Simulation experiment device for well soaking, pressure control and flowback of fracturing fluid of low-permeability coal seam

Abstract

The utility model relates to a low-permeability coal seam fracturing fluid soaking pressure control flowback simulation experiment device which comprises a coal rock core clamping subsystem, an in-situ gas-water loading subsystem, a hydraulic fracturing subsystem, a soaking process subsystem and a pressure control drainage subsystem. By means of the structure, hydraulic fracturing-soaking-pressure control flowback integrated simulation is achieved, and multi-link and multi-parameter after-pressure gas production effect influence factor analysis can be achieved; the original gas-water occurrence state of the coal seam is reduced to a certain extent, methane is injected multiple times, adsorption equilibrium is waited until the gas pressure reaches a preset critical desorption pressure value, and then high-pressure water is injected until the fluid pressure reaches the original coal seam pressure; in addition, the device can realize an integrated simulation experiment as a unified whole through one-time forming, secondary assembly for realizing part of functions is not needed, and the device is convenient and easy to use.

Description

Technical Field

[0001] This utility model relates to the field of coalbed methane development engineering technology, specifically to a simulated experimental device for controlled pressure return flow of low-permeability coalbed fracturing fluid well. Background Technology

[0002] Most coal seams in my country are characterized by low permeability, and hydraulic fracturing is commonly used to improve permeability, thereby promoting efficient surface coalbed methane development and underground gas extraction. During fracturing operations, external high-pressure fracturing fluid enters the coal seam, altering the coalbed methane-water distribution characteristics. After flowback, a large amount of fracturing fluid remains in the coal seam, affecting post-fracturing gas production. The physical properties of fracturing fluid differ significantly from those of coalbed water. Inappropriate fracturing fluid may cause coalbed water sensitivity (mainly due to clay expansion), damaging the coal seam's permeability. Furthermore, fracturing intensity (primarily pump pressure and injection volume) directly determines fracture aperture, extension length, and complexity. Excessive injection volume may severely damage the coal and rock structure, inducing the generation of large amounts of coal dust, which can also impair subsequent gas production. Currently, the impact of various fracturing process parameters on post-fracturing gas production remains unclear. It is important to note that the coal seam matrix system is rich in micro- and nano-pores. The walls of these micro- and nano-pores in coal and rock with medium to low thermal maturity are generally hydrophilic. Under capillary-driven permeation, fracturing fluid can penetrate deep into the micro- and nano-pores of the coal seam matrix, making it difficult to characterize the microscopic gas-water occurrence characteristics of the coal seam and potentially affecting the gas desorption from the matrix pores and the ability to flow into the cleavage system. Mainstream academic research indicates that shutting in the well for a period of time after fracturing (the well-shutting process) has a positive effect on post-fracturing gas production. During the well-shutting process, the fracturing fluid enters the deep part of the coal seam, increasing the original coal seam pressure and thus improving the production pressure differential for subsequent gas production. On the other hand, because the permeation capacity of micro- and nano-pores is stronger than that of medium and large pores, the fracturing fluid will spontaneously enter the micro- and nano-pores from the medium and large pores. At the same time, the gas phase in the micro- and nano-pores will enter the medium and large pores, increasing the gas saturation of the medium and large pores and thus improving post-fracturing gas production performance. However, field practice in coalbed methane blocks shows that the gas production characteristics of coalbed methane wells after the well-shutting process vary significantly, failing to consistently demonstrate the positive effects of well-shutting on post-fracturing gas production as suggested in mainstream research. Research on the impact of well-shutting on post-fracturing gas production remains weak. Refined pressure control and flowback after fracturing can suppress the high-speed non-Darcy effect at the bottom of the well and reduce rate-sensitive damage, which is crucial for achieving efficient fracturing fluid flowback and ensuring stable production in coalbed methane wells. However, most research focuses on bottom-hole pressure control after fracturing fluid flowback, with insufficient research on refined bottom-hole pressure control during the fracturing fluid flowback stage. Overall, the optimal fracturing fluid type, fracturing intensity, key well-shutting parameters, and flowback pressure control system optimization methods for maximizing post-fracturing gas production in coalbed methane remain unclear. Developing the integrated experimental equipment described in this utility model, which combines gas-water storage, hydraulic fracturing, and well-shutting flowback, can obtain fracturing fluid flowback rates and post-fracturing gas production effects under different parameter combinations. This will contribute to the theoretical breakthroughs and methodological development of system optimization, making the development of the equipment described in this utility model essential.

[0003] Existing coalbed methane (CBM) drainage experimental devices primarily focus on the controlled-pressure gas production characteristics of raw coal (coal core samples taken from the field). On the one hand, they rarely recreate the original gas-water occurrence state of the coal seam, generally conducting experiments in a single-phase gas environment and neglecting the influence of coal seam water on gas production. On the other hand, they rarely simulate the modification of coal-rock structure by hydraulic fracturing and its impact on subsequent drainage. While these experimental devices can study the controlled-pressure gas production laws of low-permeability coal seams, they cannot simulate the gas-water seepage characteristics within the composite rock mass of low-permeability coal seams and artificial fractures. Existing coal seam fracturing simulation experimental devices mainly focus on the mechanisms of fracturing initiation, propagation, and subsequent closure in coal seams, primarily studying the influence of lithology, confining pressure, and key fracturing parameters on artificial fractures. They generally do not consider subsequent well shut-in processes and controlled-pressure flowback characteristics, nor do they consider the influence of the original gas-water occurrence in the coal seam on the fracturing effect. Therefore, they cannot be used to study the impact of large-scale fracturing fluid intrusion on post-fracturing gas production. Overall, there is still a lack of indoor experimental devices capable of simulating the dynamics of in-situ fracturing and drainage of low-permeability coal seams.

[0004] In view of the above, this utility model is hereby proposed. Utility Model Content

[0005] The purpose of this invention is to provide a simulated experimental device for controlled pressure return flow of fracturing fluid in low-permeability coal seams, so as to solve the technical problems mentioned in the background.

[0006] To achieve the above objectives, this utility model provides the following technical solution:

[0007] A simulation experimental device for controlled pressure return flow of fracturing fluid in low-permeability coal seams includes a coal and rock core clamping subsystem, an in-situ gas-water loading subsystem, a hydraulic fracturing subsystem, a well-shutting process subsystem, and a controlled pressure flow production subsystem.

[0008] The coal core clamping subsystem includes a coal core, a pressure chamber, a constant temperature chamber, a second pressure sensor, and a pressure pump. The coal core is placed in the pressure chamber, which is placed in the constant temperature chamber. The pressure pump, the second pressure sensor, and the pressure chamber are connected to the pressure chamber via pipelines.

[0009] The in-situ gas-water loading subsystem includes a high-pressure methane cylinder, a water tank, and a second tee fitting. The second tee fitting is connected to the high-pressure methane cylinder, the water tank, and the coal and rock core clamping subsystem via pipelines.

[0010] The hydraulic fracturing subsystem includes a high-pressure water pump, a sealing device, and a high-pressure jet, which are connected by a high-pressure pipeline and are located in a pressurization chamber.

[0011] The well-sealing process subsystem includes a first one-way valve, a second one-way valve, and a first pressure sensor. The first one-way valve and the first pressure sensor are located on the high-pressure pipeline, and the second one-way valve is located between the second tee fitting and the coal and rock core clamping subsystem.

[0012] The pressure-controlled drainage subsystem includes a first gas-liquid separator, a methane collection cylinder, a liquid collection container, a vacuum pump, a second gas-liquid separator, a methane recovery cylinder, and a liquid recovery container. The first gas-liquid separator is connected to the methane collection cylinder and the liquid collection container, respectively. The second gas-liquid separator is connected to the vacuum pump, the methane recovery cylinder, and the liquid recovery container, respectively. The vacuum pump is connected to the coal core via a pipeline. The first gas-liquid separator is connected to the high-pressure pipeline via a first tee fitting.

[0013] Preferably, in the in-situ gas-water loading subsystem:

[0014] A first water supply pressure regulating valve, a third pressure sensor, and a first liquid flow meter are installed on the pipeline connecting the water tank and the second tee fitting.

[0015] Preferably, in the in-situ gas-water loading subsystem:

[0016] A first gas supply pressure regulating valve, a first methane flow meter, and a fourth pressure sensor are installed on the pipeline connecting the high-pressure methane cylinder and the second tee fitting.

[0017] Preferably, in the hydraulic fracturing subsystem:

[0018] A second liquid flow meter and a second water supply pressure regulating valve are also installed on the high-pressure pipeline.

[0019] Preferably, in the pressure control and drainage subsystem:

[0020] A recovery pressure regulating valve is installed on the pipeline connecting the first gas-liquid separator and the first tee fitting.

[0021] Preferably, in the pressure control and drainage subsystem:

[0022] A second methane flow meter is installed on the pipeline connecting the first gas-liquid separator and the methane collection cylinder.

[0023] Preferably, in the pressure control and drainage subsystem:

[0024] A third liquid flow meter is installed on the pipeline connected to the liquid collection container of the first gas-liquid separator.

[0025] Preferably, in the pressure control and drainage subsystem:

[0026] A third check valve is installed on the pipeline connecting the vacuum pump to the coal core.

[0027] Compared with existing technologies, the present invention provides a simulation experimental device for hydraulic fracturing, well simmering, and controlled pressure flowback of low-permeability coal seams. This device achieves integrated simulation of hydraulic fracturing, well simmering, and controlled pressure flowback, enabling analysis of factors affecting post-fracturing gas production effects across multiple stages and parameters. It also recreates the original gas-water occurrence state of the coal seam to a certain extent. This is achieved by first injecting methane multiple times and waiting for adsorption equilibrium until the gas pressure reaches the preset critical desorption pressure value, and then injecting high-pressure water until the fluid pressure reaches the original coal seam pressure. The device includes a coal core clamping subsystem, an in-situ gas-water loading subsystem, a hydraulic fracturing subsystem, a well simmering process subsystem, and a controlled pressure flowback subsystem. The experimental device is formed as a unified whole in one step, enabling integrated simulation experiments without the need for secondary assembly to achieve certain functions, making it convenient and easy to use.

[0028] To make the above-mentioned objectives, features and advantages of this utility model more apparent and understandable, preferred embodiments are described below in detail with reference to the accompanying drawings. Attached Figure Description

[0029] To more clearly illustrate the technical solutions of the embodiments of this utility model, the drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of this utility model and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0030] Figure 1 This invention provides a simulation experimental device for controlled pressure return flow of fracturing fluid in low-permeability coal seams, which is provided as an embodiment of the present invention.

[0031] The diagram is shown below:

[0032] 1. High-pressure water pump; 2. Second liquid flow meter; 3. High-pressure pipeline; 4. Second water supply pressure regulating valve; 5. First tee fitting; 6. First check valve; 7. Recovery pressure regulating valve; 8. First gas-liquid separator; 9. Second methane flow meter; 10. Third liquid flow meter; 11. Methane collection cylinder; 12. Liquid collection container; 13. First pressure sensor; 14. Sealing device; 15. High-pressure jet injector; 16. Coal core; 17. Second pressure sensor; 18. Booster pump; 9. Pressurization chamber; 20. Thermostatic chamber; 21. Second check valve; 22. Third check valve; 23. Vacuum pump; 24. Second gas-liquid separator; 25. Methane recovery cylinder; 26. Liquid recovery container; 27. Second tee fitting; 28. Third pressure sensor; 29. ​​First liquid flow meter; 30. Fourth pressure sensor; 31. First methane flow meter; 32. First water supply pressure regulating valve; 33. Water pump; 34. Water tank; 35. Gas supply pressure regulating valve; 36. High-pressure methane cylinder. Detailed Implementation

[0033] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. The components of the embodiments of the present invention described and shown in the accompanying drawings can be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0034] like Figure 1 As shown in the figure, this utility model embodiment provides a simulation experimental device for controlled pressure return flow of fracturing fluid in low-permeability coal seams, including a coal core clamping subsystem, an in-situ gas-water loading subsystem, a hydraulic fracturing subsystem, a well-shutting process subsystem, and a controlled pressure flow production subsystem.

[0035] The coal core 16 clamping subsystem includes a coal core 16, a pressurization chamber 19 for applying confining pressure to the core, a constant temperature chamber 20 for simulating the original coal seam temperature, a second pressure sensor 17 for real-time monitoring of the confining pressure of the core, and a pressurization pump 18 for providing confining pressure. The coal core 16 is placed in the pressurization chamber 19, and the pressurization chamber 19 is placed entirely in the constant temperature chamber 20. The pressurization pump 18, the second pressure sensor 17, and the pressurization chamber 19 are connected by pipelines.

[0036] The in-situ gas-water loading subsystem includes a high-pressure methane cylinder 36, a gas supply pressure regulating valve 35, a first methane flow meter 31 for monitoring gas flow, a fourth pressure sensor 30 for monitoring injected gas pressure, a water tank 34 and a water pump 33 for injecting water into the coal core 16, a first water supply pressure regulating valve 32, a first liquid flow meter 29 for monitoring water injection flow, a third pressure sensor 28 for monitoring water injection pressure, and a second one-way valve 21 for controlling the connection between the coal core 16 clamping subsystem and the in-situ gas-water loading subsystem. Each device in the subsystem is connected to a second tee fitting 27 via pipelines.

[0037] The hydraulic fracturing subsystem includes a high-pressure water pump 1, a high-pressure pipeline 3, a second liquid flow meter 2 for monitoring the injection flow rate of fracturing fluid, a second water supply pressure regulating valve 4, a first check valve 6, a first pressure sensor 13 for monitoring the injection pressure, a sealing device 14 for sealing the fracturing fluid, and a high-pressure jet injector 15 for increasing the hydraulic pressure of fracturing fluid. The above devices are connected through the high-pressure pipeline 3.

[0038] The well-sealing process subsystem includes a first check valve 6 for isolating the hydraulic fracturing subsystem from the well-sealing process, a second check valve 21 for isolating the in-situ gas-water loading subsystem from the well-sealing process, and a first pressure sensor 13 for monitoring the pressure evolution characteristics of coal and rock fluids during the well-sealing process.

[0039] The controlled pressure drainage subsystem includes a first tee fitting 5 connecting the hydraulic fracturing subsystem and the controlled pressure drainage subsystem, a recovery pressure regulating valve 7 for controlling the recovery pressure, a first gas-liquid separator 8 for separating the gas and liquid phases in the return fluid, a second methane flow meter 9 for measuring the amount of methane recovered, a third liquid flow meter 10 for measuring the flow rate of the liquid phase in the return fluid, a methane collection cylinder 11, a liquid collection container 12, a vacuum pump 23 for recovering residual fluid from the coal core 16, a third check valve 22 for connecting the coal core 16 and the vacuum pump 23, a second gas-liquid separator 24 for separating the gas and liquid phases in the residual fluid, a methane recovery cylinder 25, and a liquid recovery container 26. The "return fluid collection section" is connected to the coal core 16 via a high-pressure pipeline 3, and the "residual fluid collection section" is connected to...

[0040] The experimental system is as follows Figure 1After assembly, all valves are closed, and the integrated simulation experiment of low-permeability coalbed methane-water storage-hydraulic fracturing-well shut-off flowback begins. First, the third one-way valve 22 is opened, and the vacuum pump 23 is turned on to vacuum the coal core 16. The original fluid within the sample is recovered through the second gas-liquid separator 24, methane recovery cylinder 25, and liquid recovery container 26. Once the methane recovery cylinder 25 has stabilized, the third one-way valve 22 is closed, ending the core vacuuming stage. Next, the constant temperature chamber 20 is started, and the temperature is set to the coal seam temperature of the layer where the coal core 16 is located. Then, the pressure pump 18 is turned on to apply confining pressure to the coal core 16. The confining pressure value is monitored by a pressure sensor until it reaches the original coal seam pressure of the block where the coal core 16 is located. The pressure pump 18 is then turned off, and the confining pressure is stopped. Next, allow the coal core 16 to fully adsorb methane to its original gas content. Open the second one-way valve 21 and the gas supply pressure regulating valve 35, and slowly increase the gas supply pressure stepwise through the gas supply pressure regulating valve 35 until the gas supply pressure reaches the critical desorption pressure of the block where the coal core 16 is located. Then, stop increasing the gas supply pressure. In addition, after each pressure increase through the pressure regulating valve, wait for the pressure values ​​of the fourth pressure sensor 30 and the first pressure sensor 13 to equal the target pressure of the pressure regulating valve before proceeding to the next pressure increase. Measure the methane flow rate throughout the process using a methane flow meter, and then close the second one-way valve 21 and the gas supply pressure regulating valve 35. The methane adsorption stage of the coal core is now complete. Next, the second one-way valve 21 and the first water supply pressure regulating valve 32 are opened. Through the water supply pressure regulating valve and the water pump 33, the water injection pressure is stabilized to the original coal seam pressure of the layer where the coal core 16 is located (generally higher than the critical desorption pressure). The pressure change at both ends of the core is monitored in real time through the third pressure sensor 28 and the first pressure sensor 13. The injection water flow rate is monitored in real time through the liquid flow meter until the pressure at both ends of the core is close to equilibrium and the injection water flow rate is close to zero. It is considered that the coal core 16 has been completely restored to the original gas-water occurrence state. Then, the first water supply pressure regulating valve 32 and the second one-way valve 21 are closed. Next, start the high-pressure water pump 1, open the second water supply pressure regulating valve 4 and the first check valve 6, continuously increase the injection pressure through the pressure regulating valve, and monitor the injection water flow and injection water pressure in real time through the second liquid flow meter 2 and the first pressure sensor 13. After the injection water pressure reaches its peak, stabilize the water flow and inject water for 5 to 10 minutes. It is considered that the coal seam core has been fully fracturing. Then, close the high-pressure water pump 1, the second water supply pressure regulating valve 4 and the first check valve 6, and the hydraulic fracturing stage ends.Next, the well-sealing process begins. The first check valve 6, the second check valve 21, and the third check valve 22 are checked and closed. The well-sealing time is recorded, and the fluid pressure evolution characteristics within the coal and rock sample are monitored in real time using the first pressure sensor 13 until the preset well-sealing duration is reached, at which point the well-sealing process ends. Then, the first check valve 6 and the recovery pressure regulating valve 7 are opened. The recovery pressure is controlled through the recovery pressure regulating valve 7 to simulate the bottom-hole pressure control production conditions of a coalbed methane well. The second methane flow meter 9 and the third... Liquid flow meter 10 measures the gas phase flow and liquid phase flow of the return fluid under pressure control conditions, and collects them through methane collection cylinder 11 and liquid collection container 12. The recovery pressure is set according to a preset pressure reduction path, and the evolution characteristics of methane flow and liquid flow are recorded. The evolution curves of gas production and water production under different pressure control schemes with production time can be obtained. When the recovery pressure is reduced to the abandoned pressure of the coalbed methane well in the block where the coal core 16 is located, the first one-way valve 6 and the recovery pressure regulating valve 7 are closed, and the pressure control and drainage stage ends. Finally, the third one-way valve 22 is opened and the vacuum pump 23 is turned on to extract the residual fluid in the coal core 16. The gas phase and liquid phase in the residual fluid are separated by the second gas-liquid separator 24 and collected through methane recovery cylinder 25 and liquid recovery container 26, respectively. The methane in the methane recovery cylinder 25 can be reused.

[0041] This utility model presents an integrated simulation experimental device for low-permeability coalbed methane-water occurrence, hydraulic fracturing, and well shut-in / flowback. By employing a method of "first saturating with adsorbed gas, then injecting water under high pressure," it partially replicates the original gas-water occurrence characteristics of coal and rock. It fully considers on-site fracturing construction, well shut-in processes, and pressure-controlled drainage procedures, designing subsystems for fracturing, well shut-in processes, and pressure-controlled drainage. Furthermore, the hardware of these subsystems is interconnected to form a unified whole. By changing the valve states, it can sequentially achieve integrated indoor simulation of key fracturing, well shut-in parameters, and drainage pressure, maintaining a high degree of consistency with the construction sequence of on-site coalbed methane wells. This helps to study the multi-stage and multi-parameter influence laws of gas production in fracturing low-permeability coalbed methane wells, promotes the understanding of the relationship between fracturing fluid intrusion / flowback and post-fracturing gas production performance, and provides basic experimental test data for optimizing the fracturing-well shut-in / flowback construction scheme for low-permeability coalbed methane wells.

[0042] Compared with existing experimental devices, the innovation of the experimental device proposed in this utility model is mainly reflected in the following aspects: it realizes integrated simulation of hydraulic fracturing-well shut-in-pressure controlled drainage, enabling analysis of factors affecting gas production after fracturing in multiple stages and with multiple parameters; it restores the original gas-water occurrence state of the coal seam to a certain extent, by first injecting methane multiple times and waiting for adsorption equilibrium until the gas pressure reaches the preset critical desorption pressure value, and then injecting high-pressure water until the fluid pressure reaches the original coal seam pressure; the device includes a coal core clamping subsystem, an in-situ gas-water loading subsystem, a hydraulic fracturing subsystem, a well shut-in process subsystem, and a pressure controlled drainage subsystem. The experimental device can be formed as a unified whole in one step to realize integrated simulation experiments without the need for secondary assembly to realize some functions, making it convenient and easy to use.

[0043] In the description of this utility model, it should be noted that the terms "center," "upper," "lower," "left," "right," "vertical," "horizontal," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, or the orientation or positional relationship commonly used when the product of this utility model is in use. They are only for the convenience of describing this utility model and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this utility model. In addition, the terms "first," "second," and "third," etc., are only used to distinguish descriptions and should not be construed as indicating or implying relative importance.

[0044] Furthermore, terms such as "horizontal," "vertical," and "sag" do not imply that components must be absolutely horizontal or suspended, but rather that they can be slightly tilted. For example, "horizontal" simply means that its direction is more horizontal relative to "vertical," and does not mean that the structure must be completely horizontal, but can be slightly tilted.

[0045] In the description of this utility model, it should also be noted that, unless otherwise explicitly specified and limited, the terms "set," "install," "connect," and "link" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in this utility model based on the specific circumstances.

[0046] The above description is merely a preferred embodiment of this utility model and is not intended to limit the utility model. Various modifications and variations can be made to this utility model by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this utility model should be included within the protection scope of this utility model. It should be noted that similar reference numerals and letters in the following figures indicate similar items; therefore, once an item is defined in one figure, it does not need further definition and explanation in subsequent figures.

Claims

1. A simulation experimental device for controlled pressure flowback in low-permeability coal seam fracturing fluid wells, characterized in that, It includes a coal and rock core clamping subsystem, an in-situ gas-water loading subsystem, a hydraulic fracturing subsystem, a well shut-in process subsystem, and a pressure-controlled drainage subsystem; The coal core clamping subsystem includes a coal core, a pressure chamber, a constant temperature chamber, a second pressure sensor, and a pressure pump. The coal core is placed in the pressure chamber, which is placed in the constant temperature chamber. The pressure pump, the second pressure sensor, and the pressure chamber are connected to the pressure chamber via pipelines. The in-situ gas-water loading subsystem includes a high-pressure methane cylinder, a water tank, and a second tee fitting. The second tee fitting is connected to the high-pressure methane cylinder, the water tank, and the coal and rock core clamping subsystem via pipelines. The hydraulic fracturing subsystem includes a high-pressure water pump, a sealing device, and a high-pressure jet, which are connected by a high-pressure pipeline and are located in a pressurization chamber. The well-sealing process subsystem includes a first one-way valve, a second one-way valve, and a first pressure sensor. The first one-way valve and the first pressure sensor are located on the high-pressure pipeline, and the second one-way valve is located between the second tee fitting and the coal and rock core clamping subsystem. The pressure-controlled drainage subsystem includes a first gas-liquid separator, a methane collection cylinder, a liquid collection container, a vacuum pump, a second gas-liquid separator, a methane recovery cylinder, and a liquid recovery container. The first gas-liquid separator is connected to the methane collection cylinder and the liquid collection container, respectively. The second gas-liquid separator is connected to the vacuum pump, the methane recovery cylinder, and the liquid recovery container, respectively. The vacuum pump is connected to the coal core via a pipeline. The first gas-liquid separator is connected to the high-pressure pipeline via a first tee fitting.

2. The simulated experimental device for controlled pressure return flow of fracturing fluid in low-permeability coal seams according to claim 1, characterized in that, In the in-situ gas-water loading subsystem: A first water supply pressure regulating valve, a third pressure sensor, and a first liquid flow meter are installed on the pipeline connecting the water tank and the second tee fitting.

3. The simulated experimental device for controlled pressure return flow of fracturing fluid in low-permeability coal seams according to claim 2, characterized in that, In the in-situ gas-water loading subsystem: A first gas supply pressure regulating valve, a first methane flow meter, and a fourth pressure sensor are installed on the pipeline connecting the high-pressure methane cylinder and the second tee fitting.

4. The simulated experimental device for controlled pressure return flowback of low-permeability coal seam fracturing fluid well simmering as described in claim 3, characterized in that, In the hydraulic fracturing subsystem: A second liquid flow meter and a second water supply pressure regulating valve are also installed on the high-pressure pipeline.

5. The simulated experimental device for controlled pressure return flow of fracturing fluid in low-permeability coal seams according to claim 4, characterized in that, In the pressure control and drainage subsystem: A recovery pressure regulating valve is installed on the pipeline connecting the first gas-liquid separator and the first tee fitting.

6. The simulated experimental device for controlled pressure return flow of fracturing fluid in low-permeability coal seams according to claim 5, characterized in that, In the pressure control and drainage subsystem: A second methane flow meter is installed on the pipeline connecting the first gas-liquid separator and the methane collection cylinder.

7. The simulated experimental device for controlled pressure return flow of fracturing fluid in low-permeability coal seams according to claim 6, characterized in that, In the pressure control and drainage subsystem: A third liquid flow meter is installed on the pipeline connected to the liquid collection container of the first gas-liquid separator.

8. The simulated experimental device for controlled pressure return flow of fracturing fluid in low-permeability coal seams according to claim 7, characterized in that, In the pressure control and drainage subsystem: A third check valve is installed on the pipeline connecting the vacuum pump to the coal core.

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