A low-temperature fluid circulation fracturing coal body test system and method

Abstract

The application discloses a kind of low-temperature fluid circulation fracturing coal body test system and method.The test system simulates the mechanical response and wet modification of coal rock sample under deep in-situ stress environment by liquid CO2-water circulation fracturing and wetting modification cycle alternation effect of liquid CO2 fracturing and water wetting modification, which can study crack propagation and dynamic wetting law in the process of fracturing and wetting modification by combining with multifunctional rock dynamic monitoring module; in addition, gas-liquid two-phase permeability and pH measurement module can be combined to evaluate the change of sample permeability and acidification dissolution before and after liquid CO2-water circulation fracturing and wetting modification; therefore, the application can simulate the process of liquid CO2-water circulation fracturing and wetting modification in deep coal seam, obtain test data under different conditions, thereby optimize the number of liquid CO2 and water circulation injection, and study the influence of different injection parameters on the effect of liquid CO2-water circulation fracturing and wetting modification, to provide a basis for coal seam liquid CO2-water circulation fracturing and wetting modification dust reduction technology.

Description

Technical Field

[0001] This invention relates to the field of coal seam water injection technology for preventing mine disasters, specifically a low-temperature fluid circulation fracturing and lubricating coal body test system and method. Background Technology

[0002] Coal seam water injection technology is a proactive measure to prevent coal dust pollution and disasters such as explosions, spontaneous combustion of coal, gas outbursts, and rock bursts. Its essence is to inject a solution into the coal seam to be mined through boreholes to increase its moisture content and achieve the purpose of pre-wetting the coal body. The main functions are as follows: (1) It inhibits the primary dust in the coal body fissures and can also reduce the secondary dust generated during the coal body crushing process; (2) The increase in coal seam moisture can prevent the coal oxidation process and reduce the tendency of coal seam to spontaneous combustion; (3) During high-pressure water injection, stress is redistributed, and pressure is transferred to the deep part of the coal body. Water has a significant hindering effect on the movement of gas, reducing the amount and speed of gas in the coal. (4) Under the impact of high-pressure water flow, high-pressure water is impacted into the coal body, generating fissures that weaken the brittleness of the coal body, enhance its plasticity, change the physical and mechanical properties of the coal body, and make the coal body lose its impact tendency.

[0003] In my country, over 70% of coal seams are difficult to water-inject, characterized by high ground stress, low porosity, and low permeability. Traditional coal seam water injection techniques struggle to effectively fracturing and fully wet the coal seam, significantly limiting their applicability and injection effectiveness. To address this, the applicant has creatively proposed a concept for modifying the pore and fracture structure of coal seams through cyclic injection of liquid CO2, and for improving wetting efficiency through cyclic water injection. Based on this, related systems and methods have been designed, and four related invention patents have been published: CN114151124A, CN 114109382A, CN 114046148A, and CN115452596A. This innovative concept fully utilizes the properties of liquid CO2, such as low-temperature freeze-swelling, pressurized phase change, and acid-induced dissolution, to increase coal seam permeability, improve coal seam wetting, reduce mine dust pollution, decrease the likelihood of coal seam rockbursts and the risk of gas outbursts, and provide a new approach for developing coal seam water injection technology for enhanced wetting and dust reduction. However, the existing experimental systems are not yet able to simulate liquid CO2-induced fracturing coal bodies in real geological environments, nor can they study the effects of liquid CO2-water cycle-induced fracturing and lubricating coal bodies under the influence of related factors. Summary of the Invention

[0004] To address the numerous limitations of existing technologies, this invention provides a low-temperature fluid circulation fracturing and wetting-enhancing coal seam testing system and method. Its purpose is to utilize liquid CO2 to fracture coal seams, thereby enhancing coal permeability and improving water injection efficiency. This testing system is rational, highly operable, and provides accurate and reliable data. It allows for intelligent and automated testing, and by controlling relevant factors, it studies the influence and scientific laws of liquid CO2-water circulation fracturing and wetting-enhancing on the microstructure, mechanical properties, and wetting characteristics of coal seams. This aims to lay a theoretical and experimental foundation for the development of liquid CO2-water circulation fracturing, wetting-enhancing, and dust-reducing technology for coal seams.

[0005] To achieve the above objectives, the present invention adopts the following technical solution:

[0006] This invention provides a low-temperature fluid circulation fracturing and lubrication test system for coal bodies, including a data acquisition and control module, a liquid CO2-water circulation fracturing and lubrication module, an axial pressure-confining pressure stabilization loading module, a multi-functional coal and rock dynamic monitoring module, and a gas-liquid two-phase permeability and pH measurement module.

[0007] The liquid CO2-water circulation crack-inducing and lubricating module includes a CO2 low-temperature pressurization liquefaction device, a back pressure device, an automatic water injection device, and a vacuum drainage device.

[0008] Furthermore, the CO2 cryogenic pressurization liquefaction device includes a CO2 cylinder, a CO2-specific flow meter, a booster pump, a high-pressure piston intermediate container A, a high-pressure piston intermediate container B, a constant speed and constant pressure pump, a process switcher, and a condenser. The CO2 cylinder is connected to the air inlet at the upper end of the high-pressure piston container A via a pipeline, and a dedicated CO2 flow meter is installed on the pipeline to display the gas flow status in real time. The booster pump is connected to the liquid inlet at the lower end of the high-pressure piston intermediate container A via a pipeline, and pressurizes the CO2 to the critical state by pushing the piston upward. The exhaust (liquid) port at the upper end of the high-pressure piston intermediate container A is connected to the air (liquid) port at the upper end of the high-pressure piston intermediate container B via a pipeline. The constant speed and constant pressure pump is connected to the liquid inlet at the lower end of the high-pressure piston intermediate container B, providing stable power for the liquefaction of critical CO2. A process switcher is installed on the pipeline to realize the stable liquefaction of critical CO2 and the circulation injection of water. At the same time, an annular cold water tank is installed on the side wall of the high-pressure piston intermediate container B. The condenser is connected to the liquid inlet of the annular cold water tank via a pipeline to meet the temperature required for the liquefaction of critical CO2.

[0009] Furthermore, the backpressure device includes a backpressure intermediate container and a manual pressure regulating pump. The drain port at the upper end of the high-pressure piston intermediate container B is connected to the inlet end of the backpressure intermediate container, and the manual pressure regulating pump is connected to the backpressure intermediate container through a pipeline to provide stable pressure for it during the transport of liquid CO2, so that it remains in a liquefied state.

[0010] Furthermore, the automatic water injection device includes a constant-speed and constant-pressure pump, a process switcher, a pipeline heater, and a water flow meter. The constant-speed and constant-pressure pump is connected to the high-pressure piston intermediate container B and the water injection pipeline respectively through the process switcher. After the critical CO2 pressurization and liquefaction process is completed, the process switcher can be used to switch to the water injection pipeline to start the water injection process. The inlet end of the water injection pipeline at the lower end of the process switcher is connected to the liquid inlet of the pipeline heater, and the outlet end of the pipeline heater is connected to the outlet end of the water injection pipeline. A water flow meter is installed on it to display the water flow. The pipeline heater can quickly warm up the common pipeline section after the liquid CO2 cracking is completed, preventing the pipeline from freezing during the water injection process.

[0011] Furthermore, the vacuum drainage device includes a vacuum pump and a cryogenic high-pressure viewing window. The vacuum pump and the cryogenic high-pressure viewing window are installed on the shared pipeline section for both liquid CO2 and water injection, and are connected to the inlet end of the cryogenic high-pressure viewing window via a pipeline. After either the liquid CO2 or water injection process is completed, the residual liquid (water or liquid CO2) in the shared pipeline section is extracted, preventing liquid CO2 and water from contacting and freezing the pipeline, thus affecting the current injection process. The cryogenic high-pressure viewing window allows observation of the phase state during liquid CO2 injection and the removal of residual liquid during the vacuum drainage process.

[0012] Furthermore, on the pipeline between the CO2 cylinder and the high-pressure piston intermediate container A, a shut-off valve 1 is installed to the left of the CO2-specific flow meter; a pressure gauge 1 and a pressure relief valve 1 are installed on the high-pressure piston intermediate container A; a shut-off valve 2 and a pressure sensor 1 are installed on the pipeline between the high-pressure piston intermediate container A and the booster pump, with the pressure sensor 1 located to the right of the shut-off valve 2; a shut-off valve 3 is installed on the pipeline between the high-pressure piston intermediate container A and the high-pressure piston intermediate container B; a pressure gauge 2 and a pressure relief valve 2 are installed on the high-pressure piston intermediate container B; the process switcher is equipped with a booster valve and a water injection valve; a pressure sensor 2 is installed on the pipeline between the constant-speed constant-pressure pump and the high-pressure piston intermediate container B, located to the right of the booster valve. On the pressure valve side; a temperature sensor 1 is installed on the pipeline between the condenser and the annular cold water tank; a shut-off valve 4 and a pressure gauge 3 are installed on the pipeline between the high-pressure piston intermediate container B and the return pressure intermediate container, with the pressure gauge 3 located to the right of the shut-off valve 4; a pressure gauge 4 is installed on the pipeline between the return pressure intermediate container and the manual pressure regulating pump, and a shut-off valve 5 is installed on the liquid inlet pipeline of the manual pressure regulating pump; a temperature sensor 2 is installed on the pipeline heater; a shut-off valve 6 is installed on the pipeline between the pipeline heater and the outlet end of the water injection pipeline, located to the left of the water flow meter; on the branch pipeline between the vacuum pump and the liquid CO2 / water injection shared pipeline section, shut-off valves 7 and 8 are respectively installed at the inlet and outlet ends of the low-temperature high-pressure viewing window. In addition, all pipelines involved in the entire liquid CO2 flow process are low-temperature pipelines, and the outside of the pipelines is wrapped with heat insulation material.

[0013] The axial pressure-confining pressure stabilizing loading module includes a triaxial core clamping device, an axial pressure-confining pressure tracking loading device, and a heating and temperature control device;

[0014] Furthermore, the triaxial core clamping device includes a sample chamber, an axial pressure-confining pressure loading chamber, a left plug, a right plug, and multiple stress transmission rods, resistance transmission rods, and crack detection transmission rods. The sample chamber is located within the axial pressure-confining pressure loading chamber, and the coal core sample is placed inside the sample chamber with drilled holes at its liquid injection test end. The left and right plugs are located at the two ports of the triaxial core clamping device, respectively, to seal the ports on both sides and ensure the airtightness of the entire device. In addition, multiple stress transmission rods, resistance transmission rods, and crack detection transmission rods are arranged in a ring around the outer wall of the sample chamber, and their detection ends are all embedded in the sample chamber and in close contact with the surface of the coal core sample.

[0015] Furthermore, the axial pressure-confining pressure tracking loading device includes an axial pressure-confining pressure tracking pump. The axial pressure loading pipeline of the axial pressure-confining pressure tracking pump is connected to the axial pressure loading end of the axial pressure-confining pressure loading system, while the confining pressure loading pipeline is connected to the annular wall of the axial pressure-confining pressure loading chamber, providing power for the operation of the axial pressure-confining pressure loading chamber to simulate the stress conditions exerted on the coal core sample by the actual downhole formation.

[0016] Furthermore, the heating and temperature control device includes a heating kit. The entire triaxial core clamping device is encased in the heating kit to simulate the temperature of actual downhole formations.

[0017] Furthermore, a pressure sensor 3 is installed between the injection pipe and the left end cap of the triaxial core clamping device, with the pressure sensor 3 located near the left end cap. A shut-off valve 9 and a pressure sensor 4 are installed on the branch pipe between the axial pressure-confining pressure tracking pump and the axial pressure loading end of the axial pressure-confining pressure loading chamber, and a shut-off valve 10 and a pressure sensor 5 are installed on the branch pipe between the pump and the annular wall, with pressure sensors 4 and 5 located near the axial pressure-confining pressure loading chamber. Additionally, a pressure sensor 6 is installed on the pipe between the right end cap and the drainage outlet; a temperature sensor 3 is installed on the heating kit.

[0018] Furthermore, the crack detection transmission rods consist of eight rods, divided into two groups, arranged in a ring around the coal core sample at heights of 25mm and 75mm, with adjacent rods in the same group at a 90° angle. There are two stress transmission rods, arranged symmetrically at a height of 50mm on the coal core sample. Similarly, there are four resistance transmission rods, also arranged symmetrically at heights of 33mm and 66mm on the coal core sample. The coal core sample is a standard cylindrical sample with a diameter of 50mm and a height of 100mm. The sample has a 3mm diameter and 50mm deep borehole to simulate the injection of liquid CO2-water circulation for fracturing and lubrication of the coal body. The injection port of the injection pipe is 10mm from the bottom of the borehole. A 30mm gap is formed between the injection pipe and the inner wall of the borehole using strong adhesive. The end of the injection pipe is equipped with a quick connector for rapid connection to the fracturing pipeline.

[0019] The multi-functional coal and rock dynamic monitoring module includes a multi-dimensional signal processing computer, a multi-channel dynamic signal acquisition instrument, multiple acoustic and electrical signal amplifiers, and multiple crack detection probes, resistance probes, and stress-strain probes. The multi-dimensional signal processing computer is connected to the multi-channel dynamic signal acquisition instrument, which is connected to the crack detection probes, stress-strain probes, and resistance probes respectively through acoustic and electrical signal amplifiers. Each probe is attached to the other end of its corresponding transmission rod.

[0020] The gas-liquid two-phase permeability and pH measurement module includes a gas-liquid separator, electronic balance A, electronic balance B, waste liquid (gas) collection container, pH meter, gas-water discharge meter, gas dryer, and wet flow meter. The gas-liquid separator is connected to the drain end of the triaxial clamping device. The liquid discharged from its drain pipe flows into the waste liquid collection container in electronic balance A. The detection probe of the pH meter is immersed in the liquid in the beaker to monitor the pH value change in real time. The exhaust pipe of the gas-liquid separator is equipped with a gas-water discharge meter and a gas dryer in sequence along the gas flow direction. The gas dryer discharges the collected liquid and the dried gas into electronic balance B respectively, and the drain pipe of the dryer is equipped with a wet flow meter.

[0021] Furthermore, the exhaust pipe and the liquid discharge pipe of the gas-liquid separator are respectively equipped with a shut-off valve 11 and a shut-off valve 12.

[0022] The data acquisition and control module includes a computer and an industrial control box. The industrial control box controls the operating conditions of the booster pump, constant speed and constant pressure pump, axial pressure-confining pressure tracking pump, vacuum pump, pipeline heater, condenser, and heating kit. The computer processes the data collected by each sensor and feeds it back to the industrial control box to adjust the operating conditions of each pump and temperature control device in a timely manner. By acquiring and processing data from pressure sensor 1 in real time, the operating parameters of the booster pump can be adjusted. By acquiring and processing data from pressure sensor 2 in real time, the operating parameters of the constant speed and constant pressure pump can be adjusted. By acquiring and processing data from temperature sensor 1 in real time, the operating parameters of the condenser can be adjusted. By acquiring and processing data from temperature sensor 2 in real time, the operating parameters of the pipeline heater can be adjusted. By acquiring and processing data from pressure sensor 3 in real time, the injection pressure of the test system can be determined. Simultaneously, by acquiring and processing data from pressure sensors 4 and 5 in real time, the operating conditions of the axial pressure-confining pressure tracking pump on the axial pressure and confining pressure of the axial pressure-confining pressure loading chamber can be continuously adjusted. Finally, by acquiring and processing data from pressure sensor 6 in real time, the discharge pressure of the test system can be determined.

[0023] A test method for low-temperature fluid circulation-induced fracturing and lubrication of coal body specifically includes the following steps:

[0024] a. The coal collected on-site was processed into multiple standard cylindrical samples with a diameter of 50 mm and a height of 100 mm. A 3 mm diameter and 50 mm deep hole was drilled at the middle of the top of each sample. Before the experiment, the physicochemical properties of the coal samples were analyzed, including natural moisture content determination, computed tomography (CT), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR). The initial moisture content, porosity, volume ratio of different pore size ranges (adsorption pores, <0.1 μm, seepage pores, 0.1–100 μm, and fractures, >100 μm), and the types and contents of functional groups on the coal surface were obtained before the experiment.

[0025] b. Open the left end cap of the triaxial core clamping device and remove the axial pressure-confining pressure loading chamber. Then, select a coal sample from those that have undergone physicochemical property testing and bury the injection tube 40 mm into the pre-drilled hole in the coal sample, ensuring the injection port is 10 mm from the bottom of the hole. Simultaneously, seal the hole between the inner wall of the sample borehole and the injection tube using strong anchoring adhesive, with a sealing thickness of 30 mm. After completing the above steps, place the sample into the sample chamber within the axial pressure-confining pressure loading chamber. Then, re-fix the axial pressure-confining pressure loading chamber into the triaxial core clamping device and tighten the left end cap. Finally, connect the quick connectors at the axial pressure loading end and the annular wall of the axial pressure-confining pressure loading chamber to the two branch lines of the axial pressure-confining pressure tracking pump. Then, open the shut-off valve 9, and the axial pressure-confining pressure tracking pump controls the axial pressure-confining pressure loading mechanism to load the axial pressure onto the sample to the test preset value. After that, close the shut-off valve 9 and open the shut-off valve 10, and the axial pressure-confining pressure tracking pump controls the axial pressure-confining pressure loading mechanism to load the confining pressure onto the sample to the test preset value.

[0026] c. Before using the multifunctional coal and rock dynamic monitoring module, two crack detection probes need to be randomly selected and fixed to both ends of a cylindrical sample using medical ultrasonic coupling agent. A lead-breaking test is then conducted to calibrate the sound velocity of the sample. Additionally, calibration experiments are needed for the stress-strain probe and resistance probe to determine their calibration coefficients. After calibration, the eight crack detection probes in the multifunctional coal and rock dynamic monitoring module are divided into two groups, arranged in a ring around the sample at heights of 25mm and 75mm, with adjacent probes in the same group at a 90° angle. The four resistance probes are also divided into two groups, arranged symmetrically at heights of 33mm and 66mm, respectively. The two stress probes are arranged symmetrically at the 50mm midpoint of the sample. Finally, medical ultrasonic coupling agent is used to connect the eight crack detection probes, four resistance probes, and two stress-strain probes to the eight crack detection transmission rods, four resistance transmission rods, and two stress transmission rods, respectively.

[0027] d. After completing the above steps, before officially starting the injection of liquid CO2 to fracture the coal body, it is also necessary to determine the initial permeability of the sample. Open the shut-off valves 1, 2, 3, 4, 8, 11 and 12, and start the booster pump to control the gas in the CO2 cylinder to flow through the liquid CO2 injection pipeline and enter the borehole opened in step b for gas displacement. At the same time, the data acquisition and control module collects the gas flow rate, inlet pressure and outlet pressure monitored by the CO2 dedicated flow meter, pressure sensor 3 and pressure sensor 6, and processes the data to obtain the initial permeability of the coal core sample. The specific formula is shown in equation (1):

[0028]

[0029] In the formula: k is the permeability of the coal core sample, md; q is the gas flow rate, m 3 / s; μ is the gas dynamic viscosity coefficient at the current temperature, MPa·s; l is the length of the coal core sample, m; p0 is the standard atmospheric pressure, taken as 0.1 MPa; A is the cross-sectional area of ​​the coal core sample, m². 2 p1 is the intake pressure, MPa; p2 is the exhaust pressure, MPa.

[0030] e. After the preliminary preparations are completed, the liquid CO2-water circulation fracturing and lubricating coal body test officially begins. First, liquid CO2 is prepared: Shut-off valves 1 and 2 are opened, allowing gas from the CO2 cylinder to flow into the high-pressure piston intermediate container A. Then, the booster pump is started to push the piston upwards to increase pressure. Pressure boosting is stopped when the pointer on pressure gauge 1 is at 5-6 MPa, and then shut-off valve 1 is closed. Next, shut-off valve 3 is opened, allowing critical CO2 to flow into the high-pressure piston intermediate container B. Then, shut-off valves 2 and 3 are closed, the constant-speed, constant-pressure pump is started, the booster valve of the process switcher is opened, and the condenser is started simultaneously. The pressurization and cooling process is stopped when the pointer on pressure gauge 2 exceeds 5-6 MPa and the temperature collected by temperature sensor 1 is below 0℃. After the liquid CO2 preparation is complete, shut-off valves 4 and 5 are opened. During the liquid CO2 transport process, a manual pressure regulating pump can be operated to maintain the liquid pressure in the backpressure intermediate container, and its phase state can be observed in real time through a low-temperature, high-pressure viewing window. Next, open shut-off valve 8 to inject liquid CO2 into the fracturing sample. Simultaneously, start the multi-functional coal and rock dynamic monitoring module to observe changes in the acoustic signal detected by the crack detection probe and the electrical signal detected by the stress probe. After completing the liquid CO2 injection, close shut-off valves 4, 5, and 8. After the liquid CO2 injection process is complete, open shut-off valve 7 and start the vacuum pump to extract and clean the residual liquid CO2 in the pipeline to avoid affecting the water injection process. After observing the residual liquid cleanup through the low-temperature high-pressure viewing window, close shut-off valve 7 and the vacuum pump. Then, open the water injection valve and shut-off valve 6 of the process switcher, start the pipeline heater to warm up the shared pipeline section for liquid CO2 / water injection, and prevent the low-temperature pipeline from freezing upon contact with water. When the water in the low-temperature high-pressure viewing window is observed to be flowing normally, close the pipeline heater. Open shut-off valve 8 to allow pressurized water to further fracture and wet the sample. Simultaneously observe the changes in acoustic signals detected by the crack detection probe, the stress changes detected by the stress-strain probe, and the sample resistivity changes fed back by the resistance probe in the multi-functional coal and rock dynamic monitoring module (monitoring the sample resistivity changes allows for the monitoring of the moisture content in the sample). After water injection, close shut-off valves 6 and 8. After the water injection process is complete, open shut-off valve 7 and start the vacuum pump to extract and clean the residual water in the pipeline, avoiding any impact on the next liquid CO2 injection process. After cleaning, repeat the preparation of liquid CO2 and the liquid CO2 fracturing of the sample as described in step e. After the liquid CO2 injection process is complete, the water injection process can be repeated, and this cycle can be repeated until the set number of test cycles is reached, at which point the test is stopped. At this point, close the liquid CO2-water circulation fracturing and wetting module and the multi-functional coal and rock dynamic monitoring module.

[0031] f. Permeability determination after sample cracking and wetting; First, open stop valve 7 and start the vacuum pump to ensure that the water remaining in the pipeline during the test is cleaned up. After the test, close stop valve 7 and vacuum pump. Next, open stop valves 1, 2, 3, 4, 8, 11, and 12. Then, start the booster pump to control the gas in the CO2 cylinder to enter the borehole opened in step b of the coal core sample from the liquid CO2 pipeline, displacing the water in the sample, and start the gas-liquid separator at the same time. After the displacement is completed, record the gas / liquid mass and volume collected by electronic balance A and electronic balance B. At the same time, process the data collected by the data acquisition and control module, including the gas flow rate, inlet pressure, and outlet pressure monitored by CO2 special flow meter, pressure sensor 3, and pressure sensor 6, and use the unsteady-state gas-water relative permeability determination method to obtain the gas-water two-phase relative permeability of the coal core sample after the test. The specific calculation formulas are shown in equations (2), (3), and (4):

[0032]

[0033]

[0034]

[0035] In the formula: k rw k rg The relative permeability of the aqueous and gas phases, md; f w (S g ) represents the moisture content of the coal core sample, in %; V wi (t) is the cumulative water production at time i, in mL; V gi (t) is the cumulative gas production at time i, in mL; I is the flow capacity ratio; μ g μ w V is the dynamic viscosity coefficient of gas and water at the current temperature; where V i The formulas for calculating (t) and I are shown in (5) and (6):

[0036]

[0037]

[0038] In the formula: V i (t) represents the cumulative water vapor production at time i, in mL; ΔV wi V is the increase in water volume from time i-1 to time i, in mL; i-1 (t) is the cumulative water vapor production at time i-1, mL; Δp is the displacement pressure difference, MPa; ΔV gi Q(t) is the gas increment at atmospheric pressure over a certain time period, expressed in mL; Q(t) is the liquid (gas) production flow rate at the outlet end face of the coal core sample at time t, expressed in m³. 3 / s;Q w Δp0 is the initial flow rate of permeate from the outlet end face of the coal core sample, in mL; Δp0 is the initial driving pressure difference, in MPa; Δp(t) is the displacement pressure difference at time t, in MPa.

[0039] g. After the liquid CO2-water circulating fracturing and moisturizing coal body test, shut-off valves 9 and 10 are opened sequentially to control the axial pressure-confining pressure tracking pump to unload the axial pressure and confining pressure of the coal core sample. Then, the left plug of the triaxial core clamping device is opened, the axial pressure-confining pressure loading chamber is removed, and the sample after circulating fracturing and moisturizing is taken out of the sample chamber and cleaned. Finally, the crack propagation data obtained by the multi-functional coal and rock dynamic monitoring module and the permeability and pH data measured by the gas-liquid two-phase permeability and pH measurement module during this circulating fracturing and moisturizing process are transmitted to the data acquisition and control module.

[0040] h. After the test, the samples were tested and analyzed again, including moisture content measurement, computed tomography (CT), X-ray diffraction (XRD) and infrared spectroscopy. The changes in moisture content, porosity, volume ratio of different pore size ranges and the types and contents of functional groups on the coal surface were compared before and after the test. This comparative data can be used to evaluate the effect of liquid CO2-water circulation cracking and lubrication of coal.

[0041] i. Select another coal core sample and reset the injection pressure of liquid CO2 and water, the number of liquid CO2-water circulation injections, and the axial pressure and confining pressure values ​​of the axial pressure-confining pressure loading mechanism. Then, repeat steps a to h, recording crack propagation data, permeability measurements before and after the test, and the pH value of the liquid in the waste liquid collection container at the current set injection pressure, axial pressure, and confining pressure values ​​and the number of cycles. Repeat this process multiple times to obtain crack propagation data, permeability measurements before and after the test, and the pH value of the liquid in the waste liquid collection container for different injection pressures, axial pressures, confining pressures, and number of cycles. This provides a data analysis sample for determining the optimal implementation parameters when subsequently using liquid CO2-water circulation fracturing to wet coal seams.

[0042] The features and advantages of this invention are:

[0043] A low-temperature fluid circulation fracturing and lubricating coal body test system and method are proposed, which couples the fracturing and rock breaking effect of liquid CO2 with the lubricating and dust-reducing effect of water. It has the characteristics of high experimental efficiency, strong scalability, and applicability to various stress environments and coal and rock samples. Compared with existing technologies, this experimental system simulates the mechanical response and wetting modification of coal and rock samples under deep in-situ stress conditions subjected to alternating cycles of liquid CO2 fracturing and water wetting through a liquid CO2-water circulation fracturing and wetting enhancement module and an axial pressure-confining pressure stabilization loading module. Combined with a multi-functional coal and rock dynamic monitoring module, it can study crack propagation and dynamic wetting patterns during the fracturing and wetting process. Furthermore, by integrating gas-liquid two-phase permeability and pH measurement modules, it can be used to evaluate changes in sample permeability and acidification / dissolution before and after liquid CO2-water circulation fracturing and wetting enhancement. Therefore, this invention can simulate the liquid CO2-water circulation fracturing and wetting process in deep coal seams, obtain experimental data under different conditions, optimize the number of liquid CO2 and water circulation injections, and study the influence of different injection parameters on the liquid CO2-water circulation fracturing and wetting effect. This provides data analysis samples for determining the optimal implementation parameters when using liquid CO2-water circulation fracturing and wetting enhancement for coal seam dust reduction in subsequent practical applications. Attached Figure Description

[0044] Figure 1 This is a schematic diagram of the overall structure of the device for implementing the present invention;

[0045] Figure 2 This is a schematic diagram of the liquid CO2-water circulation crack-inducing and lubricating module in this invention;

[0046] Figure 3 This is a schematic diagram of the CO2 booster liquefaction device in this invention;

[0047] Figure 4 This is a schematic diagram of the axial compression-confining pressure stabilizing loading module in this invention;

[0048] Figure 5 This is a schematic diagram showing the layout of the crack detection transmission rod, the strain measuring instrument stress-strain probe, and the resistance probe in this invention.

[0049] Figure 6 yes Figure 5 Top view;

[0050] Figure 7 This is a schematic diagram of the structure of a multi-functional coal and rock dynamic monitoring module;

[0051] Figure 8 This is a schematic diagram of the gas-liquid two-phase permeability and pH measurement module;

[0052] In the diagram: 1. Data acquisition and control module, 1-1. Computer, 1-2. Industrial control box; 2. CO2 booster liquefaction device, 2-1. CO2 cylinder, 2-2. Shut-off valve 1, 2-3. CO2 dedicated flow meter, 2-4. High-pressure piston intermediate container A, 2-5. Pressure gauge 1, 2-6. Pressure relief valve 1, 2-7. Booster pump, 2-8. Shut-off valve 2, 2-9. Pressure sensor 1, 2-10. Shut-off valve 3, 2-11. High-pressure piston intermediate container B, 2-12. Pressure gauge 2, 2-13. Pressure relief valve 2, 2-14. Constant speed and constant pressure pump, 2-15. Process switcher ((1) Booster valve, (2) Water injection valve), 2-16. Pressure sensor 2, 2-17. 1. Annular cold water tank, 2-18; Temperature sensor 1, 2-19; ​​Condenser, 2-20; Shut-off valve 4; 3. Back pressure device, 3-1; Pressure gauge 3, 3-2; Back pressure intermediate container, 3-3; Pressure gauge 4, 3-4; Shut-off valve 5, 3-5; Manual pressure regulating pump; 4. Automatic water injection device, 4-1; Pipeline heater, 4-2; Temperature sensor 2, 4-3; Shut-off valve 6, 4-4; Water flow meter; 5. Vacuum drainage device, 5-1; Vacuum pump, 5-2; Shut-off valve 7, 5-3; Low temperature and high pressure viewing window, 5-4; Shut-off valve 8; 6. Triaxial core clamping device, 6-1; Quick connector for injection pipe, 6-2; Pressure sensor 3, 6-3; Left side plug, 6-4. Annular wall, 6-5, Axial pressure-confining pressure loading chamber, 6-6, Axial pressure loading end, 6-7, Sample chamber, 6-8, Stress transmission rod, 6-9, Resistance transmission rod, 6-10, Crack detection transmission rod, 6-11, Injection port, 6-12, Right side plug, 6-13, Pressure sensor 6, 6-14, Drain outlet; 7, Coal core sample, 7-1, Drill hole, 7-2, Strong rebar adhesive sealing section; 8, Axial pressure-confining pressure tracking loading device, 8-1, Axial pressure-confining pressure tracking pump, 8-2, Shut-off valve 9, 8-3, Shut-off valve 10, 8-4, Pressure sensor 4, 8-5, Pressure sensor 5; 9, Heating and temperature control device, 9-1, Heating kit, 9-2, Temperature sensor 3; 1 0. Multifunctional coal and rock dynamic monitoring module; 10-1. Multidimensional signal processing computer; 10-2. Multi-channel dynamic signal acquisition instrument; 10-3. Acoustic signal amplifier; 10-4. Electrical signal amplifier; 10-5. Crack detection probe; 10-6. Pressure probe; 10-7. Resistance probe; 11. Gas-liquid two-phase permeability and pH measurement module; 11-1. Gas-liquid separator; 11-2. Gas-water discharge meter; 11-3. Shut-off valve 11; 11-4. Gas dryer; 11-5. Wet flow meter; 11-6. Electronic balance B; 11-7. Shut-off valve 12; 11-8. Electronic balance A; 11-9. pH meter; 11-10. Waste liquid (gas) collection container. Detailed Implementation

[0053] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0054] like Figure 1 As shown, a low-temperature fluid circulation fracturing and lubricating coal body test system includes a data acquisition and control module (1), a liquid CO2-water circulation fracturing and lubricating module (CO2 pressurization and liquefaction device (2), back pressure device (3), automatic water injection device (4), vacuum drainage device (5)), an axial pressure-confining pressure stabilizing loading module (triaxial core clamping device (6), coal core sample (7), axial pressure-confining pressure tracking loading device (8), heating temperature control device (9)), a multi-functional coal and rock dynamic monitoring module (10), and a gas-liquid two-phase permeability and pH measurement module (11).

[0055] like Figures 2 to 3As shown, the liquid CO2-water circulation cracking and lubrication module includes a CO2 pressurization and liquefaction device (2), a back pressure device (3), an automatic water injection device (4), and a vacuum drainage device (5). In the CO2 pressurization and liquefaction device (2), the CO2 cylinder (2-1) is connected to the air inlet at the upper end of the high-pressure piston container A (2-4) through a pipeline, and a CO2-specific flow meter (2-3) is installed on the pipeline to display the gas flow status in real time; the booster pump (2-7) is connected to the liquid inlet at the lower end of the high-pressure piston intermediate container A (2-4) through a pipeline, and pressurizes the CO2 to the critical state by pushing the piston upward; the exhaust (liquid) port at the upper end of the high-pressure piston intermediate container A (2-4) is connected to the air (liquid) port at the upper end of the high-pressure piston intermediate container B (2-11) through a pipeline, and the constant speed and constant pressure pump (2- 14) Connected to the liquid inlet at the lower end of the intermediate container B (2-11) of the high-pressure piston, it provides stable power for the liquefaction of critical CO2. The pipeline is equipped with a process switcher (2-15). By switching the pressure boosting valve (2-15(1)) and the water injection valve (2-15(2)) on the process switcher (2-15) back and forth, the stable liquefaction of critical CO2 and the circulation injection of water can be achieved. At the same time, the side wall of the intermediate container B (2-11) of the high-pressure piston is equipped with an annular cold water tank (2-17). The condenser (2-19) is connected to the liquid inlet of the annular cold water tank through a pipeline to meet the temperature required for the liquefaction of critical CO2. In the back pressure device (3), the drain port at the upper end of the high pressure piston intermediate container B (2-11) is connected to the inlet end of the back pressure intermediate container (3-2), and the manual pressure regulating pump (3-5) is connected to the back pressure intermediate container (3-2) through a pipeline to provide stable pressure for it during the transport of liquid CO2, so that it remains in a liquefied state. In the automatic water injection device (4), after the critical CO2 pressurization and liquefaction process is completed, the process switcher (2-15) is switched from the pressurization valve (2-15(1)) to the water injection valve (2-15(2)) to start the water injection process. The water injection pipeline inlet end of the lower end of the process switcher (2-15) is connected to the liquid inlet of the pipeline heater (4-1), and the liquid outlet of the pipeline heater (4-1) is connected to the outlet end of the water injection pipeline. A water flow meter (4-4) is installed on it to display the water flow. The pipeline heater (4-1) can make the common pipeline section after the liquid CO2 cracking is completed quickly warm up from the low temperature, and avoid the pipeline freezing during the water injection process. In the vacuum drainage device (5), the vacuum pump (5-1) and the low-temperature high-pressure viewing window (5-3) are installed on the common pipeline section for liquid CO2 injection and water injection. They are connected to the inlet end of the low-temperature high-pressure viewing window (5-3) through the pipeline. After either liquid CO2 injection or water injection is completed, the residual liquid (water or liquid CO2) in the common pipeline section is extracted to prevent the liquid CO2 and water from freezing and freezing the pipeline, which would affect the current injection process. The low-temperature high-pressure viewing window (5-3) can observe the phase state of liquid CO2 injection and the removal of residual liquid during the vacuum drainage process.

[0056] On the pipeline between the CO2 cylinder (2-1) and the high-pressure piston intermediate container A (2-4), a shut-off valve 1 (2-2) is installed to the left of the CO2 dedicated flow meter (2-3); a pressure gauge 1 (2-5) and a pressure relief valve 1 (2-6) are installed on the high-pressure piston intermediate container A (2-4), and a shut-off valve 2 (2-8) and a pressure sensor 1 (2-9) are installed on the pipeline between it and the booster pump (2-7); a shut-off valve 3 (2-10) is installed on the pipeline between the high-pressure piston intermediate container A (2-4) and the high-pressure piston intermediate container B (2-11); a pressure gauge 2 (2-12) and a pressure relief valve 2 (2-13) are installed on the high-pressure piston intermediate container B (2-11); a pressure sensor 2 (2-16) is installed on the pipeline between the constant speed and constant pressure pump (2-14) and the high-pressure piston intermediate container B (2-11), located on the booster valve side of the process switcher; A temperature sensor 1 (2-18) is installed on the pipeline between the condenser (2-19) and the annular cold water tank (2-17); a shut-off valve 4 (2-20) and a pressure gauge 3 (3-1) are installed on the pipeline between the high-pressure piston intermediate container B (2-11) and the return pressure intermediate container (3-2); a pressure gauge 4 (3-3) is installed on the pipeline between the return pressure intermediate container (3-2) and the manual pressure regulating pump (3-5), and a shut-off valve 5 (3-4) is installed on the liquid inlet pipeline of the manual pressure regulating pump (3-5); a temperature sensor 2 (4-2) is installed on the pipeline heater (4-1); a shut-off valve 6 (4-3) is installed at the outlet end of the water injection pipeline; and shut-off valves 7 (5-2) and 8 (5-4) are installed at the inlet and outlet ends of the low-temperature high-pressure viewing window (5-3) on the branch pipeline between the vacuum pump (5-1) and the liquid injection / water injection shared pipeline section. In addition, all pipelines involved in the entire liquid CO2 flow process are cryogenic pipelines, and the outside of the pipelines are wrapped with heat insulation material.

[0057] like Figures 4 to 6As shown, the axial pressure-confining pressure stabilization loading module includes a triaxial core clamping device (6), an axial pressure-confining pressure tracking loading device (8), and a heating and temperature control device (9). In the triaxial core clamping device (6), the sample chamber (6-7) is set inside the axial pressure-confining pressure loading chamber (6-5), and the coal core sample (7) is placed inside the sample chamber (6-7). The coal core sample (7) is a standard cylindrical sample with a diameter of 50 mm and a height of 100 mm. The sample injection test end has a drill hole (7-1) with a diameter of 3 mm and a depth of 50 mm, which is used to simulate the cracking and lubrication of the coal body by liquid injection CO2-water circulation. The injection port (6-11) of the injection tube is 10mm from the bottom of the borehole. A strong anchoring adhesive (7-2) with a thickness of 30mm is used to seal the injection tube and the inner wall of the borehole. The end of the injection tube is equipped with a quick connector (6-1) for rapid connection to the fracturing pipeline. Left and right plugs (6-3 and 6-12) are located at the left and right ports of the triaxial core clamping device (6), respectively, to seal both ports and ensure the airtightness of the entire device. In addition, multiple stress transmission rods (6-8), resistance transmission rods (6-9), and crack detection transmission rods (6-10) are arranged in a ring around the outer wall of the sample chamber. Their detection ends are all embedded in the sample chamber (6-7) and in close contact with the surface of the coal core sample (7). There are two stress transmission rods (6-8), one on top and one on the bottom. The lower symmetrical arrangement is placed at the middle 50mm position of the coal core sample (7) to measure the deformation of the coal core sample when subjected to axial pressure-confining pressure loading, and to collect the stress-strain curve of the sample during the loading process; there are 4 resistance conduction rods (6-9), which are symmetrically installed at the coal core sample (7) at the height of 33mm and 66mm, to measure the change in moisture content of the coal core sample before and after the test, so as to reflect the wetting effect of the sample from the side; there are 8 crack detection conduction rods (6-10), which are evenly divided into two groups and arranged in a ring around the coal core sample (7) at the height of 25mm and 75mm, respectively, and the angle between adjacent rods in the same group is 90°, to monitor the crack expansion of the sample during the cracking process of liquid CO2, so as to reflect the cracking effect of liquid CO2 on the coal body. In the axial pressure-confining pressure tracking loading device (8), the axial pressure loading pipeline of the axial pressure-confining pressure tracking pump (8-1) is connected to the axial pressure loading end (6-6) of the axial pressure-confining pressure loading chamber (6-5), while the confining pressure loading pipeline is connected to the annular wall (6-4) of the axial pressure-confining pressure loading chamber (6-5), providing power for the operation of the axial pressure-confining pressure loading chamber to simulate the stress applied to the coal core sample by the actual underground formation. In the heating and temperature control device (9), the heating kit (9-1) wraps the entire triaxial core clamping device (6) to simulate the temperature of the actual underground formation.

[0058] A pressure sensor 3 (6-2) is installed between the quick connector of the injection pipe (6-1) and the left end cap (6-3) of the triaxial core clamping device. A shut-off valve 9 (8-2) and a pressure sensor 4 (8-4) are installed on the branch pipe between the axial pressure-confining pressure tracking pump (8-1) and the axial pressure loading end (6-6) of the axial pressure-confining pressure loading chamber. A shut-off valve 10 (6-3) and a pressure sensor 5 (8-5) are installed on the branch pipe between the pump and the annular wall (6-4). Pressure sensors 4 and 5 are both located near the axial pressure-confining pressure loading chamber. In addition, a pressure sensor 6 (6-13) is installed on the pipe between the right end cap (6-12) and the drain outlet (6-14); a temperature sensor 3 (9-2) is installed on the heating kit (9-1).

[0059] like Figure 7 As shown, the multi-functional coal and rock dynamic monitoring module (10) includes a multi-dimensional signal processing computer (10-1), a multi-channel dynamic signal acquisition instrument (10-2), an acoustic signal amplifier (10-3), an electrical signal amplifier (10-4), eight crack detection probes (10-5), four resistance probes (10-6), and two stress-strain probes (10-7). The multi-dimensional signal processing computer (10-1) is connected to the multi-channel dynamic signal acquisition instrument (10-2), which is connected to the crack detection probes (10-5), stress-strain probes (10-7), and resistance probes (10-6) through the acoustic signal amplifier (10-3) and electrical signal amplifier (10-4), respectively. Each probe is attached to the other end of its corresponding transmission rod.

[0060] like Figure 8 As shown, the gas-liquid two-phase permeability and pH measurement module (11) includes a gas-liquid separator (11-1), electronic balance A (11-8), electronic balance B (11-6), waste liquid (gas) collection container (11-10), pH meter (11-9), gas-water discharge meter (11-2), gas dryer (11-4), and wet flow meter (11-5). The gas-liquid separator (11-1) is connected to the drain end of the triaxial clamping device (6). The liquid discharged from its drain pipe flows into the waste liquid collection container (11-10) in the electronic balance A (11-8). The detection probe of the pH meter (11-9) is immersed in the liquid in the beaker to monitor the pH value change in real time. A gas-liquid separator (11-1) is equipped with a gas-water flow meter (11-2) and a gas dryer (11-4) in sequence along the gas flow direction on the exhaust pipe. The gas dryer discharges the collected liquid and the dried gas into the electronic balance B (11-6) respectively. A wet flow meter (11-5) is installed on the drain pipe of the dryer (11-4). In addition, a shut-off valve 11 (11-3) and a shut-off valve 12 (11-7) are installed on the exhaust pipe and the drain pipe of the gas-liquid separator (11-1) respectively.

[0061] The data acquisition and control module (1) includes a computer (1-1) and an industrial control box (1-2). The industrial control box (1-2) is used to control the operating conditions of the booster pump (2-8), the constant speed and constant pressure pump (2-14), the axial pressure-confining pressure tracking pump (8-1), the vacuum pump (5-1), the pipeline heater (4-1), the condenser (2-19), and the heating kit (9-1). The computer (1-1) is used to process the data collected by each sensor and feed it back to the industrial control box (1-2) to adjust the operating conditions of each pump and temperature control device in a timely manner. By collecting and processing the data from pressure sensor 1 (2-9) in real time, the operating parameters of the booster pump (2-8) can be adjusted. By collecting and processing the data from pressure sensor 2 (2-16) in real time, the operating parameters of the constant speed and constant pressure pump (2-14) can be adjusted. Adjustments are made as follows: By real-time acquisition and processing of data from temperature sensor 1 (2-18), the operating parameters of the condenser (2-19) can be adjusted; by real-time acquisition and processing of data from temperature sensor 2 (4-2), the operating parameters of the pipe heater (4-1) can be adjusted; by real-time acquisition and processing of data from pressure sensor 3 (6-2), the injection pressure of the test system can be determined; simultaneously, by real-time acquisition and processing of data from pressure sensor 4 (8-4) and pressure sensor 5 (8-5), the operating conditions of the axial pressure-confining pressure tracking pump (8-1) on the axial pressure and confining pressure of the axial pressure-confining pressure loading chamber can be continuously adjusted; finally, by real-time acquisition and processing of data from pressure sensor 6 (6-13), the discharge pressure of the test system can be determined.

[0062] A test method for low-temperature fluid circulation-induced fracturing and lubrication of coal body specifically includes the following steps:

[0063] a. The coal collected on site was processed into multiple standard cylindrical samples with a diameter of 50 mm and a height of 100 mm (7), and a hole with a diameter of 3 mm and a depth of 50 mm was drilled at the middle position of the top of each sample (7-1). Before the experiment, the physicochemical properties of the coal samples were analyzed, including natural moisture content determination test, computed tomography (CT), X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) tests, to obtain the initial moisture content, porosity, volume ratio of different pore size ranges (adsorption pores, <0.1 μm, seepage pores, 0.1~100 μm and fractures, >100 μm) and the types and contents of functional groups on the coal surface.

[0064] b. Open the left end cap (6-3) of the triaxial core clamping device (6) and remove the axial pressure-confining pressure loading chamber (6-5). Then, select a coal sample from the coal samples that have completed the physicochemical property tests and bury the injection tube 40 mm into the pre-drilled hole (7-1) of the coal sample, so that the injection port (6-11) is 10 mm from the bottom of the hole. At the same time, use strong anchoring adhesive to seal the hole (7-2) between the inner wall of the sample hole and the injection tube, with a sealing thickness of 30 mm. After completing the above steps, put the sample into the sample chamber (6-7) of the axial pressure-confining pressure loading chamber, and then fix the axial pressure-confining pressure loading chamber (6-5) back into the triaxial core clamping device (6) and tighten the left end cap (6-3). Finally, connect the quick connectors at the axial pressure loading end (6-6) and the annular wall (6-4) of the axial pressure-confining pressure loading chamber to the two branch lines of the axial pressure-confining pressure tracking pump (8-1). Then open the shut-off valve 9 (8-2), and the axial pressure-confining pressure tracking pump (8-1) controls the axial pressure-confining pressure loading mechanism to load the axial pressure on the sample (7) to the test preset value. After that, close the shut-off valve 9 (8-2) and open the shut-off valve 10 (8-3), and the axial pressure-confining pressure tracking pump (8-1) controls the axial pressure-confining pressure loading mechanism to load the confining pressure on the sample (7) to the test preset value.

[0065] c. Before using the multifunctional coal and rock dynamic monitoring module (10), two crack detection probes (10-5) need to be randomly selected and fixed to both ends of the cylindrical sample with medical ultrasonic coupling agent. Then, a lead-breaking test is carried out to calibrate the sound velocity of the sample. In addition, a calibration test is also required for the stress-strain probe and the resistance probe to determine the calibration coefficient of the probe. After calibration and calibration, the eight crack detection probes (10-5) in the multifunctional coal and rock dynamic monitoring module (10) are divided into two groups and arranged in a ring around the sample (7) at heights of 25 mm and 75 mm, respectively, with the angle between adjacent rods in the same group being 90°. The four resistance probes (10-6) are divided into two groups and arranged symmetrically above and below the sample (7) at heights of 33 mm and 66 mm, respectively. The two stress-strain probes (10-7) are arranged symmetrically above and below the sample (7) at a position of 50 mm in the middle. Finally, medical ultrasound coupling agent was used to connect 8 crack detection probes (10-5), 4 resistance probes (10-6), and 2 stress-strain probes (10-7) to 8 crack detection transmission rods (6-10), 4 resistance transmission rods (6-9), and 2 stress transmission rods (6-8), respectively.

[0066] d. After completing the above steps, before officially starting the injection of liquid CO2 to fracture the coal body, it is also necessary to determine the initial permeability of the sample (7). Open the shut-off valves 1 (2-2), 2 (2-8), 3 (2-10), 4 (2-20), 8 (5-4), 11 (11-3), and 12 (11-7), and control the gas in the CO2 cylinder (2-1) to flow through the liquid CO2 injection pipeline by starting the booster pump (2-7), and enter the borehole (7-1) opened in the coal core sample (7) in step b, to displace the gas. At the same time, process the data of gas flow, inlet pressure, and outlet pressure collected by the data acquisition and control module (1) from the CO2 dedicated flow meter (2-3), pressure sensor 3 (6-2), and pressure sensor 6 (6-13) to obtain the initial permeability of the coal core sample. The specific formula is shown in equation (1):

[0067]

[0068] In the formula: k is the permeability of the coal core sample, md; q is the gas flow rate, m 3 / s; μ is the gas dynamic viscosity coefficient at the current temperature, MPa·s; l is the length of the coal core sample, m; p0 is the standard atmospheric pressure, taken as 0.1 MPa; A is the cross-sectional area of ​​the coal core sample, m². 2 p1 is the intake pressure, MPa; p2 is the exhaust pressure, MPa.

[0069] e. After the preliminary preparations are completed, the liquid CO2-water circulation fracturing and lubrication test of the coal body officially begins. The first step is the preparation of liquid CO2: open shut-off valves 1 (2-2) and 2 (2-8) to allow gas from the CO2 cylinder (2-1) to flow into the high-pressure piston intermediate container A (2-4). Then, start the booster pump (2-7) to push the piston upwards to increase pressure. Stop pressurizing when the pointer on pressure gauge 1 (2-5) is at 5-6 MPa, and then close shut-off valve 1 (2-2). Next, open shut-off valve 3 (2-10) to allow critical CO2 to flow into the high-pressure piston intermediate container B (2-11). Then, close shut-off valves 2 (2-8) and 3 (2-10), start the constant-speed constant-pressure pump (2-14), open the booster valve (2-15(1)) of the process switcher (2-15), and simultaneously start the condenser (2-19). Stop the pressurization and cooling process when the pointer of pressure gauge 2 (2-12) is at 5-6 MPa and the temperature collected by temperature sensor 1 (2-18) is below 0℃. After the liquid CO2 is prepared, open shut-off valves 4 (2-20) and 5 (3-4). During the liquid CO2 transportation process, the manual pressure regulating pump (3-5) can be operated to maintain the liquid pressure in the back pressure intermediate container (3-2), and its phase state can be observed in real time through the low-temperature high-pressure viewing window (5-3). Next, open shut-off valve 8 (5-4) to inject liquid CO2 into the fractured sample. Simultaneously, start the multi-functional coal and rock dynamic monitoring module (10) to observe the changes in acoustic signals monitored by the crack detection probe (10-5) and the changes in electrical signals monitored by the stress-strain probe (10-7). After completing the liquid CO2 injection, close shut-off valves 4 (2-20), 5 (3-4), and 8 (5-4). After the liquid CO2 injection process is completed, open shut-off valve 7 (5-2) and start the vacuum pump (5-1) to extract and clean the residual liquid CO2 in the pipeline to avoid affecting the water injection process. After the residual liquid is cleaned, observe through the low-temperature high-pressure viewing window (5-3) and close shut-off valve 7 (5-2) and vacuum pump (5-1). Next, open the water injection valve (2-15(2)) and shut-off valve 6 (4-3) of the process switcher (2-15), and start the pipeline heater (4-1) to allow the liquid CO2 / water injection shared pipeline section to warm up, preventing the low-temperature pipeline from freezing when it comes into contact with water. When the water in the low-temperature high-pressure viewing window (5-3) is observed to flow normally, close the pipeline heater (4-1). Open the shut-off valve 8 (5-4), and the pressurized water further cracks and wets the sample. At the same time, observe the changes in the acoustic signal monitored by the crack detection probe (10-5) in the multi-functional coal and rock dynamic monitoring module (10), the stress changes monitored by the stress-strain probe (10-7), and the changes in the resistivity of the sample fed back by the resistance probe (10-6) (by monitoring the changes in the resistivity of the sample, the moisture content in the sample can be monitored). After the water injection is completed, close the shut-off valve 6 (4-3) and shut-off valve 8 (5-4).After the water injection process is completed, open the shut-off valve 7 (5-2) and start the vacuum pump (5-1) to extract and clean the residual water in the pipeline to avoid affecting the next liquid CO2 injection process. After cleaning, repeat the preparation of liquid CO2 and the liquid CO2 fracturing sample preparation in step e. After the liquid CO2 injection process is completed, the water injection process can be repeated, and this cycle is repeated until the set number of test cycles is reached and the test is stopped. At this time, turn off the liquid CO2-water circulation fracturing and lubrication module (2, 3, 4, 5) and the multi-functional coal and rock dynamic monitoring module (10).

[0070] f. Permeability determination after sample cracking and wetting: First, open shut-off valve 7 (5-2) and start vacuum pump (5-1) to ensure that any water remaining in the pipeline during the test is cleaned up. After the test, close shut-off valve 7 (5-2) and vacuum pump (5-1). Next, open shut-off valves 1 (2-2), 2 (2-8), 3 (2-10), 4 (2-20), 8 (5-4), 11 (11-3), and 12 (11-7). Then, start booster pump (2-7) to control the gas in CO2 cylinder (2-1) to enter the borehole (7-1) of the coal core sample (7) in step b through the liquid CO2 pipeline, displacing the water in the sample, and simultaneously start gas-liquid separator (11-1). After the displacement was completed, the mass and volume of gas / liquid collected by electronic balance A (11-8) and electronic balance B (11-6) were recorded. At the same time, the gas flow rate, inlet pressure and outlet pressure collected by the data acquisition and control module (1) and monitored by CO2 dedicated flow meter (2-3), pressure sensor 3 (6-2) and pressure sensor 6 (6-13) were processed. The gas-water two-phase relative permeability of the coal core sample after the test was obtained by the unsteady-state gas-water relative permeability determination method. The specific calculation formulas are shown in equations (2), (3) and (4):

[0071]

[0072]

[0073]

[0074] In the formula: k rw k rg The relative permeability of the aqueous and gas phases, md; f w (S g ) represents the moisture content of the coal core sample, in %; V wi (t) is the cumulative water production at time i, in mL; V gi (t) is the cumulative gas production at time i, in mL; I is the flow capacity ratio; μ g μ wV is the dynamic viscosity coefficient of gas and water at the current temperature; where V i The formulas for calculating (t) and I are shown in (5) and (6):

[0075]

[0076]

[0077] In the formula: V i (t) represents the cumulative water vapor production at time i, in mL; ΔV wi V is the increase in water volume from time i-1 to time i, in mL; i-1 (t) is the cumulative water vapor production at time i-1, mL; Δp is the displacement pressure difference, MPa; ΔV gi Q(t) is the gas increment at atmospheric pressure over a certain time period, expressed in mL; Q(t) is the liquid (gas) production flow rate at the outlet end face of the coal core sample at time t, expressed in m³. 3 / s;Q w Δp0 is the initial flow rate of permeate from the outlet end face of the coal core sample, in mL; Δp0 is the initial driving pressure difference, in MPa; Δp(t) is the displacement pressure difference at time t, in MPa.

[0078] g. After the liquid CO2-water circulation fracturing and moisturizing coal body test is completed, the shut-off valves 9 (8-4) and 10 (8-5) are opened in sequence to control the axial pressure-confining pressure tracking pump (8-1) to unload the axial pressure and confining pressure on the coal core sample (7). Then, the left plug (6-3) of the triaxial core clamping device is opened, the axial pressure-confining pressure loading chamber (6-5) is taken out, and the sample (7) after circulation fracturing and moisturizing is taken out from the sample chamber (6-7) and cleaned. Finally, the crack propagation data obtained by the multi-functional coal rock dynamic monitoring module (10) and the permeability and pH data measured by the gas-liquid two-phase permeability and pH measurement module (11) during this circulation fracturing and moisturizing process are transmitted to the data acquisition and control module (1).

[0079] h. After the test, the sample (7) was tested and analyzed again by measuring the moisture content, computed tomography (CT), X-ray diffraction (XRD) and infrared spectroscopy. The changes in the moisture content, porosity, volume ratio of different pore size ranges and the types and contents of functional groups on the coal surface before and after the test were compared. This comparative data can be used to evaluate the effect of liquid CO2-water circulation cracking and lubrication of coal body.

[0080] i. Select another coal core sample and reset the injection pressure of liquid CO2 and water, the number of liquid CO2-water circulation injections, and the axial pressure and confining pressure values ​​of the axial pressure-confining pressure loading mechanism. Then, repeat steps a to h, recording crack propagation data, permeability measurements before and after the test, and the pH value of the liquid in the waste liquid collection container at the current set injection pressure, axial pressure, and confining pressure values ​​and the number of cycles. Repeat this process multiple times to obtain crack propagation data, permeability measurements before and after the test, and the pH value of the liquid in the waste liquid collection container for different injection pressures, axial pressures, confining pressures, and number of cycles. This provides a data analysis sample for determining the optimal implementation parameters when subsequently using liquid CO2-water circulation fracturing to wet coal seams.

[0081] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A low-temperature fluid circulation fracturing and lubrication-enhancing coal body test system, characterized in that, It includes a data acquisition and control module (1), a liquid CO2-water circulation fracturing and lubrication module: CO2 pressurization and liquefaction device (2), back pressure device (3), automatic water injection device (4), vacuum drainage device (5), axial pressure-confining pressure stabilization loading module: triaxial core clamping device (6), coal core sample (7), axial pressure-confining pressure tracking loading device (8), heating and temperature control device (9), a multi-functional coal and rock dynamic monitoring module (10), and a gas-liquid two-phase permeability and pH measurement module (11). The liquid CO2-water circulation crack-inducing and lubricating module includes a CO2 pressurization and liquefaction device (2), a back pressure device (3), an automatic water injection device (4), and a vacuum drainage device (5). The CO2 pressurization and liquefaction device (2) includes a CO2 cylinder (2-1), a CO2-specific flow meter (2-3), a high-pressure piston intermediate container A (2-4), a booster pump (2-7), a high-pressure piston intermediate container B (2-11), a constant-speed and constant-pressure pump (2-14), a process switcher (2-15), and a condenser (2-19). The CO2 cylinder (2-1) is connected to the upper inlet of the high-pressure piston container A (2-4) via a pipeline, and a CO2-specific flow meter (2-3) is installed on the pipeline to display the gas flow status in real time. The booster pump (2-7) is connected to the lower inlet of the high-pressure piston intermediate container A (2-4) via a pipeline, and pressurizes the CO2 to a critical state by pushing the piston upward. The high-pressure piston intermediate container A (2-14) is connected to the lower inlet of the high-pressure piston intermediate container A (2-19). 4) The upper exhaust port is connected to the upper air inlet of the high-pressure piston intermediate container B (2-11) through a pipeline. The constant speed and constant pressure pump (2-14) is connected to the lower liquid inlet of the high-pressure piston intermediate container B (2-11) to provide stable power for the liquefaction of critical CO2. The pipeline is equipped with a process switcher (2-15). By switching the pressure boosting valve (2-15(1)) and the water injection valve (2-15(2)) on the process switcher (2-15) back and forth, the stable liquefaction of critical CO2 and the circulation injection of water can be achieved. At the same time, the side wall of the high-pressure piston intermediate container B (2-11) is equipped with an annular cold water tank (2-17). The condenser (2-19) is connected to the liquid inlet of the annular cold water tank through a pipeline to meet the temperature required for the liquefaction of critical CO2. The back pressure device (3) includes a back pressure intermediate container (3-2) and a manual pressure regulating pump (3-5). The drain port at the upper end of the high pressure piston intermediate container B (2-11) is connected to the inlet end of the back pressure intermediate container (3-2). The manual pressure regulating pump (3-5) is connected to the back pressure intermediate container (3-2) through a pipeline to provide stable pressure for it during the transport of liquid CO2, so that it remains in a liquefied state. The automatic water injection device (4) includes a constant speed and constant pressure pump (2-14), a process switcher (2-15), a pipeline heater (4-1), and a water flow meter (4-4). The constant speed and constant pressure pump (2-14) is connected to the high-pressure piston intermediate container B (2-11) and the water injection pipeline through the process switcher (2-15). After the critical CO2 pressurization and liquefaction process is completed, the process switcher (2-15) is switched from the pressurization valve (2-15(1)) to the water injection valve. The water injection process begins at the gate (2-15 (2)). The inlet end of the water injection pipeline at the lower end of the flow switch (2-15) is connected to the inlet of the pipeline heater (4-1). The outlet of the pipeline heater (4-1) is connected to the outlet of the water injection pipeline. A water flow meter (4-4) is installed on it to display the water flow. The pipeline heater (4-1) can quickly warm up the common pipeline section after the liquid CO2 cracking is completed, and prevent the pipeline from freezing during the water injection process. The vacuum drainage device includes a vacuum pump (5-1) and a low-temperature, high-pressure viewing window (5-3). The vacuum pump (5-1) and the low-temperature, high-pressure viewing window (5-3) are installed on the shared pipeline section for liquid CO2 injection and water injection, and are connected to the inlet end of the low-temperature, high-pressure viewing window (5-3) through the pipeline. After either the liquid CO2 injection or water injection process is completed, the residual water or liquid CO2 in the shared pipeline section is extracted to prevent the liquid CO2 from freezing and freezing the pipeline after contact with water, which would affect the current injection process. The low-temperature, high-pressure viewing window (5-3) can observe the phase state of liquid CO2 during the injection process and the removal of residual liquid during the vacuum drainage process. On the pipeline between the CO2 cylinder (2-1) and the high-pressure piston intermediate container A (2-4), a shut-off valve 1 (2-2) is installed to the left of the CO2-specific flow meter (2-3); a pressure gauge 1 (2-5) and a pressure relief valve 1 (2-6) are installed on the high-pressure piston intermediate container A (2-4), and a shut-off valve 2 (2-8) and a pressure sensor 1 (2-9) are installed on the pipeline between it and the booster pump (2-7); the high-pressure piston intermediate container A (2-4) and the high-pressure piston intermediate container A (2-4) are connected to ... A shut-off valve 3 (2-10) is installed on the pipeline between the high-pressure piston intermediate container B (2-11); a pressure gauge 2 (2-12) and a pressure relief valve 2 (2-13) are installed on the high-pressure piston intermediate container B (2-11); a pressure sensor 2 (2-16) is installed on the pipeline between the constant speed and constant pressure pump (2-14) and the high-pressure piston intermediate container B (2-11), located on the booster valve side of the process switcher; the condenser (2-19) and the annular cold water tank (2-17) are connected. Temperature sensor 1 (2-18) is installed on the pipeline between them; shut-off valve 4 (2-20) and pressure gauge 3 (3-1) are installed on the pipeline between the high-pressure piston intermediate container B (2-11) and the back pressure intermediate container (3-2); pressure gauge 4 (3-3) is installed on the pipeline between the back pressure intermediate container (3-2) and the manual pressure regulating pump (3-5), and shut-off valve 5 (3-4) is installed on the liquid inlet pipeline of the manual pressure regulating pump (3-5); temperature sensor 2 (4-2) is installed on the pipeline heater (4-1); shut-off valve 6 (4-3) is installed at the outlet end of the water injection pipeline; shut-off valve 7 (5-2) and shut-off valve 8 (5-4) are installed at the inlet and outlet ends of the low-temperature high-pressure viewing window (5-3) on the branch pipeline between the vacuum pump (5-1) and the liquid CO2 / water injection shared pipeline section; in addition, all pipelines involved in the entire liquid CO2 flow process are low-temperature pipelines, and the outside of the pipelines are wrapped with heat insulation material; The axial pressure-confining pressure stabilization loading module includes a triaxial core clamping device (6), an axial pressure-confining pressure tracking loading device (8), and a heating and temperature control device (9); the triaxial core clamping device (6) includes a sample chamber (6-7), an axial pressure-confining pressure loading chamber (6-5), a left plug (6-3), a right plug (6-12), and multiple stress transmission rods (6-8), resistance transmission rods (6-9), and crack detection transmission rods (6-10); the sample chamber (6-7) is set inside the axial pressure-confining pressure loading chamber (6-5), and the coal core sample (7) is placed inside the sample chamber (6-7), with a drill hole (7-1) at its liquid injection test end; the left plug (6-3) and the right plug (6-12) are located at the left and right ports of the triaxial core clamping device (6) respectively, used to seal the ports on both sides to ensure the airtightness of the entire device; in addition, multiple stress transmission rods ( 6-8), resistance transmission rod (6-9) and crack detection transmission rod (6-10) are arranged in a ring around the outer wall of the sample chamber, and their detection ends are buried in the sample chamber (6-7) and in close contact with the surface of the coal core sample (7); the axial pressure-confining pressure tracking loading device (8) includes an axial pressure-confining pressure tracking pump (8-1); the axial pressure loading pipeline of the axial pressure-confining pressure tracking pump (8-1) is connected to the axial pressure loading end (6-6) of the axial pressure-confining pressure loading chamber (6-5), while the confining pressure loading pipeline is connected to the annular wall (6-4) of the axial pressure-confining pressure loading chamber (6-5) to provide power for the operation of the axial pressure-confining pressure loading chamber, so as to simulate the stress situation applied to the coal core sample by the actual underground formation; the heating temperature control device (9) includes a heating kit (9-1); the entire triaxial core clamping device (6) is wrapped with a heating kit to simulate the temperature of the actual underground formation; A pressure sensor 3 (6-2) is installed between the quick connector of the injection pipe (6-1) and the left plug (6-3) of the triaxial core clamping device. A shut-off valve 9 (8-2) and a pressure sensor 4 (8-4) are installed on the branch pipe between the axial pressure-confining pressure tracking pump (8-1) and the axial pressure loading end (6-6) of the axial pressure-confining pressure loading chamber. A shut-off valve 10 (8-3) and a pressure sensor 5 (8-5) are installed on the branch pipe between the pump and the annular wall (6-4). Pressure sensors 4 and 5 are both close to the side of the axial pressure-confining pressure loading chamber. In addition, a pressure sensor 6 (6-13) is installed on the pipe between the right plug (6-12) and the drain outlet (6-14). A temperature sensor 3 (9-2) is installed on the heating kit (9-1). The multi-functional coal and rock dynamic monitoring module (10) includes a multi-dimensional signal processing computer (10-1), a multi-channel dynamic signal acquisition instrument (10-2), an acoustic signal amplifier (10-3), an electrical signal amplifier (10-4), eight crack detection probes (10-5), four resistance probes (10-6), and two stress-strain probes (10-7). The multi-dimensional signal processing computer (10-1) is connected to the multi-channel dynamic signal acquisition instrument (10-2), and the multi-channel dynamic signal acquisition instrument is connected to the crack detection probes (10-5), stress-strain probes (10-7), and resistance probes (10-6) respectively through the acoustic signal amplifier (10-3) and the electrical signal amplifier (10-4). Each probe is attached to the other end of its corresponding transmission rod. The gas-liquid two-phase permeability and pH measurement module (11) includes a gas-liquid separator (11-1), electronic balance A (11-8), electronic balance B (11-6), waste liquid gas collection container (11-10), pH meter (11-9), gas-water discharge meter (11-2), gas dryer (11-4), and wet flow meter (11-5). The gas-liquid separator (11-1) is connected to the discharge end of the triaxial clamping device (6), and the liquid discharged from its discharge pipe flows into the waste liquid collection container (11-10) in the electronic balance A (11-8). The pH meter (11-9) The detection probe is immersed in the liquid in the beaker to monitor the pH value change in real time; the exhaust pipe of the gas-liquid separator (11-1) is equipped with a gas-liquid separator flow meter (11-2) and a gas dryer (11-4) in sequence along the gas flow direction. The gas dryer discharges the liquid collected in the gas and the dried gas into the electronic balance B (11-6) respectively, and a wet flow meter (11-5) is installed on the drain pipe of the dryer (11-4); in addition, the exhaust pipe and drain pipe of the gas-liquid separator (11-1) are equipped with a shut-off valve 11 (11-3) and a shut-off valve 12 (11-7) respectively. The data acquisition and control module (1) includes a computer (1-1) and an industrial control box (1-2). The industrial control box (1-2) is used to control the operating conditions of the booster pump (2-7), the constant speed and constant pressure pump (2-14), the axial pressure-confining pressure tracking pump (8-1), the vacuum pump (5-1), the pipeline heater (4-1), the condenser (2-19), and the heating kit (9-1). The computer (1-1) is used to process the data collected by each sensor and feed it back to the industrial control box (1-2) to adjust the operating conditions of each pump and temperature control device in a timely manner. By collecting and processing the data of pressure sensor 1 (2-9) in real time, the operating parameters of the booster pump (2-7) can be adjusted. By collecting and processing the data of pressure sensor 2 (2-16) in real time, the operating parameters of the constant speed and constant pressure pump (2-14) can be adjusted. Adjustments are made as follows: By collecting and processing data from temperature sensor 1 (2-18) in real time, the operating parameters of the condenser (2-19) can be adjusted; by collecting and processing data from temperature sensor 2 (4-2) in real time, the operating parameters of the pipe heater (4-1) can be adjusted; by collecting and processing data from pressure sensor 3 (6-2) in real time, the injection pressure of the test system can be determined; at the same time, by collecting and processing data from pressure sensor 4 (8-4) and pressure sensor 5 (8-5) in real time, the operating conditions of the axial pressure-confining pressure tracking pump (8-1) on the axial pressure and confining pressure of the axial pressure-confining pressure loading chamber can be continuously adjusted; finally, by collecting and processing data from pressure sensor 6 (6-13) in real time, the discharge pressure of the test system can be determined.

2. The low-temperature fluid circulation fracturing and lubricating coal body test system according to claim 1, characterized in that, The coal core sample is a standard cylindrical sample with a diameter of 50 mm and a height of 100 mm. The sample has a drill hole with a diameter of 3 mm and a depth of 50 mm, which is used to simulate the injection of liquid CO2-water circulation into the fracturing and lubricating coal body. The injection port of the injection pipe is 10 mm away from the bottom of the drill hole. The injection pipe is sealed with strong anchoring adhesive for 30 mm between itself and the inner wall of the drill hole. The end of the injection pipe is equipped with a quick connector for quick connection to the fracturing pipeline.

3. The low-temperature fluid circulation fracturing and lubricating coal body test system according to claim 1, characterized in that, The crack detection transmission rods consist of 8 rods, which are divided into two groups and arranged in a ring around the coal core sample at heights of 25 mm and 75 mm, respectively, with an angle of 90° between adjacent rods in the same group; there are 2 stress transmission rods, which are arranged symmetrically at heights of 50 mm on the coal core sample; and there are 4 resistance transmission rods, which are also arranged symmetrically at heights of 33 mm and 66 mm on the coal core sample.

4. A test method using the low-temperature fluid circulation fracturing and lubricating coal body test system according to any one of claims 1 to 3, characterized in that, The specific steps are as follows: a. The coal blocks retrieved from the site were processed into multiple standard cylindrical samples with a diameter of 50 mm and a height of 100 mm (7), and a hole with a diameter of 3 mm and a depth of 50 mm was drilled at the middle position of the top of each sample (7-1); before the experiment, the physicochemical properties of the coal samples were analyzed, including the natural moisture content determination test, computed tomography (CT), X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) analysis, to obtain the initial moisture content, porosity, volume ratio of different pore size ranges and the types and contents of functional groups on the coal surface before the experiment; b. Open the left plug (6-3) of the triaxial core clamping device (6), remove the axial pressure-confining pressure loading chamber (6-5), and then select a coal sample from the coal samples that have completed the physicochemical property tests. Embed the injection tube 40 mm into the pre-drilled hole (7-1) of the coal sample, so that the injection port (6-11) is 10 mm from the bottom of the hole. At the same time, use strong anchoring adhesive to seal the hole (7-2) between the inner wall of the sample hole and the injection tube, with a sealing thickness of 30 mm. After completing the above steps, place the sample into the sample chamber (6-7) of the axial pressure-confining pressure loading chamber, and then re-fix the axial pressure-confining pressure loading chamber (6-5) onto the triaxial core. Inside the clamping device (6), tighten the left plug (6-3); finally, connect the quick connectors at the axial pressure loading end (6-6) and the annular wall (6-4) of the axial pressure-confining pressure loading chamber to the two branch lines of the axial pressure-confining pressure tracking pump (8-1), then open the shut-off valve 9 (8-2), the axial pressure-confining pressure tracking pump (8-1) controls the axial pressure-confining pressure loading mechanism to load the axial pressure on the sample (7) to the test preset value, then close the shut-off valve 9 (8-2), and open the shut-off valve 10 (8-3), the axial pressure-confining pressure tracking pump (8-1) controls the axial pressure-confining pressure loading mechanism to load the confining pressure on the sample (7) to the test preset value; c. Before using the multifunctional coal and rock dynamic monitoring module (10), two crack detection probes (10-5) need to be randomly selected and fixed to both ends of the cylindrical sample with medical ultrasonic coupling agent. Then, a lead-breaking test is carried out to calibrate the sound velocity of the sample. In addition, a calibration test is required for the stress-strain probe and the resistance probe to determine the calibration coefficient of the probe. After calibration and calibration, the eight crack detection probes (10-5) in the multifunctional coal and rock dynamic monitoring module (10) are divided into two groups and arranged in a ring around the sample (7) at heights of 25 mm and 75 mm, respectively. The angle between the rods is 90°; the four resistance probes (10-6) are divided into two groups and arranged symmetrically at the heights of the sample (7) at 33 mm and 66 mm respectively; the two stress-strain probes (10-7) are arranged symmetrically at the middle of the sample (7) at 50 mm; finally, medical ultrasonic coupling agent is used to connect the eight crack detection probes (10-5), four resistance probes (10-6) and two stress-strain probes (10-7) to the eight crack detection transmission rods (6-10), four resistance transmission rods (6-9) and two stress transmission rods (6-8) respectively; d. After completing the above steps, before officially starting the injection of liquid CO2 to fracture the coal body, it is also necessary to determine the initial permeability of the sample (7); open the stop valve 1 (2-2), stop valve 2 (2-8), stop valve 3 (2-10), stop valve 4 (2-20), stop valve 8 (5-4), stop valve 11 (11-3) and stop valve 12 (11-7), and control the gas in the CO2 cylinder (2-1) to flow through the liquid CO2 injection pipeline by starting the booster pump (2-7), and enter the borehole (7-1) opened in the coal core sample (7) in step b, and displace it with gas. At the same time, process the data of gas flow, inlet pressure and outlet pressure collected by the data acquisition and control module (1) and monitored by the CO2 special flow meter (2-3), pressure sensor 3 (6-2) and pressure sensor 6 (6-13) to obtain the initial permeability of the coal core sample; e. After the preliminary preparations are completed, the liquid CO2-water circulation fracturing and lubricating coal body test officially begins. The first step is the preparation of liquid CO2: open shut-off valves 1 (2-2) and 2 (2-8) to allow gas from the CO2 cylinder (2-1) to flow into the high-pressure piston intermediate container A (2-4); then start the booster pump (2-7) to push the piston upwards to increase pressure. Stop the pressurization when the pointer on pressure gauge 1 (2-5) is at 5~6 MPa, and then close shut-off valve 1 (2-2); next, open shut-off valve 3 (2-10) to allow critical CO2 to flow into the high-pressure piston intermediate container B (2-11), then close shut-off valves 2 (2-8) and 3 (2-10), and start the constant-speed, constant-pressure pump (…). 2-14), open the pressurization valve (2-15(1)) of the process switcher (2-15), and start the condenser (2-19) at the same time. When the pointer of pressure gauge 2 (2-12) is at 5~6MPa and the temperature collected by temperature sensor 1 (2-18) is below 0°C, stop the pressurization and cooling process; after the liquid CO2 is prepared, open the shut-off valve 4 (2-20) and shut-off valve 5 (3-4). During the liquid CO2 transportation process, the manual pressure regulating pump (3-5) can be operated to maintain the liquid pressure of the back pressure intermediate container (3-2), and its phase state can be observed in real time through the low temperature and high pressure viewing window (5-3); then open the shut-off valve 8 (5-4), and inject the liquid CO2 into the cracking sample, and at the same time Start the multi-functional coal and rock dynamic monitoring module (10) and observe the changes in acoustic signals monitored by the crack detection probe (10-5) and the changes in electrical signals monitored by the stress-strain probe (10-7); after completing the liquid CO2 injection, close the shut-off valves 4 (2-20), 5 (3-4) and 8 (5-4); after the liquid CO2 injection process is completed, open the shut-off valve 7 (5-2) and start the vacuum pump (5-1) to extract and clean the residual liquid CO2 in the pipeline to avoid affecting the water injection process; after the residual liquid is cleaned through the low temperature and high pressure viewing window (5-3), close the shut-off valve 7 (5-2) and the vacuum pump (5-1); then, open the process switcher (2 -15) water injection valve (2-15(2)) and shut-off valve 6 (4-3), start pipeline heater (4-1) to warm up the liquid CO2 / water injection common pipeline section, avoid the low temperature pipeline from freezing when it encounters water, observe the normal flow of water in the low temperature high pressure viewing window (5-3), then close pipeline heater (4-1); open shut-off valve 8 (5-4), pressurized water further cracks and wets the sample, at the same time observe the changes in acoustic signal monitored by crack detection probe (10-5) in multifunctional coal and rock dynamic monitoring module (10), the stress changes monitored by stress-strain probe (10-7), and the changes in sample resistivity fed back by resistance probe (10-6), so as to realize the monitoring of moisture content in the sample; After water injection is completed, shut off valves 6 (4-3) and 8 (5-4) are closed. After the water injection process is completed, shut off valve 7 (5-2) is opened and vacuum pump (5-1) is started to pump out and clean the water remaining in the pipeline to avoid affecting the next liquid CO2 injection process. After cleaning, repeat the preparation of liquid CO2 and the liquid CO2 fracturing sample in step e. After the liquid CO2 injection process is completed, the water injection process can be carried out again. This cycle is repeated until the set number of test cycles is reached and the test is stopped. At this time, the liquid CO2-water circulation fracturing and lubrication module (2, 3, 4, 5) and the multi-functional coal and rock dynamic monitoring module (10) are turned off. f. Permeability determination after sample cracking and wetting: First, open shut-off valve 7 (5-2) and start vacuum pump (5-1) to ensure that any water remaining in the pipeline during the test is cleaned up. After the test, close shut-off valve 7 (5-2) and vacuum pump (5-1). Next, open shut-off valves 1 (2-2), 2 (2-8), 3 (2-10), 4 (2-20), 8 (5-4), 11 (11-3), and 12 (11-7). Then, start booster pump (2-7) to control the gas in CO2 cylinder (2-1) to enter the step from the liquid CO2 pipeline. In the borehole (7-1) of the coal core sample (7), the water in the sample is displaced, and the gas-liquid separator (11-1) is started at the same time. After the displacement is completed, the mass and volume of gas / liquid collected by electronic balance A (11-8) and electronic balance B (11-6) are recorded. At the same time, the data of gas flow, inlet pressure and outlet pressure collected by the data acquisition and control module (1) and monitored by CO2 special flow meter (2-3), pressure sensor 3 (6-2) and pressure sensor 6 (6-13) are processed. The gas-water two-phase relative permeability of the coal core sample after the test is obtained by the non-steady-state gas-water relative permeability determination method. g. After the liquid CO2-water circulation fracturing and moisturizing coal body test is completed, the shut-off valves 9 (8-2) and 10 (8-3) are opened in sequence to control the axial pressure-confining pressure tracking pump (8-1) to unload the axial pressure and confining pressure of the coal core sample (7). Then, the left plug (6-3) of the triaxial core clamping device is opened, the axial pressure-confining pressure loading chamber (6-5) is taken out, and the sample (7) after circulation fracturing and moisturizing is taken out from the sample chamber (6-7) and cleaned. Finally, the crack propagation data obtained by the multi-functional coal rock dynamic monitoring module (10) and the permeability and pH data measured by the gas-liquid two-phase permeability and pH measurement module (11) during this circulation fracturing and moisturizing process are transmitted to the data acquisition and control module (1). h. After the test, the moisture content of the sample (7) was measured again, and the analysis was performed by computed tomography (CT), X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR). The changes in the moisture content, porosity, volume ratio of different pore sizes, and types and contents of functional groups on the coal surface were compared before and after the test. This comparative data can be used to evaluate the effect of liquid CO2-water circulation cracking and lubrication of coal. i. Select another coal core sample and reset the injection pressure of liquid CO2 and water, the number of times liquid CO2-water circulation is added, and the axial pressure and confining pressure values ​​of the axial pressure-confining pressure loading mechanism. Then, repeat steps a to h, and record the crack propagation data, permeability measurement data before and after the test, and pH value of the liquid in the waste liquid collection container at the current set injection pressure, axial pressure, and confining pressure values ​​and the number of cycles. Repeat this process multiple times to obtain the crack propagation data, permeability measurement data before and after the test, and pH value of the liquid in the waste liquid collection container at different injection pressures, axial pressures, confining pressures, and number of cycles of liquid CO2-water circulation cracking and lubrication of coal core samples. This provides a basis for the coal seam liquid CO2-water circulation cracking, lubrication, and dust reduction technology.

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