A design method for synchronous fracturing and weakening of roof and top coal in two-hard super-thick coal seam

Abstract

This application relates to a design method for simultaneous fracturing and weakening of the roof and top coal of two hard, extra-thick coal seams. It proposes a design method for the borehole layout parameters and fracturing parameters of the roof and top coal under two hard conditions in fully mechanized caving mining. This method effectively destroys the integrity of the hard roof and hard top coal and weakens their mechanical strength, thereby increasing the top coal release rate during fully mechanized caving mining. At the same time, it achieves roof cutting and pressure relief, avoiding the manifestation of strong mine pressure caused by large-area roof overhang leading to sudden fracture. Furthermore, the method of this application is simple to operate, safe, efficient, and has wide applicability.

Description

Technical Field

[0001] This application relates to the field of coal mining technology, specifically to a design method for simultaneous fracturing and weakening of the roof and top coal of two hard, extra-thick coal seams. Background Technology

[0002] The main factors affecting the releaseability and migration patterns of top coal are the degree of top coal fragmentation and roof conditions. The fragmentation and migration of top coal are directly related to the properties of the roof. Under the condition of coexistence of hard top coal and hard roof (two hard conditions), methods to weaken the hard roof while improving the releaseability of top coal are still under exploration.

[0003] Currently, the main artificial auxiliary measures to improve the risk of top coal caving in fully mechanized longwall mining faces include blasting pre-fracturing and hydraulic fracturing. Among these, blasting pre-fracturing technology is relatively mature and widely used, but its underground construction process is complex, its safety is poor, and dust control after blasting is difficult, making it unsuitable for large-scale regional applications. Hydraulic fracturing of the roof or top coal has proven effective in engineering trials in recent years, especially in addressing the problem of strong roof pressure. However, when facing a combination of hard roof and hard top coal in the stope, how to simultaneously solve the problems of strong roof pressure and difficult top coal caving requires further research into adaptive design methods. Summary of the Invention

[0004] To overcome at least one deficiency in the prior art, this application provides a design method for simultaneous fracturing and weakening of the roof and top coal of two hard, extra-thick coal seams.

[0005] Firstly, a design method for simultaneously fracturing and weakening the roof and top coal of two hard, extra-thick coal seams is provided, including:

[0006] Based on the stratigraphic parameters and physical and mechanical parameters of each rock stratum in the longwall mining face, a synchronous fracturing geological-engineering model is established. Each rock stratum includes the coal seam, the immediate roof composed of sandy mudstone, and the basic roof composed of siltstone and fine sandstone. The geological profile of the synchronous fracturing geological-engineering model, from bottom to top, is a combination of the coal seam, immediate roof, and basic roof.

[0007] Based on the synchronous fracturing geological-engineering model, the fracturing pump injection rate and fracturing pump injection duration were set, and fracturing design software was used to determine the fracture propagation size when boreholes were arranged in different rock strata.

[0008] Based on the crack propagation size, determine the rock strata corresponding to the borehole layout when the crack is fully extended in the longitudinal direction, and use them as the optimal rock strata for borehole layout.

[0009] Determine the optimal fracturing pumping time when the fracture length reaches a preset value when the borehole is placed in the optimal borehole arrangement rock layer;

[0010] Multiple directional long boreholes are drilled in the optimal borehole layout rock strata in the mining area. Each completed directional long borehole is subjected to segmented fracturing according to the optimal fracturing pumping time and fracturing pumping discharge rate, so as to achieve synchronous fracturing of the coal seam roof and top coal.

[0011] In one embodiment, the longitudinal extension of the crack includes: the upper end of the crack extending to the upper interface of the main roof, and the lower end of the crack extending to the lower interface of the top coal.

[0012] In one embodiment, determining the optimal fracturing pumping time when the fracture length reaches a preset value when the borehole is arranged in the optimal borehole arrangement rock formation includes:

[0013] The boreholes are arranged in the optimal borehole arrangement rock strata, and the optimal fracturing pumping time is determined when the fracture length reaches the preset value based on the relationship between fracture length and time.

[0014] In one embodiment, formation-related parameters include lithology, thickness, and minimum horizontal principal stress; physical and mechanical parameters include elastic modulus, Poisson's ratio, porosity, permeability, and filtration coefficient.

[0015] In one embodiment, the method further includes:

[0016] Once the staged fracturing time reaches the optimal fracturing pumping time, adjust the fracturing pumping rate to reduce the pumping pressure below the fracture extension pressure, and continue pumping for a certain period of time to ensure that the top coal is fully saturated with water.

[0017] In one embodiment, the method further includes:

[0018] Multiple drilling sites are arranged in the transport roadway or return air roadway of the longwall mining face. The distance between two adjacent directional long boreholes in each drilling site is twice the preset value of the fracture length.

[0019] Compared with the prior art, this application has the following beneficial effects: The synchronous fracturing and weakening design method for the roof and top coal of two hard extra-thick coal seams proposed in this application provides a design method for the drilling layout parameters and fracturing parameters of the roof and top coal under two hard conditions in fully mechanized caving mining. This effectively destroys the integrity of the hard roof and hard top coal and weakens their mechanical strength, improves the top coal release rate during fully mechanized caving mining, and achieves roof cutting and pressure relief, avoiding the manifestation of strong mine pressure caused by large-area roof suspension and sudden fracture. Furthermore, the method of this application is simple to operate, safe and efficient, and has wide applicability. Attached Figure Description

[0020] This application can be better understood by referring to the description given below in conjunction with the accompanying drawings, which, together with the detailed description below, are incorporated in and form part of this specification. In the drawings:

[0021] Figure 1 A flowchart illustrating the design method for simultaneous fracturing and weakening of the roof and top coal of two hard, extra-thick coal seams according to an embodiment of this application is shown.

[0022] Figure 2 A geological profile of the simultaneous fracturing geological-engineering model is shown;

[0023] Figure 3 The diagram shows the range of fracture height propagation in top coal seam with borehole arrangement.

[0024] Figure 4 This diagram shows the fracture height propagation range of a borehole arrangement in the direct top fracturing configuration;

[0025] Figure 5 The diagram shows the range of fracture height propagation in the borehole arrangement at the base.

[0026] Figure 6 A schematic diagram showing the relationship between fracture height and fracture pumping time is shown.

[0027] Figure 7 A schematic diagram showing the relationship between fracture length and fracture pumping time is shown.

[0028] Figure 8 The plan view of the layout of long boreholes for simultaneous fracturing of the roof and top coal of the longwall mining face is shown.

[0029] Figure 9 A cross-sectional view of the arrangement of long boreholes for simultaneous fracturing of the roof and top coal in the longwall mining face is shown. Detailed Implementation

[0030] Exemplary embodiments of the present application will be described below with reference to the accompanying drawings. For clarity and brevity, not all features of the actual embodiments are described in the specification. However, it should be understood that many embodiment-specific decisions can be made in the development of any such actual embodiment to achieve the developer’s specific objectives, and these decisions may vary as the embodiments differ.

[0031] It should also be noted that, in order to avoid obscuring this application with unnecessary details, only the device structure closely related to the solution according to this application is shown in the accompanying drawings, while other details that are not closely related to this application are omitted.

[0032] It should be understood that this application is not limited to the described embodiments by virtue of the following description with reference to the accompanying drawings. In this document, embodiments may be combined with each other, features may be substituted or borrowed between different embodiments, and one or more features may be omitted in one embodiment, where feasible.

[0033] This application provides a method for simultaneously fracturing and weakening the roof and top coal of two hard, extra-thick coal seams. Figure 1 A flowchart illustrating the design method for simultaneous fracturing and weakening of the roof and top coal of two hard, extra-thick coal seams according to an embodiment of this application is shown. See [link to relevant documentation]. Figure 1 The methods include:

[0034] Step S1: Based on the stratigraphic parameters and physical and mechanical parameters of each rock stratum in the longwall mining face, establish a synchronous fracturing geological-engineering model. Each rock stratum includes the coal seam, the immediate roof composed of sandy mudstone, and the basic roof composed of siltstone and fine sandstone. The geological profile of the synchronous fracturing geological-engineering model from bottom to top is a combination of the coal seam, immediate roof, and basic roof.

[0035] Specifically, formation-related parameters include lithology, thickness, and minimum horizontal principal stress; physical and mechanical parameters include elastic modulus, Poisson's ratio, porosity, permeability, and filtration coefficient.

[0036] Here, the roof includes the immediate roof and the basic roof. The synchronous fracturing geological-engineering model is the basis for conducting numerical simulations to obtain the optimal synchronous fracturing design parameters. The model reflects the geomechanical properties and borehole engineering properties contained in the special combination of hard roof and hard top coal.

[0037] The conventional method for creating geological models for fracturing design involves providing a vertical stratigraphic profile of the coal seam and its overlying strata, encompassing various lithologies, and obtaining the physical and mechanical parameters corresponding to all different lithologies. However, this approach often struggles to obtain data for each lithological level due to the complexity and variability of the lithologies and limitations of technical means. Numerical modeling, on the other hand, simplifies the complex geological conditions of the actual situation, improving efficiency and feasibility in problem-solving. This embodiment addresses the challenge of the coexistence of a difficult-to-collapse top coal seam and a difficult-to-collapse roof during fully mechanized longwall mining. Therefore, this embodiment simplifies the geological profile of the simultaneous fracturing geological-engineering model to a combination of the coal seam, immediate roof, and basic roof. Figure 2 A geological profile of the synchronous fracturing geological-engineering model is shown.

[0038] The coal seam is an extra-thick and hard coal seam, consisting of a lower section that can be directly mined and an upper section that is difficult to collapse during mining. Its mechanical strength is greater than that of ordinary coal seams, but weaker than that of a hard roof. The roof is a combination of different lithological strata above the coal seam. Based on mechanical strength characteristics and mining stability, the different lithological strata combinations of the roof are simplified into an immediate roof and a basic roof, which are represented in the model. The immediate roof is a stratum with relatively weak mechanical strength that is easy to collapse after the top coal collapses and does not transmit horizontal stress. The basic roof is a stratum with relatively strong mechanical strength that is difficult to collapse even after the top coal and immediate roof collapses and can transmit horizontal stress. Based on the specific lithological strata and their stability during mining, the immediate roof is a stratum adjacent to the coal seam that can collapse in time. Its lithology is mudstone, shale, sandy mudstone, or thin-layered sandstone, with a thickness of 1–3 m. The basic roof is a hard, thick stratum above the immediate roof that is difficult to collapse. Its lithology is thick-layered sandstone or limestone, with a thickness greater than 2 m.

[0039] Step S2: Based on the synchronous fracturing geological-engineering model, set the fracturing pump injection rate and fracturing pump injection duration, and use fracturing design software to determine the fracture propagation size when the boreholes are arranged in different rock strata. That is, obtain the fracture propagation size when the boreholes are arranged in the coal seam, the immediate roof, and the main roof respectively. The fracture propagation size includes the fracture height and the fracture length.

[0040] Here, borehole-related parameters are set in the synchronous fracturing geological-engineering model, including borehole strata, borehole azimuth, and borehole diameter. Table 1 shows the borehole-related parameters.

[0041] Table 1. Drilling-related parameters

[0042] Drilling type directional drilling Borehole strata Coal seam / immediate roof / basic roof Drilling azimuth / ° 165 Drilling diameter / mm 120

[0043] The fracturing design software can be MFrac Suite. Relevant data from the geological-engineering model are input into the fracturing design software, the fracturing pump injection rate and duration are set, and simulation calculations are performed to obtain the fracture propagation size under different injection durations. The fracturing pump injection rate is determined based on the performance of the fracturing pump and the actual working conditions in the coal mine, and is generally not greater than 1.0 m. 3 / min, the fracturing pump injection time is generally 30 to 60 minutes.

[0044] In this step, a pseudo-3D fracturing fracture propagation model is selected in the fracturing design software. After initiation, the open-hole fracturing fracture extends symmetrically on both sides. Fracture wall roughness and friction are not considered. The wellbore hydraulic calculation model is based on empirical formulas. According to the pseudo-3D fracturing fracture propagation model, the following relationships exist between fracture height, fracture half-length, fracture width, fracture injection rate, and fracture injection time:

[0045]

[0046]

[0047] Among them, h f x is the crack height. f The crack length is half the length of the crack, w is the crack width, and q is the crack width. i S represents the fracture injection rate, t represents the fracture injection duration, and S represents the injection volume. p For initial filtration loss, C L denoted as the filtration coefficient, h as the thickness of the formation where the borehole is located, β as the fluid filtration range factor, and erfc as the complementary error function.

[0048] Step S3: Based on the crack propagation size, determine the rock strata corresponding to the borehole layout when the crack is fully extended in the longitudinal direction, and use them as the optimal borehole layout rock strata.

[0049] Here, "fully extended longitudinally" means that the upper end of the crack extends to the upper interface of the main roof and the lower end extends to the lower interface of the top coal. In other words, when the crack extends upwards into the main roof and downwards into the top coal, simultaneously disrupting the structural integrity of the roof and top coal, it is considered to have fully extended longitudinally. The upper and lower end depths of the crack can be determined based on its height. Specifically, due to the different mechanical properties and stress differences of the top coal, immediate roof, and main roof, the extension height of the fracturing crack varies in the upward and downward directions. Let the upper interface depth of the main roof be D1, the lower interface depth of the top coal be D2, the borehole initiation depth be D, the upper half of the fracturing crack height be H1, and the lower half height be H2. Then, the upper end depth of the longitudinally extended fracturing crack should be less than the upper interface depth of the main roof, i.e., H1 > D - D1, and the lower end depth should be greater than the lower interface depth of the top coal, i.e., H2 > D2 - D.

[0050] Step S4: Determine the optimal fracturing pumping time when the fracture length reaches the preset value when the borehole is arranged in the optimal borehole arrangement rock layer.

[0051] Due to differences in formation stress levels along the vertical and horizontal axes, fractures extend at varying degrees in both height and length directions. After a certain period of fracturing pumping, the vertical extension of the fractures becomes sufficient and tends to stabilize, while fracture growth primarily occurs along the length. Based on sufficient fracture height extension, the fracturing pumping time is adjusted to ensure the fracture length reaches a preset value. Therefore, this step requires determining the optimal fracturing pumping time when the fracture length reaches the preset value.

[0052] Specifically, this method can obtain the relationship between fracture height and fracture pumping time when the borehole is arranged in the optimal borehole arrangement rock formation, as well as the relationship between fracture length and fracture pumping time. By obtaining the relationship between fracture height and fracture pumping time, the fracture pumping time and fracture length corresponding to the fracture height being sufficient for propagation can be determined. Then, based on the relationship between fracture length and fracture pumping time, the optimal fracturing pumping time when the fracture length reaches the preset value can be determined.

[0053] Step S5: Construct multiple directional long boreholes in the optimal borehole layout rock strata in the mining area. Perform segmented fracturing on each completed directional long borehole according to the optimal fracturing pumping time and fracturing pumping discharge rate to achieve synchronous fracturing of the coal seam roof and top coal.

[0054] Here, multiple drilling sites are arranged in the transport roadway or return air roadway of the longwall mining face, and the distance between two adjacent directional long boreholes in each drilling site is twice the preset value of the fracture length.

[0055] This embodiment proposes a design method for the drilling layout parameters and fracturing parameters of the roof and top coal under two hard conditions in fully mechanized longwall mining. This method effectively destroys the integrity of the hard roof and hard top coal and weakens their mechanical strength, thereby increasing the top coal release rate during fully mechanized longwall mining. At the same time, it achieves roof cutting and pressure relief, avoiding the manifestation of strong mine pressure caused by large-area roof suspension and sudden fracture. Furthermore, the method of this application is simple to operate, safe and efficient, and has wide applicability.

[0056] Furthermore, once the duration of the staged fracturing reaches the optimal fracturing pumping duration, the fracturing pumping rate is reduced to lower the pumping pressure below the fracture extension pressure, and pumping continues for a certain duration to ensure that the top coal is fully saturated with water.

[0057] Here, in combination with the needs of top coal weakening and dust reduction in top coal caving mining, during the fracturing operation, after the pre-designed fracturing pump injection time is reached, the pump injection rate is reduced so that the pump injection pressure is lowered below the fracture extension pressure, and the fracturing fracture stops expanding. The pump injection time is increased, and the injected liquid is slowly filtered out in the original fracture and enters the coal seam, fully saturating the coal seam with water, increasing the moisture content, and further weakening the strength of the top coal. At the same time, the coal dust concentration in fully mechanized caving mining is reduced.

[0058] In one embodiment, for a coal seam in a working face, the burial depth is between 900 and 1000 m, the dip angle is between 1° and 3°, and the coal seam thickness is between 10.27 m and 20.56 m, with an average of 16.00 m. This is considered an extra-thick coal seam with a thick and hard roof. The designed length of the working face's transport roadway and return air roadway is 2110 m, the cut-out length is 240 m, the roadway azimuth is 165°, and the fully mechanized top-coal caving method is used for mining, with a coal cutting height of 4 m and a coal release height of 8 m. Due to the presence of hard coal and a thick, hard roof, the top coal is difficult to cavitate and the roof is difficult to collapse during fully mechanized top-coal caving, resulting in a low coal recovery rate. At the same time, the movement of the hard roof leads to significant mine pressure manifestation problems. To ensure safe production at the working face, it is necessary to carry out fracturing and weakening of the hard coal and thick, hard roof.

[0059] Data analysis revealed the lithological distribution of the coal seam roof. The roof lithology adjacent to the coal seam is mudstone or sandy mudstone, with extremely poor to relatively good integrity and low rock strength. Above it, the lithology is mainly fine sandstone, medium sandstone, siltstone, and silty mudstone, which are widely developed throughout the area, with good rock integrity and relatively high rock strength and hardness.

[0060] Based on the results of underground stress testing in coal mines, minimum horizontal principal stress data were obtained at different locations in the coal seam and roof. In addition, samples were taken from the top coal and roof strata of the working face to prepare standard rock specimens. Physical and mechanical property tests were conducted to obtain data on the elastic modulus, Poisson's ratio, density, porosity, permeability, and filtration coefficient of different lithologies. Table 2 shows the relevant formation parameters, and Table 3 shows the physical and mechanical parameters.

[0061] Table 2. Stratigraphic Parameters

[0062] rock strata Lithology Thickness / m Minimum horizontal principal stress / MPa Basic top fine-grained sandstone 25 15.06 Just top Sandy mudstone 3 14.65 Top coal coal 16 14.27

[0063] Table 3 Physical and Mechanical Parameters

[0064]

[0065] In step S2, firstly, directional long boreholes are arranged in the top coal seam at a distance of -5m from the top surface (with the upper coal-rock interface as the boundary, upward is positive and downward is negative). The relevant data from the established fracturing geological-engineering model are then input into the MFrac Suite fracturing design software, and the pump injection rate is set to 0.7m³ / h. 3 The pumping speed is 1 / min, the pumping time is 55 minutes, and the total hydraulic volume pumped is 38.5 m³. 3 The crack height was obtained through simulation calculations for that pumping duration. Figure 3 The diagram shows the range of fracture height extension in top coal seam with borehole arrangement.

[0066] Then, the arrangement of the directional long borehole was changed to be placed in the immediate top, 1.5m from the top of the coal seam (with the upper coal-rock interface as the boundary, upward is positive and downward is negative). The same fracturing pump injection rate and injection duration as above were set, and numerical calculations were performed to obtain the corresponding fracturing fracture propagation height. Figure 4 A diagram showing the fracture height propagation range in a borehole arrangement at the direct top of the fracturing site is presented.

[0067] Finally, the arrangement of the directional long borehole was changed to be placed in the main top layer, 15m from the top of the coal seam (with the upper coal-rock interface as the boundary, upward is positive and downward is negative). The same fracturing pump injection rate and injection duration as above were set, and numerical calculations were performed to obtain the corresponding fracturing fracture propagation height. Figure 5 The diagram shows the range of fracture height propagation in the borehole arrangement at the base.

[0068] In step S3, the simulation results of the borehole arrangement in the top coal show that the upper half of the fracture is 8.6m high and the lower half is 11.2m high (with the borehole position as the central axis). It can be seen that the fracture extends fully in the top coal, but does not extend fully in the main roof, which does not meet the condition that the burial depth of the upper end of the fracture is less than the burial depth of the upper interface of the main roof.

[0069] Simulation results of borehole arrangement in the immediate roof show that the upper half of the fracturing fracture is 12.3m high and the lower half is 26.7m high. It extends fully in the top coal, but not fully in the main roof, and does not meet the condition that the burial depth of the upper end of the fracture is less than the burial depth of the upper interface of the main roof.

[0070] Simulation results of borehole arrangement in the main roof show that the upper half of the fracturing fracture is 13.2m high and the lower half is 47.5m high. It extends fully in the top coal and also fully in the main roof, satisfying the condition that the lower end of the fracture is buried deeper than the lower interface of the top coal and the upper end of the fracture is buried deeper than the upper interface of the main roof.

[0071] Therefore, the optimal borehole arrangement is the top of the rock stratum, with the borehole 15m away from the upper coal-rock interface of the coal seam.

[0072] In step S4, when the borehole is positioned 15m above the coal-rock interface at the base of the coal seam, the pumping rate is 0.7m³ / min. 3 When the injection rate is 1 / min and the injection time is 55 min, the fracturing design software can be used to obtain the relationship between fracture height and injection time, and the relationship between fracture length and injection time. Figure 6 A schematic diagram showing the relationship between fracture height and fracture pumping time is shown. Figure 7 A schematic diagram illustrating the relationship between fracture length and fracture pumping time is shown. According to... Figure 6It can be seen that after 15 minutes of pumping, the fracture height growth slows down, until after 55 minutes, the fracture height expansion size meets the requirements; according to Figure 7 At 55 minutes, the fracture length was 37m. In this embodiment, the designed fracture length is 40m (preset fracture length value). Therefore, the pumping time needs to be increased to 65 minutes to make the fracture length reach 40m. That is, the optimal fracturing pumping time is 65 minutes.

[0073] In step S5, multiple directional long boreholes are drilled in the basic roof of the mining area. For each completed directional long borehole, the optimal fracturing pumping time (65 min) and fracturing pumping flow rate (0.7 m³ / min) are determined. 3 The coal seam roof and top coal are simultaneously fractured in stages ( / min). Here, multiple drilling sites are arranged in the transport roadway or return air roadway of the longwall face. Multiple directional long boreholes are constructed in each drilling site. The distance between two adjacent directional long boreholes is twice the fracture length, i.e., 80m. Figure 8 The plan view showing the layout of long boreholes for simultaneous fracturing of the roof and top coal of the longwall face is shown. Figure 9 A cross-sectional view of the arrangement of long boreholes for simultaneous fracturing of the roof and top coal in the longwall mining face is shown.

[0074] In summary, in the above embodiments, during the working face advancement process, for the hard roof and top coal, the drilling and fracturing parameters arranged in the basic roof are preferred to carry out segmented fracturing, which weakens the hard roof, reduces the roof overhang area and the impact and influence of roof pressure on the working face, and at the same time, the basic roof fracturing cracks extend to the top coal, and the injected liquid enters the coal body with the cracks. The top coal cut by the cracks and softened by the liquid is more likely to break under the action of the advance support pressure of the mining area, thereby improving the top coal release rate.

[0075] The above descriptions are merely various embodiments of this application, but the scope of protection of this application is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in this application should be included within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.

Claims

1. A two-hard extra-thick coal seam roof and top coal synchronous fracturing weakening design method, characterized in that, include: Based on the stratigraphic parameters and physical and mechanical parameters of each rock stratum in the longwall mining face, a synchronous fracturing geological-engineering model is established. Each rock stratum includes a coal seam, a direct roof composed of sandy mudstone, and a basic roof composed of siltstone and fine sandstone. The geological profile of the synchronous fracturing geological-engineering model, from bottom to top, is a combination of the coal seam, the direct roof, and the basic roof. Based on the synchronous fracturing geological-engineering model, the fracturing pump injection rate and fracturing pump injection duration are set, and fracturing design software is used to determine the fracture propagation size when boreholes are arranged in different rock strata. Based on the crack propagation size, determine the rock strata corresponding to the borehole arrangement when the crack is fully extended in the longitudinal direction, and use them as the optimal rock strata for borehole arrangement. Determine the optimal fracturing pumping time when the fracture length reaches a preset value when the boreholes are arranged in the optimal borehole arrangement rock strata; Multiple directional long boreholes are constructed in the optimal borehole layout rock strata in the mining area. Each completed directional long borehole is subjected to segmented fracturing according to the optimal fracturing pumping time and the fracturing pumping discharge rate to achieve synchronous fracturing of the coal seam roof and top coal. The longitudinal extension of the crack includes: the upper end of the crack extending to the upper interface of the main roof, and the lower end of the crack extending to the lower interface of the top coal.

2. The method as described in claim 1, characterized in that, in, Determining the optimal fracturing pumping time when the fracture length reaches a preset value when the borehole is arranged in the optimal borehole arrangement rock formation includes: The boreholes are arranged in the optimal borehole arrangement rock stratum, and the optimal fracturing pumping time is determined when the fracture length reaches the preset value based on the relationship between fracture length and time.

3. The method as described in claim 1, characterized in that, The formation-related parameters include lithology, thickness, and minimum horizontal principal stress; the physical and mechanical parameters include elastic modulus, Poisson's ratio, porosity, permeability, and filtration coefficient.

4. The method as described in claim 1, characterized in that, The method further includes: Once the duration of the staged fracturing reaches the optimal fracturing pumping duration, the fracturing pumping rate is adjusted to reduce the pumping pressure below the fracture extension pressure, and pumping continues for a certain duration to ensure that the top coal is fully saturated with water.

5. The method as described in claim 1, characterized in that, The method further includes: Multiple drilling sites are arranged in the transport roadway or return air roadway of the longwall mining face, and the distance between two adjacent directional long boreholes in each drilling site is twice the preset value of the fracture length.

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