Methods for eliminating fracturing islands in the roof and floor of soft, outburst-prone coal seams through borehole drilling and drainage.

Abstract

This invention discloses a method for eliminating fracturing islands in the roof and floor of a fractured and soft outburst-prone coal seam through drilling and drainage. The method involves two stages: the first stage uses directional long boreholes in the roof for hydraulic fracturing, constructing a network of fracturing fractures within each fracturing section of the target coal seam via hydraulic perforation, and then implementing networked gas extraction to enhance regional permeability; the second stage addresses fracturing islands formed between adjacent fracturing sections during the segmented hydraulic fracturing of the roof by using floor boreholes and high-pressure water injection to forcibly remove fragmented, water-bearing coal from the target coal seam, followed by networked gas extraction to eliminate these fracturing islands. This invention utilizes roof hydraulic fracturing and bottom drainage roadway drilling for synergistic coal seam gas control. High-pressure water injection forcibly removes the coal-water mixture accumulated at the bottom of the coal seam, effectively eliminating fracturing islands and gas extraction blind spots, achieving safe and efficient permeability enhancement and gas extraction.

Description

Technical Field

[0001] This invention relates to the field of coal seam gas extraction technology, and more specifically to a method for eliminating gas outbursts by drilling holes in the roof and floor of soft, fractured coal seams to eliminate fracturing islands. Background Technology

[0002] With the gradual depletion of shallow coal resources, coal mining depth is increasing at a rate of 10 to 25 meters per year. Deep coal seams are increasingly affected by high ground stress, high gas pressure, low permeability, and strong mining disturbances, leading to a growing risk of coal and gas outbursts and other coal-rock dynamic disasters. Crushed and soft outburst-prone coal seams refer to structural coal seams formed by intense tectonic compression, folding deformation, and interlayer sliding and kneading, with a low firmness coefficient. Soft coal seams with a strength of no more than 0.8 have a broken and loose physical structure, are easily deformed, and have poor permeability. Under deep mining conditions, they are more prone to coal and gas outbursts and other disasters. Moreover, the fracture network formed by conventional permeability enhancement technology in soft coal seams is difficult to maintain stably in the long term, resulting in poor permeability enhancement and gas extraction effects.

[0003] To address the challenge of gas control in soft, fractured coal seams prone to outbursts, segmented hydraulic fracturing of the roof has been employed to achieve regional permeability enhancement, yielding promising engineering results. This technology leverages the well-formed rock strata by drilling long directional boreholes within the coal seam roof. Segmented fracturing is then used to achieve remote, regional fracturing, often employing proppant such as silica sand to maintain the fracture network connecting the roof and the coal seam, thus enhancing regional permeability. However, during hydraulic fracturing, soft, fractured coal seams are highly susceptible to the "stress shadow" effect caused by the opening of fractures. This results in high stress concentration in the area between two fracturing sections, leading to even lower permeability in that region. This can create drainage blind spots during later stages of mining, posing a potential risk of gas hazard.

[0004] Furthermore, due to the influence of its own weight stress, the large amount of water injected into the coal seam by hydraulic fracturing will continuously migrate and accumulate downwards. While wetting the coal seam, this also causes stress transfer, subjecting the coal body in the area between the two fracturing sections to stress disturbance caused by water wetting. Under the combined effect of stress concentration caused by fracturing and stress disturbance caused by water wetting, the area between the two fracturing sections, especially the lower-middle part near the floor, will form fracturing islands and extraction blind zones. Because the gas migration channels in the coal seam are blocked in this area, it is difficult to extract the gas through the fracture network formed by segmented hydraulic fracturing of the roof, resulting in the accumulation of a large amount of high-concentration gas, significantly increasing the risk of gas disasters, and even coal and gas outbursts. Summary of the Invention

[0005] To overcome the shortcomings of the existing technology, this invention provides a method for eliminating fracturing islands in the top and bottom plates of soft, outburst-prone coal seams by drilling and pressurizing the gas-water mixture accumulated at the bottom of the coal seam under high pressure. This eliminates fracturing islands and blind spots in gas extraction, avoids the risk of dynamic disasters such as coal and gas outbursts, provides technical support for the safe and efficient mining of deep coal, and achieves safe and efficient permeability enhancement and gas extraction in coal seams.

[0006] The present invention adopts the following technical solution to achieve its objective.

[0007] This invention relates to a method for eliminating hydraulic fracturing islands in the roof and floor of soft, outburst-prone coal seams. The method involves two stages: a roof hydraulic fracturing stage and a floor hydraulic drainage stage. The roof hydraulic fracturing stage employs a segmented hydraulic fracturing method using long directional boreholes. These boreholes are drilled in the roof strata of the target coal seam, dividing the coal seam into multiple fracturing segments along its strike. Hydraulic perforations are then used to sequentially perform segmented hydraulic fracturing, forming a network of fracturing fractures. After the segmented hydraulic fracturing is completed, networked gas extraction is performed on each fracturing segment. During the hydraulic fracturing operation, fracturing islands will form in the area between adjacent fracturing segments of the target coal seam. The fracturing island; the bottom plate hydraulic forced drainage stage refers to the process of forcibly draining the fragmented water-bearing coal body in the target coal seam through bottom plate boreholes using high-pressure water injection; in the bottom drainage roadway within the bottom strata directly below the area between adjacent fracturing sections of the target coal seam, water injection boreholes and pressure relief boreholes are drilled upwards through the strata to the target coal seam; high-pressure water is injected into the area between adjacent fracturing sections of the target coal seam through the water injection boreholes, and the fragmented water-bearing coal body formed by the water injection is discharged along the pressure relief boreholes. After the forced drainage of the fragmented water-bearing coal body is completed, the water injection boreholes and pressure relief boreholes are connected for gas extraction, thereby eliminating the fracturing island in the area between adjacent fracturing sections of the target coal seam.

[0008] The method for eliminating fracturing islands in the top and bottom plates of soft and brittle outburst-prone coal seams is characterized by the following: in the middle area of ​​adjacent fracturing sections of the same target coal seam, the water injection holes and pressure relief holes are arranged in a row along the transverse direction, forming two rows of water injection holes and one row of pressure relief holes, with the row of pressure relief holes located between the two rows of water injection holes.

[0009] The method for eliminating fracturing islands in the top and bottom plates of soft, outburst-prone coal seams using drilling and pressure relief is characterized by the following: in the middle area of ​​adjacent fracturing sections of the same target coal seam, the water injection boreholes and pressure relief boreholes are grouped in groups of three, and the three boreholes in the group are arranged along the coal seam direction, in the following order: the first water injection borehole, the pressure relief borehole, and the second water injection borehole, so that the two water injection boreholes are symmetrically arranged with the pressure relief borehole as the axis.

[0010] The method for eliminating fracturing islands in the top and bottom plates of soft, outburst-prone coal seams using drilling pressure drainage is characterized by the following: the final position of the water injection borehole reaches the top surface of the target coal seam, and the final position of the pressure relief borehole is located from the bottom surface of the target coal seam upwards to one-third of the target coal thickness.

[0011] The characteristic of this invention's method for eliminating fracturing islands in the roof and floor of soft, outburst-prone coal seams through borehole pressure drainage is that it sets the water injection pressure of the water injection borehole. Not less than 8 MPa.

[0012] The method for eliminating fracturing islands in the top and bottom plates of soft and fractured coal seams using borehole pressure drainage is characterized by the following: during the hydraulic drainage stage of the bottom plate, when the actual mass of soft and fractured coal discharged from the pressure relief borehole reaches the critical mass of soft and fractured coal discharged from the pressure relief borehole, the hydraulic drainage is completed, high-pressure water injection is stopped, and the system is moved sequentially to the next borehole group or the middle area of ​​the adjacent fracturing section of the target coal seam. The hydraulic drainage of the bottom plate is repeated until all fracturing sections are covered.

[0013] The characteristic of this invention's method for eliminating fracturing islands in the top and bottom plates of soft, outburst-prone coal seams via borehole pressure discharge is that the actual mass of soft, fracturing coal discharged from the pressure relief borehole is calculated using the following method:

[0014] The soft coal mass discharged through the pressure relief borehole naturally accumulates into a conical coal pile, and the actual mass is calculated using equation (1):

[0015] (1);

[0016] In formula (1):

[0017] For the first With the In the middle region of the fracturing section, the first The actual mass of the soft coal mass discharged from the pressure relief borehole;

[0018] For the first With the In the middle region of the fracturing section, the first The density of the soft coal mass discharged from the pressure relief borehole;

[0019] and They were respectively in the second With the In the middle region of the fracturing section, the first The base radius and cone height of the conical coal pile that discharges broken and soft coal in the pressure relief borehole;

[0020] The critical mass of soft coal mass discharged from the pressure relief borehole is calculated using equation (2):

[0021] (2);

[0022] In formula (2): For the first With the In the middle region of the fracturing section, the first The critical mass of soft coal mass discharged from the pressure relief borehole; The pressure relief coefficient refers to the first... With the In the middle region of the fracturing section, when eliminating stress islands and gas drainage blind zones, the ratio of the mass of coal body to be discharged from the pressure relief borehole to the mass of coal body in the discharge area. Target coal seam density; The effective radius of influence of the water injection borehole; The horizontal segment spacing for directional long boreholes; The target coal seam thickness; The target is the porosity of the coal seam.

[0023] Compared with existing technologies, the beneficial effects of this invention are as follows:

[0024] 1. This invention adopts a method of hydraulic fracturing of the top plate and hydraulic drainage of the bottom plate for the elimination of outbursts in soft and fragile coal seams. This method greatly enhances the permeability of the coal body, effectively overcomes the "stress shadow" effect, eliminates the fracturing islands and gas drainage blind zones in the coal body between two adjacent fracturing sections, and significantly improves the permeability enhancement effect of soft and fragile coal seams.

[0025] 2. The present invention forms a main extraction channel by drilling long holes in the top plate and a near-end extraction channel by drilling through layers in the bottom plate. Together, they constitute a coordinated extraction system of "far-near combination" and "top-bottom linkage", which significantly improves the gas extraction efficiency.

[0026] 3. The loose and easily deformable structure of fragmented and soft outburst-prone coal seams makes them prone to collapse during high-pressure water injection due to lateral stress imbalance, and the wetting range is limited. In this invention, the three-hole group design, with the water injection holes symmetrically arranged around the pressure relief hole as the axis, ensures overall stress balance, significantly improving the stability of the high-pressure water injection holes. Simultaneously, the symmetrically distributed water injection holes ensure sufficient wetting of the coal body in the middle area between adjacent fracturing sections, greatly reducing the coal discharge resistance and the probability of blockage in the pressure relief hole, thereby improving coal discharge efficiency. Furthermore, when high-pressure water is injected simultaneously into the two water injection holes on both sides, a bidirectional squeezing effect is created on the fragmented and soft water-bearing coal body, causing the coal body to converge towards the middle pressure relief hole, thus expanding the effective coal discharge range of the pressure relief hole to a certain extent.

[0027] 4. Critical mass in this invention It takes into account parameters such as coal seam density and porosity to quantify the minimum coal discharge quality for eliminating fracturing islands and gas drainage blind zones. This ensures the effectiveness of fracturing and drainage in soft, outburst-prone coal seams while avoiding gas disaster hazards and engineering waste caused by insufficient or excessive coal discharge. Attached Figure Description

[0028] Figure 1 This is a schematic elevation view of the drilling arrangement on the top and bottom plates of the soft, outburst-protruding coal seam in this invention.

[0029] Figure 2 This is a schematic diagram of the planar arrangement of boreholes in the top and bottom plates of a soft, outburst-protruding coal seam in this invention.

[0030] Figure 3 The first in the same borehole group in this invention With the Schematic diagram of the stress concentration zone in the middle area of ​​the fracturing section.

[0031] Figure 4 The first in the same borehole group in this invention With the Schematic diagram of the wetted zone in the middle area of ​​the fracturing section.

[0032] Figure 5 The first in the same borehole group in this invention With the Schematic diagram of the wetted zone in the middle of the fracturing section.

[0033] Figure 6 The first in the same borehole group in this invention With the Schematic diagram of the coal discharge zone in the middle area of ​​the fracturing section.

[0034] Numbers in the diagram: 1. Roof directional long borehole; 2. Roof strata; 3. Target coal seam; 4. Floor strata; 5. Bottom drainage roadway; 6. Water injection borehole; 7. Pressure relief borehole; 8. High-pressure hose; 9. High-pressure valve; 10. Borehole blowout preventer; 11. Fracturing pump; 12. Fracturing mesh; 13. i With the i +1 Stress concentration zone in the middle region of the fracturing section; 14th i With the i +1 Wetting zone in the middle area of ​​the fracturing section; 15th i With the i +1 Fracturing section, middle area coal discharge zone. Detailed Implementation

[0035] See Figure 1 and Figure 2 In this embodiment, the method for eliminating fracturing islands in the top and bottom plates of a soft, outburst-prone coal seam by drilling and flushing is carried out in two stages: the first stage is the hydraulic fracturing stage of the top plate, and the second stage is the hydraulic flushing stage of the bottom plate.

[0036] In the roof hydraulic fracturing stage, a segmented hydraulic fracturing method using directional long boreholes is employed. A directional long borehole 1 is drilled in the roof strata 2 of the target coal seam 3, dividing the target coal seam 3 into multiple fracturing segments along the coal seam strike. Segmented hydraulic fracturing is then carried out sequentially using hydraulic perforations to form a fracturing network 12. After the segmented hydraulic fracturing is completed, networked gas extraction is performed for each fracturing segment. During the hydraulic fracturing operation, fracturing islands will form in the area between adjacent fracturing segments of the target coal seam.

[0037] The bottom hydraulic forced drainage stage refers to the process of forcibly draining the fragmented water-bearing coal body in the target coal seam through bottom boreholes using high-pressure water injection. Specifically, in the bottom drainage roadway 5 within the bottom strata 4 directly below the area between adjacent fracturing sections of the target coal seam 3, water injection boreholes 6 and pressure relief boreholes 7 are drilled upwards through the strata. High-pressure water is injected into the area between adjacent fracturing sections of the target coal seam through the water injection boreholes 6. The resulting fragmented water-bearing coal body, moistened by the water injection, is discharged along the pressure relief boreholes 7. After the forced drainage of the fragmented water-bearing coal body is completed, the water injection boreholes 6 and 7 are connected for gas extraction, thereby eliminating fracturing islands in the area between adjacent fracturing sections of the target coal seam 3.

[0038] In practice, the corresponding technical measures also include:

[0039] Within the intermediate region of adjacent fracturing sections of the same target coal seam 3, water injection boreholes 6 and pressure relief boreholes 7 are arranged in transverse rows, forming two rows of water injection boreholes and one row of pressure relief boreholes. The row of pressure relief boreholes is located between the two rows of water injection boreholes. Within the intermediate region of adjacent fracturing sections of the same target coal seam 3, water injection boreholes 6 and pressure relief boreholes 7 are grouped in sets of three, arranged along the coal seam strike, in the following order: first water injection borehole, pressure relief borehole, and second water injection borehole, with the two water injection boreholes symmetrically arranged about the pressure relief borehole 7 as an axis. The termination point of water injection borehole 6 reaches the top surface of the target coal seam, and the termination point of pressure relief borehole 7 is located from the bottom surface of the target coal seam 3 upwards to one-third of the target coal thickness. The water injection pressure of the water injection boreholes is set. P i Not less than 8 MPa.

[0040] During the hydraulic drainage stage of the bottom plate, when the actual mass of the soft coal body discharged from the pressure relief borehole 7 reaches the critical mass of the soft coal body discharged from the pressure relief borehole, the hydraulic drainage is completed, high-pressure water injection is stopped, and the machine is moved sequentially to the next borehole group or the middle area of ​​the adjacent fracturing section of the target coal seam. The hydraulic drainage of the bottom plate is repeated until all fracturing sections are covered.

[0041] In this embodiment, the actual mass of the soft coal mass discharged from the pressure relief borehole is calculated using the following method:

[0042] The soft coal mass discharged through the pressure relief borehole naturally accumulates into a conical coal pile, and the actual mass is calculated using equation (1):

[0043] (1);

[0044] In formula (1):

[0045] For the first With the In the middle region of the fracturing section, the first The actual mass of the soft coal mass discharged from the pressure relief borehole; For the first With the In the middle region of the fracturing section, the first The density of the soft coal mass discharged from the pressure relief borehole was obtained through on-site testing. and They were respectively in the second With the In the middle region of the fracturing section, the first The base radius and cone height of the conical coal pile that discharged the soft coal mass from the pressure relief borehole were obtained through on-site measurement.

[0046] In this embodiment, the critical mass of soft coal mass discharged from the pressure relief borehole is calculated using equation (2):

[0047] (2);

[0048] In formula (2): For the first With the In the middle region of the fracturing section, the first The critical mass of soft coal mass discharged from the pressure relief borehole; The pressure relief coefficient refers to the first... With the In the middle area of ​​the fracturing section, when eliminating stress islands and gas drainage blind zones, the ratio of the mass of coal body to be discharged from the pressure relief borehole to the mass of coal body in the discharge area is obtained through trial discharge based on the actual geological conditions of the mine. The target coal seam density was obtained in the laboratory according to the recommended method in GB / T 23561.3-2009. The effective influence radius of the water injection borehole is approximately equal for all water injection boreholes in the target coal seam. The horizontal segment spacing for directional long boreholes; The target coal seam thickness; The target coal seam porosity was obtained through laboratory testing according to the recommended method in GB / T23561.4-2009.

[0049] Figure 3 This illustrates the first borehole in the same borehole group. With the In the stress concentration zone 13 in the middle of the fracturing section, during the segmented hydraulic fracturing of the roof, the target coal seam 3 experiences a "stress shadowing" effect due to its interaction with the roof hydraulic fracturing operation, leading to the stress concentration zone 13. i and i The coal seam in the middle area of ​​the +1 fracturing section is subjected to high stress concentration, forming a stress concentration zone.

[0050] Figure 4 This illustrates the first borehole in the same borehole group. With the Wetting zone 14 in the middle of the fracturing section Figure 5 This illustrates the first borehole in the same borehole group. i With the i A profile of the wetting zone 14 in the middle region of the +1 fracturing section; in the bottom drainage roadway 5 directly below the area between adjacent fracturing sections of the target coal seam 3, water injection boreholes 6 and pressure relief boreholes 7 are drilled upwards into the target coal seam 3, respectively, using a "row-arranged, symmetrical, three-hole group" borehole layout pattern to ensure that high-pressure water is evenly injected from both sides into the middle region between the two adjacent fracturing sections, so as to fully wet the coal body in the middle region between the two adjacent fracturing sections, forming the first With the The wetted zone 14 in the middle of the fracturing section; the coal structure in this area will be more loose. The energy carried during the stress transmission process, in addition to causing plastic deformation of the coal, also needs to overcome the frictional resistance between coal particles, which deteriorates the stress transmission path and efficiency, and alleviates the stress concentration effect in the inter-section area to a certain extent.

[0051] Figure 6 This illustrates the first borehole in the same borehole group. With the In the middle section of the fracturing zone, coal discharge area 15, due to the influence of its own weight stress, water injected into the coal seam by hydraulic fracturing and high-pressure water injection will accumulate below. Under the combined effect of stress concentration caused by fracturing and stress disturbance caused by water wetting, the first... With the In the middle area of ​​the fracturing section, there will still be fracturing islands and extraction blind zones in the coal body near the bottom plate. Therefore, it is necessary to use the pressure relief borehole 7 to force the coal body in the near-bottom plate area to be discharged. The coal body within the effective height coverage range of the pressure relief borehole constitutes the coal discharge zone.

[0052] In practice, during high-pressure water injection, water is injected into the coal seam through a high-pressure hose 8 via a fracturing pump 11. The injection pressure and flow rate are precisely controlled by a high-pressure valve 9. A blowout prevention device 10 is installed at the opening of the pressure relief borehole 7. The high-pressure water injection process not only meets the water injection needs of different areas of the soft coal seam, but also quickly captures the gas gushing out of the pressure relief borehole, preventing it from spreading in the roadway and causing gas exceedances, thus ensuring the continuity and safety of the high-pressure water injection operation.

[0053] During the high-pressure water injection process, the first With the The first section of the middle area n In a borehole group, the coal mass discharged from a single cross-layer pressure relief borehole has an approximate conical shape, and the volume of the discharged coal mass is... for: Based on this, the first With the Middle area of ​​the segment, the first In a borehole group, the actual mass of broken and soft coal mass discharged from a single cross-layer pressure relief borehole for:

[0054] ;

[0055] No. With the Range 14 of the wetted zone in the middle of the fracturing section for:

[0056] ;

[0057] Considering the porosity and fractures in soft coal seams, the first With the The coal volume of the wetted zone 14 in the middle of the fracturing section for:

[0058] ;

[0059] Combined with the effective length of the bottom plate through-layer pressure relief drilling , h=H / 3, determine the number i With the i +1 Fracturing Section Middle Region Coal Discharge Zone 15 Coal Body Volume for:

[0060] ;

[0061] Based on this, the first With the The first section of the middle area Critical mass of soft coal mass discharged from decompression boreholes in borehole group for:

[0062] .

[0063] It will be apparent to those skilled in the art that this invention is not limited to the operational details described above. The order of fracturing, the specific arrangement of boreholes, and the fracturing parameters can all be adjusted according to actual geological and engineering conditions, and these changes all fall within the scope of protection of the claims of this invention.

Claims

1. A method for eliminating fracturing islands in the roof and floor of a soft, outburst-prone coal seam through borehole drilling for outburst suppression, characterized by: For the soft and explosive coal seam, the construction is carried out in two stages: the first stage is the roof hydraulic fracturing stage, and the second stage is the floor hydraulic drainage stage. The roof hydraulic fracturing stage is carried out by using the method of segmented hydraulic fracturing with directional long boreholes. Roof directional long boreholes (1) are drilled in the roof strata (2) of the target coal seam (3) to divide the target coal seam (3) into multiple fracturing segments along the coal seam plane. The segmented hydraulic fracturing is carried out sequentially by hydraulic perforation to form a fracturing network (12). After the segmented hydraulic fracturing is completed, networked gas extraction is carried out for each fracturing segment. During the hydraulic fracturing operation, fracturing islands are formed in the area between adjacent fracturing segments of the target coal seam. The floor hydraulic drainage stage refers to the use of high-pressure water injection. The method involves forcibly removing the loose, water-bearing coal body from the target coal seam through bottom boreholes. This is achieved by drilling water injection holes (6) and pressure relief holes (7) into the target coal seam (3) in a bottom drainage roadway (5) within the bottom strata (4) directly below the area between adjacent fracturing sections of the target coal seam (3). High-pressure water is injected into the area between adjacent fracturing sections of the target coal seam through the water injection holes (6). The loose, water-bearing coal body formed by the water injection is then discharged along the pressure relief holes (7). After the forced removal of the loose, water-bearing coal body, the water injection holes (6) and pressure relief holes (7) are connected for gas extraction, thereby eliminating fracturing islands in the area between adjacent fracturing sections of the target coal seam (3).

2. The method for eliminating fracturing islands in the roof and floor of soft, outburst-prone coal seams by drilling and drainage as described in claim 1, characterized in that: in In the middle area of ​​adjacent fracturing sections of the same target coal seam, the water injection boreholes and pressure relief boreholes are arranged in a row along the transverse direction, forming two rows of water injection boreholes and one row of pressure relief boreholes. The row of pressure relief boreholes is located between the two rows of water injection boreholes.

3. The method for eliminating fracturing islands in the roof and floor of soft, outburst-prone coal seams by drilling and drainage as described in claim 1, characterized in that: in In the middle area of ​​adjacent fracturing sections of the same target coal seam (3), the water injection borehole (6) and the pressure relief borehole (7) are grouped into three holes. The three holes in the group are arranged along the coal seam direction, in the following order: the first water injection borehole, the pressure relief borehole and the second water injection borehole, so that the two water injection boreholes (6) are arranged symmetrically with the pressure relief borehole (7) as the axis.

4. The method for eliminating fracturing islands in the roof and floor of soft, outburst-prone coal seams by drilling and drainage as described in claim 1, characterized in that: The final position of the water injection borehole (6) reaches the top surface of the target coal seam, and the final position of the pressure relief borehole (7) is located from the bottom surface of the target coal seam upwards to one-third of the target coal thickness.

5. The method for eliminating fracturing islands in the roof and floor of soft, outburst-prone coal seams by drilling and drainage as described in claim 1, characterized in that: Set the water injection pressure of the water injection borehole. Not less than 8 MPa.

6. The method for eliminating fracturing islands in the roof and floor of soft, outburst-prone coal seams by drilling and drainage as described in claim 1, characterized in that: in During the hydraulic drainage stage of the bottom plate, when the actual mass of the soft coal body discharged from the pressure relief borehole (7) reaches the critical mass of the soft coal body discharged from the pressure relief borehole, the hydraulic drainage is completed, the high-pressure water injection is stopped, and the machine moves sequentially to the next borehole group or the middle area of ​​the adjacent fracturing section of the target coal seam, and repeats the hydraulic drainage of the bottom plate until all fracturing sections are covered.

7. The method for eliminating fracturing islands in the roof and floor of soft, outburst-prone coal seams by drilling and drainage according to claim 6, characterized in that: The actual mass of soft coal fragments discharged from the pressure relief borehole is calculated using the following method: The soft coal mass discharged through the pressure relief borehole naturally accumulates into a conical coal pile, and the actual mass is calculated using equation (1): （1） ； In formula (1): For the first With the In the middle region of the fracturing section, the first The actual mass of the soft coal mass discharged from the pressure relief borehole; For the first With the In the middle region of the fracturing section, the first The density of the soft coal mass discharged from the pressure relief borehole; and They were respectively in the second With the In the middle region of the fracturing section, the first The base radius and cone height of the conical coal pile that discharges broken and soft coal in the pressure relief borehole; The critical mass of soft coal mass discharged from the pressure relief borehole is calculated using equation (2): （2） ； In formula (2): For the first With the In the middle region of the fracturing section, the first The critical mass of soft coal mass discharged from the pressure relief borehole; The pressure relief coefficient refers to the first... With the In the middle region of the fracturing section, when eliminating stress islands and gas drainage blind zones, the ratio of the mass of coal body to be discharged from the pressure relief borehole to the mass of coal body in the discharge area. Target coal seam density; The effective radius of influence of the water injection borehole; The horizontal segment spacing for directional long boreholes; The target coal seam thickness; The target is the porosity of the coal seam.

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