MOFs (Metal-Organic Frameworks) material-based liquid CO2 fracturing coalbed methane exploitation anti-reflection method and experimental device

Abstract

The invention discloses an MOFs material-based liquid CO2 fracturing coalbed methane exploitation anti-reflection method and an experimental device, and the method comprises the following steps: 1) preparing an MOFs material, and modifying MOFs with a modifier; (2) constructing a liquid CO2 fracturing system containing the MOFs material; (3) fracturing the liquid CO2 and injecting the MOFs; (4) the permeability increasing effect of the coal seam is maintained, and long-term storage of CO2 is achieved; and 5) recycling the MOFs material. According to the experimental device, a liquid CO2 pressurization injection system is connected with an MOFs material distribution circulation system and a true triaxial loading and temperature control system, the true triaxial loading and temperature control system is connected with a gas separation and recovery system, and the gas separation and recovery system is connected with the MOFs material distribution circulation system. The data acquisition and control system is respectively connected with the liquid CO2 pressurization injection system, the MOFs material distribution circulation system, the true triaxial loading and temperature control system and the gas separation and recovery system. The problems that in the prior art, the crack stability is poor, a long-term effective flow guide channel is difficult to form, the utilization rate of CO2 in a coal seam is low, and the collection rate of CH4 is low are solved.

Description

Technical Field

[0001] This invention belongs to the field of permeability enhancement technology for coalbed methane extraction, specifically relating to a method for enhancing permeability in coalbed methane extraction using liquid CO2 fracturing based on MOFs materials, and also relating to an experimental device for enhancing permeability in coalbed methane extraction using liquid CO2 fracturing based on MOFs materials. Background Technology

[0002] Coalbed methane (CBM), as an important unconventional natural gas resource, mainly exists in the pore and fracture structure of coal seams in an adsorbed or free state.

[0003] However, the high adsorption capacity of low-permeability coal seams, insufficient microfracture development, and insufficient gas desorption power have long limited the efficiency of coalbed methane extraction. Currently, coalbed methane permeability enhancement methods mainly include hydraulic fracturing, acid fracturing, and CO2-enhanced fracturing, but certain technical bottlenecks remain. Hydraulic fracturing easily induces water-locking effects in the coal seam, inhibiting gas desorption; acid fracturing can corrode coal seam minerals and improve permeability, but may cause irreversible damage to the coal structure; CO2-enhanced fracturing promotes gas release through CO2 adsorption-desorption and utilizes CO2 phase change expansion to generate fracture propagation, but the fracture stability is poor, making it difficult to form long-term effective flow channels. Therefore, optimizing CO2 fracturing permeability enhancement technology, improving the long-term support capacity of fractures, further enhancing the utilization rate of CO2 in the coal seam, and further enhancing the CH4 collection rate are key research directions in the current field of coalbed methane extraction.

[0004] MOFs (metal-organic frameworks) have shown great potential in the field of gas storage and separation due to their ultra-high specific surface area, tunable pore size and excellent gas adsorption capacity. Summary of the Invention

[0005] The purpose of this invention is to provide a method and experimental apparatus for improving the permeability of coalbed methane extraction using liquid CO2 fracturing with MOFs materials. This invention solves the problems of poor fracture stability, difficulty in forming long-term effective flow channels, low utilization rate of CO2 in coal seams, and low collection rate of CH4 in the existing technology.

[0006] The technical solution adopted in this invention is a method for enhancing the permeability of coalbed methane extraction using liquid CO2 fracturing based on MOFs materials, implemented according to the following steps: Step 1: Prepare MOF materials and modify MOFs with modifiers; Step 2: Construct a liquid CO2 fracturing system containing MOFs materials; Step 3: Liquid CO2 fracturing and MOF injection; Step 4: Maintain the permeability enhancement effect of the coal seam and achieve long-term CO2 sequestration; Step 5: Recycle MOF materials.

[0007] The invention is further characterized by: Step 1 specifically includes the following steps: Step 1.1: Select a metal ion source and an organic ligand in a molar ratio of 1:8-16, dissolve them in deionized water or an organic solvent to form a homogeneous and transparent precursor solution. Step 1.2: Seal the precursor solution in a reaction vessel and react at 100-180℃ for 12-24 hours. After the reaction, centrifuge to separate, wash to remove impurities, and dry under vacuum. Step 1.3: Use a modifier to modify the surface of MOFs materials to form a composite material.

[0008] Step 2 specifically includes the following steps: Step 2.1: The CO2 is compressed and cooled below the critical temperature using a high-pressure pump system to form liquid CO2; Step 2.2: Disperse the modified MOF material in liquid CO2 at a mass fraction of 0.1%-1.0%, and use circulating stirring or ultrasonic homogenization to make the MOF material uniformly suspended in liquid CO2; Step 2.3: Adjust the system temperature, pressure, and MOF content; test the system viscosity, flowability, and adsorption performance.

[0009] Step 3 specifically includes the following steps: Step 3.1: Connect the liquid CO2 fracturing system to the wellbore and check the valve sealing, pressure sensor and temperature monitoring device for proper functioning. Step 3.2: Inject the liquid CO2 mixture into the target coal seam according to the designed discharge capacity, and control the injection pressure within the range of 10-25 MPa; Step 3.3: Liquid CO2 vaporizes within the coal seam, fracturing the coal body to create fractures and pores; Step 3.4: MOFs materials enter the cracks and pores, capture CO2, and form a stable adsorption layer on the crack walls.

[0010] Step 4 specifically includes the following steps: Step 4.1: After fracturing is completed, gradually increase the pressure of the liquid CO2 fracturing system and monitor the gas flow rate and CO2 content in the coal seam; Step 4.2: Start the extraction system, extract CH4, record the changes in CH4 content, flow rate and pressure, and analyze the production increase effect; Step 4.3: Detect CO2 concentration, flow rate, and leakage through an online monitoring system, and evaluate the CO2 fixation capacity and sequestration rate of MOFs in the coal seam using numerical simulation and adsorption equilibrium model.

[0011] Step 5 specifically includes the following steps: Step 5.1: Desorb and regenerate the MOF particles recovered from the coal seam gas by heating or depressurization to release the adsorbed CO2 for reuse. Step 5.2: After the coal seam pressure stabilizes, liquid CO2 fracturing can be repeated to establish a periodic permeability enhancement-storage monitoring system.

[0012] Another technical solution adopted in this invention is an experimental device for improving the permeability of coalbed methane extraction using liquid CO2 fracturing based on MOFs materials. The device includes a liquid CO2 pressurization injection system and a data acquisition and control system. The liquid CO2 pressurization injection system is connected to the MOFs material distribution and circulation system and the true triaxial loading and temperature control system. The true triaxial loading and temperature control system is connected to the gas separation and recovery system. The gas separation and recovery system is connected to the MOFs material distribution and circulation system. The data acquisition and control system is connected to the liquid CO2 pressurization injection system, the MOFs material distribution and circulation system, the true triaxial loading and temperature control system, and the gas separation and recovery system, respectively.

[0013] Another feature of the present invention is that: The liquid CO2 pressurization and injection system includes a first CO2 storage bottle. The outlet end of the first CO2 storage bottle is connected in sequence to a pressure reducing valve, a booster, a temperature detector, and a dynamic mixer. The dynamic mixer is equipped with an anti-backflow device. The dynamic mixer is also connected to a true triaxial loading and temperature control system. A booster is connected between the dynamic mixer and the true triaxial loading and temperature control system. The MOFs material distribution and circulation system includes an ultrasonic disperser and a surface modifier. The surface modifier is connected to the outlet of the dynamic mixer, and the ultrasonic disperser is connected to the inlet of the surface modifier. A precision filter is installed inside the dynamic mixer.

[0014] The true triaxial loading and temperature control system includes a true triaxial loading device, in which a coal sample is placed and a solid-liquid separation filter is placed at the bottom. The true triaxial loading device is also connected to a crack propagation monitoring system. The bottom of the true triaxial loading device is also connected to a liquid recovery device and a low-pressure heating desorption chamber. The liquid recovery device is equipped with a filter and a water level gauge. The low-pressure heating desorption chamber is connected to a high-temperature resistant chamber. The bottom of the high-temperature resistant chamber is equipped with a first heating device. The low-pressure heating desorption chamber is equipped with a pressure and temperature detector. The outlet of the low-pressure heating desorption chamber is connected to an ultrasonic disperser.

[0015] The gas separation and recovery system includes a first CH4 storage bottle and a second CO2 storage bottle. The outlets of the first CH4 storage bottle and the second CO2 storage bottle are connected to a first gas recovery device. The first gas recovery device is equipped with a first MOF (Medium-Oxide-Fuel). SThe adsorption tower is connected to a first gas detector between the first CH4 storage bottle and the first gas recovery device. The second CO2 storage bottle is connected to the first gas recovery device in sequence to the first gas detector, the first liquefaction and purification device and the first compression pump. The outlet end of the first gas recovery device is connected to a low-pressure heating desorption chamber. The gas separation and recovery system also includes a third CO2 storage bottle and a second CH4 storage bottle. The outlets of both the third CO2 and CH4 storage bottles are connected to a second gas recovery device. A second gas detector, a second liquefaction and purification device, and a second compression pump are sequentially connected between the third CO2 storage bottle and the second gas recovery device. A second gas detector is connected between the second CH4 storage bottle and the second gas recovery device. A second MOF (Medium-Oxide-Fuel) is installed in the second gas recovery device. S The adsorption tower, the outlet end of the second gas recovery device, and the true triaxial loading device.

[0016] The beneficial effects of this invention are: This invention achieves synergistic optimization of permeability enhancement and gas desorption in coalbed methane extraction via liquid CO2 fracturing by introducing modified ZIF-8 particles from type A materials. The modified ZIF-8 particles form a stable support structure within the fracture, improving the long-term conductivity of the fracture and preventing permeability attenuation caused by fracture closure. Simultaneously, their highly selective adsorption properties enhance CO2 retention in the coal seam, and the use of modified ZIF particles from type B materials improves the CO2 replacement efficiency for CH4, thereby increasing the coalbed methane recovery rate.

[0017] To further verify the permeability enhancement mechanism of the combined action of MOFs and CO2, this invention designed and developed an experimental system for MOFs-CO2 combined fracturing permeability enhancement and gas desorption. By integrating a CO2 injection system, a MOF particle dispersion system, a true triaxial loading system, a fracture propagation monitoring system, and a gas collection and analysis system, the system systematically studied the effects of different injection temperatures, pressures, MOF particle concentrations, and confining pressures on fracture morphology, gas desorption kinetics, and CO2 sequestration characteristics, and optimized key process parameters. This research overcomes the limitations of traditional hydraulic fracturing and single CO2 fracturing, realizing the multiple synergistic effects of MOFs-CO2 combined fracturing in coalbed methane extraction, including permeability enhancement, gas desorption, and carbon sequestration. It provides an innovative technical approach for permeability enhancement and green energy development in low-permeability coal seams. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of the experimental device for improving the permeability of coalbed methane extraction using liquid CO2 fracturing based on MOFs materials, according to the present invention.

[0019] In the diagram, 1. First CO2 storage bottle, 2. Pressure reducing valve, 3. Pressure booster, 4. Temperature detector, 5. Anti-backflow device, 6. CO2 molecule, 7. Dynamic mixer, 8. Precision filter, 9. Ultrasonic disperser, 10. Surface modifier, 11. CH4 molecule, 12. First MOFs material feeding device, 13. Water molecule, 14. Second MOFs material feeding device, 15. First valve, 16. Second valve, 17. Third valve, 18. Fourth valve, 19. First booster pump, 20. Second booster pump, 21. True triaxial loading device, 22. Coal sample, 23. Solid-liquid separation filter screen, 24. Liquid recovery device, 25. Filter screen, 26. Water level gauge, 27. Low-pressure heating desorption chamber, 28. First heating device, 29. High-temperature resistant chamber body, 30. Stirrer, 31. Pressure and temperature detector, 32. First CH4 storage bottle, 33. Second CO2 storage bottle, 34. Fifth valve, 35. Sixth valve, 36. Seventh valve, 37. Eighth valve, 38. Ninth valve, 39. Tenth valve, 40. First gas detector, 41. Second gas detector, 42. First liquefaction and purification device, 43. Second liquefaction and purification device, 44. First compression pump, 45. Second compression pump, 46. First MOF S Adsorption tower, 47. Second MOF S 48. Adsorption tower, 49. First gas recovery device, 50. Second gas recovery device, 51. Second heating device, 52. Third heating device, 53. Third CO2 storage bottle, 54. Second CH4 storage bottle, 55. Crack propagation monitoring system. Detailed Implementation

[0020] The following detailed description is provided in conjunction with specific implementation methods.

[0021] Example 1 This invention relates to a method for enhancing the permeability of coalbed methane extracted using liquid CO2 fracturing based on MOF materials, which is implemented according to the following steps: Step 1: Prepare MOF materials and modify MOFs with modifiers; Step 2: Construct a liquid CO2 fracturing system containing MOFs materials; Step 3: Liquid CO2 fracturing and MOF injection; Step 4: Maintain the permeability enhancement effect of the coal seam and achieve long-term CO2 sequestration; Step 5: Recycle MOF materials.

[0022] Example 2 Compared to Example 1, in this example, step 1: prepare MOFs materials and modify MOFs with modifiers. The purpose is to obtain MOFs materials with high specific surface area, excellent adsorption performance and good mechanical stability, so as to provide a basis for subsequent liquid CO2 permeation enhancement and storage.

[0023] Specifically, it includes the following steps: Step 1.1: Preparation of precursor solution 1) Select a metal ion source (such as Zn) 2+ Cu 2+ Z 4+ (etc.) and organic ligands (such as terephthalic acid, imidazole compounds); 2) Dissolve in deionized water or organic solvents (DMF, ethanol, etc.) by molar ratio to form a homogeneous and transparent solution.

[0024] Specifically as follows: Metal ion source: Zinc nitrate hexahydrate Zn(NO3)2·6H2O; Organic ligand: 2-methylimidazole; Typical molar ratio: (organic ligand: metal ion source = 8-16:1); Solvent: Methanol (MeOH) or deionized water / methanol = 1:1; Specific formulation (nano ZIF-8, high L:M ratio): Solution A: Weigh 2.97 g Zn(NO3)2·6H2O (10 mmol) and dissolve it in 100 mL of methanol; Solution B: Weigh 13.14 g of 2-Melm (160 mmol) and dissolve it in 100 mL of methanol; Procedure: Quickly pour B into A, magnetically stir at room temperature for 30-60 minutes (or overnight) to form an emulsion; allow to stand, centrifuge and wash (wash twice with methanol and once with ethanol), then place in a vacuum dryer at 60℃.

[0025] Increasing the ratio of 2-methylimidazole to the metal ion source, for example, to 16:1, will result in smaller particle size and more developed pores; decreasing the ratio, for example, to 8:1, will result in larger particle size and better suitability for proppant applications.

[0026] For aqueous green synthesis: maintain 2-methylimidazole:Zn = 8-16:1, replace methanol with an equal volume of deionized water, and stir at room temperature or 40 degrees Celsius for 1-4 hours.

[0027] Step 1.2: Solvothermal synthesis; 1) Seal the precursor solution in a reaction vessel and react at 100-180℃ for 12-24 hours; 2) After the reaction, centrifuge to separate the components, wash to remove impurities, and then dry under vacuum conditions.

[0028] Step 1.3: Surface modification and composite; 1) Surface modification of MOF materials is performed using modifiers such as amino groups, carboxyl groups, fluorides, or graphene; 2) Form a composite material with both high adsorption and pressure resistance to improve stability in a high-pressure liquid CO2 environment.

[0029] Specifically, solution-based amination... Solvent: Anhydrous toluene or ethanol; Mixing ratio: ZIF-8:APTES mass ratio 1:0.05-0.15; Conditions: Reflux at 60-80 degrees Celsius for 2-6 hours; Washing / Drying: Ultrasonic wash twice with ethanol, wash once with acetone; then vacuum dry at 60 degrees Celsius for 6 hours; Loading amount: Controlled at 3-8 wt%.

[0030] Example 3 Compared to Example 1, step 2 of this embodiment involves constructing a liquid CO2 fracturing system containing MOFs materials. The objective is to construct such a system to effectively enhance coal seam porosity and achieve CO2 fixation and adsorption. Specifically, the following steps are included: Step 2.1: Liquid CO2 Acquisition and Storage; 1) CO2 is compressed and cooled to below the critical temperature (temperature < -56.6℃, pressure > 5.2MPa) using a high-pressure pump system to form liquid CO2; 2) Store in a high-pressure insulated container to ensure stable temperature and pressure.

[0031] Step 2.2: MOF material dispersion; 1) The modified MOF material was dispersed in liquid CO2 at a mass fraction of 0.1%-1.0%; 2) Use circulating stirring or ultrasonic homogenization to make it uniformly suspended and distributed in liquid CO2.

[0032] Step 2.3: Optimization of fracturing fluid performance; 1) Adjust the system temperature, pressure and MOF content; the temperature and pressure are (temperature < -56.6℃, pressure > 5.2MPa), and the MOF content is 0.1%-1.0%.

[0033] 2) Test the viscosity, flowability and adsorption performance of the system to ensure that it can achieve both effective crack propagation and high CO2 adsorption efficiency.

[0034] Example 4 Compared to Example 1, step 3 of this embodiment involves liquid CO2 fracturing and MOF injection. The purpose is to utilize the rapid vaporization and expansion effect of liquid CO2 and the adsorption properties of MOF materials to achieve coal seam fracture propagation and permeability enhancement. Specifically, it includes the following steps: Step 3.1: Wellhead Connection and System Inspection; 1) Connect the liquid CO2 fracturing system to the wellbore; 2) Check the valve sealing and whether the pressure sensor and temperature monitoring device are working properly.

[0035] Step 3.2: Liquid CO2 injection; 1) Inject the liquid CO2 mixture into the target coal seam according to the designed discharge rate (2-5 m / min); 2) Control the injection pressure within the range of 10-25MPa to ensure deep penetration of liquid CO2.

[0036] Step 3.3: Vaporization expansion and fracture propagation; 1) Liquid CO2 rapidly vaporizes and expands within the coal seam, generating instantaneous high-pressure impact; 2) Low temperature causes thermal shrinkage cracks in the coal body, which overlap with stress-induced cracks to form a complex crack network.

[0037] Step 3.4: Oriented embedding of MOF materials; 1) During the gasification process, MOF materials are carried into the cracks and pores; 2) Its porous structure can capture CO2 and form a stable adsorption layer on the crack wall to prevent crack closure.

[0038] Example 5 Compared to Example 1, step 4 of this embodiment aims to maintain the coal seam permeability enhancement effect and achieve long-term CO2 sequestration. Specifically, it includes the following steps: Step 4.1: Pressure release and venting; 1) Gradually reduce the system pressure after fracturing is completed; 2) Monitor the gas flow rate and CO2 content in the coal seam to prevent secondary damage caused by sudden depressurization.

[0039] Step 4.2: Coalbed methane extraction; 1) Start the extraction system to extract CH4; 2) Record changes in CH4 content, flow rate, and pressure, and analyze the yield increase effect.

[0040] Step 4.3: Sealing, monitoring, and evaluation; 1) Detect CO2 concentration, flow rate, and leakage through an online monitoring system; 2) The CO2 fixation capacity and sequestration rate of MOFs in coal seams were evaluated using numerical simulation and adsorption equilibrium model.

[0041] Example 6 Compared to Example 1, step 5 of this embodiment aims to recycle MOF materials, thereby achieving material recycling, system reuse, and improved economic efficiency. Specifically, it includes the following steps: Step 5.1: MOF material regeneration 1) Desorption and regeneration of MOF particles recovered from coal seam produced gas; 2) Use heating or depressurization to release the adsorbed CO2 for reuse.

[0042] Step 5.2: Cyclic fracturing and long-term monitoring 1) Liquid CO2 fracturing can be repeated after the coal seam pressure stabilizes; 2) Establish a periodic permeability enhancement-storage monitoring system to achieve continuous and efficient mining and stable storage.

[0043] Example 7 This invention constructs an experimental device for enhancing the permeability of coalbed methane (CBM) through liquid CO2 fracturing based on MOFs (Metal-Oxide-Fracturing Materials). It investigates the synergistic mechanism of MOFs during liquid CO2 fracturing and their impact on coal seam fracture propagation and gas desorption behavior. The aim is to address issues in CBM extraction such as low coal seam permeability, fracture closure, low gas desorption rate, and the water-locking effect and low carbon dioxide sequestration efficiency associated with traditional hydraulic fracturing, thereby improving CBM recovery. The overall structure of the MOF-based liquid CO2 fracturing CBM enhancement experimental device comprises five functional systems: a liquid CO2 pressurization and injection system, a MOFs material distribution and circulation system, a true triaxial loading and temperature control system, a gas separation and recovery system, and a data acquisition and control system.

[0044] The liquid CO2 pressurization injection system consists of a first CO2 storage bottle 1, a pressure reducing valve 2, a pressure booster 3, a temperature detector 4, an anti-backflow device 5, a dynamic mixer 7, a precision filter 8, a first valve 15, a second valve 16, a third valve 17, a fourth valve 18, a first pressure booster pump 19, and a second pressure booster pump 20. The liquid CO2 pressurization injection system is mainly used to release, stabilize, and boost the pressure of high-pressure CO2 from the first CO2 storage bottle 1, and control its flow state to achieve stable delivery and controllable injection of liquid CO2. The pressure reducing valve 2 is used to control the initial pressure and avoid system overload; the pressure booster 3 pressurizes the CO2 to the set fracturing pressure range (3~10MPa); the dynamic mixer 7 is used in conjunction to ensure that CO2 and MOF suspended particles are fully mixed to form a stable and uniform fracturing fluid; the anti-backflow device 5 prevents gas from the coal sample from flowing back and contaminating the system; and the temperature detector 4 monitors the CO2 state changes in real time to prevent premature phase change. The liquid CO2 pressurized injection system ultimately enables liquid CO2 to enter the coal sample of the true triaxial loading system under controlled conditions, completing the initial fracturing and permeation process.

[0045] The MOFs material distribution and circulation system consists of an ultrasonic disperser 9, a surface modifier 10, a first MOFs material feeding device 12, a second MOFs material feeding device 14, and a precision filter 8. The first MOFs material feeding device 12 contains ZIF-8 material with CH4 molecules 11 on the upper layer and water molecules 13 on the lower layer. The second MOFs material feeding device 14 contains CO2. The core of the MOFs material distribution and circulation system is that the ultrasonic disperser 9 uniformly disperses the MOFs, and the surface modifier 10 regulates the surface chemical properties of the MOFs to modify them into type A (CO2-loving) and type B (CH4-loving) materials, achieving two types of targeted adsorption properties: CO2-loving and CH4-loving. The precision filter 8 ensures uniform particle distribution, avoids clogging, and filters impurities. Through the MOFs material distribution and circulation system, the recycling and ratio control of MOFs materials (A:B = 1:1~2:1) can be achieved, maintaining long-term stable operation of the system.

[0046] The true triaxial loading and temperature control system includes a true triaxial loading device 21 (composed of a true triaxial loading box, a confining pressure boosting pump, and an axial loading device), a coal sample 22, a solid-liquid separation filter 23, and a crack propagation monitoring system 54. The true triaxial loading and temperature control system simulates real-world stress conditions to achieve three-dimensional loading and crack control of the coal sample. Acoustic emission sensors, an infrared thermal imager, a high-precision CT scanner, and the crack propagation monitoring system 54 can monitor the crack propagation path and stress concentration areas in real time; the solid-liquid separation filter 23 is used to drain excess fluid.

[0047] The gas separation and recovery system includes a first CH4 storage bottle 32, a second CH4 storage bottle 53, a second CO2 storage bottle 33, a third CO2 storage bottle 52, a fifth valve 34, a sixth valve 35, a seventh valve 36, a seventh valve 37, a eighth valve 38, a ninth valve 39, a tenth valve, a first gas detector 40, a second gas detector 41, a low-pressure heating desorption chamber 27, a high-temperature resistant chamber 29, a stirrer 30, a pressure and temperature detector 31, a first compression pump 44, a second compression pump 45, a first MOFs adsorption tower 46, a second MOFs adsorption tower 47, a first gas recovery device 48, a second gas recovery device 49, a first heating device 28, a second heating device 50, a third heating device 51, a first liquefaction and purification device 42, a second liquefaction and purification device 43, a liquid recovery device 24, a filter screen 25, and a water level gauge 26. The gas separation and recovery system is mainly responsible for the diversion, adsorption, separation, and purification of the fracture product gases. A stirrer 30 maintains uniform flow within the sample; a pressure and temperature detector 31 feeds back to the data acquisition module to ensure automatic adjustment and safety control during the loading process; a first heating device 28 and a high-temperature resistant chamber 29 are used to regulate the temperature, allowing the adsorbed gas in the MOFs to be released; when the gas enters the adsorption tower through the filter and exhaust channel, CO2 and CH4 are automatically identified; by controlling the fifth valve 34 to the tenth valve 39, different flow paths can be guided at different stages to achieve selective recovery; the second heating device 50 and the third heating device 51 heat up and regenerate the MOFs material during the desorption stage, and introduce the desorbed gas into the first compression pump 44 and the second compression pump 45 for purification through the first liquefaction purification device 42 and the second liquefaction purification device 43; the first gas detector 40 and the second gas detector 41 monitor the gas flow rate and concentration changes in real time to ensure the accuracy and safety of the recovery process; the first gas recovery device 48 and the second gas recovery device 49 recover the gas that did not participate in the reaction or the gas produced by the reaction, including the first MOFs. S Adsorption tower 46, second MOF S The adsorption tower 47, the second heating device 50, and the third heating device 51 are used for the adsorption, separation, and regeneration of CO2 and CH4, respectively; the liquid recovery device 24 and the water level gauge 26 are used to collect waste liquid and liquid suspended matter and monitor the fluid volume changes in real time.

[0048] The data acquisition and control system consists of a temperature sensor 4, a pressure and temperature sensor 31, a first gas detector 40, a second gas detector 41, and a fracture propagation monitoring system 54 (including a data acquisition and control terminal, a signal processing module, and a computer control platform). It monitors key parameters in real time, such as liquid CO2 injection pressure, flow rate, temperature field changes in the MOF mixture, fracture propagation rate, and gas concentration. Through the linkage between the data acquisition and control terminal and the computer program, it achieves automated experimental control and closed-loop regulation; it enables real-time data recording, curve plotting, and visualization of experimental results, providing quantitative support for fracturing mechanism analysis.

[0049] Specifically, the outlet of the first CO2 storage bottle 1 is connected to the inlet of the pressure reducing valve 2 via a gas guide pipe. The outlet of the pressure reducing valve 2 is connected to the booster 3 via a gas guide pipe. The booster 3 adjusts the high-pressure CO2 to a pressure range suitable for injection into the mixer. The outlet of the booster 3 is connected to the temperature detector 4 via a gas guide pipe. The temperature detector 4 monitors the temperature of the CO2 in the gas guide pipe in real time. The outlet of the temperature detector 4 is connected to the dynamic mixer 7 via a gas guide pipe. The dynamic mixer 7 stores and mixes the reactants. The outlet of the ultrasonic disperser 9 is connected to the surface modifier 10 via a gas guide pipe. The surface modifier 10 performs chemical modification on the MOFs material. The outlet of the surface modifier 10 is connected to the surface modifier via a gas guide pipe. Connect the dynamic mixer 7; the outlet of the dynamic mixer 7 is connected to the fourth valve 18 via a gas guide pipe. The function of the fourth valve 18 is to control the flow rate. The outlet of the fourth valve 18 is connected to the first booster pump 19 via a gas guide pipe. The function of the first booster pump 19 is to adjust the high-pressure CO2 and MFOs mixture to a pressure range suitable for injection into the true triaxial equipment. The outlet of the first booster pump 19 is connected to the second booster pump 20 via a gas guide pipe. The function of the second booster pump 20 is to repressurize the high-pressure CO2 and MFOs mixture to a pressure range suitable for injection into the true triaxial equipment. The outlet of the second booster pump 20 is connected to the true triaxial loading device 21 via a gas guide pipe. The function of the true triaxial loading device 21 is to simulate coal seam conditions and achieve C O2 fracturing and CH4 displacement: The outlet of the true triaxial loading device 21 is connected to the ninth valve 38 via a gas guide pipe. The function of the ninth valve 38 is to control the gas flow rate. The outlet of the ninth valve 38 is connected to the second gas recovery device 49 via a gas guide pipe. The function of the second gas recovery device 49 is to recover unreacted CO2 and CH4 gases. The outlet of the second gas recovery device 49 is connected to the second gas detector 41 via a gas guide pipe. The function of the second gas detector 41 is to monitor the gas flow rate. The outlet of the second gas detector 41 is connected to the tenth valve 39 via a gas guide pipe. The function of the tenth valve 39 is to control the gas flow rate. The outlet of the tenth valve 39 is connected to the second CH4 storage bottle 53 via a gas guide pipe. The CH4 storage bottle is used to store the recovered CH4 gas. The outlet of the second gas recovery device 49 is connected to the second compression pump 45 through a gas guide pipe. The function of the second compression pump 45 is to compress gaseous CO2 into liquid CO2. The outlet of the second compression pump 45 is connected to the second liquefaction and purification device 43 through a gas guide pipe. The function of the second liquefaction and purification device 43 is to purify the recovered CO2. The outlet of the second liquefaction and purification device 43 is connected to the second gas detector 41 through a gas guide pipe. The function of the second gas detector 41 is to monitor the gas flow rate. The outlet of the second gas detector 41 is connected to the third CO2 storage bottle 52 through a gas guide pipe. The function of the third CO2 storage bottle 52 is to store the recovered CO2 gas.The outlet of the true triaxial loading device 21 is connected to the fracture propagation monitoring system 54 via a line. The function of the fracture propagation monitoring system 54 is to monitor the fracturing effect and fracture propagation in real time. The outlet of the true triaxial loading device 21 is connected to the liquid recovery device 24 via a conduit. The function of the liquid recovery device 24 is to store reaction waste liquid. The outlet of the true triaxial loading device 21 is connected to the low-pressure heating desorption chamber 27 via a conduit. The function of the low-pressure heating desorption chamber 27 is to separate gas from MOF particles. The outlet of the low-pressure heating desorption chamber 27 is connected to the ultrasonic disperser 9 via a conduit. The function of the ultrasonic disperser 9 is to disperse MOF particles. The outlet of the low-pressure heating desorption chamber 27 is connected to the first gas recovery device 48 via a gas guide pipe. The function of the first gas recovery device 48 is to recover unreacted CO2 gas and CH4 gas. The outlet of the first gas recovery device 48 is connected to the first gas detector via a gas guide pipe. 40. The function of the first gas detector 40 is to monitor the gas flow rate. The outlet of the first gas detector 40 is connected to the first CH4 storage bottle 32 via a gas guide pipe. The function of the first CH4 storage bottle 32 is to store the recovered CH4 gas. The outlet of the first gas recovery device 48 is connected to the first gas detector 40 via a gas guide pipe. The function of the first gas detector 40 is to monitor the gas flow rate. The outlet of the first gas detector 40 is connected to the first compression pump 44 via a gas guide pipe. The outlet of the first compression pump 44 is connected to the first liquefaction and purification device 42 via a gas guide pipe. The function of the first liquefaction and purification device 42 is to purify the recovered CO2. The outlet of the first liquefaction and purification device 42 is connected to the first gas detector 40 via a gas guide pipe. The function of the first gas detector 40 is to monitor the gas flow rate. The outlet of the first gas detector 40 is connected to the second CO2 storage bottle 33 via a gas guide pipe.

[0050] The experimental setup of this invention can simulate actual geological conditions in coal seams and analyze the effects of MOF (Metal-Oxide-Fractions) ratio, CO2 injection pressure, and temperature on fracture network evolution and gas desorption processes. The liquid CO2 pressurization injection system is responsible for delivering liquid CO2 to the experimental setup under different temperatures (-10~10℃), injection pressures (5~50MPa), and flow rates (50~500ml / min) to simulate the phase changes and permeability enhancement effects of CO2 during coal seam fracturing. The MOF material distribution and circulation system ensures uniform distribution of ZIF-8 particles in the CO2 fluid and mainly consists of a MOF particle storage chamber, an ultrasonic disperser, and particle delivery pipelines, ensuring that the particles enter the coal seam fractures with the CO2, thus improving the fracture conductivity. The true triaxial loading and temperature control system simulates the deep geological environment of coal seams by applying different confining pressures (5~30MPa) to ensure that the experimental conditions match the actual stress state of the coal seam. The data acquisition and control system uses acoustic emission monitoring, high-precision CT scanning, and infrared thermography to record in real time the processes of fracture initiation, propagation, branching, and MOF particle filling in the fractures, and quantitatively analyzes the support effect of MOF particles in the fractures. The gas separation and recovery system is used to measure the gas desorption rate and recovery rate during the CO2 replacement of CH4 process, and uses an online gas analyzer, isothermal adsorption device, and gas collection tank for precise monitoring.

[0051] During the experimental setup, CO2 is first cooled and pressurized to a liquid state in a high-pressure storage tank, then injected into a dynamic mixer via a flow control valve. Simultaneously, MOFs material, pulverized by an ultrasonic stirrer, is fed into a surface modifier. The processed MOFs, materials A and B, are injected into the dynamic mixer in a specific ratio according to different coal ranks, and thoroughly mixed with the liquid CO2. This mixture forms a "suspended carrier phase" and is injected into the true triaxial coal sample cavity. The liquid CO2 expands and cools within the coal body due to phase change, creating an instantaneous temperature stress field that promotes rapid expansion of coal fractures. Simultaneously, a mixed ZIF-8 material is injected into the coal sample fractures via a distribution pump. The CO2-loving ZIF-8 material forms a microscale support framework within the fractures, preventing fracture closure, and simultaneously enriches and locally fixes liquid and gaseous CO2 under low-temperature, high-pressure conditions. The CH4-loving ZIF-8 material rapidly adsorbs CH4 released by CO2 at the fracture-matrix interface and is recovered with the permeate flow, achieving immediate capture and enrichment of methane.

[0052] The gas separation and recovery system features a two-stage adsorption bed. The first layer consists of CO2-loving MOFs to intercept residual CO2 and promote re-adsorption. The second layer consists of CH4-loving MOFs to collect methane released from the coal. Controlled by a gas concentration sensor and an automatic switching valve, the methane collection bottle opens when the CH4 concentration reaches its peak, enabling precise collection. Solid MOF particles that have absorbed CH4 are separated by a solid-liquid separation filter and enter a low-pressure, heated desorption chamber. Through heating or depressurization, the MOF materials are regenerated and recycled, releasing CO2 and CH4 gases. These gases then enter the gas separation and recovery system for separation and collection. Some gases are directly collected through a true triaxial device. Liquid waste, after being separated by a solid-liquid separation filter, is collected in a liquid recovery unit.

[0053] This experimental setup consists of three parts: a fracture propagation experiment, a gas desorption experiment, and a CO2 sequestration stability experiment. These experiments investigate the role of MOF particles in fracture support and CO2 replacement of CH4, respectively. In the fracture propagation experiment, by controlling the injection temperature (-30~10℃), pressure (2~8MPa), and MOF material ratio (CO-loving:CH4-loving = 1:1~1:3), combined with acoustic emission monitoring and CT scanning, the morphology, length, and number of branches of the fractures under different experimental conditions were analyzed. In the gas desorption experiment, the effects of CO2 replacement pressure (2~15MPa) and temperature (-10~30℃) on gas desorption kinetics were investigated, and data were recorded using an online gas analyzer and an isothermal adsorption device. This setup can optimize fracture propagation and gas desorption efficiency for different coal quality conditions, providing experimental data support for subsequent analysis of fracture propagation patterns and energy migration mechanisms.

[0054] The core mechanism of the combined effect of MOFs and CO2 is reflected in three aspects. First, MOF particles act as fracture proppant, filling fracture voids, effectively preventing fracture closure, and improving coal seam permeability. Second, the porous structure on the surface of MOF particles provides additional adsorption sites for CO2 to replace CH4, increasing CO2 permeability in the coal seam, increasing the CH4 release rate, and improving coalbed methane recovery.

[0055] Compared to traditional CO2 fracturing coalbed methane technology, this invention exhibits significant advantages in fracture support and gas desorption. The introduction of MOF particles not only improves the long-term conductivity of fractures but also enhances the kinetics of CO2 replacing CH4. The innovation of this experimental setup lies in its integration of a multi-functional system encompassing CO2 fracturing, MOF particle dispersion, triaxial loading, fracture propagation monitoring, and gas analysis. This allows the experiment to simulate a real coal seam environment, systematically evaluating the permeability-enhancing effect of the MOF-CO2 composite system in coalbed methane extraction. It not only achieves the organic combination of liquid CO2 fracturing and MOF functional materials but also constructs a recyclable gas desorption-adsorption-recovery process, providing an innovative experimental and engineering approach for increasing the production of low-permeability coalbed methane and for CO2 resource sequestration.

Claims

1. A method for enhancing the permeability of coalbed methane extraction using liquid CO2 fracturing based on MOFs materials, characterized in that, The specific steps are as follows: Step 1: Prepare MOF materials and modify MOFs with modifiers; Step 2: Construct a liquid CO2 fracturing system containing MOFs materials; Step 3: Liquid CO2 fracturing and MOF injection; Step 4: Maintain the permeability enhancement effect of the coal seam and achieve long-term CO2 sequestration; Step 5: Recycle MOF materials.

2. The method for enhancing permeability in coalbed methane extraction using liquid CO2 fracturing based on MOFs materials according to claim 1, characterized in that, Step 1 specifically includes the following steps: Step 1.1: Select a metal ion source and an organic ligand in a molar ratio of 1:8-16, dissolve them in deionized water or an organic solvent to form a homogeneous and transparent precursor solution. Step 1.2: Seal the precursor solution in a reaction vessel and react at 100-180℃ for 12-24 hours. After the reaction, centrifuge to separate, wash to remove impurities, and dry under vacuum. Step 1.3: Use a modifier to modify the surface of MOFs materials to form a composite material.

3. The method for enhancing permeability in coalbed methane extraction using liquid CO2 fracturing based on MOFs materials according to claim 1, characterized in that, Step 2 specifically includes the following steps: Step 2.1: The CO2 is compressed and cooled below the critical temperature using a high-pressure pump system to form liquid CO2; Step 2.2: Disperse the modified MOF material in liquid CO2 at a mass fraction of 0.1%-1.0%, and use circulating stirring or ultrasonic homogenization to make the MOF material uniformly suspended in liquid CO2; Step 2.3: Adjust the system temperature, pressure, and MOF content; test the system viscosity, flowability, and adsorption performance.

4. The method for enhancing permeability in coalbed methane extraction using liquid CO2 fracturing based on MOFs materials according to claim 1, characterized in that, Step 3 specifically includes the following steps: Step 3.1: Connect the liquid CO2 fracturing system to the wellbore and check the valve sealing, pressure sensor and temperature monitoring device for proper functioning. Step 3.2: Inject the liquid CO2 mixture into the target coal seam according to the designed discharge capacity, and control the injection pressure within the range of 10-25 MPa; Step 3.3: Liquid CO2 vaporizes and fracturing the coal seam to generate fractures and pores; Step 3.4: MOFs materials enter the cracks and pores, capture CO2, and form a stable adsorption layer on the crack walls.

5. The method for enhancing permeability in coalbed methane extraction using liquid CO2 fracturing based on MOFs materials according to claim 1, characterized in that, Step 4 specifically includes the following steps: Step 4.1: After fracturing is completed, gradually increase the pressure of the liquid CO2 fracturing system and monitor the gas flow rate and CO2 content in the coal seam; Step 4.2: Start the extraction system, extract CH4, record the changes in CH4 content, flow rate and pressure, and analyze the production increase effect; Step 4.3: Detect CO2 concentration, flow rate, and leakage through an online monitoring system, and evaluate the CO2 fixation capacity and sequestration rate of MOFs in the coal seam using numerical simulation and adsorption equilibrium model.

6. The method for enhancing permeability in coalbed methane extraction using liquid CO2 fracturing based on MOFs materials according to claim 1, characterized in that, Step 5 specifically includes the following steps: Step 5.1: Desorb and regenerate the MOF particles recovered from the coal seam gas by heating or depressurization to release the adsorbed CO2 for reuse. Step 5.2: After the coal seam pressure stabilizes, liquid CO2 fracturing can be repeated to establish a periodic permeability enhancement-storage monitoring system.

7. An experimental apparatus for improving the permeability of coalbed methane extraction using liquid CO2 fracturing based on MOFs materials, employing any of the methods for improving the permeability of coalbed methane extraction using liquid CO2 fracturing based on MOFs materials according to claims 1-6, characterized in that... It includes a liquid CO2 pressurization and injection system and a data acquisition and control system. The liquid CO2 pressurization and injection system is connected to the MOFs material distribution and circulation system and the true triaxial loading and temperature control system. The true triaxial loading and temperature control system is connected to the gas separation and recovery system. The gas separation and recovery system is connected to the MOFs material distribution and circulation system. The data acquisition and control system is connected to the liquid CO2 pressurization and injection system, the MOFs material distribution and circulation system, the true triaxial loading and temperature control system, and the gas separation and recovery system, respectively.

8. The experimental device for improving the permeability of coalbed methane extraction based on MOFs materials using liquid CO2 fracturing according to claim 7, characterized in that, The liquid CO2 pressurization injection system includes a first CO2 storage bottle (1), and the outlet end of the first CO2 storage bottle (1) is connected in sequence to a pressure reducing valve (2), a booster (3), a temperature detector (4) and a dynamic mixer (7). The dynamic mixer (7) is equipped with an anti-backflow device (5). The dynamic mixer (7) is also connected to a true triaxial loading and temperature control system. The booster (3) is connected between the dynamic mixer (7) and the true triaxial loading and temperature control system. The MOFs material distribution and circulation system includes an ultrasonic disperser (9) and a surface modifier (10). The surface modifier (10) is connected to the outlet end of the dynamic mixer (7), and the ultrasonic disperser (9) is connected to the inlet end of the surface modifier (10). A precision filter (8) is provided inside the dynamic mixer (7).

9. The experimental device for improving permeability in coalbed methane extraction based on MOFs materials using liquid CO2 fracturing, as described in claim 7 or 8, is characterized in that... The true triaxial loading and temperature control system includes a true triaxial loading device (21), in which a coal sample (22) is placed and a solid-liquid separation filter (23) is placed at the bottom. The true triaxial loading device (21) is also connected to a crack propagation monitoring system (54). The bottom end of the true triaxial loading device (21) is also connected to a liquid recovery device (24) and a low-pressure heating desorption chamber (27). The liquid recovery device (24) is equipped with a filter screen (25) and a water level gauge (26). The low-pressure heating desorption chamber (27) is connected to a high-temperature resistant chamber (29). The bottom of the high-temperature resistant chamber (29) is equipped with a first heating device (28). The low-pressure heating desorption chamber (27) is equipped with a pressure and temperature detector (4). The outlet end of the low-pressure heating desorption chamber (27) is connected to an ultrasonic disperser (9).

10. The experimental device for improving the permeability of coalbed methane extraction based on MOFs materials using liquid CO2 fracturing according to claim 7, characterized in that, The gas separation and recovery system includes a first CH4 storage bottle (32) and a second CO2 storage bottle (33). The outlets of the first CH4 storage bottle (32) and the second CO2 storage bottle (33) are connected to a first gas recovery device (48). A first MOF is installed in the first gas recovery device (48). S The adsorption tower (46) is connected to the first gas detector (40) between the first CH4 storage bottle (32) and the first gas recovery device (48). The second CO2 storage bottle (33) is connected to the first gas detector (40), the first liquefaction purification device (42) and the first compression pump (44) in sequence. The outlet end of the first gas recovery device (48) is connected to the low-pressure heating desorption chamber (27). The gas separation and recovery system also includes a third CO2 storage bottle (52) and a second CH4 storage bottle (53). The outlets of the third CO2 storage bottle (52) and the second CH4 storage bottle (53) are connected to a second gas recovery device (49). A second gas detector (41), a second liquefaction purification device (43), and a second compression pump (45) are sequentially connected between the third CO2 storage bottle (52) and the second gas recovery device (49). A second gas detector (41) is connected between the second CH4 storage bottle (53) and the second gas recovery device (49). A second MOF is installed in the second gas recovery device (49). S The adsorption tower (47), the outlet end of the second gas recovery device (49) and the true triaxial loading device (21).

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