturbance of pressure waves continuously refreshes the reaction interface, effectively slowing down the process of newly formed carbonate minerals clogging the pores, and significantly improving the maximum storage capacity and service life of a single working face.

[0036] (5) This invention uses coal gangue as the main aggregate of artificial reservoirs to achieve a three-in-one system of "gangue backfilling - carbon sequestration - heat extraction", transforming coal mine waste into a carrier for carbon sequestration and geothermal development, thus avoiding land occupation and environmental pollution caused by surface stockpiling of gangue. In addition, an intelligent monitoring and exit mechanism is constructed based on the triple thresholds of hydraulic depletion, chemical depletion and thermodynamic depletion, which judges the system status in real time and switches the working face in a timely manner to ensure the economical and efficient operation of the process, providing a full life cycle solution for green mining of deep coal mines.

[0037] (6) According to calculations, using the method of this invention, 20-80 kg of CO2 can be fixed per cubic meter of backfill, while geothermal energy can be recovered for heating or power generation in the mining area; zero external freshwater consumption significantly reduces operating costs; gravity self-driven injection reduces power consumption; and the three-stage backfilling uses coal gangue to replace cement and other cementing materials, further reducing costs. The overall carbon footprint is reduced by more than 60% compared to traditional processes. Attached Figure Description

[0038] Figure 1 The solubility curve of CO2 in water;

[0039] Figure 2 A schematic diagram of a deep coal mine filling space carbon dioxide sequestration and heat recovery synergy system based on mine water circulation;

[0040] Figure 2 The components are as follows: 1. High-permeability injection zone; 2. Medium-permeability reaction zone; 3. Low-permeability collection zone; 4. Gas-liquid mixing device; 5. Micro / nano bubble generator; 6. CO2 storage tank; 7. Injection pipeline network; 8. Pulse generator; 9. Injection well; 10. Extraction well; 11. Heat exchange station; 12. Collection system; 13. Extraction pump; 14. Cyclone desander; 15. Data acquisition and control system. Detailed Implementation

[0041] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0042] Unless otherwise specified, the technical means used in the following embodiments are all conventional means well known to those skilled in the art, and the experimental methods without specific conditions are all conventional methods in the art.

[0043] In the description of this invention, terms such as "top," "bottom," "inner," "outer," "upper," and "lower," which indicate orientation or positional relationship, are used only based on the orientation shown in the accompanying drawings for the purpose of describing this invention, and are not intended to indicate or imply that the device referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0044] Example 1

[0045] This embodiment provides a method for the coordinated development of carbon dioxide sequestration and geothermal extraction in deep artificial reservoirs. Through an integrated closed-loop process of "production-charging-injection-sequestration-production," it achieves the synergistic effect of in-situ CO2 mineralization and sequestration, deep geothermal extraction, and mine water resource recycling. The specific steps are as follows:

[0046] 1. Constructing heterogeneous artificial reservoirs

[0047] In the target goaf, a three-stage graded coal gangue backfilling process of "coarse-medium-fine" is adopted. Three functional zones are constructed sequentially from the injection end to the extraction end to create a customized pore flow field suitable for CO2 mineralization and heat exchange.

[0048] (1) High-permeability injection zone (near the injection end, i.e., one end of the injection well): filled with large-diameter coal gangue (50~100 mm) to form a high-permeability zone (permeability is about 2×10 under 6 MPa axial pressure). -9 ~3×10 -9 The extension length accounts for 15% to 20% of the total length. The function of this area is to withstand the hydraulic impact of the injected fluid and to quickly and evenly distribute the fluid injected at a single point to the entire working surface cross section, avoiding the formation of a single short-circuit channel.

[0049] Intermediate Permeability Reaction Zone (Middle Section): Filled with mixed-size coal gangue (20~50 mm) to form a medium permeability zone (permeability approximately 1×10⁻⁶ under 6 MPa axial pressure). -10 ~2×10 -10 This area, which accounts for 65% to 70% of the total length, is the main site for fluid transport and in-situ CO2 mineralization, providing ample contact area and reaction time.

[0050] Low-permeability collection zone (near the extraction end): Filled with fine-grained coal gangue (<20 mm) to form a low-permeability zone (permeability is approximately 2×10⁻⁶ under 6 MPa axial pressure). -10 ~3×10 -10 This area, which extends 10% to 15% of the total length, serves as a dense filter layer to intercept suspended particles and detached minerals generated by the front-end reaction, preventing blockage of the extraction pipeline.

[0051] The three zones are designed with a gradual transition to avoid local eddies or short circuits caused by sudden changes in permeability.

[0052] (2) Preparation of low-temperature carbon-rich fluid

[0053] High-temperature mine water (typically 40-50℃) gushing from deep mines is pumped to a surface heat exchange station via an extraction system. After heat energy is extracted through a heat exchanger, the cooled mine water enters a gas-liquid mixing device. Simultaneously, industrially captured CO2 gas (purity ≥95%) is introduced into the gas-liquid mixing device at a pressure of 1.2-2.0 MPa. The solubility curve of CO2 in water (…) Figure 1 As can be seen, the lower the temperature and the higher the pressure, the greater the solubility of CO2. Taking all factors into consideration, the temperature of the mine water extracted by the heat exchanger needs to be reduced to 15-20℃. Before being introduced into the gas-liquid mixing device, the CO2 gas is first cut into micro-nano bubbles of 10 nm-50 μm using a micro-nano bubble generator, and then uniformly dissolved in the low-temperature mine water. Due to the high solubility of CO2 under low temperature and high pressure conditions, a weakly acidic, high-density, low-temperature carbon-rich fluid with a pH of 3.0-5.0 and a density slightly greater than that of formation water can be prepared. In this state, CO2 exists entirely in a dissolved state, with no free bubbles.

[0054] (3) Gravity self-driving and pulse oscillation injection

[0055] The low-temperature carbon-rich fluid prepared in step (2) is injected into the artificial reservoir constructed in step (1) from the injection end through the downhole injection network.

[0056] Gravity-driven injection: Utilizing the hydrostatic pressure of the hydraulic column in deep mines (the pressure increases by approximately 1 MPa for every 100 m increase in depth), automatic fluid seepage can be achieved with little or no auxiliary pressurization. The bottom hole pressure is dynamically controlled within 60% to 80% of the fracturing pressure of the filling body and the surrounding rock of the top and bottom plates, ensuring that the integrity of the formation is not compromised.

[0057] Pulsed oscillation injection: Based on stable injection, a low-frequency high-pressure pulse generator is used to apply a pressure pulse lasting 5-15 minutes every 2-6 hours. The pulse pressure amplitude is 1.2-1.5 times the stable injection pressure. The output pulse frequency of the pulse generator is 0.01-0.1 Hz, and the pulse pressure amplitude is 2-8 MPa. This pulse, through the cyclic disturbance of the pressure wave, effectively flushes the large pore network inside the packing material that is about to form scale, delays the blockage of fluid channels by carbonate deposits, and significantly extends the storage life.

[0058] (4) In-situ thermal activation and solid waste mineralization

[0059] The injected low-temperature, carbon-rich fluid disperses rapidly in the high-permeability injection zone and then enters the medium-permeability reaction zone. The medium-permeability reaction zone is located in a deep geothermal environment (generally 35-60℃), where the fluid is gradually heated. This thermal activation accelerates the formation of calcium and magnesium alkaline ions (CaO) in the coal gangue. 2+ Mg 2+ The dissolution of HCO3 in the fluid - CO3 2-These ions react with each other to form solid carbonate minerals such as calcium carbonate and magnesium carbonate, which are deposited on the surface of coal gangue particles and between pores.

[0060] This mineralization reaction is exothermic (approximately 60-100 kJ of heat is released per mole of CO2), and in conjunction with deep geothermal energy, it further increases the fluid temperature. Simultaneously, CO2 transforms from a dissolved state into solid minerals, achieving permanent geological sequestration without the risk of leakage.

[0061] (5) Extraction and thermal energy extraction of heat-rich decarbonized fluids

[0062] The decarbonized, high-temperature mine water continues to move towards the extraction end. As it passes through the low-permeability collection zone, suspended particles are intercepted and filtered by the fine-particle-size packing material. In the extraction end collection system, the fluid is pumped to the surface heat exchange station by the extraction pump.

[0063] The temperature of the extracted fluid is usually 20-30°C higher than that of the injection end, and the concentration of dissolved inorganic carbon (DIC) is reduced by 30%-50%. After the high-temperature fluid extracts heat energy through the heat exchange system, the temperature drops to 15-20°C. After undergoing cyclone sand removal and sedimentation clarification treatment, it is sent back to the gas-liquid mixing device in step (2) for recycling, realizing a closed-loop circulation of mine water with zero external discharge.

[0064] (6) Intelligent monitoring and exit of the system lifecycle

[0065] The system is equipped with an intelligent monitoring platform to collect real-time data on parameters such as bottom-hole pressure, injection flow rate, fluid temperature, and DIC concentration from injection wells, as well as return water temperature, return water DIC concentration, and return water pH from extraction wells. When at least one of the following three depletion thresholds is met, the system automatically determines that the storage potential of the current filling area has been essentially exhausted, stops injection, and switches to the next filling face:

[0066] ①Hydraulic depletion: When maintaining the upper limit of injection pressure, the injection flow rate decreases to less than 10% of the initial economic flow rate;

[0067] ② Chemical depletion: The DIC concentration in the return water of the extraction well tends to be consistent with that at the injection end, and the pH value of the return water remains acidic (indicating that the mineralization reaction has basically stopped).

[0068] ③ Thermodynamic depletion: The temperature of the return water in the extraction well continues to drop and falls below 30℃ (indicating that geothermal energy has been fully extracted).

[0069] 2. Supporting systems

[0070] This embodiment also provides a collaborative system for implementing the above method, such as... Figure 2 As shown, it includes the following units:

[0071] ① Heterogeneous artificial reservoir unit: It is formed by filling the target goaf with three-stage graded coal gangue in three stages: coarse-medium-fine. It includes a high-permeability injection zone 1, a medium-permeability reaction zone 2 and a low-permeability collection zone 3 from the injection end to the extraction end.

[0072] ② Gas-liquid mixing unit: includes a gas-liquid mixing device 4, whose inlet is connected to the mine water circulation pipeline and the CO2 storage tank 6 respectively, for fully mixing low-temperature mine water and CO2 to prepare carbon-rich fluid. A micro-nano bubble generator 5 is also provided between the CO2 storage tank 6 and the gas-liquid mixing device 4.

[0073] ③ Injection unit (located at the injection end): includes an injection pipeline network 7 connected to the outlet of the gas-liquid mixing unit, a pulse generator 8 (pulse water pump or hydraulic pulse generator) located in the pipeline network, and an injection well 9.

[0074] ④ Extraction unit: including liquid collection system 12, extraction pump 13 and extraction well 10 installed at the extraction end.

[0075] ⑤ Heat exchange unit: including heat exchange station 11 (plate heat exchanger or shell and tube heat exchanger) located on the ground, connected to the outlet of liquid collection system 12, used to extract the heat energy of the pumped fluid and send the cooled mine water back to the gas-liquid mixing unit.

[0076] ⑥ Water treatment unit: including a hydrocyclone sand separator 14 and a matching sedimentation tank ( Figure 2 (Not shown in the image) is connected to the outlet of heat exchange station 11 to remove suspended particulate matter from the extracted fluid. The mine water (cooled) after the suspended matter has been removed is returned to the gas-liquid mixing unit for reuse, so its outlet is connected to the gas-liquid mixing unit.

[0077] ⑦ Intelligent monitoring unit: including sensors (pressure sensor, flow meter, temperature sensor) installed in injection well 9, sensors (flow meter, thermometer, pH meter, DIC analyzer) installed in extraction well 10, and data acquisition and control system 15 (PLC or DCS). The data acquisition and control system 15 receives signals from each sensor and determines the system exit time according to the preset depletion threshold, and automatically controls valve switching.

[0078] Example 2

[0079] This embodiment uses a goaf filling working face in a deep coal mine as the implementation object. This mine has the following characteristics: a mining depth of approximately 800 m, an original deep rock temperature of approximately 45℃ (belonging to a secondary heat hazard zone), and coal gangue as the main aggregate for the filling material. The distance between the injection well and the extraction well is 200 m. The specific implementation method is as follows:

[0080] 1. Construction of heterogeneous artificial reservoirs in goaf areas

[0081] After the first working face (200 m strike length) in this mining area was completed, gangue backfilling was immediately carried out. The underground sorting system sieved the coal gangue into three grades according to particle size:

[0082] Large particle size (50~100 mm) is used in hypertonic injection zones;

[0083] Medium particle size (20~50 mm) is used in the intermediate osmosis reaction zone;

[0084] Fine particle size (5~20 mm) is used in low-touch liquid collection areas.

[0085] The filling sequence proceeds from the injection end towards the extraction end. Specifically: at the end near the injection well, a 30 m (15%) length of large-diameter coal gangue forms a high-permeability injection zone to withstand fluid impact and prevent the formation of short-circuit channels; in the middle of the working face, a 140 m (70%) length of mixed-diameter coal gangue forms a medium-permeability reaction zone, serving as the main site for fluid migration and in-situ mineralization; at the end near the extraction well, a 30 m (15%) length of fine-diameter coal gangue forms a low-permeability collection zone, constituting a dense filter layer to intercept minerals and suspended particles detached from the front-end reaction. The thickness of each filling layer is determined by the working face mining height; in this embodiment, it is 3.5 m. A compaction mechanism behind the filling hydraulic support is used for initial compaction at a pressure of 2.0 MPa to connect with the roof.

[0086] 2. Configuration of Low-Temperature Carbon-Rich Fluid Preparation System

[0087] Construct heat exchange stations and CO2 gas-liquid mixing stations on the ground:

[0088] The heat exchange station uses two plate heat exchangers (one for use and one for standby), with a designed heat exchange capacity of 3.0 MW, which can reduce the temperature of mine water from 45℃ to 20℃.

[0089] The gas-liquid mixing station includes a 15 m³ volume 3 A closed high-pressure mixing tank (gas-liquid mixing device) is designed to mix at a pressure of 1.5 MPa.

[0090] The CO2 gas source comes from a nearby CO2 storage tank (99% purity) and is delivered to the gas-liquid mixing device through a micro-nano bubble generator (0.1 μm pore size).

[0091] 3. System startup and injection parameters

[0092] First, mine water (45℃) is pumped from the existing central water tank in the mining area to the heat exchange station, cooled to about 20℃, and then sent to the gas-liquid mixing device. The CO2 pipeline valve is opened, and CO2 is introduced into the gas-liquid mixing device at a rate of 1800~2000 kg / h. The mixture is circulated using a static mixer for 3~5 minutes to prepare a high-density carbon-rich fluid (density slightly greater than that of formation water) with a pH of about 4.5 and a dissolved CO2 concentration of 22~25 g / L.

[0093] The injection end is located on the side of the transport roadway, utilizing an existing grouting borehole converted into an injection well. The well bottom depth is 800 m, and the hydrostatic pressure of the liquid column is approximately 8.0 MPa. An injection pump (variable frequency control) delivers the carbon-rich fluid at a rate of 80 m... 3 An initial flow rate of / h is injected into an artificial reservoir at a depth of 800 m. The fluid automatically seeps into the filling body. To prevent overpressure damage to the top plate, the wellhead regulating valve controls the bottom hole pressure at approximately 12 MPa (the fracturing pressure of the surrounding rock at this location is 16 MPa, accounting for 75%).

[0094] The pulse generator uses a hydraulic pulse generator with the following parameters: pulse interval of 4 hours, pulse duration of 10 minutes, pulse frequency of 0.01~0.1 Hz, and pulse amplitude of 3.0 MPa. The pressure wave cleanses the large pore network inside the filling material that is about to form scale, maintaining the fluid flow capacity of the channels.

[0095] 4. Monitoring of in-situ mineralization and thermal energy extraction processes

[0096] The system was simulated to run continuously for 90 days, with a comprehensive data extraction performed every 15 days. Key data are shown in Table 1 below.

[0097] Table 1 System Operation Monitoring Data

[0098]

[0099] As shown in Table 1, during the initial stage of system operation (the first 30 days), the temperature at the extraction end can reach 47-48℃. This is because the deep protolith thermal field (45℃) and the strong chemical exothermic reaction during carbonate mineralization have a superimposed effect. With the continuous injection of 20℃ low-temperature fluid, the reservoir heat is continuously extracted, resulting in a non-linear decreasing trend of the extraction end temperature over time.

[0100] During the first 30 days of operation, a large number of fresh, active calcium and magnesium sites were exposed on the surface of the coal gangue, resulting in an extremely rapid carbon fixation reaction rate and a DIC reduction rate consistently above 50%. As the reaction progressed, the generated solid carbonate minerals formed a dense passivation layer on the particle surface, leading to a gradual decrease in the DIC consumption rate in the later stages. Even as the absolute amount of mineralized products accumulated and deposited in some micropore throats, the effective permeability within the filling mass continued to decrease slowly over time. This resulted in the injection flow rate, initially 80 m³ / h, decreasing under the premise of maintaining a constant bottomhole safety injection pressure (12 MPa). 3 / h gradually decreased to 38.6 m on day 90. 3 / h, exhibiting a slow and controllable flow decay pattern.

[0101] Heat recovery calculations: During the first 90 days of operation, the average temperature difference was approximately 22.4℃, and the average extraction flow rate was approximately 62.0 m³. 3 The cumulative heat gain per hour is approximately: 62.0 × 1000 × 22.4 × 4.18 × 24 × 90 ≈ 1.25 × 10⁻⁶ 10 kJ, equivalent to approximately 427 tons of standard coal, can power about 35,000 m³ of the mining area. 2 The building provides heating in winter.

[0102] CO2 sequestration capacity calculation: The total cumulative CO2 injected over 90 days is based on an average DIC value of 24.2 g / L at the injection end and an average injection flow rate of 63.4 m³ / L. 3 Based on a per-hour calculation, the cumulative CO2 injection is approximately: 24.2 × 63.4 × 24 × 90 / 1000 ≈ 3315 kg (approximately 3.32 tons). Note: This is basic simulation data; the average DIC reduction rate is approximately 45%, and the estimated CO2 storage is approximately 1.5 tons. Scaled up to actual engineering dimensions (working face length 200 m, which can be extended to 280 m in actual projects), it is estimated that a single working face can store approximately 800-1000 tons of CO2.

[0103] Injection and extraction balance: The extraction flow rate is always slightly lower than the injection flow rate (the difference is about 1~3 m³). 3 The figure ( / h) indicates that some fluid is retained in the pores of the filling body, which is consistent with the trend of the pores being gradually filled by minerals, and the retention amount is within a controllable range.

[0104] 5. Water Treatment and Circulation

[0105] The extraction end liquid collection system is located in the return airway and uses a gravity-flow collection trough to collect the water. After treatment in an inclined tube sedimentation tank (retention time 1.0 h) and a hydrocyclone desander (separating particles >20 μm), the suspended solids concentration in the mine water is <5 mg / L, meeting the injection requirements. The treated mine water is then piped back to the surface heat exchange station, where it is cooled again to approximately 20°C before entering a mixing tank for reuse. No external fresh water is added during system operation, achieving a fully closed-loop zero-discharge system.

[0106] 6. System Exit and Transition

[0107] After 180 days (6 months) of operation, the intelligent monitoring system reported the following alarms:

[0108] When the injection pressure was maintained at 12 MPa, the injection flow rate dropped to less than 8 m³ / s. 3 / h, lower than the initial flow rate of 80 m³ / h 3 When the flow rate reaches 10% per hour, the system controller automatically determines that the pores in the current filling area have been blocked by minerals, meeting the depletion threshold, and issues a transfer command. The central control system automatically closes the injection pipeline valve, and on-site operators seal the injection well and extraction well. The injection-production pipeline network is then switched to the next newly completed artificial reservoir unit in the mining area (which has already completed three-stage filling), restarting a new cycle.

[0109] 7. Summary of Results

[0110] Table 2 Summary of Results

[0111]

[0112] As shown in Table 2, this embodiment fully verifies the feasibility of the method of the present invention in deep thermal hazard mining areas (geothermal temperature 45℃). The system operation cycle is about 6 months, and it achieves the synergistic effect of CO2 mineralization and storage (average storage rate of about 45%), geothermal extraction (average temperature difference of 22.4℃) and closed-loop recycling of mine water (zero external water supply), which has good promotion value.

[0113] The embodiments described above are only some, not all, of the embodiments of the present invention. The detailed description of the embodiments of the present invention is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments. The scope of protection of the present invention is determined by the scope claimed in the claims. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

Claims

1. A method for coordinating carbon dioxide sequestration and thermal recovery in deep artificial reservoirs, characterized in that, Includes the following steps: Step 1: In the target goaf, a three-stage graded coal gangue backfilling process of "coarse-medium-fine" is adopted to construct a high-permeability injection zone, a medium-permeability reaction zone, and a low-permeability collection zone in sequence along the direction from the injection end to the extraction end, forming an artificial reservoir with a heterogeneous pore flow field; wherein, the injection end is the end closer to the injection well, and the extraction end is the end closer to the extraction well; Step 2: The high-temperature mine water extracted from the deep mine is cooled to 15~20℃ after heat energy is extracted by the ground heat exchange system to obtain low-temperature mine water; then the low-temperature mine water is mixed with captured CO2 in the form of micro-nano bubbles in a gas-liquid mixing device to prepare low-temperature carbon-rich mine water with a pH value of 3.0~5.

0. Step 3: The low-temperature carbon-rich mine water obtained in Step 2 is injected into the artificial reservoir constructed in Step 1 through the injection end pipeline network. The injection process adopts a low-frequency high-pressure pulse oscillation method, and gravity self-driven injection is achieved by utilizing the gravity of the deep well fluid column. The bottom hole pressure is dynamically controlled to be 60%~80% of the fracturing pressure of the filling body and the surrounding rock of the top and bottom plates. The low-frequency high-pressure pulse oscillation injection method specifically involves applying a pressure pulse lasting 5 to 15 minutes every 2 to 6 hours, with the pulse pressure amplitude being 1.2 to 1.5 times the stable injection pressure. Step 4: The injected low-temperature carbon-rich mine water seeps into the intermediate infiltration reaction zone of the artificial reservoir and is heated by deep geothermal energy. The thermal activation accelerates the dissolution of calcium and magnesium alkaline ions in the coal gangue, which react with CO2 to form carbonate minerals, thus achieving in-situ mineralization and sequestration of CO2. At the same time, the exothermic reaction of mineralization and the geothermal temperature work together to raise the temperature of the mine water. Step 5: Collect the mine water whose temperature has increased after the mineralization reaction in Step 4 at the extraction end, and pump it to the ground heat exchange station to extract heat energy. After cooling, the mine water is returned to the gas-liquid mixing device in Step 2 for recycling after solid-liquid separation. The temperature of the mine water collected at the extraction end is 20-30°C higher than the temperature of the injected water at the injection end in step three, and its dissolved inorganic carbon concentration is 30%-50% lower than that of the injected water at the injection end. Step 6: Monitor the bottom pressure of the injection well, the injection flow rate, and the dissolved inorganic carbon concentration and temperature of the return water from the extraction well in real time. When at least one of the thresholds for hydraulic depletion, chemical depletion, or thermodynamic depletion is met, stop injecting low-temperature carbon-rich mine water into the current artificial reservoir and switch to injecting into another artificial reservoir.

2. The method for coordinating carbon dioxide sequestration and thermal recovery in deep artificial reservoirs according to claim 1, characterized in that, In step one, of the total length of the artificial reservoir along the direction from the injection end to the extraction end, the extension length of the high-permeability injection zone accounts for 15% to 20% of the total length and is filled with large-particle coal gangue with a particle size of 50 to 100 mm; the extension length of the medium-permeability reaction zone accounts for 65% to 70% of the total length and is filled with mixed-particle coal gangue with a particle size of 20 to 50 mm; and the extension length of the low-permeability collection zone accounts for 10% to 15% of the total length and is filled with fine-particle coal gangue with a particle size of less than 20 mm.

3. The method for coordinating carbon dioxide sequestration and thermal recovery in deep artificial reservoirs according to claim 1, characterized in that, The mixing pressure in the gas-liquid mixing device described in step two is controlled at 1.2~2.0 MPa; the density of the low-temperature carbon-rich mine water is greater than that of the formation water.

4. The method for coordinating carbon dioxide sequestration and thermal recovery in deep artificial reservoirs according to claim 1, characterized in that, The threshold for hydraulic depletion in step six is ​​set as follows: when maintaining the upper limit of injection pressure, the injection flow rate decreases to less than 10% of the initial economic flow rate; the threshold for chemical depletion is set as follows: the dissolved inorganic carbon concentration in the return water of the extraction well tends to be consistent with that at the injection end and the pH value of the return water remains acidic; the threshold for thermodynamic depletion is set as follows: the temperature of the return water of the extraction well continues to decrease and falls below 30°C.

5. A deep artificial reservoir carbon dioxide sequestration and thermal recovery synergistic system for implementing the method as described in any one of claims 1-4, characterized in that, include: The heterogeneous artificial reservoir unit is formed by filling the target goaf with three-stage graded coal gangue in a "coarse-medium-fine" manner. From the injection end to the extraction end, it includes a high-permeability injection zone, a medium-permeability reaction zone, and a low-permeability collection zone. A gas-liquid mixing unit includes a gas-liquid mixing device, the inlet of which is connected to a mine water circulation pipeline and a CO2 gas source, respectively, for mixing low-temperature mine water with CO2 to form low-temperature carbon-rich mine water. The injection unit includes an injection pipeline network, a pulse generator, and an injection well located at the injection end; the injection pipeline network is connected to the outlet of the gas-liquid mixing unit; the pulse generator is located on the injection pipeline network and is used to apply low-frequency high-pressure pulse oscillations to the injected fluid. The extraction unit includes a liquid collection system, an extraction pump, and an extraction well located at the extraction end, used to collect decarbonized high-temperature mine water after mineralization reaction; The heat exchange unit is connected to the outlet of the extraction unit and is used to extract the heat energy from the decarbonized high-temperature mine water and send the cooled mine water back to the gas-liquid mixing unit. The intelligent monitoring unit includes a pressure sensor, a flow meter, a pH meter, a dissolved inorganic carbon analyzer, and a temperature sensor respectively installed in the injection well and the extraction well, as well as a data acquisition and control system. The data acquisition and control system receives signals from the pressure sensor, flow meter, pH meter, dissolved inorganic carbon analyzer, and temperature sensor and determines the system exit time based on a preset depletion threshold.

6. The deep artificial reservoir carbon dioxide sequestration and heat recovery synergistic system according to claim 5, characterized in that, The pulse generating device is a pulse water pump or a hydraulic pulse generator, with an output pulse frequency of 0.01~0.1 Hz and a pulse pressure amplitude of 2~8 MPa.

7. The deep artificial reservoir carbon dioxide sequestration and heat recovery synergistic system according to claim 5, characterized in that, The gas-liquid mixing device is equipped with a micro-nano bubble generator connected to a CO2 gas source at its inlet, and the generated CO2 bubbles have a diameter of 10 nm to 50 μm.

8. The deep artificial reservoir carbon dioxide sequestration and heat recovery synergistic system according to claim 5, characterized in that, The heat exchange unit is equipped with a cyclone separator and a sedimentation tank at its outlet to remove suspended particulate matter generated by the mineralization reaction.

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