A method for improving the air permeability of ultra-deep buried high-in-situ stress soft pulverized coal seams

Abstract

The present invention discloses a method for improving the air permeability of an ultra-deep buried high-in-situ stress soft pulverized coal seam, and belongs to the technical field of permeability enhancement of low-permeability coal seams; specifically, a borehole with a certain structure is first established to form a low-frequency microwave transmission pipeline extending above the bottom plate of a target coal seam, and a medium-frequency microwave transmission pipeline extending below the top plate of the target coal seam, low-frequency microwaves are emitted above the bottom plate of the target coal seam for heating to form low-frequency microwave energy to expand coal seam cracks, and then medium-frequency microwaves are emitted in the coal seam below the top plate of the target coal seam to form medium-frequency microwave energy to expand coal seam cracks, and then refrigerant is introduced to cool the target coal seam; cyclic heating and cooling operations are performed to form a network of cracks; the present invention improves the overall air permeability and stability of ultra-deep buried high-in-situ stress soft pulverized coal seams; and solves the problem that deep buried high-in-situ stress low-permeability soft coal seams are not easy to enhance permeability.

Description

Technical Field

[0001] The invention belongs to the technical field of permeability enhancement of low-permeability coal seams, and relates to a method for improving the permeability of ultra-deep buried high-in-situ stress soft pulverized coal seams. Background Art

[0002] Many practical factors will affect the permeability enhancement effect of coal seams. For soft pulverized coal seams with high ground stress and low permeability, it is necessary to expand the cracks in the coal seams to generate a complete fracture network to increase permeability. At present, the permeability enhancement technologies at home and abroad mainly include hydraulic methods, blasting fracturing methods, water injection or gas displacement methods, electrochemical methods, etc. These methods have greatly improved the permeability enhancement effect of ordinary coal seams. However, due to the strong adsorption of gas by soft coal seams with low permeability, the hydraulic method has water locking properties that lead to crack closure. The blasting fracturing method and the electrochemical method and other permeability enhancement methods will increase the gas pressure in the coal seam and increase safety risks.

[0003] Geostress has an important influence on the mining, crack formation and permeability increase of coal seams. The change of geostress will directly affect the physical structure of coal seams, and then affect the mining effect of coal seams. The permeability of coal seams in high geostress areas is poor. Stress produces a certain pressure on coal seams, which makes it difficult to form cracks, and gas is difficult to discharge. Gas extraction is difficult and gas accumulation is easy to form, which increases the risk of gas explosion. When the stress of coal seams is reduced by external force, the cracks will open or expand again, and the permeability of coal seams can be enhanced. The crack expansion technology can effectively expand the cracks of coal seams, improve the permeability of coal seams, reduce the stress of coal seams, and increase the flow channel of gas in coal seams. When the stress of coal seams is concentrated, it may cause coal seams to collapse, causing safety hazards. Therefore, it is necessary to study a way to increase the permeability of ultra-deep buried high geostress soft pulverized coal seams, increase the permeability of coal seams, and reduce the risk of gas explosion. Summary of the invention

[0004] The present invention overcomes the shortcomings of the prior art and proposes a method for improving the air permeability of ultra-deep buried high-ground stress soft pulverized coal seams. The method uses a low-expansion polyurethane coal-saving agent to improve the support capacity of the coal seam roof and floor. At the same time, medium and low frequency microwaves are used to alternately heat the coal seam and Freon (R-134a) is used to lower the temperature of the coal seam to form a temperature difference, thereby improving the overall permeability and stability of the coal seam; solving the problem that deep buried high-ground stress low-permeability soft coal seams are not easy to increase permeability.

[0005] In order to achieve the above object, the present invention is achieved through the following technical solutions:

[0006] A method for improving the air permeability of ultra-deep buried high-in-situ stress soft pulverized coal seams, comprising the following steps:

[0007] S1. A hexagonal structure is formed on the land surface, and the positions of the first boreholes are determined at the vertex and the center respectively; an equilateral triangle structure is formed between two adjacent first boreholes and the first borehole in the center; the center of each equilateral triangle is determined as the position of the second borehole; the first borehole is used as a low-frequency microwave heating point, and the second borehole is used as a medium-frequency microwave heating point;

[0008] S2. Drill holes at the determined drilling positions, so that the depth of the first borehole reaches above the target coal seam floor, and the depth of the second borehole reaches below the target coal seam roof; the bottoms of all first boreholes are connected, and the bottoms of all second boreholes are connected; and at the same time, coal-saving agents are injected through the first boreholes and the second boreholes to reinforce the roof and bottom plates;

[0009] S3. After the coal-saving agent takes effect, the transmission pipeline is laid: the transmission pipeline laid in the first borehole is a low-frequency microwave transmission pipeline, and the transmission pipeline laid in the second borehole is a medium-frequency microwave transmission pipeline; one of the low-frequency microwave transmission pipelines is connected to the ground refrigeration system, and the tops of the other low-frequency microwave transmission pipelines are sealed; similarly, one of the medium-frequency microwave transmission pipelines is connected to the ground refrigeration system, and the tops of the other medium-frequency microwave transmission pipelines are sealed;

[0010] S4. Arrange a low-frequency microwave transmitter in the low-frequency microwave transmission pipeline, and arrange an intermediate-frequency microwave transmitter in the intermediate-frequency microwave transmission pipeline;

[0011] S5, start the low-frequency microwave transmitter in the low-frequency microwave transmission pipeline, the microwave frequency is 300MHz~500MHz, heat the coal seam at the bottom plate of the target coal seam, make the microwave reach the target coal seam and form low-frequency microwave energy to expand the coal seam cracks;

[0012] S6, turn off the low-frequency microwave transmitter, turn on the medium-frequency microwave transmitter in the medium-frequency microwave transmission pipeline, adjust the microwave frequency of the medium-frequency microwave transmitter to 1 GHz to 3 GHz, heat the roof of the target coal seam so that the microwave reaches the target coal seam and forms medium-frequency microwave energy to expand the coal seam cracks;

[0013] S7, shut down the medium frequency microwave transmitter, release Freon to the low frequency microwave transmission pipeline and the medium frequency microwave transmission pipeline respectively through the ground refrigeration system to cool the target coal seam, and shut down the ground refrigeration system when the temperature drops to higher than or close to the temperature of the target coal seam before microwave heating;

[0014] S8. Loop through steps S5 to S7 to fully expand the cracks in the target coal seam, and then extract the gas.

[0015] Preferably, the coal-saving agent is a low-expansion polyurethane coal-saving agent.

[0016] Preferably, the low-frequency microwave transmission pipeline and the medium-frequency microwave transmission pipeline are both made of aluminum waveguide tubes.

[0017] Preferably, the flow rate of Freon is 0.2 m / s to 0.3 m / s, and the flow rate is 5 L / min to 10 L / min.

[0018] Preferably, the Freon is R-134a.

[0019] Preferably, in steps S5 to S7, the temperature and crack expansion of the target coal seam are monitored in real time by a microseismic monitoring system.

[0020] The beneficial effects of the present invention compared with the prior art are as follows:

[0021] 1. This method establishes the drilling method by arranging the heating points in a triangular shape, which ensures the accuracy of the distance and the sufficient microwave energy to heat the pulverized coal seam to enhance the effect of crack expansion. The different depths of pipeline laying ensure that the medium and low frequency microwaves can act on coal seams of different depths when heating in a multi-point manner, and also ensure that the heat can be fully absorbed when cooling the coal seam. At the same time, it also provides convenience for the low-expansion polyurethane coal-saving agent to support the target coal seam roof and target coal seam floor.

[0022] 2. This method uses microwave crack expansion technology. Microwave energy heats the coal seam to induce changes in the internal stress of the coal seam. Low-frequency microwave energy and medium-frequency microwave energy are alternately combined to heat the pulverized coal seam to promote crack expansion. Medium-frequency microwaves can balance the penetration depth and energy distribution. Low-frequency microwaves have a longer action time, and the combination of low-frequency and medium-frequency microwaves can shorten the time, achieving effective crack expansion in both deep and shallow coal seams, promoting the formation of a crack network in the coal seam cracks, and comprehensively improving the permeability of the coal seam.

[0023] 3. This method uses microwave energy cracking technology and does not require the use of other chemical substances. At the same time, after the polyurethane takes effect, its active ingredients have little pollution to the environment and will not produce other additional harmful and corrosive gases. The safe working environment for construction workers is guaranteed, which is more in line with environmental protection concepts and provides technical support and guarantee for mining.

[0024] 4. This method selects aluminum waveguide tubes as the material for transmitting microwave energy. Since the coal seam is an ultra-deep buried pulverized coal seam, the aluminum waveguide tube has strong corrosion resistance and is more suitable for long-distance underground transmission. At the same time, the inner wall material of the aluminum waveguide tube has good thermal conductivity, electrical conductivity and low loss, which not only ensures that the microwave energy is efficiently transmitted to the target coal seam with minimal loss during the transmission process, but also ensures the optimal heat transfer effect during the transmission process using Freon (R-134a) as a coal seam coolant.

[0025] 5. This method uses a real-time monitoring system to monitor the production and expansion of cracks in real time, and adjust the frequency of microwaves in time to prevent the excessive expansion of coal seam cracks and collapse, thereby protecting the fragility of the coal seam. Using monitoring data to control the frequency of microwave irradiation of the coal seam can ensure the stability of the crack expansion process and avoid excessive damage to the coal seam; the monitoring system can also monitor the flow and velocity of Freon (R-134a) in the coal seam, and timely control the temperature change of the coal seam to prevent the sudden collapse of the coal seam.

[0026] 6. Compared with traditional polyurethane materials, the low-expansion polyurethane coal-saving agent used in this method is more suitable for soft pulverized coal seams in terms of expansion rate and air permeability improvement, and can play a good supporting role while preventing cracks from closing, with less impact on the coal seam structure; after being injected into the borehole, it quickly solidifies in the coal seam, and the solidified polyurethane coal-saving agent has a high compressive strength, which can firmly bond the cracks in the coal seam and enhance the stability of the overall structure of the coal seam. At the same time, it can also play a supporting role in high-temperature environments, and will not affect its supporting effect in the coal seam during the subsequent heating and cooling of the coal seam, thus ensuring the stability of the coal seam.

[0027] 7. The coal seam cooling agent used in this method is Freon (R-134a). Compared with other types of Freon, Freon (R-134a) is less toxic, safer for the human body and more suitable for industrial use. Freon (R-134a) has a good refrigeration effect and can efficiently complete the refrigeration cycle; Freon (R-134a) is converted into gas and liquid through a condenser in the ground refrigeration system and stored for repeated use. Freon (R-134a) has stable chemical properties, will not react with the closed circulation conduit of the refrigeration system, and will not pollute the coal seam. It is highly safe and environmentally friendly.

[0028] 8. This method uses microwave energy to increase the temperature of the coal seam, and uses Freon (R-134a) as a coal seam refrigerant to reduce the temperature of the coal seam. The internal structure of the coal seam will expand and contract due to temperature changes, and the internal stress will change and redistribute accordingly, so that the cracks in the coal seam will be expanded, thereby improving the permeability of the coal seam. High and low temperatures can also change the cracks in the coal seam, reduce gas content, improve coal seam safety, reduce safety hazards, and will not affect the surrounding environment, which is more energy-saving and environmentally friendly. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 is a position distribution map of the first borehole and the second borehole in the present invention;

[0030] Figure 2 It is a cross-sectional schematic diagram of the location of the low expansion polyurethane coal-saving agent action area in the target coal seam;

[0031] Figure 3It is a schematic cross-sectional diagram of the low-frequency microwave energy expanding coal seam fractures in the target coal seam;

[0032] Figure 4 It is a schematic cross-sectional diagram of the coal seam fractures expanded by medium frequency microwave energy in the target coal seam;

[0033] Figure 5 It is a schematic cross-sectional view of the coal seam area after liquid Freon (R-134a) acts on the target coal seam to form Freon (R-134a) cooling and expanding cracks.

[0034] Numbers in the figure:

[0035] 1-first borehole; 2-second borehole; 3-low-frequency microwave transmission pipeline; 4-target coal seam roof; 5-action area of ​​low-expansion polyurethane coal-saving agent; 6-medium-frequency microwave transmission pipeline; 7-target coal seam floor; 8-first storage tank; 9-second storage tank; 10-low-frequency microwave energy expands coal seam cracks; 11-pulverized coal seam area; 12-medium-frequency microwave energy expands coal seam cracks; 16-coal seam area after Freon (R-134a) cools and expands cracks. DETAILED DESCRIPTION

[0036] In order to make the technical problems, technical solutions and beneficial effects to be solved by the present invention clearer, the present invention is further described in detail in conjunction with the embodiments and the accompanying drawings. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention. The technical solutions of the present invention are described in detail below in conjunction with the embodiments and the accompanying drawings, but the scope of protection is not limited thereto.

[0037] Example 1

[0038] For pulverized coal seams with a burial depth of 700m, see Figure 1-5 This embodiment proposes a method for improving the air permeability of ultra-deep buried high-in-situ stress soft pulverized coal seams, and the specific steps are as follows:

[0039] S1, a hexagonal structure is formed on the land surface, and the positions of the first boreholes 1 are determined at the vertex and the center respectively; the borehole distance S between adjacent first boreholes 1 is 50m, and an equilateral triangle structure is formed between two adjacent first boreholes 1 and the center first borehole 1; the center of each equilateral triangle is determined as the position of the second borehole 2; the first borehole 1 is used as a low-frequency microwave heating point, and the second borehole 2 is used as a medium-frequency microwave heating point; the distance between adjacent second boreholes 2 is R1, and the distance between adjacent first boreholes 1 and second boreholes 2 is R2 (see Figure 1 ).

[0040] S2. Drill holes at the determined drilling positions, so that the depth of the first borehole 1 reaches above the target coal seam floor 7 with a buried depth of about 690m, and the depth of the second borehole 2 reaches below the target coal seam roof 4 with a buried depth of about 650m; and the bottoms of the seven first boreholes 1 are connected, and the bottoms of the six second boreholes 2 are connected; at the same time, low-expansion polyurethane coal-saving agent is injected through the first borehole 1 and the second borehole 2 to reinforce the target coal seam roof 4 and the target coal seam floor 7, forming a low-expansion polyurethane coal-saving agent action area 5 (see Figure 2 ), enhance the supporting effect to ensure the stability of the target coal seam between the target coal seam roof 4 and the target coal seam floor 7, and the target coal seam is the pulverized coal seam area 11.

[0041] S3. After the low-expansion polyurethane coal-saving agent takes effect, the transmission pipeline is laid. The material of the transmission pipeline is selected from aluminum waveguide tubes. During the laying, ensure that the transmission pipeline is in good contact with the coal seam. The transmission pipeline laid in the first borehole 1 is a low-frequency microwave transmission pipeline 3, and the transmission pipeline laid in the second borehole 2 is a medium-frequency microwave transmission pipeline 6. One of the low-frequency microwave transmission pipelines 3 is connected to the first storage tank 8 of the ground refrigeration system, and the tops of the remaining low-frequency microwave transmission pipelines 3 are closed. Similarly, one of the medium-frequency microwave transmission pipelines 6 is connected to the second storage tank 9 of the ground refrigeration system, and the tops of the remaining medium-frequency microwave transmission pipelines 6 are closed.

[0042] S4. A low-frequency microwave transmitter is arranged in the low-frequency microwave transmission pipeline 3, and a medium-frequency microwave transmitter is arranged in the medium-frequency microwave transmission pipeline 6. Since the depths of the low-frequency microwave transmission pipeline 3 and the medium-frequency microwave transmission pipeline 6 are different, it can be ensured that microwave energies of different frequencies can play a role in coal seams of different depths. These positions can ensure that there will be no "cold zone" in the coal seams between each drilling distance, and ensure that the crack expansion in the target coal seam can form a crack network.

[0043] S5, start the low-frequency microwave transmitter in the low-frequency microwave transmission pipeline 3, adjust the microwave frequency to 400MHz, and heat the coal seam at the bottom plate 7 of the target coal seam. The microwave of this frequency can penetrate the coal seam buried at a depth of 690m. After the low-frequency microwave is emitted, the temperature and crack expansion of the target coal seam are monitored in real time through the existing microseismic monitoring system. When it is detected that the microwave reaches the target coal seam, the heating time is controlled to 15 minutes, so that the low-frequency microwave energy expands the coal seam cracks 10 (see Figure 3 ).

[0044] S6, turn off the low-frequency microwave transmitter, turn on the medium-frequency microwave transmitter in the medium-frequency microwave transmission pipeline 6, adjust the microwave frequency of the medium-frequency microwave transmitter to 1.5 GHz, heat the target coal seam roof 4, and monitor the microwave to expand the cracks in the direction of the target coal seam. The heating time is controlled to be about 15 minutes, forming medium-frequency microwave energy to expand the coal seam cracks 12 (see Figure 4 ).

[0045] S7, turn off the medium-frequency microwave transmitter, then open the first storage tank 8 and the second storage tank 9 of the ground refrigeration system, and release Freon (R-134a) from the first storage tank 8 and the second storage tank 9 to the low-frequency microwave transmission pipeline 3 and the medium-frequency microwave transmission pipeline 6 respectively; Freon (R-134a) flows through the coal seam along the closed pipeline to cool the coal seam;

[0046] The flow rate of Freon (R-134a) is controlled to be 0.3m / s and the flow rate is 5L / min to ensure that Freon (R-134a) can be evenly distributed in the target coal seam. At the same time, the temperature change of the target coal seam is detected. When the temperature drops to a temperature higher than or close to the temperature of the target coal seam before microwave heating, the ground refrigeration system is turned off. After Freon (R-134a) is circulated back to the ground refrigeration system, a high and low temperature cycle process is completed. At this time, the coal seam area after Freon (R-134a) cools down and expands the cracks is gradually formed in the target coal seam 16 (see Figure 5 ).

[0047] S8. Loop through steps S5 to S7, and monitor the expansion of fractures in the target coal seam, and monitor whether the gas can be discharged from the target coal seam along the fracture network; after a through fracture network is formed in the target coal seam, extract the gas; during the gas extraction process, due to the expansion of fractures in the target coal seam, the gas emission rate is accelerated, and the gas extraction strategy is adjusted in time; regularly monitor the working status of the gas extraction system and evaluate the gas flow and concentration.

[0048] Example 2

[0049] For pulverized coal seams with a burial depth of 1000m, see Figure 1-5 , This embodiment proposes a method for improving the permeability of ultra-deep buried high-in-situ stress soft pulverized coal seam. Compared with embodiment 1, in embodiment 2, the burial depth of the target coal seam roof 4 is 950m, and the burial depth of the target coal seam floor 7 is 990m;

[0050] In step S5, the microwave frequency emitted by the low-frequency microwave transmitter is 300 MHz, and the heating time is controlled to be 30 minutes;

[0051] In step S6, the microwave frequency emitted by the medium frequency microwave transmitter is 1 GHz, and the heating time is controlled to be 30 minutes;

[0052] In step S7, the flow rate of Freon (R-134a) is controlled to be 0.2 m / s and the flow rate is 10 L / min;

[0053] The rest are the same as in Example 1.

[0054] It should be noted that the target coal seams described in Examples 1 and 2 are all soft and powdery coal seams and are coal seams in ultra-deep buried high-stress areas. The coal seams have low strength, are difficult to drill, have high gas content, are thick, have extremely low permeability coefficients, and have harsh implementation conditions. It is difficult to achieve the ideal permeability enhancement effect using existing conventional permeability enhancement methods.

[0055] Embodiments 1 and 2 of the present invention partially enhance the supporting effect of the target coal seam roof 4 and the target coal seam floor 7 through low-expansion polyurethane coal-saving agent, and utilize the thermal expansion and contraction effect generated by alternating microwave energy and liquid Freon (R-134a) to enhance the air permeability of the target coal seam; selecting an equilateral triangle well layout method can effectively achieve microwave expansion of existing cracks in the target coal seam and the generation of new cracks, while also preventing the reinforcement effect of the low-expansion polyurethane coal-saving agent on the target coal seam roof 4 and the target coal seam floor 7 from being affected.

[0056] The excellent compressive resistance of the low-expansion polyurethane coal-saving agent is more suitable for this type of coal seam to enhance the coal seam support. The low-expansion polyurethane coal-saving agent has a low expansion rate and has obvious effects on soft coal seams. It will not produce high internal pressure after expansion. At the same time, it has a high safety factor, rapid curing ability and good supporting force. The low expansion characteristic can prevent the closure of cracks in the target coal seam, and can reinforce the target coal seam to form a stable support system, thereby improving the support capacity of the target coal seam roof 4 and the target coal seam floor 7.

[0057] The temperature of the target coal seam is raised by microwave energy, and the coal body expands due to the heat, causing cracks to expand. Liquid Freon (R-134a) then absorbs heat and converts from liquid to gas to lower the temperature of the coal seam. The coal body shrinks and the cracks further expand, thereby increasing the number of cracks and connectivity in the coal seam as a whole, thereby improving the permeability of the coal seam.

[0058] In the present invention, in order to promote the expansion of coal seam cracks, medium and low frequency microwave energy are selected alternately. The low frequency microwave energy can effectively penetrate the coal seam to promote the expansion of cracks in the deeply buried coal seam. The medium frequency microwave can control the heating of the upper coal seam while improving the permeability of the coal seam. The combination of the two can meet the demand for improving the permeability of the coal seam. At the same time, the microwave energy is carried out in a multi-point heating manner, which can make the heating and crack expansion effects of the target coal seam more uniform, improve the connectivity of the crack network in the target coal seam, and reduce the risk of local overheating causing gas explosion.

[0059] The low-frequency microwave transmission pipeline 3 and the medium-frequency microwave transmission pipeline 6 are made of aluminum waveguide tubes, which have strong corrosion resistance and thermal conductivity, can effectively reduce the loss of microwave energy during transmission, and can ensure the heat conduction effect of the refrigerant Freon (R-134a), and will not chemically react with the low-expansion polyurethane coal-saving agent. They are suitable for long-distance underground transmission in ultra-deep coal seams, ensuring the long-term stability and safety of the material.

[0060] Select Freon (R-134a) with a lower boiling point, which has a stronger refrigeration performance and can quickly remove the heat in the target coal seam during the circulation process, promote the expansion of cracks, and improve the permeability of the target coal seam. After the refrigeration effect, Freon (R-134a) changes from liquid to gas and returns to the ground refrigeration system, where it is recooled to liquid for repeated circulation.

[0061] Microseismic monitoring technology is used in the process of microwave energy expanding coal seam cracks. It is possible to monitor the expansion of target coal seam cracks in real time and switch between medium and low frequency microwaves in time, so as to achieve efficient monitoring and ensure safety. At the same time, the system and maintenance costs of microseismic monitoring are low.

[0062] The above content is a further detailed description of the present invention in combination with a specific preferred embodiment. It cannot be determined that the specific embodiments of the present invention are limited to this. For ordinary technicians in the technical field to which the present invention belongs, several simple deductions or substitutions can be made without departing from the present invention, which should be regarded as belonging to the present invention and the scope of patent protection determined by the submitted claims.

Claims

1. A method for improving the air permeability of ultra-deep buried high-in-situ stress soft pulverized coal seams, characterized in that: The following steps are involved: S1. A hexagonal structure is formed on the soil surface, with the vertex and the center respectively being the positions of the first boreholes (1); an equilateral triangle structure is formed between two adjacent first boreholes (1) and the center first borehole (1); the center of each equilateral triangle is determined as the position of the second borehole (2); the first borehole (1) is used as a low-frequency microwave heating point, and the second borehole (2) is used as a medium-frequency microwave heating point; S2. Drilling is performed at the determined drilling positions, so that the depth of the first drilling hole (1) reaches above the target coal seam floor (7), and the depth of the second drilling hole (2) reaches below the target coal seam roof (4); the bottoms of all the first drilling holes (1) are connected, and the bottoms of all the second drilling holes (2) are connected; at the same time, a coal-saving agent is injected through the first drilling hole (1) and the second drilling hole (2) to reinforce the target coal seam roof (4) and the target coal seam floor (7); S3, after the coal-saving agent takes effect, the transmission pipeline is laid: the transmission pipeline laid in the first borehole (1) is a low-frequency microwave transmission pipeline (3), and the transmission pipeline laid in the second borehole (2) is a medium-frequency microwave transmission pipeline (6); one of the low-frequency microwave transmission pipelines (3) is connected to the ground refrigeration system, and the tops of the other low-frequency microwave transmission pipelines (3) are closed; one of the medium-frequency microwave transmission pipelines (6) is connected to the ground refrigeration system, and the tops of the other medium-frequency microwave transmission pipelines (6) are closed; S4, arranging a low-frequency microwave transmitter in the low-frequency microwave transmission pipeline (3), and arranging a medium-frequency microwave transmitter in the medium-frequency microwave transmission pipeline (6); S5, starting the low-frequency microwave transmitter in the low-frequency microwave transmission pipeline (3), with the microwave frequency of 300 MHz to 500 MHz, to heat the coal seam at the bottom plate (7) of the target coal seam, so that the microwave reaches the target coal seam and forms low-frequency microwave energy to expand the coal seam fissures (10); S6, turning off the low-frequency microwave transmitter, turning on the medium-frequency microwave transmitter in the medium-frequency microwave transmission pipeline (6), adjusting the microwave frequency of the medium-frequency microwave transmitter to 1 GHz to 3 GHz, heating the target coal seam roof (4) so ​​that the microwave reaches the target coal seam, and forming medium-frequency microwave energy to expand coal seam fissures (12); S7, turning off the medium frequency microwave transmitter, releasing Freon to the low frequency microwave transmission pipeline (3) and the medium frequency microwave transmission pipeline (6) through the ground refrigeration system, cooling the target coal seam, and turning off the ground refrigeration system when the temperature drops to a temperature higher than or close to the temperature of the target coal seam before microwave heating; S8. Loop through steps S5 to S7 to fully expand the cracks in the target coal seam, and then extract the gas.

2. The method for improving the air permeability of ultra-deep buried high-in-situ stress soft pulverized coal seams according to claim 1, characterized in that: The coal-saving agent is a low-expansion polyurethane coal-saving agent.

3. The method for improving the air permeability of ultra-deep buried high-in-situ stress soft pulverized coal seams according to claim 1, characterized in that: The low-frequency microwave transmission pipeline (3) and the medium-frequency microwave transmission pipeline (6) are both made of aluminum waveguide tubes.

4. The method for improving the air permeability of ultra-deep buried high-in-situ stress soft pulverized coal seams according to claim 1, characterized in that: The flow rate of Freon is 0.2 m / s ~0.3m / s, and the flow rate is 5L / min~10L / min.

5. A method for improving the air permeability of ultra-deep buried high-in-situ stress soft pulverized coal seams according to claim 1 or 4, characterized in that: The freon is R-134a.

6. The method for improving the air permeability of ultra-deep buried high-in-situ stress soft pulverized coal seams according to claim 1, characterized in that: In steps S5 to S7, the temperature and crack expansion of the target coal seam are monitored in real time by a microseismic monitoring system.

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