Device and method for enhancing gas extraction from low-gas coal seams by coupling ultrasonic excitation with nitrogen injection and pressurization

Abstract

The present invention discloses a device and method for enhancing gas extraction from low-gas coal seams by coupling ultrasonic excitation with nitrogen injection and pressurization. The device comprises an enhanced permeability borehole and an investigation borehole, each borehole being equipped with an extraction screen. An integrated device for coupling ultrasonic excitation with nitrogen injection and pressurization is also installed in the extraction screen of the enhanced permeability borehole. The integrated device for coupling ultrasonic excitation with nitrogen injection and pressurization comprises a guide protection cone head, the rear end of which is respectively connected to an ultrasonic transducer, a nitrogen injection pipeline, and the front end of a high-pressure corrosion-resistant water pipe. The present invention addresses two aspects: improving the permeability of coal seams and increasing the driving force for coal seam gas flow. Ultrasonic excitation is used to transform the fracture structure of the coal body. On this basis, nitrogen injection and pressurization are used to enhance the "driving force" for gas flow in the coal seam, thereby increasing the gas flow effect and enhancing gas extraction efficiency. This method can effectively solve the problem of "not being able to extract" gas in low-gas mines and significantly improve the gas extraction efficiency of low-gas coal seams.

Description

Technical Field

[0001] The present invention belongs to the technical field of coal seam gas extraction, and in particular relates to a device and method for enhancing gas extraction from low-gas coal seams by coupling ultrasonic excitation with nitrogen injection and pressurization. Background Art

[0002] As the level of mechanization and capacity in my country's mines continues to improve, some low-gas mines are experiencing a sharp increase in absolute gas emission from the working face during high-intensity mining. This leads to a sharp rise in gas concentrations and occasional over-limit events, seriously impacting safe and efficient mine production. Initial coal seam gas content and gas pressure in these low-gas mines are low. Most pre-drainage boreholes show normal sealing and negative borehole pressure, but the extracted gas concentration is extremely low, typically meaning no gas is extracted. The on-site solution to the high absolute gas emission and over-limit risks during mining in low-gas mines is to suspend mining operations, then exhaust the gas through ventilation systems, and then resume mining operations after gas concentrations in the working face and tunnels drop below 0.2%. This approach seriously impacts the coordinated layout and balance of the mine's "injection-extraction-excavation-mining" system, increases atmospheric pollution from gas exhaust, and can even trigger gas disasters. Therefore, how to solve the problem of a sharp increase in the absolute gas outburst volume at the working face during high-intensity mining in low-gas mines has become an issue that must be paid attention to in the prevention and control of mine gas disasters.

[0003] Increasing the permeability of coal seams is an effective method for improving gas extraction efficiency. Currently, common permeability enhancement methods are divided into physical and chemical permeability enhancement. However, when applied to low-gas mines with low gas content and gas pressure, chemical permeability enhancement technology is ineffective and often causes coal seam contamination. The application of physical permeability enhancement technology also struggles to achieve the expected results. This is because while technologies such as hydraulic fracturing, ultrasonic excitation, and pre-splitting blasting can effectively transform coal reservoirs and form effective fracture channels for gas flow, they cannot solve problems such as low gas pressure and insufficient gas flow dynamics. This shows that simply increasing coal seam permeability cannot meet the goal of efficient extraction in low-gas mines. Hydraulic fracturing is a commonly used technique for increasing coal seam permeability. However, over 50% of my country's low-gas, low-permeability coal seams are soft. These seams are highly water-sensitive and, when cemented, can easily block gas flow channels. Consequently, hydraulic fracturing often fails to achieve the desired permeability-enhancing effect. Furthermore, hydraulic fracturing consumes significant amounts of freshwater resources and requires the treatment of large amounts of flowback wastewater during subsequent coal mining, which is difficult and environmentally unfriendly. Pre-splitting blasting permeability enhancement technology is effective in hard coal, but is often less effective in soft coal and may even induce secondary hazards. Ultrasonic excitation, as an emerging permeability enhancement technology, offers advantages such as concentrated energy, strong penetration, and a pollution-free design. It can effectively improve coal seam pore structure, increase pore connectivity, and promote the desorption of adsorbed gas. Summary of the Invention

[0004] The present invention aims to provide a solution to the current problem of low-gas mines experiencing a sharp increase in gas concentration at the working face during high-intensity mining, which affects safe and efficient production in the mines. By considering both improving coal seam permeability and increasing the driving force of coal seam gas flow, the present invention aims to efficiently solve the problem of low-gas mines being unable to extract gas.

[0005] To this end, the technical solution adopted by the present invention is: a device for enhancing gas extraction from low-gas coal seams by coupling ultrasonic excitation with nitrogen injection and pressurization, comprising an anti-permeability borehole arranged in the coal seam and at least two investigation boreholes arranged around the anti-permeability borehole, each borehole being installed with an extraction screen, and an integrated device for coupling ultrasonic excitation with nitrogen injection and pressurization being installed in the extraction screen of the anti-permeability borehole, all borehole openings being sealed with high pressure, and all boreholes being connected to a gas extraction device outside the borehole via a negative pressure extraction pipeline;

[0006] The integrated device for coupling ultrasonic excitation and nitrogen injection and pressurization includes a guide protection conical head, the rear end of which is respectively connected to the front ends of an ultrasonic transducer, a nitrogen injection pipeline and a high-pressure corrosion-resistant water pipe, and the installation of the ultrasonic transducer, the nitrogen injection pipeline and the high-pressure corrosion-resistant water pipe should ensure that they do not exceed the maximum cross-section of the guide protection conical head, and the ultrasonic reflection port of the ultrasonic transducer, the nitrogen injection nozzle of the nitrogen injection pipeline and the water injection port of the high-pressure corrosion-resistant water pipe all extend outside the extraction screen; the ultrasonic transducer is connected to the ultrasonic excitation control box outside the anti-permeability borehole through a pressure-resistant and waterproof cable, the nitrogen injection pipeline is connected to a nitrogen injection storage tank with a pressurization buffer box outside the anti-permeability borehole, and the high-pressure corrosion-resistant water pipe is connected to a water tank with a water pump outside the anti-permeability borehole;

[0007] The negative pressure extraction pipeline, nitrogen injection pipeline, and high-pressure corrosion-resistant water pipe are all provided with control valves.

[0008] As a preferred embodiment of the above scheme, the high-pressure sealing used in the anti-reflection drilling is rapid high-pressure sealing, and the high-pressure sealing used in the inspection drilling is high-pressure capsule sealing.

[0009] More preferably, there are two investigation holes arranged around the anti-reflection drilling hole, and the two investigation holes are respectively located on the left and right sides of the anti-reflection drilling hole.

[0010] It is further preferred that the boost buffer tank is connected to the compressor, and the nitrogen injection storage tank, the boost buffer tank, and the nitrogen injection high-pressure pump truck are connected in sequence on the nitrogen injection pipeline outside the permeability-enhancing borehole, and the control valve is arranged on the nitrogen injection pipeline between the nitrogen injection tank and the boost buffer tank, and the boost buffer tank and the compressor are respectively equipped with pressure gauges.

[0011] Further preferably, the control valve on the high-pressure corrosion-resistant water pipe is arranged between the water pump and the water tank, and the negative pressure extraction pipeline is respectively provided with a control valve corresponding to each borehole.

[0012] At the same time, the present invention also provides a method for enhancing gas extraction from low-gas coal seams by coupling ultrasonic excitation and nitrogen injection and pressurization, including the above-mentioned device for enhancing gas extraction from low-gas coal seams by coupling ultrasonic excitation and nitrogen injection and pressurization. The method for enhancing gas extraction from low-gas coal seams by coupling ultrasonic excitation and nitrogen injection and pressurization includes the following steps:

[0013] S1. Survey and sample the coal seam, transport it back to the laboratory in a sealed manner, and complete sample preparation;

[0014] S2. Conduct ultrasonic excitation seepage test, coal seam displacement test and permeability enhancement test on coal samples to determine the optimal ultrasonic frequency f1, coal seam displacement pressure P1 and permeability enhancement distance R respectively;

[0015] S3. Constructing extraction boreholes on-site into the coal seam, including an anti-permeability borehole and at least two investigation boreholes disposed around the anti-permeability borehole, using the anti-permeability distance R of the coal seam determined in step S2 as the observation distance between the edge of the anti-permeability borehole and the edge of the investigation borehole; monitoring the gas flow rate along the length of the anti-permeability borehole and determining the location and number of fixed-point anti-permeability areas;

[0016] S4. Install drainage screens in all boreholes, seal all borehole openings with high pressure, and connect the boreholes to the gas drainage device outside the boreholes through negative pressure drainage pipelines;

[0017] S5. Place an integrated device for coupling ultrasonic excitation and nitrogen injection pressure boosting into the anti-reflection borehole, ensure that the ultrasonic transducer is arranged within any fixed anti-reflection area, connect the pressure-resistant and waterproof cable to the ultrasonic excitation control box outside the anti-reflection borehole, connect the nitrogen injection pipeline to the nitrogen injection storage tank with a pressurized buffer tank outside the anti-reflection borehole, and connect the high-pressure corrosion-resistant water pipe to the water tank with a water pump outside the anti-reflection borehole;

[0018] S6. Start the water pump and the control valve on the high-pressure corrosion-resistant water pipe to pump clean water into the anti-permeability borehole. When observing the water backflow from the borehole, stop the water injection and close the control valve on the high-pressure corrosion-resistant water pipe.

[0019] S7, turning on the ultrasonic excitation control box, adjusting the ultrasonic frequency to f1, applying the ultrasonic transducer to the coal seam, and ultrasonically exciting the fixed-point anti-reflection area. After the ultrasonic excitation is completed, turning off the ultrasonic control box;

[0020] S8, turning on the compressor, the nitrogen injection high-pressure pump truck, and the control valve on the nitrogen injection pipeline, maintaining the nitrogen injection pressure at P1, and allowing nitrogen to reach the designated anti-permeability area along the nitrogen injection pipeline. The nitrogen injection time t1 is determined according to Formula 1;

[0021]

[0022] S9. Close the control valve, compressor and nitrogen injection high-pressure pump truck on the nitrogen injection pipeline;

[0023] S10, moving the integrated device for coupling ultrasonic excitation and nitrogen injection and pressurization to the next fixed-point anti-reflection area, repeating steps S6-S9), and performing coupling "permeation increase + pressurization" on the next fixed-point anti-reflection area; until the coupling "permeation increase + pressurization" of all fixed-point anti-reflection areas is completed;

[0024] S11. After the "infiltration + pressurization" operation is completed, open the control valve on the negative pressure extraction pipeline to extract gas.

[0025] Preferably, in step S7, the ultrasonic excitation duration is 30-50 minutes.

[0026] Preferably, the "permeability enhancement + pressure enhancement" operation is performed on each fixed-point permeability enhancement area from deep to shallow in the permeability enhancement drill hole.

[0027] The beneficial effects of the present invention: The gas extraction efficiency of low-gas coal seams is determined by the combined effects of permeability and internal gas pressure gradient. The present invention starts from two aspects: improving the permeability of coal seams and increasing the driving force of coal seam gas flow, and proposes coupling ultrasonic excitation and nitrogen injection and pressurization to enhance gas extraction from low-gas coal seams. Ultrasonic excitation is used to transform the fracture structure of the coal body, promote mutual communication between fractures, and increase the desorption amount of adsorbed gas. On this basis, nitrogen injection and pressurization are used to enhance the "driving force" of gas flow in the coal seam, increase the gas flow effect, and enhance gas extraction efficiency. It can effectively solve the problem of "not being able to extract" gas in low-gas mines, and can greatly improve the gas extraction efficiency of low-gas coal seams. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 It is a structural schematic diagram of the present invention.

[0029] Figure 2 This is a schematic diagram of the structure of an integrated device for coupling ultrasonic excitation and nitrogen injection pressurization. DETAILED DESCRIPTION

[0030] The present invention will be further described below by way of examples and in conjunction with the accompanying drawings:

[0031] Combine Figure 1 — Figure 2 As shown, a device for enhancing gas extraction from low-gas coal seams by coupling ultrasonic excitation with nitrogen injection and pressurization is mainly composed of an anti-permeability borehole 2, an investigation borehole 3, a guide protection cone head 4, a fixed-point anti-permeability area 5, a high-pressure corrosion-resistant water pipe 9, a nitrogen injection pipeline 10, an extraction screen 11, a rapid high-pressure sealing hole 12, a high-pressure capsule sealing hole 13, a control valve 14, a negative pressure extraction pipeline 15, a gas extraction device 16, a compressor 17, a nitrogen injection storage tank 18, a pressurization buffer box 19, a nitrogen injection high-pressure pump truck 20, a pressure-resistant and waterproof cable 21, an ultrasonic excitation control box 22, a water pump 23, an ultrasonic transducer 24, and a water tank 25.

[0032] Both the anti-permeability borehole 2 and the investigation borehole 3 are set in the coal seam 1. There are at least two investigation boreholes 3, which are arranged around the anti-permeability borehole 2. When there are two investigation boreholes 3, they are located on the left and right sides of the anti-permeability borehole 2. A drainage screen 11 is installed in each borehole. All boreholes are connected to a gas extraction device 16 outside the borehole via a negative pressure extraction pipeline 15 for subsequent gas extraction.

[0033] An integrated device for coupling ultrasonic excitation and nitrogen injection pressurization is installed within the extraction screen 11 of the permeability-enhancing borehole 2. All borehole openings are sealed with high pressure. Preferably, the high-pressure sealing used in the permeability-enhancing borehole 2 is rapid high-pressure sealing 12, while the high-pressure sealing used in the investigation borehole 3 is high-pressure capsule sealing 13.

[0034] The integrated device for coupling ultrasonic excitation and nitrogen injection pressurization includes a guide protection conical head 4, the rear end of which is respectively connected to the front end of the ultrasonic transducer 24, the nitrogen injection pipeline 10, and the high-pressure corrosion-resistant water pipe 9. The ultrasonic transducer 24, the nitrogen injection pipeline 10, and the high-pressure corrosion-resistant water pipe 9 should be installed so as not to exceed the maximum cross-section of the guide protection conical head 4. The guide protection conical head 4 is tapered, with a small front end and a large rear end. The small end is convenient for guidance during insertion, and the large rear end is used to protect the ultrasonic transducer 24, the nitrogen injection pipeline 10, and the high-pressure corrosion-resistant water pipe 9.

[0035] The ultrasonic reflection port of the ultrasonic transducer 24, the nitrogen injection nozzle of the nitrogen injection pipeline 10, and the water injection port of the high-pressure corrosion-resistant water pipe 9 all extend outside the extraction screen 11. The nitrogen injection nozzle 8 is provided at the front end of the nitrogen injection pipeline 10. The ultrasonic transducer 24 is connected to the ultrasonic excitation control box 22 outside the anti-permeability borehole 2 via a pressure-resistant and waterproof cable 21. The nitrogen injection pipeline 10 is connected to the nitrogen injection storage tank 18 with a pressurized buffer tank 19 outside the anti-permeability borehole 2. The high-pressure corrosion-resistant water pipe 9 is connected to the water tank 25 with a water pump 23 outside the anti-permeability borehole 2.

[0036] Control valves 14 are provided on the negative pressure extraction pipeline 15 , the nitrogen injection pipeline 10 , and the high-pressure corrosion-resistant water pipe 9 .

[0037] Specifically, the booster buffer tank 19 is connected to the compressor 17. The nitrogen injection tank 18, booster buffer tank 19, and nitrogen injection high-pressure pump truck 20 are sequentially connected on the nitrogen injection pipeline 10 outside the anti-permeability borehole 2. A control valve 14 is installed on the nitrogen injection pipeline 10 between the nitrogen injection tank 18 and the booster buffer tank 19. The booster buffer tank 19 and compressor 17 are each equipped with a pressure gauge. The control valve 14 on the high-pressure corrosion-resistant water pipe 9 is installed between the water pump 23 and the water tank 25. The negative pressure extraction pipeline 15 is equipped with a control valve 14 for each borehole.

[0038] A method for enhancing gas extraction from low-gas coal seams by coupling ultrasonic excitation with nitrogen injection and pressurization, using the above-mentioned device for enhancing gas extraction from low-gas coal seams by coupling ultrasonic excitation with nitrogen injection and pressurization, comprises the following steps:

[0039] S1. Survey and sample coal seam 1, transport it back to the laboratory in a sealed manner, and complete sample preparation.

[0040] S2. Conduct ultrasonic excitation seepage test, coal seam displacement test and permeability enhancement test on coal samples to determine the optimal ultrasonic frequency f1, coal seam displacement pressure P1 and permeability enhancement distance R respectively.

[0041] S3. Construct extraction boreholes on site into the coal seam 1, including an anti-permeability borehole 2 and at least two investigation boreholes 3 arranged around the anti-permeability borehole 2. The anti-permeability distance R of the coal seam determined in step S2 is used as the observation distance between the edge of the anti-permeability borehole 2 and the edge of the investigation borehole 3; monitor the gas flow along the length direction in the anti-permeability borehole 2 and determine the position and number of fixed-point anti-permeability areas 5, such as 3-5, but not limited to this.

[0042] S4. Install extraction screens 11 in all boreholes, seal all borehole openings with high pressure, and connect the boreholes to the gas extraction device 16 outside the boreholes through negative pressure extraction pipelines 15.

[0043] S5. Place an integrated device for coupling ultrasonic excitation and nitrogen injection pressurization into the anti-reflection borehole 2, and ensure that the ultrasonic transducer 24 is arranged in any fixed anti-reflection area 5, connect the pressure-resistant and waterproof cable 21 to the ultrasonic excitation control box 22 outside the anti-reflection borehole 2, connect the nitrogen injection pipeline 10 to the nitrogen injection storage tank 18 with the pressurization buffer box 19 outside the anti-reflection borehole 2, and connect the high-pressure corrosion-resistant water pipe 9 to the water tank 25 with the water pump 23 outside the anti-reflection borehole 2.

[0044] S6. Open the water pump 23 and the control valve 14 on the high-pressure corrosion-resistant water pipe 9 to pump clean water into the anti-permeability borehole 2. When observing the backflow of water from the inspection borehole 3, stop the water injection and close 23 and the control valve 14 on the high-pressure corrosion-resistant water pipe 9.

[0045] S7. Turn on the ultrasonic excitation control box 22, adjust the ultrasonic frequency to f1, and apply ultrasonic excitation to the coal seam 1 through the ultrasonic transducer 24 to the fixed-point anti-reflection area 5. After the ultrasonic excitation is completed, turn off the ultrasonic control box 22. Preferably, the ultrasonic excitation lasts for 30-50 minutes.

[0046] S8, start the compressor 17, the nitrogen injection high-pressure pump truck 20 and the control valve 14 on the nitrogen injection pipeline 10, maintain the nitrogen injection pressure at P1, and allow the nitrogen to reach the fixed-point anti-reflection area 5 along the nitrogen injection pipeline 10. The nitrogen injection time t1 is determined according to Formula 1;

[0047]

[0048] S9. Close the control valve 14, the compressor 17 and the nitrogen injection high-pressure pump truck 20 on the nitrogen injection pipeline 10.

[0049] S10. Move the integrated device for coupling ultrasonic excitation and nitrogen injection and pressurization to the next fixed-point anti-reflection area 5, repeat steps S6-S9, and couple the next fixed-point anti-reflection area 5 with "increase in penetration + pressurization" until the coupling of "increase in penetration + pressurization" of all fixed-point anti-reflection areas 5 is completed.

[0050] S11. After the "infiltration + pressurization" operation is completed, the control valve 14 on the negative pressure extraction pipeline 15 is opened to perform gas extraction.

[0051] Preferably, the "permeability enhancement + pressurization" operation is performed from deep to shallow in the permeability enhancement borehole 2 in each of the fixed permeability enhancement areas 5. Natural fractures 6 exist in the coal seam 1, and permeability enhancement fractures 7 are generated after the coupling of ultrasonic excitation and nitrogen injection pressurization.

[0052] Low-gas coal seams have low internal permeability, low gas content, and low gas pressure. Ultrasonic excitation is organically combined with nitrogen injection to increase the permeability and pressure of low-gas coal seams. As an emerging permeability enhancement technology, ultrasonic excitation has the advantages of concentrated energy, strong penetration ability, and no pollution. It can effectively improve the pore structure of coal seams, increase pore connectivity, and promote the desorption of adsorbed gas in coal seams. Nitrogen is an inert gas and also a weakly adsorbed gas. Considering both safety and economy, nitrogen is the optimal gas for gas injection and pressurization. Therefore, the present invention proposes a device and technology for enhancing gas extraction from low-gas coal seams by coupling ultrasonic excitation with nitrogen injection and pressurization. The mechanical vibration effect, cavitation effect, and thermal effect generated by ultrasonic excitation act together on the coal body, changing the pore structure of the coal body, promoting gas desorption from the coal body surface, and generating a large number of microcracks inside the coal body. Secondary cracks and primary cracks are constantly connected, forming more gas flow channels. High-pressure nitrogen is then injected into the coal seam to provide sufficient power for internal gas migration and achieve efficient gas extraction.

[0053] The coupling effect of ultrasonic excitation and nitrogen injection pressurization is manifested in two aspects. On the one hand, ultrasonic excitation can change the pore and fracture structure of the coal seam, increasing the amount of free gas in the coal body. The micropores and mesopores of the coal body gradually connect to form macropores or fractures, and the fractures are interconnected. The increase in the number of macropores and fractures in the coal seam increases the permeability of the coal seam. The coal fracture system is the main channel for gas seepage, and gas seepage obeys Darcy's law:

[0054]

[0055] Where q is the gas seepage velocity, m / s; k D is the coal permeability, m 2; p is the gas pressure, MPa; μ is the dynamic viscosity, Pa.s.

[0056] As can be seen from Formula 2, the gas seepage velocity is proportional to the pressure gradient. Therefore, injecting high-pressure nitrogen into the coal seam increases the pore pressure of the coal body, effectively compensating for the problem of insufficient gas seepage power.

[0057] On the other hand, when ultrasonic excitation acts on the coal body, a large number of cracks are formed inside the coal body for gas flow. The crack opening greatly affects the gas flow efficiency. According to the coal body dual-medium effective stress equation (Formula 3), the crack opening is a dynamic parameter determined by the external stress and the internal pressure of the coal body. Under normal circumstances, the coal seam gas pressure is low, the driving force for gas flow inside the coal seam is insufficient, the crack opening is reduced, and the coal seam permeability is reduced. When high-pressure nitrogen is injected, the gas pressure inside the coal seam props up the coal body, effectively preventing the crack opening from decreasing and ensuring the smooth flow of gas channels.

[0058]

[0059] Where, is the effective stress, MPa; σ ij is the external stress on the coal body, MPa; α f and α p are the effective coefficients of cracks and pores respectively; p f and p p are the gas pressures in fractures and pores, respectively.

Claims

1. A method for enhancing gas extraction from low-gas coal seams by coupling ultrasonic excitation with nitrogen injection and pressurization, characterized by: A device for enhancing gas extraction from low-gas coal seams by coupling ultrasonic excitation and nitrogen injection and pressurization is disclosed. The device comprises an anti-permeability borehole (2) arranged in a coal seam (1) and at least two investigation boreholes (3) arranged around the anti-permeability borehole (2). A extraction screen (11) is installed in each borehole. An integrated device for coupling ultrasonic excitation and nitrogen injection and pressurization is also installed in the extraction screen (11) of the anti-permeability borehole (2). All borehole openings are sealed with high pressure, and all boreholes are connected to a gas extraction device (16) outside the borehole through a negative pressure extraction pipeline (15). The integrated device for coupling ultrasonic excitation and nitrogen injection and pressurization comprises a guide protection cone head (4), the rear end of which is connected to an ultrasonic transducer (24), The nitrogen injection pipeline (10) is connected to the front end of the high-pressure corrosion-resistant water pipe (9), and the installation of the ultrasonic transducer (24), the nitrogen injection pipeline (10) and the high-pressure corrosion-resistant water pipe (9) should ensure that they do not exceed the maximum cross-section of the guide protection cone head (4), and the ultrasonic reflection port of the ultrasonic transducer (24), the nitrogen injection nozzle of the nitrogen injection pipeline (10) and the water injection port of the high-pressure corrosion-resistant water pipe (9) all extend outside the extraction screen (11); the ultrasonic transducer (24) is connected to the ultrasonic excitation control box (22) outside the anti-permeability borehole (2) through a pressure-resistant waterproof cable (21), the nitrogen injection pipeline (10) is connected to the nitrogen injection storage tank (18) with a pressurized buffer box (19) outside the anti-permeability borehole (2), and the high-pressure corrosion-resistant water pipe (9) is connected to the water tank (25) with a water pump (23) outside the anti-permeability borehole (2); The negative pressure extraction pipeline (15), the nitrogen injection pipeline (10), and the high-pressure corrosion-resistant water pipe (9) are all provided with a control valve (14); The method for enhancing gas extraction from low-gas coal seams by coupling ultrasonic excitation with nitrogen injection and pressurization comprises the following steps: S1. Survey and sample the coal seam (1), transport it back to the laboratory in a sealed manner, and complete sample preparation; S2. Conduct ultrasonic excitation seepage test, coal seam displacement test and permeability enhancement test on coal samples to determine the optimal ultrasonic frequency f1, coal seam displacement pressure P1 and permeability enhancement distance R respectively; S3, constructing extraction boreholes on site into the coal seam (1), including an anti-permeability borehole (2) and at least two investigation boreholes (3) arranged around the anti-permeability borehole (2), using the anti-permeability distance R of the coal seam determined in step S2 as the observation distance between the edge of the anti-permeability borehole (2) and the edge of the investigation borehole (3); monitoring the gas flow along the length direction in the anti-permeability borehole (2) and determining the position and number of the fixed-point anti-permeability areas (5); S4. Install extraction screens (11) in all boreholes, seal all borehole openings with high pressure, and connect the boreholes to the gas extraction device (16) outside the boreholes through negative pressure extraction pipelines (15); S5. Place an integrated device for coupling ultrasonic excitation and nitrogen injection pressure boosting in the anti-reflection borehole (2), and ensure that the ultrasonic transducer (24) is arranged in any fixed anti-reflection area (5), connect the pressure-resistant and waterproof cable (21) to the ultrasonic excitation control box (22) outside the anti-reflection borehole (2), connect the nitrogen injection pipeline (10) to the nitrogen injection storage tank (18) with the pressure boosting buffer box (19) outside the anti-reflection borehole (2), and connect the high-pressure corrosion-resistant water pipe (9) to the water tank (25) with the water pump (23) outside the anti-reflection borehole (2); S6, opening the water pump (23) and the control valve (14) on the high-pressure corrosion-resistant water pipe (9), pumping clean water into the anti-permeability borehole (2), and when observing the backflow of water from the inspection borehole (3), stopping the water injection, closing the water pump (23) and the control valve (14) on the high-pressure corrosion-resistant water pipe (9); S7, turning on the ultrasonic excitation control box (22), adjusting the ultrasonic frequency to f1, and applying ultrasonic excitation to the coal seam (1) through the ultrasonic transducer (24), so as to perform ultrasonic excitation on the fixed point anti-reflection area (5), and turning off the ultrasonic excitation control box (22) after the ultrasonic excitation is completed; S8, turning on the compressor (17), the nitrogen injection high-pressure pump truck (20) and the control valve (14) on the nitrogen injection pipeline (10), maintaining the nitrogen injection pressure at P1, and allowing the nitrogen to reach the fixed-point anti-permeability region (5) along the nitrogen injection pipeline (10). The nitrogen injection time t1 is determined according to Formula 1; Formula 1 S9, closing the control valve (14), the compressor (17) and the nitrogen injection high-pressure pump truck (20) on the nitrogen injection pipeline (10); S10, moving the integrated device for coupling ultrasonic excitation and nitrogen injection and pressurization to the next fixed-point anti-reflection area (5), repeating steps S6-S9, and performing coupling "permeation increase + pressurization" on the next fixed-point anti-reflection area (5); until the coupling "permeation increase + pressurization" of all fixed-point anti-reflection areas (5) is completed; S11. After the "infiltration + pressurization" operation is completed, the control valve (14) on the negative pressure extraction pipeline (15) is opened to perform gas extraction.

2. The method for enhancing gas extraction from low-gas coal seams by coupling ultrasonic excitation with nitrogen injection and pressurization according to claim 1 is characterized in that: The high-pressure sealing method used in the anti-reflection drilling hole (2) is a rapid high-pressure sealing method (12), and the high-pressure sealing method used in the inspection drilling hole (3) is a high-pressure capsule sealing method (13).

3. The method for enhancing gas extraction from low-gas coal seams by coupling ultrasonic excitation with nitrogen injection and pressurization according to claim 1 is characterized in that: There are two inspection boreholes (3) arranged around the anti-reflection borehole (2), and the two inspection boreholes (3) are respectively located on the left and right sides of the anti-reflection borehole (2).

4. The method for enhancing gas extraction from low-gas coal seams by coupling ultrasonic excitation with nitrogen injection and pressurization according to claim 1 is characterized in that: The boosting buffer tank (19) is connected to the compressor (17), and the nitrogen injection storage tank (18), the boosting buffer tank (19), and the nitrogen injection high-pressure pump truck (20) are sequentially connected on the nitrogen injection pipeline (10) outside the permeability-enhancing borehole (2), and the control valve (14) is arranged on the nitrogen injection pipeline (10) between the nitrogen injection storage tank (18) and the boosting buffer tank (19), and the boosting buffer tank (19) and the compressor (17) are respectively equipped with pressure gauges.

5. The method for enhancing gas extraction from low-gas coal seams by coupling ultrasonic excitation with nitrogen injection and pressurization according to claim 1 is characterized in that: The control valve (14) on the high-pressure corrosion-resistant water pipe (9) is arranged between the water pump (23) and the water tank (25), and the negative pressure extraction pipeline (15) is respectively provided with a control valve (14) corresponding to each borehole.

6. The method for enhancing gas extraction from low-gas coal seams by coupling ultrasonic excitation with nitrogen injection and pressurization according to claim 1 is characterized in that: In step S7, the ultrasonic excitation duration is 30-50 minutes.

7. The method for enhancing gas extraction from low-gas coal seams by coupling ultrasonic excitation with nitrogen injection and pressurization according to claim 1 is characterized in that: The "permeability enhancement + pressure enhancement" operation is carried out in each fixed-point permeability enhancement area (5) from deep to shallow in the permeability enhancement borehole (2).

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