A coal seam tunnel permeability enhancement system based on supercritical CO2-pulse water composite injection and a permeability enhancement method thereof

Abstract

The application discloses a coal seam tunnel permeability enhancement system based on supercritical CO2-pulse water composite injection and a permeability enhancement method thereof, and belongs to the technical field of high ground stress and low permeability coal seam tunnel gas extraction and permeability enhancement. The system comprises a supercritical CO2 injection unit, a pulse water injection unit, an intelligent monitoring and control system and a rotating nozzle assembly. The application forms a multi-fracture network through the high permeability of supercritical CO2, and then uses the high-frequency impact of pulse water to enlarge the fracture opening and enhance the connectivity, while reducing the water consumption to reduce the water lock effect, so that it is adapted to the permeability enhancement demand of high ground stress and low permeability coal seams. The application uses supercritical CO2 and pulse water to synergistically enhance permeability, effectively enlarges the permeability enhancement radius, and has obvious effect improvement compared with the traditional hydraulic fracturing, and the connectivity of the fracture network is high. In the application, the rotating injection of pulse water enables the uniform distribution of energy, and avoids the hole wall collapse caused by excessive fracturing.

Description

Technical Field

[0001] This invention belongs to the field of gas extraction and permeability enhancement technology in high-stress, low-permeability coal seam tunnels. Specifically, it is a coal seam tunnel permeability enhancement system and method based on supercritical CO2-pulse water composite injection. Background Technology

[0002] Deep coal seams are characterized by high pressure (high ground stress, high gas pressure, and high fracture water pressure), high temperature, and low permeability. Therefore, the core challenge in gas extraction through tunnels in deep coal seams lies in improving the permeability of the coal seam. Traditional permeability enhancement technologies are difficult to effectively enhance the permeability of coal seams with high ground stress, high gas pressure, and low permeability.

[0003] Specifically, if only hydraulic fracturing is used, the coal seam can only be fractured at a pressure ≥30MPa. However, high pressure easily leads to borehole wall collapse, and a single hydraulic fracturing can only form 1-2 main fractures with a small expansion range. Furthermore, because the water after hydraulic fracturing remains in the pores of the coal seam, the permeability of the coal seam decreases, making the subsequent gas drainage efficiency significantly affected by the water-locking effect (the attenuation value is as high as 40% after 6 months). If only supercritical CO2 is used, although multiple fractures can be formed, the fracture opening is small, the channel stability is poor, and it is easy to close. While pulsed water can be used to improve permeability in a small area of ​​the borehole, its radius of action is small and cannot penetrate deep into the coal seam. Moreover, high-pressure pulsed water can cause the soft coal seam to become muddy, which may actually reduce permeability. Field engineering tests show that the gas drainage capacity of traditional technology is only 0.1~0.3 m³ / min, and the drainage time to reach the standard is more than 45 days, which cannot meet the needs of rapid tunnel construction.

[0004] Therefore, developing a composite permeability enhancement technology that combines "wide-range fracturing, strong interconnected fractures, and low water-locking impact" is key to solving the problem of deep coal seam gas control. Summary of the Invention

[0005] The purpose of this invention is to address the problems existing in the prior art by providing a coal seam tunnel permeability enhancement system and method based on supercritical CO2-pulsed water composite injection. This invention utilizes the high permeability of supercritical CO2 to form a multi-fracture network, and then leverages the high-frequency impact of pulsed water to expand the fracture aperture and enhance connectivity, while simultaneously reducing water consumption to mitigate the water-locking effect, thus adapting it to the permeability enhancement requirements of high-stress, low-permeability coal seams.

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

[0007] In one aspect, the present invention provides a coal seam tunnel permeability enhancement system based on supercritical CO2-pulsed water composite injection, comprising a supercritical CO2 injection unit, a pulsed water injection unit, an intelligent monitoring and control system, and a rotating nozzle assembly.

[0008] The supercritical CO2 injection unit includes a supercritical CO2 generator, a supercritical CO2 injection pipe, and a leak-proof structure. The supercritical CO2 generator includes a CO2 storage tank, a heater, and a high-pressure pump. The CO2 storage tank is connected to the heater, the heater is connected to the high-pressure pump, and the high-pressure pump is connected to the injection end of the supercritical CO2 injection pipe. The leak-proof structure includes expanded graphite, which is sealed in the gap between the supercritical CO2 injection pipe and the borehole.

[0009] The pulsed water injection unit includes a high-pressure pulsed water pump and a pulsed water injection pipe; one end of the high-pressure pulsed water pump is connected to an external water source, and the other end is connected to the injection end of the pulsed water injection pipe, which is inserted and fixed inside the supercritical CO2 injection pipe.

[0010] The intelligent monitoring and control system includes an intelligent monitoring unit and an intelligent control unit; the intelligent monitoring unit includes various parameter monitoring sensors; the intelligent control unit includes a PLC controller and a monitor, the PLC controller and the monitor are connected, and the monitor is connected to the intelligent monitoring unit, the supercritical CO2 injection unit, and the pulsed water injection unit.

[0011] The rotary nozzle assembly includes a pulsed water-driven rotary nozzle, which is rotatably connected to the bottom of the supercritical CO2 injection pipe and the pulsed water injection pipe via a rotary joint. The high frequency and high pressure of the pulsed water can drive the pulsed water-driven rotary nozzle to rotate automatically, and the rotation speed is adjusted by the flow rate of the pulsed water. Several nozzles are evenly distributed around the circumference of the pulsed water-driven rotary nozzle, and the spray angle of the nozzles covers the entire borehole.

[0012] Furthermore, the supercritical CO2 injection pipe is made of Φ32mm stainless steel with a pressure resistance rating of ≥40MPa, and its inner wall is polished to reduce flow resistance during the injection process.

[0013] Furthermore, the high-pressure pulse water pump uses a three-plunger reciprocating pump to regulate the pulse water, setting the maximum pressure to 25MPa, adjusting the pulse frequency range to 10~15Hz, and setting the water flow rate to 10~15L / min; the pulse water injection pipe uses a Φ16mm stainless steel pipe, and the pulse water injection pipe is coaxially set with the supercritical CO2 injection pipe.

[0014] Furthermore, various parameter monitoring sensors, including pressure sensors, flow sensors, and temperature sensors, are all connected to the monitor. Among them, pressure sensors and temperature sensors are installed on the supercritical CO2 injection pipe to monitor the pressure and temperature of supercritical CO2 in real time, and pressure sensors and flow sensors are installed on the pulsed water injection pipe to monitor the pressure and flow of pulsed water in real time. The PLC controller is a PLC controller equipped with a fuzzy control algorithm.

[0015] Further, the rotation speed of the pulsed hydraulic-driven rotary nozzle is controllably adjustable within 0 - 30 r / min; twelve nozzles are evenly distributed, the angle between adjacent nozzles is 30°, and the nozzle outlet diameter is 2 mm.

[0016] In another aspect of the present invention, a permeability enhancement method for the above-described coal seam tunnel permeability enhancement system based on supercritical CO2 - pulsed water compound injection is provided, including the following steps:

[0017] S1. Use a drilling rig to drill holes in the coal seam of the permeability enhancement area.

[0018] S2. Lower the supercritical CO2 injection pipe and the pulsed water injection pipe coaxially arranged therein into the borehole, install a rotary nozzle assembly at the end of the pipe body, and simultaneously use expanded graphite material to seal the gap between the pipe body and the borehole inlet end.

[0019] S3. Connect the supercritical CO2 injection unit, the pulsed water injection unit, and the intelligent monitoring and control system (connect the CO2 storage tank, heater, high-pressure pump, supercritical CO2 injection pipe, high-pressure pulsed water pump, pulsed water injection pipe, pressure sensor, flow sensor, temperature sensor, PLC controller, monitor), set the injection pressure and temperature parameters of the supercritical CO2 injection unit, and set the pressure and operating frequency of the pulsed water injection unit.

[0020] S4. Conduct a leak test, introduce compressed air at 2 MPa, and test the pressure drop within 30 min. If the pressure drop within 30 min is less than 1%, the seal is considered qualified.

[0021] S5. Conduct parameter debugging. Heat the supercritical CO2 injection unit to 35°C and pressurize it to 22 MPa, observe and ensure the stability of the supercritical state, and adjust the flow rate of the supercritical CO2 injection unit; set the frequency, pressure, and flow rate of the pulsed water injection unit, test the rotary drilling speed of the rotary nozzle assembly driven by the pulsed water flow rate; calibrate the intelligent monitoring and control system and check the stability of data transmission.

[0022] S6. Start the supercritical CO2 injection unit, inject supercritical CO2 into the coal seam, and stabilize the injection pressure at 22 MPa, observing the opening of the coal seam fissures; when stable and dense fissures are formed, conduct composite permeability enhancement, start the pulsed water injection unit, inject pulsed water, and make the rotary nozzle assembly rotate driven by the pulsed water. At this time, under the synergistic action of supercritical CO2 and pulsed water, the coal seam fissures expand and the permeability enhancement radius expands.

[0023] S7. Adjust the parameters in real time. When the hole wall pressure exceeds the critical value of 48 MPa, reduce the supercritical CO2 injection rate.

[0024] S8. After a stable fracture network is formed, the supercritical CO2 pressure is gradually reduced to 10 MPa, and the pulse water pressure is simultaneously reduced to 10 MPa to avoid rapid pressure reduction that could lead to pore collapse.

[0025] S9. Shut down the supercritical CO2 injection unit and the pulsed water injection unit, and recover the residual supercritical CO2 through a conventional recovery device to avoid direct venting.

[0026] Furthermore, in step S1, the drilling rig used is a ZDY-4500L drilling rig with a borehole diameter of Φ113mm, a drilling speed of 30r / min, a thrust of 280kN, and compressed air for slag removal.

[0027] Furthermore, in step S5, the flow rate of the supercritical CO2 injection unit is adjusted to 8 L / min; the frequency of the pulse water injection unit is set to 15 Hz, the pressure to 25 MPa, and the flow rate to 15 L / min; the rotational speed of the rotating nozzle assembly is tested to be 30 r / min.

[0028] Furthermore, in step S6, supercritical CO2 is injected into the coal seam at a rate of 8 L / min; the frequency of the injected pulse water is 15 Hz, the pressure is 25 MPa, and the flow rate is 15 L / min; the rotational speed of the rotating nozzle assembly is 30 r / min.

[0029] Furthermore, in step S7, the supercritical CO2 injection rate is reduced to 6 L / min.

[0030] The core innovation of this invention lies in the following: 1) This invention designs a "supercritical CO2-pulsed water composite permeability enhancement system" to achieve synergistic effects of "multi-fracture fracturing + wide-range fracture widening," thereby expanding the permeability enhancement radius. 2) This invention optimizes the parameter matching between supercritical CO2 and pulsed water, enhancing both fracturing aperture and fracture stability. 3) This invention integrates an intelligent monitoring and control module, adapting to coal seam parameters in real time, preventing borehole collapse, and reducing the water-locking effect.

[0031] In summary, compared with the prior art, the present invention has the following beneficial effects:

[0032] 1) This invention utilizes supercritical CO2 and pulsed water to synergistically enhance permeability, effectively expanding the permeability radius and significantly improving the effect compared to traditional hydraulic fracturing, while also achieving high connectivity of the fracture network.

[0033] 2) The amount of water used in the pulsed water in this invention is relatively small, which reduces the side effects of water lock effect and improves the stability of extraction.

[0034] 3) In this invention, the rotational injection of pulsed water ensures uniform energy distribution, avoiding the collapse of the borehole wall caused by excessive fracturing.

[0035] 4) This invention has intelligent monitoring and dynamic adjustment functions, which can match coal seam parameters and fracturing permeability enhancement parameters in real time, ensuring that permeability enhancement and safety can be carried out in parallel. Attached Figure Description

[0036] To more clearly illustrate the technical solutions of the specific embodiments of the present invention, the accompanying drawings used in the specific embodiments will be briefly introduced below. In the drawings, the elements or parts are not necessarily drawn to actual scale.

[0037] Figure 1 A schematic diagram of the overall structure of the antireflective system for invention.

[0038] Figure 2 This is a schematic diagram of the rotating nozzle assembly in the anti-reflection system of the present invention.

[0039] Figure 3 This is a schematic diagram of the underground well layout, drilling, and permeability enhancement method of the present invention.

[0040] In the diagram: 1-CO2 storage tank; 2-heater; 3-high pressure pump; 4-supercritical CO2 injection pipe; 5-high pressure pulse water pump; 6-pulse water injection pipe; 7-motor; 8-PLC controller; 9-monitor; 10-pulse hydraulically driven rotary nozzle; 11-nozzle; 12-rotary joint; 13-pressure sensor and temperature sensor; 14-initial anti-permeability fracture network; 15-composite anti-permeability fracture network; 16-expanded graphite; 17-first rock stratum; 18-second rock stratum; 19-third rock stratum; 20-roof; 21-target coal seam; 22-floor. Detailed Implementation

[0041] The embodiments of the technical solution of the present invention will now be described in detail with reference to the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solution of the present invention, and are therefore merely examples and should not be used to limit the scope of protection of the present invention. It should be noted that, unless otherwise stated, the technical or scientific terms used in this application should have the ordinary meaning understood by those skilled in the art to which this invention pertains. Example 1

[0042] like Figure 1 and Figure 2 As shown, this embodiment provides a coal seam tunnel permeability enhancement system based on supercritical CO2-pulsed water composite injection, including a supercritical CO2 injection unit, a pulsed water injection unit, an intelligent monitoring and control system, and a rotating nozzle assembly.

[0043] The supercritical CO2 injection unit includes a supercritical CO2 generator, a supercritical CO2 injection pipe 4, and a leak-proof structure. The supercritical CO2 generator includes a CO2 storage tank 1, a heater 2, and a high-pressure pump 3. The CO2 storage tank 1 is connected to the heater 2, the heater 2 is connected to the high-pressure pump 3, and the high-pressure pump 3 is connected to the injection end of the supercritical CO2 injection pipe 4. The high-pressure pump 3 is driven by a motor 7. The leak-proof structure includes expanded graphite 16, which is sealed within the gap between the supercritical CO2 injection pipe 4 and the borehole. The supercritical CO2 injection pipe 4 is made of Φ32mm stainless steel with a pressure resistance rating of ≥40MPa, and its inner wall is polished to reduce flow resistance during the injection process.

[0044] The pulsed water injection unit includes a high-pressure pulsed water pump 5 and a pulsed water injection pipe 6. One end of the high-pressure pulsed water pump 5 is connected to an external water source, and the other end is connected to the injection end of the pulsed water injection pipe 6. The high-pressure pulsed water pump 5 is driven by a motor 7, and the pulsed water injection pipe 6 is inserted and fixed inside the supercritical CO2 injection pipe 4. The high-pressure pulsed water pump 5 uses a three-plunger reciprocating pump to regulate the pulsed water, with a maximum pressure of 25MPa, an adjustable pulse frequency range of 10~15Hz, and a water flow rate of 10~15L / min. The pulsed water injection pipe 6 is made of Φ16mm stainless steel and is coaxially arranged with the supercritical CO2 injection pipe 4.

[0045] The intelligent monitoring and control system includes an intelligent monitoring unit and an intelligent control unit. The intelligent monitoring unit includes various parameter monitoring sensors. The intelligent control unit includes a PLC controller 8 and a monitor 9, which are connected. The monitor 9 is connected to the intelligent monitoring unit, the supercritical CO2 injection unit, and the pulsed water injection unit. The various parameter monitoring sensors include pressure sensors, flow sensors, and temperature sensors, all of which are connected to the monitor 9. Pressure sensors and temperature sensors 13 are installed on the supercritical CO2 injection pipe 4 to monitor the pressure and temperature of the supercritical CO2 in real time. Pressure sensors and flow sensors are installed on the pulsed water injection pipe 6 to monitor the pressure and flow rate of the pulsed water in real time. The PLC controller 8 is a PLC controller equipped with a fuzzy control algorithm.

[0046] The rotary sprinkler assembly includes a pulsed hydraulic-driven rotary sprinkler 10. The pulsed hydraulic-driven rotary sprinkler 10 is hermetically and rotationally connected to the bottoms of the supercritical CO2 injection pipe 4 and the pulsed water injection pipe 6 through a rotary joint 12. Moreover, the pulsed hydraulic-driven rotary sprinkler 10 is connected to both the supercritical CO2 injection pipe 4 and the pulsed water injection pipe 6. The high-frequency and high-pressure pulsed water can drive the pulsed hydraulic-driven rotary sprinkler 10 to rotate automatically, and the rotation speed is adjusted by the flow rate of the pulsed water. The maximum rotation speed of the pulsed hydraulic-driven rotary sprinkler 10 can be adjusted to 30 r / min. Twelve nozzles 11 are evenly distributed along the circumference of the pulsed hydraulic-driven rotary sprinkler 10. The included angle between adjacent nozzles 11 is 30°. The nozzle diameter of the nozzles 11 is 2 mm, and the spraying angles of the twelve nozzles 11 cover the entire borehole. Embodiment 2

[0047] This embodiment provides a permeability enhancement method for the coal seam tunnel permeability enhancement system based on supercritical CO2-pulsed water composite injection described in Embodiment 1. As Figure 3 shown, it includes the following steps:

[0048] S1. Use a drill rig to drill holes in the coal seam of the permeability enhancement area.

[0049] In this step, the drill rig uses a ZDY-4500L drill rig, the borehole diameter is Φ113 mm, the drilling rotation speed is 30 r / min, the propulsion force is 280 kN, and the compressed air is used for slag discharge. Drill through the first rock layer 17, the second rock layer 18, and the third rock layer 19 in sequence until reaching the target coal seam 21 between the roof 20 and the floor 22.

[0050] S2. Lower the supercritical CO2 injection pipe 4 and the pulsed water injection pipe 6 coaxially arranged therein into the borehole, install a rotary sprinkler assembly at the end of the pipe body, and at the same time use expanded graphite 16 material to seal the gap between the pipe body and the borehole inlet end.

[0051] S3. Connect the supercritical CO2 injection unit, the pulsed water injection unit, and the intelligent monitoring and control system (the CO2 storage tank 1, the heater 2, the high-pressure pump 3, the supercritical CO2 injection pipe 4, the high-pressure pulsed water pump 5, the pulsed water injection pipe 6, the pressure sensor, the flow sensor, the temperature sensor, the PLC controller 8, and the monitor 9 are connected), set the injection pressure and temperature parameters of the supercritical CO2 injection unit, and set the pressure and operating frequency of the pulsed water injection unit.

[0052] S4. Conduct a leakage test, introduce 2 MPa of compressed air, and test the pressure drop degree within 30 min. If the pressure drop within 30 min is less than 1%, it is considered that the seal is qualified.

[0053] S5. Perform parameter debugging: heat the supercritical CO2 injection unit to 35℃ and pressurize it to 22MPa, observe and ensure the supercritical state is stable, and adjust the flow rate of the supercritical CO2 injection unit; set the frequency, pressure and flow rate of the pulse water injection unit, and test the rotary drilling speed driven by the pulse water flow of the rotary nozzle assembly; calibrate the intelligent monitoring and control system and check whether the data transmission is stable.

[0054] In this step, the flow rate of the supercritical CO2 injection unit was adjusted to 8 L / min; the frequency of the pulse water injection unit was set to 15 Hz, the pressure to 25 MPa, and the flow rate to 15 L / min; the rotational speed of the rotating nozzle assembly was measured to be 30 r / min.

[0055] S6. Activate the supercritical CO2 injection unit to inject supercritical CO2 into the coal seam, stabilize the injection pressure at 22 MPa, and observe the coal seam fracture aperture. When stable and dense fractures are formed, perform composite permeability enhancement by activating the pulsed water injection unit and injecting pulsed water. The rotating nozzle assembly rotates under the drive of the pulsed water. At this time, under the synergistic effect of supercritical CO2 and pulsed water, the coal seam fractures expand and the permeability enhancement radius expands.

[0056] In this step, supercritical CO2 is injected into the coal seam at a rate of 8 L / min; the frequency of the injected pulsed water is 15 Hz, the pressure is 25 MPa, and the flow rate is 15 L / min; the rotational speed of the rotating nozzle assembly is 30 r / min.

[0057] S7. Adjust parameters in real time. When the borehole wall pressure exceeds the critical value of 48MPa, reduce the supercritical CO2 injection rate.

[0058] In this step, the supercritical CO2 injection rate is reduced to 6 L / min.

[0059] S8. After a stable fracture network is formed, the supercritical CO2 pressure is gradually reduced to 10 MPa, and the pulse water pressure is simultaneously reduced to 10 MPa to avoid rapid pressure reduction that could lead to pore collapse.

[0060] In this step, the stable fracture network includes an initial anti-permeability fracture network 14 and a composite anti-permeability fracture network 15. The initial anti-permeability fracture network 14 is located within the supercritical CO2 diffusion range, and the composite anti-permeability fracture network 15 is located within the pulsed water expansion fracture region.

[0061] S9. Shut down the supercritical CO2 injection unit and the pulsed water injection unit, and recover the residual supercritical CO2 through a conventional recovery device to avoid direct venting. Example 3

[0062] In this example, a tunnel is constructed through a low-permeability coal seam with a burial depth of 980m. The permeability coefficient of this coal seam is only 0.001m² / (MPa²・d), the ground stress is 44.42MPa, the gas pressure is 1.8MPa, and the firmness coefficient f=0.6-0.8. It belongs to the "three highs and one low" coal seam.

[0063] Based on Examples 1 and 2, the anti-reflection system is used for anti-reflection construction, which specifically includes the following steps:

[0064] 1) Preparation stage

[0065] For drilling operations, a ZDY-4500L drilling rig was used to drill a Φ113mm hole. The drilling parameters were: rotation speed 30r / min, thrust 280kN, and compressed air for slag removal.

[0066] Injection pipe arrangement: The Φ32mm supercritical CO2 injection pipe 4 (including the coaxially set Φ16mm pulse water injection pipe 6) is lowered into the borehole. A rotating nozzle assembly is installed at the end of the pipe, and the gap between the pipe body and the borehole inlet end is sealed with expanded graphite 16 material.

[0067] Equipment Connections: Connect the supercritical CO2 injection unit, pulsed water injection unit, and intelligent monitoring and control system;

[0068] Leakage test: Pass 2MPa compressed air through, and the pressure drop should be ≤0.02MPa after 30 minutes to ensure a qualified seal.

[0069] 2) Debugging parameters

[0070] Supercritical CO2 injection unit commissioning: Heat to 35℃, pressurize to 22MPa to ensure supercritical stability, and control the flow rate to 8L / min;

[0071] Pulse water injection unit debugging: set frequency 15Hz, pressure 25MPa, flow rate 15L / min, and test the rotation speed of the rotary nozzle assembly at 30r / min;

[0072] Intelligent monitoring and control system debugging: calibrate the pressure sensor (error ±0.05MPa) and flow sensor (error ±0.05L / min) to ensure normal data transmission.

[0073] 3) Supercritical CO2-induced fracturing stage (30 min)

[0074] The supercritical CO2 injection unit was activated, and supercritical CO2 was injected into the coal seam at a rate of 8 L / min, with the pressure stabilizing at 22 MPa. Observation with a borehole inspection instrument revealed the formation of micro-fractures with a density of 5 fractures / m (aperture 0.3~0.5 mm). The borehole wall pressure was monitored at 38~42 MPa, with no abnormal fluctuations.

[0075] 4) Composite anti-reflection stage (60 min)

[0076] Simultaneously start the pulsed water injection unit with a frequency of 15 Hz, a pressure of 25 MPa, and a flow rate of 15 L / min; the rotating nozzle assembly rotates at 30 r / min driven by pulsed water. Supercritical CO2 and pulsed water act synergistically, expanding the fracture aperture to 1.8 mm and the expansion radius to 12 m.

[0077] 5) Real-time parameter adjustment

[0078] When the local hole wall pressure is monitored to reach 48 MPa, reduce the supercritical CO2 injection rate to 6 L / min.

[0079] 6) Final stage (10 min)

[0080] Gradually reduce the supercritical CO2 pressure (from 22 MPa to 10 MPa) and simultaneously reduce the pulsed water pressure (from 25 MPa to 10 MPa); shut down the equipment and recover the residual CO2 (recover it into the storage tank through the recovery pump) to avoid direct emission. Example 4

[0081] In this example, take the coal seam permeability enhancement and gas extraction in the tunnel of a low-permeability coal seam section at a burial depth of 1150 m as an example.

[0082] Based on Example 1 and Example 2, carry out permeability enhancement and gas extraction construction through the said permeability enhancement system, which specifically includes the following steps:

[0083] 1) Drill holes (Φ113 mm) in the coal seam of this permeability enhancement area and set the drilling parameters (drilling speed: 30 r / min, thrust: 280 kN).

[0084] 2) Lower the Φ32 mm supercritical CO2 injection pipe 4 and the Φ16 mm pulsed water injection pipe 6 inside it into the drill hole, install a rotating nozzle assembly at the end of the pipe, and use expanded graphite 16 material to seal the gap between the pipe body and the drill hole inlet end.

[0085] 3) Connect the supercritical CO2 injection unit, pulsed water injection unit, and intelligent monitoring and control system, set the injection pressure and temperature parameters of the supercritical CO2 injection unit, and set the pressure and operating frequency of the pulsed water injection unit.

[0086] 4) Conduct a leakage test, introduce 2 MPa of compressed air, and test the pressure drop degree within 30 min. If the pressure drop within 30 min is less than 1%, the seal is qualified.

[0087] 5) Parameter adjustment: Heat the supercritical CO2 injection unit to 35℃ and pressurize it to 22MPa. Observe and ensure that the supercritical state is stable. Adjust the flow rate of the supercritical CO2 injection unit to 8L / min. Set the frequency of the pulse water injection unit to 15Hz, the pressure to 25MPa, and the flow rate to 15L / min. Test the rotation speed of the rotating nozzle assembly to 30r / min.

[0088] 6) Activate the supercritical CO2 injection unit and inject supercritical CO2 into the coal seam at a rate of 8 L / min. Maintain a stable injection pressure of 22 MPa. Observe the coal seam fracture aperture through a borehole inspection instrument. When stable and dense fractures are formed, perform composite permeability enhancement by activating the pulsed water injection unit and injecting pulsed water at a flow rate of 15 L / min. This causes the rotating nozzle assembly to rotate under the drive of the pulsed water. At this time, under the synergistic effect of supercritical CO2 and pulsed water, the coal seam fractures expand and the permeability enhancement radius increases.

[0089] 7) Adjust parameters in real time. When the borehole wall pressure exceeds the critical value of 48MPa, reduce the supercritical CO2 injection rate to 6L / min.

[0090] 8) After a stable fracture network (5m / fracture) is formed, the supercritical CO2 pressure is gradually reduced to 10MPa, and the pulse water pressure is simultaneously reduced to 10MPa to avoid rapid pressure reduction that could cause pore collapse.

[0091] 9) Shut down the supercritical CO2 injection unit and the pulsed water injection unit, and recover the residual supercritical CO2 through a conventional recovery device to avoid direct venting.

[0092] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention, and they should all be covered within the scope of the claims and specification of the present invention.

Claims

1. A coal seam tunnel permeability enhancement system based on supercritical CO2-pulsed water composite injection, characterized in that: It includes a supercritical CO2 injection unit, a pulsed water injection unit, an intelligent monitoring and control system, and a rotating nozzle assembly; The supercritical CO2 injection unit includes a supercritical CO2 generator, a supercritical CO2 injection pipe (4), and a leak-proof structure. The supercritical CO2 generator includes a CO2 storage tank (1), a heater (2), and a high-pressure pump (3). The CO2 storage tank (1) is connected to the heater (2), the heater (2) is connected to the high-pressure pump (3), and the high-pressure pump (3) is connected to the injection end of the supercritical CO2 injection pipe (4). The leak-proof structure includes expanded graphite (16), which is sealed in the gap between the supercritical CO2 injection pipe (4) and the borehole. The pulse water injection unit includes a high-pressure pulse water pump (5) and a pulse water injection pipe (6); one end of the high-pressure pulse water pump (5) is connected to an external water source and the other end is connected to the injection end of the pulse water injection pipe (6), and the pulse water injection pipe (6) is inserted and fixed inside the supercritical CO2 injection pipe (4); The intelligent monitoring and control system includes an intelligent monitoring unit and an intelligent control unit; the intelligent monitoring unit includes various parameter monitoring sensors; the intelligent control unit includes a PLC controller (8) and a monitor (9), the PLC controller (8) and the monitor (9) are connected, and the monitor (9) is connected to the intelligent monitoring unit, the supercritical CO2 injection unit and the pulse water injection unit; The rotary nozzle assembly includes a pulsed water-driven rotary nozzle (10), which is connected to the bottom of the supercritical CO2 injection pipe (4) and the pulsed water injection pipe (6) via a rotary joint (12). The pulsed water-driven rotary nozzle (10) can be automatically rotated by the high frequency and high pressure of the pulsed water, and the rotation speed is adjusted by the flow rate of the pulsed water. Several nozzles (11) are evenly distributed around the pulsed water-driven rotary nozzle (10), and the spray angle of the nozzles (11) covers the entire borehole.

2. The coal seam tunnel permeability enhancement system based on supercritical CO2-pulsed water composite injection according to claim 1, characterized in that: The supercritical CO2 injection pipe (4) is made of Φ32mm stainless steel pipe with a pressure resistance rating of ≥40MPa. Its inner wall is polished to reduce flow resistance during the injection process.

3. The coal seam tunnel permeability enhancement system based on supercritical CO2-pulsed water composite injection according to claim 2, characterized in that: The high-pressure pulse water pump (5) uses a three-plunger reciprocating pump to regulate the pulse water, sets the maximum pressure to 25MPa, adjusts the pulse frequency range to 10~15Hz, and sets the water flow rate to 10~15L / min; the pulse water injection pipe (6) is made of Φ16mm stainless steel pipe, and the pulse water injection pipe (6) is coaxially set with the supercritical CO2 injection pipe (4).

4. The coal seam tunnel permeability enhancement system based on supercritical CO2-pulsed water composite injection according to claim 3, characterized in that: Various parameter monitoring sensors include pressure sensors, flow sensors and temperature sensors. All pressure sensors, flow sensors and temperature sensors are connected to the monitor (9). Among them, pressure sensors and temperature sensors are installed on the supercritical CO2 injection pipe (4) to monitor the pressure and temperature of supercritical CO2 in real time. Pressure sensors and flow sensors are installed on the pulse water injection pipe (6) to monitor the pressure and flow of pulse water in real time. The PLC controller (8) adopts a PLC controller (8) equipped with a fuzzy control algorithm.

5. A coal seam tunnel permeability enhancement system based on supercritical CO2-pulsed water composite injection according to claim 4, characterized in that: The rotational speed of the pulsed hydraulic-driven rotary nozzle (10) is controllably adjustable within 0 - 30 r / min; twelve nozzles (11) are evenly distributed, the angle between adjacent nozzles (11) is 30°, and the nozzle diameter of the nozzle (11) is 2 mm.

6. The permeability enhancement method for a coal seam tunnel permeability enhancement system based on supercritical CO2-pulsed water composite injection as described in claim 5, characterized in that, It includes the following steps: S1. Use a drilling rig to drill holes in the coal seam in the permeability enhancement area; S2. Lower the supercritical CO2 injection pipe (4) and the pulsed water injection pipe (6) coaxially arranged therein into the drilled hole, install a rotary nozzle assembly at the end of the pipe body, and at the same time use expanded graphite (16) material to seal the gap between the pipe body and the inlet end of the drilled hole; S3. Connect the supercritical CO2 injection unit, the pulsed water injection unit, and the intelligent monitoring and control system, set the injection pressure and temperature parameters of the supercritical CO2 injection unit, and set the pressure and operating frequency of the pulsed water injection unit; S4. Conduct a leakage test, introduce compressed air at 2 MPa, and test the pressure drop within 30 min. If the pressure drop within 30 min is less than 1%, the seal is considered qualified; S5. Conduct parameter debugging, heat up the supercritical CO2 injection unit to 35 °C, pressurize it to 22 MPa, observe and ensure the stability of the supercritical state, and adjust the flow rate of the supercritical CO2 injection unit; Set the frequency, pressure, and flow rate of the pulsed water injection unit, and test the rotary drilling speed of the rotary nozzle assembly driven by the pulsed water flow rate; Calibrate the intelligent monitoring and control system and check the stability of data transmission; S6. Start the supercritical CO2 injection unit, inject supercritical CO2 into the coal seam, stabilize the injection pressure at 22 MPa, and observe the opening degree of the coal seam fissures; When stable and dense fissures are formed, conduct composite permeability enhancement, start the pulsed water injection unit, inject pulsed water, and make the rotary nozzle assembly rotate under the drive of the pulsed water. At this time, under the synergistic action of supercritical CO2 and pulsed water, the coal seam fissures expand and the permeability enhancement radius expands; S7. Adjust the parameters in real time. When the hole wall pressure exceeds the critical value of 48 MPa, reduce the supercritical CO2 injection rate; S8. After a stable fissure network is formed, gradually reduce the supercritical CO2 pressure to 10 MPa and synchronously reduce the pulsed water pressure to 10 MPa to avoid pore collapse caused by rapid pressure reduction; S9. Shut down the supercritical CO2 injection unit and the pulsed water injection unit, and recover the residual supercritical CO2 through a conventional recovery device to avoid direct evacuation.

7. The permeability enhancement method for a coal seam tunnel permeability enhancement system based on supercritical CO2-pulsed water composite injection according to claim 6, characterized in that: In step S1, the drilling rig uses a ZDY-4500L drilling rig, the drilling hole diameter is Φ113 mm, the drilling rotational speed is 30 r / min, the propulsion force is 280 kN, and air is used for slag removal.

8. The permeability enhancement method for a coal seam tunnel permeability enhancement system based on supercritical CO2-pulsed water composite injection according to claim 6, characterized in that: In step S5, adjust the flow rate of the supercritical CO2 injection unit to 8 L / min; Set the frequency of the pulsed water injection unit to 15 Hz, the pressure to 25 MPa, and the flow rate to 15 L / min; the rotational speed of the rotary nozzle assembly is tested to be 30 r / min.

9. The permeability enhancement method for a coal seam tunnel permeability enhancement system based on supercritical CO2-pulsed water composite injection according to claim 6, characterized in that: In step S6, inject supercritical CO2 into the coal seam at a rate of 8 L / min; the frequency of the injected pulsed water is 15 Hz, the pressure is 25 MPa, and the flow rate is 15 L / min; the rotational speed of the rotary nozzle assembly is 30 r / min.

10. The permeability enhancement method for a coal seam tunnel permeability enhancement system based on supercritical CO2-pulsed water composite injection according to claim 6, characterized in that: In step S7, reduce the supercritical CO2 injection rate to 6 L / min.

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