A synergistic hydraulic permeability enhancement and semi-permeable membrane enhanced gas extraction system and extraction method thereof

Abstract

This invention discloses a synergistic hydraulic permeability enhancement and semi-permeable membrane-enhanced gas extraction system and method, belonging to the field of low-permeability coal seam gas extraction technology. The system comprises a dual-casing structure, hydraulic permeability enhancement drive, semi-permeable membrane negative pressure gradient extraction, and recovery circulation systems. The dual-casing structure mainly consists of a main water injection pipe and multiple outer casings, a main extraction pipe and multiple inner casings. The outer casings are equipped with jet nozzles, and the inner casings have gas extraction holes covered by a polytetrafluoroethylene (PTFE) semi-permeable membrane. The dual-casing structure system combines permeability enhancement and extraction functions. Precise hydraulic permeability enhancement is achieved through the outer casings, while the inner casings utilize the selective permeability characteristics of the semi-permeable membrane to enhance gas extraction, thus solving the problems of short permeability enhancement efficiency, low extraction efficiency, and poor sealing performance in traditional technologies. This invention is suitable for efficient gas extraction and coal seam permeability enhancement in high-stress, low-permeability coal seams, ensuring safe coal mine production and improving coalbed methane resource utilization.

Description

Technical Field

[0001] This invention belongs to the field of low-permeability coal seam gas extraction technology, specifically a gas extraction system and extraction method that combines hydraulic permeability enhancement and semi-permeable membrane enhancement. Background Technology

[0002] As coal mining depth continues to increase, the proportion of high-stress, low-permeability coal seams is rising year by year. These coal seams have sparse pore and fracture development, poor connectivity, complex gas occurrence, and high desorption and diffusion resistance, resulting in low efficiency of traditional gas extraction technology and becoming a core problem for safe coal mine production.

[0003] In existing technologies, hydraulic fracturing is the mainstream method for improving the permeability of low-permeability coal seams. It improves the permeability of coal seams by forming a fracture network through high-pressure fluid impact on the coal body. However, it has the following drawbacks: First, the fractures are prone to close due to stress rebound after fracturing, resulting in a short period of effectiveness in improving permeability. Second, the sealing performance between the extraction pipeline and the coal body is poor, which can easily lead to gas leakage and air backflow, resulting in a decrease in extraction concentration. Third, coal slag and accumulated liquid can easily clog the pipeline during the extraction process, affecting the continuity of extraction.

[0004] Semi-permeable membrane materials, due to their selective permeability, allow only specific gases to pass through, making them a mature technology in fluid separation. However, current technologies have not integrated them with extraction pipe structures to achieve a unified "permeability enhancement-extraction-sealing" function. Traditional single-pipe or open-hole extraction methods cannot simultaneously meet the fracture expansion requirements of hydraulic permeability enhancement and the directional and efficient requirements of gas extraction, making it difficult to solve the core pain points of "difficult permeability enhancement, poor sealing, and low extraction efficiency" in low-permeability coal seams. Summary of the Invention

[0005] The purpose of this invention is to solve the problems existing in the prior art and to provide a synergistic hydraulic permeability enhancement and semi-permeable membrane enhanced gas extraction system and extraction method.

[0006] This invention is achieved through the following technical solution:

[0007] One aspect of the present invention provides a synergistic hydraulic permeability enhancement and semi-permeable membrane enhanced gas extraction system, comprising a dual-casing structure system, a hydraulic permeability enhancement drive system, a semi-permeable membrane negative pressure gradient extraction system, and a recovery and circulation system.

[0008] The dual-casing structure system includes multiple outer casings, multiple inner casings, a main water injection pipe, and a main extraction pipe. Several jet nozzles are evenly distributed on the bottom wall of each outer casing. Multiple inner casings are coaxially nested inside their respective outer casings. Multiple sets of circumferential gas extraction holes are evenly distributed along the height of each inner casing wall, with each set including multiple gas extraction holes evenly distributed around the circumference of the inner casing. Each inner casing wall has a corresponding circumferential gas extraction hole... Each group of gas extraction holes is fixedly connected to a ring of PTFE semi-permeable membrane, which completely covers all the gas extraction holes in each group. The extraction main pipe is located inside the water injection main pipe and is connected to the top openings of multiple outer sleeves. The extraction main pipe is also connected to the top openings of multiple inner sleeves. A solenoid valve is installed on the extraction main pipe near the top opening of each inner sleeve. The water injection main pipe and multiple outer sleeves form an enhanced permeability channel, and the extraction main pipe and multiple inner sleeves form an extraction channel.

[0009] The hydraulic permeation enhancement drive system includes a storage tank, a high-pressure water injection pump, a frequency converter control cabinet, and a flow sensor. The storage tank is connected to the high-pressure water injection pump via a pipeline, the high-pressure water injection pump is connected to the main water injection pipe, the frequency converter control cabinet is electrically connected to the high-pressure water injection pump, and the flow sensor is installed inside the outer casing.

[0010] The semi-permeable membrane negative pressure gradient extraction system includes a gas-slag-water separator, a negative pressure extraction pump, a gas storage tank, and a gas concentration sensor. The gas storage tank is connected to the gas-slag-water separator via a pipeline, the gas-slag-water separator is connected to the negative pressure extraction pump via a pipeline, the negative pressure extraction pump is connected to the extraction main pipe, and the gas concentration sensor is installed inside the inner sleeve.

[0011] The recycling system includes a residue collection tank, a liquid recovery pump, and a liquid buffer tank. The residue collection tank is connected to the gas-slag-water separator via a pipeline. One end of the liquid recovery pump is connected to the gas-slag-water separator via a pipeline, and the other end of the liquid recovery pump is connected to the liquid buffer tank via a pipeline. The liquid buffer tank is connected to the storage tank via a pipeline.

[0012] As a preferred technical solution, the system of the present invention also includes a remote control box, which is connected to the solenoid valve, flow sensor, gas concentration sensor, frequency converter control cabinet, negative pressure extraction pump, and liquid recovery pump.

[0013] As a preferred technical solution, both the outer sleeve and the inner sleeve are made of high-strength wear-resistant steel pipes.

[0014] Another aspect of the present invention provides a gas extraction method for the above-described synergistic hydraulic permeability enhancement and semi-permeable membrane enhanced gas extraction system, comprising the following steps:

[0015] S1. Drill a hole into the target coal seam, with the bottom of the hole reaching inside the coal seam; then slowly lower the outer casing and inner casing as a whole into the hole.

[0016] S2. Close the extraction channel, open the permeability enhancement channel, start the high-pressure water injection pump, and inject the fracturing fluid in the storage tank into each outer casing through the water injection main pipe. Finally, it is sprayed out through the jet nozzle. The sprayed high-pressure water jet fracturing the surrounding coal seam causes the surrounding coal seam to form initial fractures.

[0017] S3. Maintain the pressure and flow rate at which the initial fractures formed for at least 30 minutes to ensure uniform fracture formation. At the same time, the high-pressure water jets create slots in the coal seam around the borehole, further improving fracture connectivity.

[0018] S4. Gradually reduce the pressure and flow rate until the high-pressure water pump is stopped and the permeability enhancement channel is closed.

[0019] S5. After the excess fracturing fluid in the outer casing is fully discharged with the jet nozzle, open the extraction channel and carry out the extraction operation.

[0020] S6. Start the negative pressure extraction pump and stabilize the extraction negative pressure value at -0.02 MPa to slowly discharge the free gas in the fractured fissures; then gradually increase the negative pressure to -0.05 MPa and maintain it for a preset time to form a stable negative pressure field in the coal seam fractures to accelerate the conversion of adsorbed gas to free gas; then further increase the negative pressure to -0.08 MPa and maintain it until extraction is completed.

[0021] S7. After extraction is completed, gradually increase the negative pressure value of the negative pressure extraction pump back to 0 MPa, stop the negative pressure extraction pump, and close the extraction channel.

[0022] As a preferred technical solution, in step S2 above, closing the extraction channel involves controlling all solenoid valves to be in the closed state via a remote control box; the high-pressure water injection pump performs gradient pressure and flow increase.

[0023] Phase 1: Set the target pressure to 5MPa, control the flow rate to 50L / min, maintain for 5-10 minutes, the pressure drop should be ≤1MPa, and there should be no sudden pressure drop, then proceed to Phase 2;

[0024] Phase Two: Set the target pressure to 10 MPa, control the flow rate to 60 L / min, maintain for 5-10 minutes, with a pressure drop of 1 MPa < 2 MPa, forming micro-cracks, and proceed to Phase Three;

[0025] Phase 3: Set the target pressure to 15 MPa, control the flow rate to 70 L / min, maintain for 5-10 minutes, pressure drop between 2 MPa and 3 MPa, and a small amount of gas overflows, then proceed to Phase 4;

[0026] Stage 4: Set the target pressure to 20 MPa, control the flow rate to 80 L / min, maintain for 5-10 minutes, and when the pressure drops by more than 3 MPa, the initial cracks will form.

[0027] As a preferred technical solution, in step S5 above, opening the extraction channel is achieved by controlling all solenoid valves to be in the open state via a remote control box.

[0028] As a preferred technical solution, in step S6 above, during the extraction process, the gas and a small amount of entrained gas-liquid-solid mixed medium in the coal seam fractures enter the annular space between the outer and inner casings through the jet nozzles on the outer casing, then enter the inner casing through the polytetrafluoroethylene semi-permeable membrane and gas extraction holes, and finally enter the extraction main pipe and then the gas-slag-water separator. The gas and gas-liquid-solid mixed medium undergo three-phase separation of gas, liquid, and slag through the gas-slag-water separator: the separated gas is transported to the gas storage tank from the gas outlet of the gas-slag-water separator; the separated liquid is pressurized and transported to the liquid buffer tank by the liquid recovery pump from the liquid outlet of the gas-slag-water separator, and then returned to the storage tank from the liquid buffer tank to realize the recovery and reuse of fracturing fluid; the separated coal slag / solid residue is discharged into the residue collection box from the slag discharge end of the gas-slag-water separator.

[0029] As a preferred technical solution, in step S7 above, the liquid recovery pump is stopped, and the liquid buffer tank and the liquid storage tank complete the recovery cycle; the gas inlet of the gas storage tank is closed and sealed; and the residual material collection box is transferred and disposed of.

[0030] As a preferred technical solution, in step S2 above, if the pressure drop is still ≤3MPa after 5-10 minutes in stage four, then the next stage of gradient pressure and flow increase with an amplitude of 5MPa and 10L / min will continue until the current stage is maintained for 5-10 minutes and the pressure drop is >3MPa, thus forming an initial crack.

[0031] This invention features a dual-tube structure that combines permeability enhancement and gas extraction functions. The outer casing achieves precise hydraulic permeability enhancement, while the inner casing utilizes the selective permeability of a semi-permeable membrane to strengthen gas extraction. This solves the problems of short-term permeability enhancement, low extraction efficiency, and poor sealing performance associated with traditional technologies. It is suitable for efficient gas extraction and permeability enhancement in high-stress, low-permeability coal seams, ensuring safe coal mine production and improving coalbed methane resource utilization.

[0032] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0033] 1) In this invention, the outer casing is used for high-pressure hydraulic fracturing, which can concentrate pressure to expand a uniform fracture network; the inner casing with a polytetrafluoroethylene semi-permeable membrane undertakes the extraction operation; during the fracturing and permeability enhancement stage, the extraction channel is closed to ensure the fracture expansion effect; during the extraction stage, the permeability enhancement channel is closed, and the polytetrafluoroethylene semi-permeable membrane on the inner casing guides the flow, which solves the problem of "interference in the extraction channel during fracturing and easy closure of fractures during extraction" in traditional single-tube systems.

[0034] 2) In this invention, the fracturing and permeability enhancement stage adopts a gradient pressure and flow increase method. The advantages of this method are: ① By loading in stages from low-pressure pre-fracture to medium-pressure expansion to high-pressure penetration, the original microfractures are opened first and the local stress concentration is weakened, avoiding the risks of wellbore instability, coal body spalling, or roof / floor penetration caused by one-time high pressure; ② The pressure decay at each stage is used as the online criterion for fracture initiation and expansion, realizing controllable fracturing with "monitoring and adjustment at the same time", preventing blind overpressure and ineffective injection; ③ It promotes the gradual evolution of fractures from microfractures to main fractures and their more uniform distribution around the well, improving fracture connectivity, permeability enhancement radius, and subsequent pumping and diversion capacity.

[0035] 3) In this invention, a polytetrafluoroethylene semi-permeable membrane is used on the inner casing, which is permeable to gas and can significantly block liquids and particles from entering the extraction hole, thereby reducing the risk of coal slag and accumulated liquid entering the inner casing and causing blockage, and improving the continuity and stability of extraction.

[0036] 4) In this invention, the recycling system can treat and reuse a small portion of the accumulated liquid mixed in during the extraction process as fracturing fluid, thereby reducing the consumption of fresh water and reagents. Attached Figure Description

[0037] The accompanying drawings, which are provided to further illustrate the invention and form part of this application, illustrate exemplary embodiments of the invention and are used to explain the invention, but do not constitute an undue limitation of the invention.

[0038] Figure 1 This is a flowchart illustrating the system structure and process flow of the present invention.

[0039] Figure 2 This is a partial cross-sectional view of the double-sleeve structure in the system of the present invention.

[0040] In the diagram: 1. Residual material collection box; 2. Gas-slag-water separator; 3. Negative pressure extraction pump; 4. Solenoid valve; 5. Gas storage tank; 6. Liquid storage tank; 7. High-pressure water injection pump; 8. Variable frequency control cabinet; 9. Jet nozzle; 10. Outer sleeve; 11. Inner sleeve; 12. Liquid recovery pump; 13. Flow sensor; 14. Gas concentration sensor; 15. Gas extraction hole; 16. Remote control box; 17. Liquid buffer tank; 18. Water injection main pipe; 19. Extraction main pipe; 20. Polytetrafluoroethylene semi-permeable membrane; 21. Rock stratum one; 22. Rock stratum two; 23. Coal seam roof; 24. Coal seam; 25. Coal seam floor. Detailed Implementation

[0041] To enable those skilled in the art to better understand the present invention, the present invention will be further described clearly and completely below with reference to the accompanying drawings and embodiments. It should be noted that, unless otherwise specified, the embodiments and features in the embodiments of this application can be combined with each other. Example 1

[0042] like Figure 1 and Figure 2 As shown, this embodiment provides a synergistic hydraulic permeability enhancement and semi-permeable membrane enhanced gas extraction system, including a dual-casing structure system, a hydraulic permeability enhancement drive system, a semi-permeable membrane negative pressure gradient extraction system, a recovery and circulation system, and a remote control box 16.

[0043] The dual-casing structure system includes multiple outer casings 10, multiple inner casings 11, a main water injection pipe 18, and a main extraction pipe 19. Both the outer casings 10 and inner casings 11 are made of high-strength, wear-resistant steel pipes. Several jet nozzles 9 are evenly distributed on the bottom wall of each outer casing 10. Multiple inner casings 11 are coaxially nested inside their respective outer casings 10. Multiple sets of circumferential gas extraction holes are evenly distributed along the height of the wall of each inner casing 11. Each set of circumferential gas extraction holes includes multiple gas extraction holes 15 evenly distributed around the circumference of the inner casing 11. Each inner casing 11 has... A polytetrafluoroethylene (PTFE) semi-permeable membrane 20 is fixedly connected to each group of circumferential gas extraction holes, and the PTFE semi-permeable membrane 20 completely covers the multiple gas extraction holes 15 of each group; the extraction main pipe 19 is set inside the water injection main pipe 18, and the water injection main pipe 18 is connected to the top opening of multiple outer sleeves 10, and the extraction main pipe 19 is connected to the top opening of multiple inner sleeves 11; a solenoid valve 4 is installed on the extraction main pipe 19 near the top opening of each inner sleeve 11; the water injection main pipe 18 and multiple outer sleeves 10 form a permeability enhancement channel, and the extraction main pipe 19 and multiple inner sleeves 11 form an extraction channel.

[0044] The hydraulic permeability enhancement drive system includes a storage tank 6, a high-pressure water injection pump 7, a frequency converter control cabinet 8, and a flow sensor 13. The storage tank 6 is connected to the high-pressure water injection pump 7 through a pipeline. The high-pressure water injection pump 7 is connected to the main water injection pipe. The frequency converter control cabinet 8 is electrically connected to the high-pressure water injection pump 7. The frequency converter control cabinet 8 is used to adjust the high-pressure water injection pump 7 to achieve water injection flow and pressure control. The flow sensor 13 is installed inside the outer casing 10 to monitor the fracturing fluid flow.

[0045] The semi-permeable membrane negative pressure gradient extraction system includes a gas-slag-water separator 2, a negative pressure extraction pump 3, a gas storage tank 5, and a gas concentration sensor 14. The gas storage tank 5 is connected to the gas-slag-water separator 2 through a pipeline, the gas-slag-water separator 2 is connected to the negative pressure extraction pump 3 through a pipeline, the negative pressure extraction pump 3 is connected to the extraction main pipe, and the gas concentration sensor 14 is installed in the inner sleeve 11 to monitor the extraction gas concentration.

[0046] The recycling system includes a residue collection tank 1, a liquid recovery pump 12, and a liquid buffer tank 17. The residue collection tank 1 is connected to the gas-slag-water separator 2 via a pipeline. One end of the liquid recovery pump 12 is connected to the gas-slag-water separator 2 via a pipeline, and the other end of the liquid recovery pump 12 is connected to the liquid buffer tank 17 via a pipeline. The liquid buffer tank 17 is connected to the storage tank 6 via a pipeline.

[0047] The remote control box 16 is connected to the solenoid valve 4, flow sensor 13, gas concentration sensor 14, frequency converter control cabinet 8, negative pressure extraction pump 3, and liquid recovery pump 12. After receiving the signals collected by the flow sensor 13 and gas concentration sensor 14, the remote control box 16 adjusts and controls the solenoid valve 4, frequency converter control cabinet 8, negative pressure extraction pump 3, and liquid recovery pump 12. Example 2

[0048] This embodiment takes a gas-outburial-prone soft coal seam with a burial depth of approximately 980m and a thickness of approximately 5m as an example, and uses the synergistic hydraulic permeability enhancement and semi-permeable membrane enhanced gas extraction system described in Example 1 for extraction, such as... Figure 1 and Figure 2 As shown, the sampling method includes the following steps:

[0049] S1. Drill a hole into the target coal seam. The bottom of the hole passes through rock stratum 1 21 and rock stratum 22 in sequence and reaches the coal seam 24 located between the coal seam roof 23 and the coal seam floor 25. Then, the outer sleeve 10 and inner sleeve 11 are slowly lowered into the hole as a whole.

[0050] S2. Close the extraction channel (control all solenoid valves 4 to be closed via remote control box 16), open the permeability enhancement channel, start the high-pressure water injection pump 7, inject the fracturing fluid in the storage tank 6 into each outer casing 10 through the water injection main pipe 18, and finally spray it out through the jet nozzle 9. The sprayed high-pressure water flow fracturing the surrounding coal seam 24 causes the surrounding coal seam 24 to form initial fractures.

[0051] During the above-mentioned permeability enhancement process, the high-pressure water injection pump 7 is controlled and adjusted by the frequency converter control cabinet 8 to achieve gradient pressure and flow increase, as detailed below:

[0052] Phase 1: Set the target pressure to 5MPa, control the flow rate to 50L / min, maintain for 5min, the pressure drops by 0.7MPa, and there is no sudden drop in pressure, then proceed to Phase 2;

[0053] Phase Two: Set the target pressure to 10 MPa, control the flow rate to 60 L / min, maintain for 5 minutes, the pressure drops by 1.6 MPa, forming micro-cracks, and proceed to Phase Three;

[0054] Phase 3: Set the target pressure to 15 MPa, control the flow rate to 70 L / min, maintain for 5 minutes, the pressure drops by 2.8 MPa, a small amount of gas overflows, and proceed to Phase 4;

[0055] Phase 4: Set the target pressure to 20 MPa, control the flow rate to 80 L / min, maintain for 5 minutes, the pressure drops by 3.2 MPa, and the initial cracks form.

[0056] S3. Maintain a pressure of 20 MPa and a flow rate of 80 L / min for 40 minutes to ensure uniform fracture formation. At the same time, the high-pressure water jet creates cuts in the coal seam 24 around the borehole, further improving fracture connectivity.

[0057] S4. Gradually reduce the pressure and flow rate until the high-pressure water pump 7 is stopped and the permeability enhancement channel is closed.

[0058] S5. After the excess fracturing fluid in the outer casing 10 is fully discharged with the jet nozzle 9, open the extraction channel (control all solenoid valves 4 to be in the open state through the remote control box 16) and carry out the extraction operation.

[0059] S6. Start the negative pressure extraction pump 3 and stabilize the extraction negative pressure value at -0.02 MPa to slowly discharge the free gas in the fracture after fracturing; then gradually increase the negative pressure to -0.05 MPa and maintain it for a preset time to form a stable negative pressure field in the coal seam fracture to accelerate the conversion of adsorbed gas to free gas. At the same time, start the liquid recovery pump 12; then further increase the negative pressure to -0.08 MPa and maintain it until extraction is completed.

[0060] During the extraction process, the gas and a small amount of gas-liquid-solid mixture in the fractures of coal seam 24 enter the annular space between the outer casing 10 and the inner casing 11 through the jet nozzle 9 on the outer casing 10, and then enter the inner casing 11 through the polytetrafluoroethylene semi-permeable membrane 20 and the gas extraction hole 15. After entering the extraction main pipe 19 from the inner casing 11, it enters the gas-slag-water separator 2. The gas and gas-liquid-solid mixture undergo three-phase separation of gas, liquid and slag through the gas-slag-water separator 2: the separated gas is transported to the gas storage tank 5 from the gas outlet of the gas-slag-water separator 2; the separated liquid is pressurized and transported to the liquid buffer tank 17 through the liquid recovery pump 12 from the liquid outlet of the gas-slag-water separator 2, and then returned to the storage tank 6 from the liquid buffer tank 17 to realize the recovery and reuse of fracturing fluid; the separated coal slag / solid residue is discharged into the residue collection box 1 from the slag discharge end of the gas-slag-water separator 2.

[0061] S7. After extraction is completed, gradually increase the negative pressure value of the negative pressure extraction pump 3 to 0 MPa, stop the negative pressure extraction pump 3, and close the extraction channel; stop the liquid recovery pump 12, and complete the recovery cycle of the liquid buffer tank 17 and the liquid storage tank 6; close the gas inlet of the gas storage tank 5 and seal it; transfer and dispose of the residual material collection box 1. Example 3

[0062] This embodiment takes a gas-outburial-prone hard coal seam with a burial depth of approximately 1030m and a thickness of approximately 6m as an example, and uses the synergistic hydraulic permeability enhancement and semi-permeable membrane enhanced gas extraction system described in Example 1 for extraction, such as... Figure 1 and Figure 2 As shown, the sampling method includes the following steps:

[0063] S1. Drill a hole into the target coal seam. The bottom of the hole passes through rock stratum 1 21 and rock stratum 22 in sequence and reaches the coal seam 24 located between the coal seam roof 23 and the coal seam floor 25. Then, the outer sleeve 10 and inner sleeve 11 are slowly lowered into the hole as a whole.

[0064] S2. Close the extraction channel (control all solenoid valves 4 to be closed via remote control box 16), open the permeability enhancement channel, start the high-pressure water injection pump 7, inject the fracturing fluid in the storage tank 6 into each outer casing 10 through the water injection main pipe 18, and finally spray it out through the jet nozzle 9. The sprayed high-pressure water flow fracturing the surrounding coal seam 24 causes the surrounding coal seam 24 to form initial fractures.

[0065] During the above-mentioned permeability enhancement process, the high-pressure water injection pump 7 is controlled and adjusted by the frequency converter control cabinet 8 to achieve gradient pressure and flow increase, as detailed below:

[0066] Phase 1: Set the target pressure to 5MPa, control the flow rate to 50L / min, maintain for 10min, the pressure drops by 0.5MPa, and there is no sudden drop in pressure, then proceed to Phase 2;

[0067] Phase Two: Set the target pressure to 10 MPa, control the flow rate to 60 L / min, maintain for 10 minutes, and the pressure will drop by 1.5 MPa to form micro-cracks, then proceed to Phase Three;

[0068] Phase 3: Set the target pressure to 15 MPa, control the flow rate to 70 L / min, maintain for 10 minutes, the pressure drops by 2.7 MPa, a small amount of gas overflows, and then proceed to Phase 4;

[0069] Phase 4: Set the target pressure to 20 MPa, control the flow rate to 80 L / min, maintain for 10 minutes, the pressure drops by 3 MPa, and gas still overflows in small amounts, then proceed to Phase 5;

[0070] Phase 5: Set the target pressure to 25 MPa, control the flow rate to 90 L / min, maintain for 10 min, and the pressure will drop by 4 MPa, at which point the initial crack will form.

[0071] S3. Maintain a pressure of 25 MPa and a flow rate of 90 L / min for 40 minutes to ensure uniform fracture formation. At the same time, the high-pressure water jet creates slits in the coal seam 24 surrounding the borehole, further improving fracture connectivity.

[0072] S4. Gradually reduce the pressure and flow rate until the high-pressure water pump 7 is stopped and the permeability enhancement channel is closed.

[0073] S5. After the excess fracturing fluid in the outer casing 10 is fully discharged with the jet nozzle 9, open the extraction channel (control all solenoid valves 4 to be in the open state through the remote control box 16) and carry out the extraction operation.

[0074] S6. Start the negative pressure extraction pump 3 and stabilize the extraction negative pressure value at -0.02 MPa to slowly discharge the free gas in the fracture after fracturing; then gradually increase the negative pressure to -0.05 MPa and maintain it for a preset time to form a stable negative pressure field in the coal seam fracture to accelerate the conversion of adsorbed gas to free gas. At the same time, start the liquid recovery pump 12; then further increase the negative pressure to -0.08 MPa and maintain it until extraction is completed.

[0075] During the extraction process, the gas and a small amount of gas-liquid-solid mixture in the fractures of coal seam 24 enter the annular space between the outer casing 10 and the inner casing 11 through the jet nozzle 9 on the outer casing 10, and then enter the inner casing 11 through the polytetrafluoroethylene semi-permeable membrane 20 and the gas extraction hole 15. After entering the extraction main pipe 19 from the inner casing 11, it enters the gas-slag-water separator 2. The gas and gas-liquid-solid mixture undergo three-phase separation of gas, liquid and slag through the gas-slag-water separator 2: the separated gas is transported to the gas storage tank 5 from the gas outlet of the gas-slag-water separator 2; the separated liquid is pressurized and transported to the liquid buffer tank 17 through the liquid recovery pump 12 from the liquid outlet of the gas-slag-water separator 2, and then returned to the storage tank 6 from the liquid buffer tank 17 to realize the recovery and reuse of fracturing fluid; the separated coal slag / solid residue is discharged into the residue collection box 1 from the slag discharge end of the gas-slag-water separator 2.

[0076] S7. After extraction is completed, gradually increase the negative pressure value of the negative pressure extraction pump 3 to 0 MPa, stop the negative pressure extraction pump 3, and close the extraction channel; stop the liquid recovery pump 12, and complete the recovery cycle of the liquid buffer tank 17 and the liquid storage tank 6; close the gas inlet of the gas storage tank 5 and seal it; transfer and dispose of the residual material collection box 1.

[0077] The embodiments described above are merely examples of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention.

Claims

1. A synergistic hydraulic permeability enhancement and semi-permeable membrane enhanced gas extraction system, characterized in that: It includes a dual-casing structure system, a hydraulically enhanced permeability drive system, a semi-permeable membrane negative pressure gradient extraction system, and a recovery and circulation system; The dual-casing structure system includes multiple outer casings (10), multiple inner casings (11), a main water injection pipe (18), and a main extraction pipe (19); several jet nozzles (9) are evenly distributed on the bottom wall of each outer casing (10); multiple inner casings (11) are coaxially nested inside the corresponding multiple outer casings (10); multiple sets of circumferential gas extraction holes are evenly distributed along the height direction on the wall of each inner casing (11), and each set of circumferential gas extraction holes includes multiple gas extraction holes (15) evenly distributed around the circumference of the inner casing (11); the wall of each inner casing (11) is uniformly fixed at the position corresponding to each set of circumferential gas extraction holes. A polytetrafluoroethylene semi-permeable membrane (20) is fixedly connected, and the polytetrafluoroethylene semi-permeable membrane (20) completely covers the multiple gas extraction holes (15) of each group; the extraction main pipe (19) is set inside the water injection main pipe (18), the water injection main pipe (18) is connected to the top opening of multiple outer sleeves (10), and the extraction main pipe (19) is connected to the top opening of multiple inner sleeves (11); a solenoid valve (4) is installed on the extraction main pipe (19) near the top opening of each inner sleeve (11); the water injection main pipe (18) and multiple outer sleeves (10) form a permeability enhancement channel, and the extraction main pipe (19) and multiple inner sleeves (11) form an extraction channel; The hydraulic permeation drive system includes a storage tank (6), a high-pressure water injection pump (7), a frequency converter control cabinet (8), and a flow sensor (13). The storage tank (6) is connected to the high-pressure water injection pump (7) through a pipeline. The high-pressure water injection pump (7) is connected to the main water injection pipe. The frequency converter control cabinet (8) is electrically connected to the high-pressure water injection pump (7). The flow sensor (13) is installed inside the outer casing (10). The semi-permeable membrane negative pressure gradient extraction system includes a gas-slag-water separator (2), a negative pressure extraction pump (3), a gas storage tank (5), and a gas concentration sensor (14); the extraction main pipe (19) is connected to the gas-slag-water separator (2) through a pipeline, the gas-slag-water separator (2) is connected to the negative pressure extraction pump (3) through a pipeline, and the negative pressure extraction pump (3) is connected to the gas storage tank (5) through a pipeline; the gas concentration sensor (14) is installed in the inner sleeve (11) to monitor the extraction gas concentration; The recycling system includes a residue collection tank (1), a liquid recovery pump (12), and a liquid buffer tank (17). The residue collection tank (1) is connected to the gas-slag-water separator (2) through a pipeline. One end of the liquid recovery pump (12) is connected to the gas-slag-water separator (2) through a pipeline, and the other end of the liquid recovery pump (12) is connected to the liquid buffer tank (17) through a pipeline. The liquid buffer tank (17) is connected to the storage tank (6) through a pipeline.

2. The gas extraction system for synergistic hydraulic permeability enhancement and semi-permeable membrane reinforcement according to claim 1, characterized in that: It also includes a remote control box (16), which is connected to the solenoid valve (4), flow sensor (13), gas concentration sensor (14), frequency converter (8), negative pressure extraction pump (3), and liquid recovery pump (12), respectively.

3. The gas extraction system for synergistic hydraulic permeability enhancement and semi-permeable membrane reinforcement according to claim 1, characterized in that: Both the outer sleeve (10) and the inner sleeve (11) are made of high-strength wear-resistant steel pipe.

4. The extraction method of a gas extraction system with synergistic hydraulic permeability enhancement and semi-permeable membrane reinforcement as described in claim 2, characterized in that, Includes the following steps: S1. Drill a hole into the target coal seam, with the bottom of the hole reaching inside the coal seam (24); then slowly lower the outer casing (10) and inner casing (11) into the hole as a whole. S2. Close the extraction channel, open the permeability enhancement channel, start the high-pressure water injection pump (7), inject the fracturing fluid in the storage tank (6) into each outer casing (10) through the water injection main pipe (18), and finally spray it out through the jet nozzle (9). The sprayed high-pressure water flow fracturing the surrounding coal seam (24) causes the surrounding coal seam (24) to form initial fractures. S3. Maintain the pressure and flow rate at the initial fracture formation time for at least 30 minutes to ensure uniform fracture formation. At the same time, the high-pressure water jet will form a cut in the coal seam (24) around the borehole, further improving the fracture connectivity. S4. Gradually reduce the pressure and flow rate until the high-pressure water pump (7) is stopped and the permeability channel is closed; S5. After the excess fracturing fluid in the outer casing (10) is fully discharged with the jet nozzle (9), open the extraction channel and carry out the extraction operation. S6. Start the negative pressure extraction pump (3) and stabilize the extraction negative pressure value at -0.02 MPa to slowly discharge the free gas in the fracture after fracturing; then gradually increase the negative pressure to -0.05 MPa and maintain it for a preset time to form a stable negative pressure field in the coal seam fracture to accelerate the conversion of adsorbed gas to free gas; then further increase the negative pressure to -0.08 MPa and maintain it until extraction is completed; S7. After the extraction is completed, gradually raise the negative pressure value of the negative pressure extraction pump (3) back to 0 MPa, stop the negative pressure extraction pump (3), and close the extraction channel.

5. The extraction method of a gas extraction system with synergistic hydraulic permeability enhancement and semi-permeable membrane reinforcement according to claim 4, characterized in that: In step S2, the extraction channel is closed by controlling all solenoid valves (4) to be in the closed state through the remote control box (16); the high-pressure water injection pump (7) performs gradient pressure and flow increase: Phase 1: Set the target pressure to 5MPa, control the flow rate to 50L / min, maintain for 5-10 minutes, the pressure drop should be ≤1MPa, and there should be no sudden pressure drop, then proceed to Phase 2; Phase Two: Set the target pressure to 10 MPa, control the flow rate to 60 L / min, maintain for 5-10 minutes, with a pressure drop of 1 MPa < 2 MPa, forming micro-cracks, and proceed to Phase Three; Phase 3: Set the target pressure to 15 MPa, control the flow rate to 70 L / min, maintain for 5-10 minutes, pressure drop between 2 MPa and 3 MPa, and a small amount of gas overflows, then proceed to Phase 4; Stage 4: Set the target pressure to 20 MPa, control the flow rate to 80 L / min, maintain for 5-10 minutes, and when the pressure drops by more than 3 MPa, the initial cracks will form.

6. The extraction method of a gas extraction system with synergistic hydraulic permeability enhancement and semi-permeable membrane reinforcement according to claim 5, characterized in that: In step S5, opening the extraction channel is achieved by controlling all solenoid valves (4) to be in the open state through the remote control box (16).

7. The extraction method of a gas extraction system with synergistic hydraulic permeability enhancement and semi-permeable membrane reinforcement as described in claim 6, characterized in that: In step S6, during the extraction process, the gas and a small amount of entrained gas-liquid-solid mixture in the coal seam (24) fissures enter the annular space between the outer casing (10) and the inner casing (11) through the jet nozzle (9) on the outer casing (10), then enter the inner casing (11) through the polytetrafluoroethylene semi-permeable membrane (20) and the gas extraction hole (15), and then enter the extraction main pipe (19) from the inner casing (11) and then enter the gas-slag-water separator (2); the gas and gas-liquid-solid mixture pass through the gas-slag-water separator. The separator (2) performs three-phase separation of gas, liquid and slag: the separated gas is transported from the gas outlet of the gas-slag-water separator (2) to the gas storage tank (5); the separated liquid is pressurized from the liquid outlet of the gas-slag-water separator (2) and transported to the liquid buffer tank (17) by the liquid recovery pump (12), and then returned from the liquid buffer tank (17) to the storage tank (6) to realize the recovery and reuse of fracturing fluid; the separated coal slag / solid residue is discharged from the slag discharge end of the gas-slag-water separator (2) into the residue collection box (1).

8. The extraction method of a gas extraction system with synergistic hydraulic permeability enhancement and semi-permeable membrane reinforcement according to claim 7, characterized in that: In step S7, the liquid recovery pump (12) is stopped, the liquid buffer tank (17) and the liquid storage tank (6) complete the recovery cycle; the gas storage tank (5) is closed and sealed; the residual material collection box (1) is transferred and disposed of.

9. The extraction method of a gas extraction system with synergistic hydraulic permeability enhancement and semi-permeable membrane reinforcement according to claim 5, characterized in that: In stage four, if the pressure drop remains ≤3MPa after 5-10 minutes, the next stage of gradient pressure and flow increase with an amplitude of 5MPa and 10L / min continues until the current stage is maintained for 5-10 minutes and the pressure drop is >3MPa, thus forming the initial fracture.

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