Gas extraction method for thick coal seam by using directional multi-branch long borehole gas drive flow increasing

Abstract

The application discloses a thick coal seam gas extraction method applying directional multi-branch long borehole gas drive flow increasing, which comprises the following steps: A: coal seam geological condition investigation; B: completing the construction of the bedding extraction borehole and the directional multi-branch long borehole, and pre-extracting under negative pressure before mining; C: selecting a directional multi-branch long borehole as a first gas injection borehole, and implementing a long injection short extraction mode; D: after the first gas injection borehole gas injection is completed, a long extraction short extraction mode is implemented; then a second gas injection borehole is selected to implement the long injection short extraction mode; E: a third, a fourth, and so on, gas injection borehole is selected to implement the long injection short extraction mode according to the method in the step D; the first round of gas extraction is completed; F: according to the method in the step E, the second round, the third round, and so on, of gas extraction is continuously completed until the gas in the thick coal seam is fully extracted through the gas drive flow increasing mode. The application can effectively solve the problem of low gas extraction efficiency of the upper horizon of the existing thick coal seam.

Description

Technical Field

[0001] This invention relates to the field of underground coal mine gas extraction technology, and in particular to a method for gas extraction from thick coal seams using directional multi-branch long borehole gas-driven flow enhancement. Background Technology

[0002] Methane gas, as a clean energy source associated with coal seams, is widely used in power generation, residential gas supply, and vehicle fuel. Efficient extraction of coal seam methane is a fundamental measure for preventing gas explosions and coal and gas outbursts in underground coal mines. High-methane coal seams generally exhibit poor permeability and low permeability. Traditional negative pressure extraction techniques face challenges such as rapid extraction attenuation and unsatisfactory extraction results. While existing methods like hydraulic fracturing, hydraulic perforation, hydraulic slotting, and loosening blasting can improve coal seam permeability, they face the problem of reservoir pressure decline in the later stages of extraction, preventing a fundamental improvement in coal seam permeability.

[0003] With the development of science and technology, the successful surface coalbed methane injection and displacement test has provided a new approach to gas extraction. This technology can not only improve the extraction speed of mixed gas flow, but also solve the problem of gas reservoir pressure drop in the later stage of extraction. It has received widespread attention due to its safety, efficiency and environmental friendliness.

[0004] The invention patent with patent number 201911262514.3, entitled "A Method for Alternating Surface and Underground Extraction of Coal Seam Gas," discloses a method for alternating surface and underground extraction of coal seam gas. First, a row of vertical wells is drilled at the corresponding surface location of the working face. After hydraulic fracturing and supercritical CO2 fracturing are performed sequentially, coal seam gas is extracted from the surface wells. Then, multiple rows of in-seam extraction boreholes are drilled within the working face. When the surface well gas extraction enters its depletion phase, the surface gas extraction equipment is dismantled, and the surface well gas injection system is connected. This invention involves intermittent gas injection for surface gas displacement of coal seam gas, followed by extraction through underground in-seam extraction boreholes, until the coal seam reaches the recovery standard. This method fully leverages the advantages of surface vertical drilling fracturing, high displacement and extraction efficiency, and low extraction cost of underground in-seam extraction boreholes. It rationally plans the spatiotemporal connection between surface vertical wells and underground in-seam extraction boreholes for gas extraction, converting depleted vertical extraction wells into gas injection wells, improving the utilization rate of surface vertical wells, reducing gas extraction costs, and promoting effective gas management in mines with tight mining schedules. Currently, underground gas displacement often uses a method of injecting gas through a central borehole and extracting through two side boreholes (one injection, two extractions). However, due to the large number of in-seam extraction boreholes (often hundreds) during working face construction, the gas-driven flow enhancement operation is labor-intensive, and sequential gas displacement suffers from low efficiency. Furthermore, while horizontal gas injection and displacement in in-seam drainage boreholes primarily address the low permeability of thin and medium-thick coal seams, near-horizontal in-seam drilling cannot effectively cover gas drainage in upper strata of thick and extra-thick coal seams. In the field of gas drainage, coal seams with a thickness of 3.5 to 6 meters during underground mining are considered thick coal seams; those exceeding 6 meters are considered extra-thick coal seams. Therefore, there is an urgent need to propose a new method for gas injection and displacement to increase flow and drain gas from thick and extra-thick coal seams.

[0005] With the continuous development of geological directional drilling technology, directional multi-branch drilling technology has been widely applied in underground coal mines due to its advantages such as high-precision construction of long boreholes and flexible changes in borehole angles and strata. However, currently, it is mostly concentrated on using directional multi-branch long boreholes for traditional negative pressure extraction. Directional multi-branch long boreholes can cover dozens of in-seam extraction boreholes laterally. If the two technologies of in-seam extraction borehole gas injection and displacement and directional multi-branch long boreholes are combined—that is, directional multi-branch long borehole gas injection and in-seam short borehole extraction—it can not only effectively solve the problem of gas extraction from upper strata in thick coal seams, but also greatly improve the efficiency of gas extraction through gas injection displacement. Based on this, this invention proposes a method for gas extraction using directional multi-branch long boreholes for gas injection displacement and flow enhancement in thick and extra-thick coal seams. Summary of the Invention

[0006] The purpose of this invention is to provide a method for gas extraction from thick coal seams using directional multi-branch long boreholes with gas-driven flow enhancement. This method combines directional multi-branch long boreholes with in-seam extraction boreholes to form a complete gas injection displacement flow enhancement system for gas extraction from thick coal seams, effectively solving the problem of low gas extraction efficiency in existing thick coal seam upper strata.

[0007] The present invention adopts the following technical solution:

[0008] A method for gas extraction from thick coal seams using directional multi-branch long borehole gas-driven flow enhancement includes the following steps:

[0009] A: Conduct an investigation into the geological conditions and gas occurrence of the coal seam in the pre-extraction coal mine to determine the coal seam thickness, stratum undulation, and original coal seam gas content;

[0010] B: Based on the geological conditions of the coal seam and the actual site conditions, determine the location, size, stratum, and number of in-seam extraction boreholes and directional multi-branch long boreholes respectively, and then complete the construction of in-seam extraction boreholes and directional multi-branch long boreholes respectively; after the construction is completed, seal the boreholes and connect them to the gas extraction pipeline system, and use the gas extraction pipeline system to perform pre-mining negative pressure extraction of gas in the upper strata;

[0011] C: Parameters are monitored in real time using the extraction parameter measuring device in the extraction system. When the coal seam gas concentration value extracted by the in-seam extraction borehole is lower than the set first threshold and the pure extraction flow rate is lower than the set second threshold, a directional multi-branch long borehole is selected as the first gas injection borehole and connected to the underground gas injection pipeline system. Gas is injected into the directional multi-branch long borehole using the underground gas injection pipeline system, while gas is simultaneously pumped back from the in-seam extraction borehole. This is the long injection short extraction mode.

[0012] D: Real-time monitoring of the pressure and continuous gas injection time within the directional multi-branch long borehole serving as the first gas injection borehole. When the coal seam gas concentration value extracted by the in-seam extraction borehole is lower than the set fourth threshold and the gas mixing flow rate is higher than the set fifth threshold, the fracturing development and gas injection displacement of the borehole are completed. Then, long injection and short extraction continue until the set time, at which point gas injection and displacement into the directional multi-branch long borehole serving as the gas injection borehole are stopped. Then, the directional multi-branch long borehole is switched to extraction mode, i.e., long extraction and short extraction mode is implemented.

[0013] Subsequently, another directional multi-branch long borehole was selected as the second gas injection borehole and connected to the downhole gas injection pipeline system. Gas was injected into the directional multi-branch long borehole, which served as the second gas injection borehole, using the downhole gas injection pipeline system. At the same time, gas was pumped back from the in-seam extraction borehole, which is to implement the long injection and short extraction mode. Then, the change in the amount of gas extracted in the area of ​​the in-seam extraction borehole covered by the directional multi-branch long borehole, which served as the second gas injection borehole, was monitored in real time.

[0014] E: Following the method in step D, when the coal seam gas concentration value detected by the in-seam extraction borehole is lower than the set fourth threshold and the gas mixing flow rate is higher than the set fifth threshold, the directional multi-branch long borehole, which is used as the second gas injection borehole, will be switched to the long extraction short extraction mode; and the directional multi-branch long borehole, which is used as the third gas injection borehole, will be selected, and then the long injection short extraction mode will be implemented.

[0015] Similarly, following the above method, after treating all directional multi-branch long boreholes as a single gas injection borehole, the first round of gas extraction is completed.

[0016] F: Following the method in step E, continue to complete the second, third, ..., nth rounds of gas extraction until the gas in the thick coal seam has been fully extracted through gas-driven flow enhancement.

[0017] In step B, the arrangement of gas-driven flow enhancement in thick coal seam gas boreholes using directional multi-branch long boreholes includes several borehole groups consisting of several in-seam extraction boreholes and at least three directional multi-branch long boreholes.

[0018] The multiple in-seam extraction boreholes are arranged along the inclined direction of the working face, and the borehole depth is determined according to the length of the inclined direction of the working face. The spacing of the in-seam extraction boreholes is determined according to the coal seam gas extraction radius. Multiple directional multi-branch long boreholes are arranged along the upper strata of the thick coal seam, and the directional borehole depth is determined according to the length of the working face in the strike direction. The stratum arrangement of the boreholes is determined according to the coal seam thickness.

[0019] The directional multi-branch long borehole and the in-seam extraction borehole are arranged in a cross shape.

[0020] In step C, the in-seam drainage borehole is opened, and a multi-parameter gas monitoring instrument connected to the gas drainage main pipe is used to observe and record in real time the single-hole negative pressure, mixed flow rate, and methane concentration data displayed by the multi-parameter gas monitoring instrument. When the above data remain stable at three consecutive detection time points, the relevant values ​​are recorded and used as the coal seam gas concentration value. After a set time of gas drainage, when the recorded coal seam gas concentration value is lower than the set first threshold and the pure drainage flow rate drops below the set second threshold, If it is determined that single negative pressure extraction has no significant effect, then select a directional multi-branch long borehole as the first gas injection borehole and stop gas extraction from that directional multi-branch long borehole. Then connect the directional multi-branch long borehole to the downhole compressed air system or air compressor system, start the switch valve to inject gas into the directional multi-branch long borehole as the first gas injection borehole, and simultaneously pump it back from the in-seam extraction borehole. Subsequently, monitor the change in gas extraction volume in the area of ​​the in-seam extraction borehole covered by the directional multi-branch long borehole as the first gas injection borehole in real time.

[0021] The first threshold is 90% of the initial value of gas concentration extraction; the extraction pure flow rate refers to the pure methane flow rate, and the value of the extraction pure flow rate is the product of the monitored methane concentration and the mixed flow rate; the second threshold is 80% of the initial value of the pure methane flow rate extraction; the initial values ​​of gas concentration and pure methane flow rate extraction refer to the data recorded when the gas concentration and pure methane flow rate of all in-seam extraction boreholes in the group remain stable after three consecutive monitoring sessions during the initial stage of single negative pressure extraction.

[0022] In step C, the condition for determining that the negative pressure, mixed flow rate, and methane concentration data of the single hole of the extraction borehole remain stable is that the change range of the negative pressure, mixed flow rate, and methane concentration of the extraction borehole is simultaneously less than a set third threshold, which can be set to 10%.

[0023] In step D, the fourth threshold is 70% of the coal seam gas concentration before gas injection begins; the fifth threshold is 150% of the gas mixed flow rate before gas injection begins; the gas concentration and mixed flow rate before gas injection begin refer to the data recorded when the concentration and mixed flow rate of all in-seam extraction boreholes in the group remain stable after three consecutive monitoring sessions at the beginning of gas injection.

[0024] All in-seam extraction boreholes covered by directional multi-branch long boreholes along the strike are set as a group, and gas parameters are monitored and calculated in batches on a group basis.

[0025] This invention involves arranging directional, multi-branched long boreholes along the upper strata of a thick coal seam, injecting gas into these boreholes and their branches, and extracting the gas from the in-seam extraction boreholes in the lower strata. This creates a complete gas-driven loop system consisting of the directional long boreholes, multi-branched boreholes, and in-seam extraction boreholes. The permeability of the upper coal seam is improved, and the gas in the upper coal seam flows down into the in-seam extraction boreholes following the gas flow. This method can effectively solve the problem of low gas extraction efficiency in the upper strata of thick and extra-thick coal seams.

[0026] Traditional in-seam borehole gas drive often adopts the "one injection, two extraction" mode, which can only pre-fracture two boreholes each time, resulting in low work efficiency. The directional multi-branch long borehole used in this invention can reach a length of more than 500m, which can cover more than 100 in-seam boreholes. The permeability enhancement and pre-fracturing of coal body area is wide and large, which greatly improves the efficiency of gas drive compared with traditional in-seam borehole. Attached Figure Description

[0027] Figure 1 This is a schematic diagram of the process of the present invention;

[0028] Figure 2 This is a schematic diagram of the gas-driven flow enhancement process in this invention;

[0029] Figure 3 This is a top view of the overall borehole distribution in this invention;

[0030] Figure 4 This is a partially enlarged schematic diagram of the drilling process in this invention. Detailed Implementation

[0031] The present invention will now be described in detail with reference to the accompanying drawings and embodiments:

[0032] like Figures 1 to 4 As shown, the thick coal seam gas extraction method using directional multi-branch long borehole gas-driven flow enhancement described in this invention includes the following steps:

[0033] A: Conduct an investigation into the geological conditions and gas occurrence of the coal seam in the coal mine to be pre-extracted, and determine the basic parameters such as coal seam thickness, stratum undulation, and original coal seam gas content. These parameters are helpful for the subsequent construction and extraction process.

[0034] B: Based on the geological conditions of the coal seam and the actual site conditions, determine the location, size, stratum, and number of in-seam extraction borehole 3 and directional multi-branch long boreholes respectively, and then complete the construction of in-seam extraction borehole 3 and directional multi-branch long boreholes respectively; after the construction is completed, seal the boreholes and connect them to the gas extraction pipeline system, and use the gas extraction pipeline system to perform pre-mining negative pressure pre-extraction of gas in the upper strata;

[0035] like Figure 4 As shown, in this embodiment, the layout of the thick coal seam gas borehole using directional multi-branch long borehole gas-driven flow enhancement includes several borehole groups consisting of several in-seam extraction boreholes 3 and at least three directional multi-branch long boreholes. The in-seam extraction boreholes 3 are arranged along the inclined direction of the working face 1, with the borehole depth determined according to the length of the inclined direction of the working face, and the spacing between the in-seam extraction boreholes 3 determined according to the coal seam gas extraction radius. The directional multi-branch long boreholes are arranged along the upper strata of the thick coal seam, with the directional borehole depth determined according to the length of the working face strike direction, and the strata arrangement of the boreholes determined according to the coal seam thickness. The directional multi-branch long boreholes are constructed using a directional geological drilling rig 2, and each directional multi-branch long borehole consists of a directional long borehole 4 and a directional branch borehole 5.

[0036] In this embodiment, since the directional multi-branch long boreholes are arranged along the strike direction of the working face, and the in-seam extraction boreholes 3 are arranged along the inclined direction of the working face, the two form a cross shape, as shown below. Figure 4 As shown, all the in-seam extraction boreholes 3 covered by the directional multi-branch long borehole along the strike are set as a group, and the gas parameters can be monitored and calculated in batches as a group.

[0037] After completing the construction of the in-seam drainage borehole 3 and the directional multi-branch long borehole, due to the high gas content and concentration in the coal seam boreholes, conventional negative pressure drainage could be used to extract the gas present in the coal seam near the boreholes. To prevent the gas in the boreholes from surging into the roadway and to ensure a safe working environment in the roadway, the boreholes were sealed and the gas drainage pipeline system was connected. The gas drainage pipeline system was then used for pre-mining negative pressure pre-drainage of the in-seam drainage borehole 3 and the upper-seam directional multi-branch borehole. The method of sealing the boreholes and connecting the gas drainage pipeline system is a conventional technique in this field. The gas drainage pipeline system mainly includes a gas drainage main pipe 6, gas drainage branch pipes 7, a gas multi-parameter monitor 8, a compressed air main pipe 9, an air inlet pipe 10, a booster pump 11, a pressure gauge 12, and switch valves 13, etc., with the connections as follows: Figure 2 As shown, it will not be elaborated further here.

[0038] In this invention, the borehole layout for thick coal seam gas drive with directional multi-branch long boreholes, as determined above, can maximize the efficiency of borehole layout, save on borehole construction, and provide technical reference and standard guidance for the large-scale application of displacement technology.

[0039] C: The extraction parameter measuring device in the extraction system is used to monitor the parameters in real time. When the coal seam gas concentration value extracted by the in-seam extraction borehole 3 is lower than the set first threshold and the pure extraction flow rate is lower than the set second threshold, a directional multi-branch long borehole is selected as the first gas injection borehole and connected to the underground gas injection pipeline system. Gas is injected into the directional multi-branch long borehole using the underground gas injection pipeline system, and at the same time, gas is drawn back from the in-seam extraction borehole 3. The long inlet and short outlet form a complete loop system, that is, the long injection and short extraction mode is implemented.

[0040] In step C, during the initial stage of extraction, as gas is continuously extracted from the borehole, the gas concentration and flow rate in the coal seam will gradually decrease. The in-seam extraction borehole 3 is opened, and the gas multi-parameter monitoring instrument 8 (i.e., the extraction parameter measuring device) connected to the gas extraction main pipe is used to observe and record in real time the single-hole negative pressure, mixed flow rate, and methane concentration data displayed by the gas multi-parameter monitoring instrument 8 for the in-seam extraction borehole 3. When the above data remain stable at three consecutive detection time points, the relevant values ​​are recorded and used as the coal seam gas concentration value. After a set time of gas extraction, when the recorded coal seam gas concentration value is lower than the set first threshold (indicating that the coal seam gas concentration does not change significantly), and the pure extraction flow rate drops below the set second threshold, it is determined that... Single negative pressure extraction has no significant effect. At this point, a directional multi-branch long borehole is selected as the first gas injection borehole, and gas extraction from this directional multi-branch long borehole is stopped. Then, the directional multi-branch long borehole is connected to the downhole compressed air system or air compressor system (i.e., the downhole gas injection pipeline system), and the switch valve 1313 is activated to inject gas into the directional multi-branch long borehole, which serves as the first gas injection borehole. At the same time, gas is extracted from the in-seam extraction borehole 3, which implements the long injection short extraction mode. Subsequently, the change in gas extraction volume in the area covered by the in-seam extraction borehole 3, which serves as the first gas injection borehole, is monitored in real time.

[0041] In this embodiment, the gas concentration value is not based on the change range of a single borehole data as a reference indicator. Instead, the average gas concentration of all in-seam extraction boreholes 3 monitored at a certain moment within the group is calculated. The first threshold is 90% of the initial gas concentration extraction value. The extraction pure flow rate refers to the pure methane flow rate, and the value of the extraction pure flow rate is the product of the monitored methane concentration and the mixed flow rate. Similarly, in this invention, the determination of whether to start the gas injection mode is not based on the change range of a single in-seam extraction borehole 3 data. Instead, the average value of all in-seam extraction borehole 3 data monitored at a certain moment within the group is taken as the determination basis. The second threshold can be set to 80% of the initial methane pure flow rate extraction value. The determination condition for the stability of the single-hole negative pressure, mixed flow rate, and methane concentration data of the in-seam extraction borehole 3 is that the change range of the single-hole negative pressure, mixed flow rate, and methane concentration of the extraction borehole is simultaneously less than the set third threshold, which can be set to 10%.

[0042] In this embodiment, the initial values ​​of gas concentration and methane pure flow rate refer to the data recorded when the gas concentration and methane pure flow rate of all in-seam extraction boreholes in the group remain stable after three consecutive monitoring sessions during the initial stage of single negative pressure extraction. The stability standard is that the change in the three monitoring data is less than 10%.

[0043] D: Real-time monitoring of the pressure and continuous gas injection time in the directional multi-branch long borehole serving as the first gas injection borehole. When the coal seam gas concentration value extracted by the in-seam extraction borehole 3 is lower than the set fourth threshold and the gas mixing flow rate is higher than the set fifth threshold, the fracturing development and gas injection displacement of the borehole are completed. Then, long injection and short extraction continue until the set time, and then gas injection displacement into the directional multi-branch long borehole serving as the gas injection borehole is stopped. Then, the directional multi-branch long borehole is switched to extraction mode, that is, long extraction and short extraction mode is implemented.

[0044] Subsequently, another directional multi-branch long borehole was selected as the second gas injection borehole and connected to the downhole gas injection pipeline system. Gas was injected into the directional multi-branch long borehole, which served as the second gas injection borehole, using the downhole gas injection pipeline system. At the same time, gas was pumped back from the in-seam extraction borehole 3, which is to implement the long injection and short extraction mode. Then, the change in gas extraction volume in the area covered by the directional multi-branch long borehole, which served as the second gas injection borehole, was monitored in real time.

[0045] In step D, with gas extraction under the long injection-short extraction mode, after gas fracturing, if the directional multi-branch long borehole and the in-seam extraction borehole 3 are connected, the gas concentration in the coal seam will significantly decrease after a period of fracturing development, and the gas mixing flow rate will significantly increase. The in-seam extraction borehole 3 is opened, and the gas multi-parameter monitoring instrument 8 (i.e., extraction parameter measuring device) connected to the gas extraction main pipe is used to observe and record in real time the single-hole negative pressure, mixing flow rate, and methane concentration data displayed by the gas multi-parameter monitoring instrument 8 for the in-seam extraction borehole 3. When the above data remain stable at three consecutive detection time points, the values ​​are recorded as the coal seam gas concentration value.

[0046] In this embodiment, the coal seam gas concentration value is not based on the variation range of a single borehole data as a reference indicator, but rather on the average gas concentration of all in-seam extraction boreholes 3 monitored at a certain moment within the group. The fourth threshold is 70% of the coal seam gas concentration value before gas injection begins. The gas mixing flow rate is not based on the variation range of a single borehole data as a reference indicator, but rather on the average gas mixing flow rate of all in-seam extraction boreholes 3 monitored at a certain moment within the group. The fifth threshold can be set to 150% of the gas mixing flow rate value before gas injection begins.

[0047] In this embodiment, the gas concentration and mixed flow rate values ​​before the start of gas injection refer to the data recorded when the concentration and mixed flow rate of all in-seam extraction boreholes in the group remain stable after three consecutive monitoring sessions at the start of gas injection. The stability standard is that the change in the three monitoring data is less than 10%.

[0048] E: Following the method in step D, when the coal seam gas concentration value of the monitored in-seam extraction borehole 3 is lower than the set fourth threshold and the gas mixing flow rate is higher than the set fifth threshold, the directional multi-branch long borehole, which is used as the second gas injection borehole, will be switched to the long extraction short extraction mode; and the directional multi-branch long borehole, which is used as the third gas injection borehole, will be selected, and then the long injection short extraction mode will be implemented.

[0049] Similarly, following the above method, after treating all directional multi-branch long boreholes as a single gas injection borehole, the first round of gas extraction is completed.

[0050] F: Following the method in step E, continue to complete the second, third, ..., nth rounds of gas extraction until the gas in the thick coal seam has been fully extracted through gas-driven flow enhancement.

Claims

1. A method for gas extraction from thick coal seams using directional multi-branch long borehole gas-driven flow enhancement, characterized in that, The steps are as follows: A: Conduct an investigation into the geological conditions and gas occurrence of the coal seam in the pre-extraction coal mine to determine the coal seam thickness, stratum undulation, and original coal seam gas content; B: Based on the geological conditions of the coal seam and the actual site conditions, determine the location, size, stratum, and number of in-seam extraction boreholes and directional multi-branch long boreholes respectively, and then complete the construction of in-seam extraction boreholes and directional multi-branch long boreholes respectively; after the construction is completed, seal the boreholes and connect them to the gas extraction pipeline system, and use the gas extraction pipeline system to perform pre-mining negative pressure extraction of gas in the upper strata; C: Parameters are monitored in real time using the extraction parameter measuring device in the extraction system. When the coal seam gas concentration value extracted by the in-seam extraction borehole is lower than the set first threshold and the pure extraction flow rate is lower than the set second threshold, a directional multi-branch long borehole is selected as the first gas injection borehole and connected to the underground gas injection pipeline system. Gas is injected into the directional multi-branch long borehole using the underground gas injection pipeline system, while gas is simultaneously pumped back from the in-seam extraction borehole. This is the long injection short extraction mode. D: Real-time monitoring of the pressure and continuous gas injection time within the directional multi-branch long borehole serving as the first gas injection borehole. When the coal seam gas concentration value extracted by the in-seam extraction borehole is lower than the set fourth threshold and the gas mixing flow rate is higher than the set fifth threshold, the fracturing development and gas injection displacement of the borehole are completed. Then, long injection and short extraction continue until the set time, at which point gas injection and displacement into the directional multi-branch long borehole serving as the gas injection borehole are stopped. Then, the directional multi-branch long borehole is switched to extraction mode, i.e., long extraction and short extraction mode is implemented. Subsequently, another directional multi-branch long borehole was selected as the second gas injection borehole and connected to the downhole gas injection pipeline system. Gas was injected into the directional multi-branch long borehole, which served as the second gas injection borehole, using the downhole gas injection pipeline system. At the same time, gas was pumped back from the in-seam extraction borehole, which is to implement the long injection and short extraction mode. Then, the change in the amount of gas extracted in the area of ​​the in-seam extraction borehole covered by the directional multi-branch long borehole, which served as the second gas injection borehole, was monitored in real time. E: Following the method in step D, when the coal seam gas concentration value detected by the in-seam extraction borehole is lower than the set fourth threshold and the gas mixing flow rate is higher than the set fifth threshold, the directional multi-branch long borehole, which is used as the second gas injection borehole, will be switched to the long extraction short extraction mode; and the directional multi-branch long borehole, which is used as the third gas injection borehole, will be selected, and then the long injection short extraction mode will be implemented. Similarly, following the above method, after treating all directional multi-branch long boreholes as a single gas injection borehole, the first round of gas extraction is completed. F: Following the method in step E, continue to complete the second, third, ..., nth rounds of gas extraction until the gas in the thick coal seam has been fully extracted through gas-driven flow enhancement.

2. The method for gas extraction from thick coal seams using directional multi-branch long borehole gas-driven flow enhancement as described in claim 1, characterized in that: In step B, the layout of the thick coal seam gas boreholes using directional multi-branch long borehole gas-driven flow enhancement includes several borehole groups, each group consisting of several in-seam extraction boreholes and at least three directional multi-branch long boreholes.

3. The method for gas extraction from thick coal seams using directional multi-branch long borehole gas-driven flow enhancement as described in claim 1, characterized in that: Multiple in-seam extraction boreholes are arranged along the inclined direction of the working face, and the borehole depth is determined according to the length of the inclined direction of the working face. The spacing of the in-seam extraction boreholes is determined according to the coal seam gas extraction radius. Multiple directional multi-branch long boreholes are arranged along the upper strata of the thick coal seam, and the directional borehole depth is determined according to the length of the working face strike direction. The stratum arrangement of the boreholes is determined according to the coal seam thickness.

4. The method for gas extraction from thick coal seams using directional multi-branch long borehole gas-driven flow enhancement as described in claim 1, characterized in that: The directional multi-branch long borehole and the in-seam extraction borehole are arranged in a cross shape.

5. The method for gas extraction from thick coal seams using directional multi-branch long borehole gas-driven flow enhancement according to claim 2, characterized in that: In step C, the in-seam extraction borehole is opened and the gas multi-parameter monitoring instrument connected to the gas extraction main pipe is used to observe and record the single-hole negative pressure, mixed flow rate and methane concentration data of the in-seam extraction borehole in real time. When the above data remain stable at three consecutive detection time points, the relevant values ​​are recorded and used as the coal seam gas concentration value. After a set time of gas extraction, when the recorded coal seam gas concentration value is lower than the set first threshold and the extraction flow rate drops below the set second threshold, it is determined that single negative pressure extraction has no significant effect. At this time, a directional multi-branch long borehole is selected as the first gas injection borehole and gas extraction from the directional multi-branch long borehole is stopped. Then, the directional multi-branch long borehole is connected to the underground compressed air system or air compressor system, and the switch valve is activated to inject gas into the directional multi-branch long borehole, which serves as the first gas injection borehole, while simultaneously pumping back from the in-seam extraction borehole. Subsequently, the change in the gas extraction volume within the in-seam extraction borehole area covered by the directional multi-branch long borehole, which serves as the first gas injection borehole, is monitored in real time.

6. The method for gas extraction from thick coal seams using directional multi-branch long borehole gas-driven flow enhancement as described in claim 5, characterized in that: The first threshold is 90% of the initial value of gas concentration extraction; the extraction pure flow rate refers to the pure methane flow rate, and the value of the extraction pure flow rate is the product of the monitored methane concentration and the mixed flow rate; the second threshold is 80% of the initial value of the pure methane flow rate extraction; the initial values ​​of gas concentration and pure methane flow rate extraction refer to the data recorded when the gas concentration and pure methane flow rate of all in-seam extraction boreholes in the group remain stable after three consecutive monitoring sessions during the initial stage of single negative pressure extraction.

7. The method for gas extraction from thick coal seams using directional multi-branch long borehole gas-driven flow enhancement according to claim 5, characterized in that: In step C, the condition for determining that the negative pressure, mixed flow rate, and methane concentration data of the single hole of the in-seam extraction borehole remain stable is that the change range of the negative pressure, mixed flow rate, and methane concentration of the single hole of the extraction borehole is simultaneously less than a set third threshold, which is set to 10%.

8. The method for gas extraction from thick coal seams using directional multi-branch long borehole gas-driven flow enhancement according to claim 2, characterized in that: In step D, the fourth threshold is 70% of the coal seam gas concentration before gas injection begins; the fifth threshold is 150% of the gas mixed flow rate before gas injection begins; the gas concentration and mixed flow rate before gas injection begin refer to the data recorded when the concentration and mixed flow rate of all in-seam extraction boreholes in the group remain stable after three consecutive monitoring sessions at the beginning of gas injection.

9. The method for gas extraction from thick coal seams using directional multi-branch long borehole gas-driven flow enhancement as described in claim 1, characterized in that: All in-seam extraction boreholes covered by directional multi-branch long boreholes along the strike are set as a group, and gas parameters are monitored and calculated in batches on a group basis.

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