A method and system for hydraulic fracturing of a roof overhang at the end of a coal face

Abstract

The application discloses a kind of coal face end head suspended roof hydraulic fracturing construction method and system equipment, comprising the following steps: S1, test coal mine working face roof physical mechanics property, determine the firmness of roof rock, assess the area of end head suspended roof;S2, by theoretical analysis, numerical calculation explores water injection rate, principal stress difference, fracturing sequence, borehole inclination these several different influence factors on hard roof cracking damage weakening, provide reference for hydraulic fracturing field construction design;S3, determine the working face end head suspended roof hydraulic fracturing construction parameter;S4, according to the construction parameter determined to carry out hydraulic fracturing construction.The application uses hydraulic fracturing method, high-pressure fluid is injected into target hard rock layer, so as to produce crack or make natural crack restart, transform rock structure, form crack network system, achieve the purpose of weakening end head hard roof and timely collapse, reduce roof pressure intensity, ensure the safety of working face.

Description

Technical Field

[0001] This invention relates to the field of coal mining technology, specifically to a method and system equipment for hydraulic fracturing with the roof suspended at the end of a coal mining face. Background Technology

[0002] Hard roofs are thick, stable, and hard rock strata such as limestone, sandstone, and conglomerate that lie directly above coal seams. Hard roofs are characterized by their thickness, high integrity, high strength, dense structure, underdeveloped joints and fissures, and strong self-bearing capacity. The Protodyakonov coefficient of hard roof rocks is generally between 5 and 8, and in some cases even exceeds 10. In my country, coal seams with hard roofs account for approximately 30% of the total coal seams, distributed across more than 50% of the country's mining areas. Over 40% of working faces have hard, difficult-to-collapse roofs under intense pressure, especially those with thin, directly exposed hard roofs, which are even more widely distributed.

[0003] During coal mine face mining, it is common to encounter situations where a hard, thick roof at the roadway end or side is difficult to collapse, resulting in a large area of ​​suspended roof. This hard, thick roof is a major cause of intense mining-induced pressure and roadway deformation. If the suspended roof problem is not addressed, it can easily lead to safety hazards. A sudden roof failure can cause a large area of ​​sudden roof pressure, and stress disturbance can easily trigger impact pressure accidents, resulting in both safety issues and economic losses. Summary of the Invention

[0004] To address the aforementioned technical shortcomings, the purpose of this invention is to provide a method and system equipment for hydraulic fracturing of the overhead roof at the end of a coal mining face.

[0005] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0006] This invention provides a hydraulic fracturing construction method for a coal mining face with overhead suspension, characterized by comprising the following steps:

[0007] S1. Test the physical and mechanical properties of the roof of the coal mine working face, determine the hardness of the roof rock, and assess the area of ​​the overhang at the end.

[0008] S2. Through theoretical analysis and numerical calculation, we explore the effects of several influencing factors, such as water injection rate, principal stress difference, fracturing sequence, and borehole inclination angle, on the weakening of fracturing damage in hard roofs, and provide a design basis for the on-site construction design of hydraulic fracturing.

[0009] S3. Determine the hydraulic fracturing construction parameters for the suspended end of the working face;

[0010] S4. Carry out hydraulic fracturing construction according to the determined construction parameters.

[0011] Preferably, the mechanical properties described in step S1 include uniaxial compressive strength, tensile strength, and shear strength.

[0012] Preferably, step S2 specifically includes the following steps:

[0013] S2.1 Establish a segmented hydraulic fracturing numerical model;

[0014] S2.2 Analyze the high-pressure fluid injection process of segmented hydraulic fracturing, and observe the expansion law of each fracture segment, the seepage range after injection, and the changes in roof collapse.

[0015] S2.3 Analyze the sensitivity factors of segmented hydraulic fracturing. That is, construct a segmented hydraulic fracturing numerical model and conduct simulation experiments on four sensitivity factors: water injection rate, stress difference, fracturing sequence, and borehole inclination angle. Analyze the influence of each sensitivity factor on the fracture propagation law of hydraulic fracturing. For different sensitivity factors, use the single variable method to conduct simulation experiments. Finally, obtain the fracture propagation law under the influence of different factors.

[0016] Preferably, step S2.1 includes the following steps:

[0017] S2.1.1. Based on the rock strata distribution and borehole layout diagram, a numerical calculation model was established using discrete element software;

[0018] S2.1.2 Determine the parameters and boundary conditions of the model, group and assign parameters to different lithologies, and assign different mechanical parameters to simulate the propagation of cracks in different rock masses.

[0019] Preferably, step S2.2 includes the following steps:

[0020] S2.2.1 After the model is established, four fracturing sections of hole L are selected to simulate the water injection process. After each section of hole L is fracturing, the simulation results are statistically analyzed.

[0021] S2.2.2 Determining the Permeability Ratio and Number of Fractures: Introducing the parameter "Permeability Ratio," denoted as Ks, defined as the ratio of the area of ​​influence of each fracturing section after water injection to the total area of ​​the model. The formula for the permeability ratio is:

[0022]

[0023] In the formula, S1 is the area of ​​influence after water injection in the fracturing section, and S is the total area of ​​the model;

[0024] Measure the area of ​​the blue region in the simulation results, and then divide it by the total area of ​​the numerical model to obtain the permeability ratio of each fracturing section after water injection.

[0025] The number of fractures generated in the model after water injection in each fracturing section was counted, including tensile fractures, shear fractures, and total fractures.

[0026] Preferably, step S3 specifically includes:

[0027] S3.1 Determine the fracturing water pressure. The theoretical pressure for hydraulic fracturing is calculated using the following formula:

[0028] P1=1.3(3σ3-σ1+R t );

[0029] Where: P1 -- estimated fracturing pressure required for direct hydraulic fracturing, MPa;

[0030] σ1 -- Maximum principal stress at the fracturing point, MPa;

[0031] σ3 -- Minimum principal stress at the fracturing point, MPa;

[0032] Rt -- Tensile strength of the rock strata above the fracturing point, MPa;

[0033] S3.2 Determine the fracturing time. The formula for calculating the fracturing time is as follows:

[0034]

[0035] In the formula: α -- radius of the crack;

[0036] B -- the width of the crack;

[0037] H -- Crack height;

[0038] n -- the number of cracks;

[0039] Q -- Flow rate of the fracturing unit;

[0040] S3.3 Determine the borehole spacing based on the elliptical shape of the hydraulic fracturing fracture and the displacement field relationship of the crack under plane stress in linear elastic fracture mechanics:

[0041]

[0042] In the formula, σ is the hydraulic pressure inside the crack; E is the elastic modulus of the coal and rock; and a is the length of the crack.

[0043] The fracturing fracture can be considered as the synthesis of many consecutive Type I through cracks arranged along the fracture length. Applying Equation 4 to any crack yields the fracture width equation:

[0044]

[0045] Then, by the principle of volume conservation, we can obtain:

[0046]

[0047] In the formula, H is the seam height in the Z direction, which should be a variable. Considering that this model would have no solution if variable analysis were performed, we assume that the seam heights are equal, and a simplified analysis yields:

[0048]

[0049] Replacing σ(x) with the average value σ and rearranging, we can obtain the crack radius as:

[0050]

[0051] Inputting Q, T, E, σ, and H, the borehole spacing d is obtained as follows:

[0052]

[0053] In the formula, a is the radius of the hydraulic crack; k is the crack superposition coefficient.

[0054] Preferably, step S4 includes:

[0055] S4.1. Use a drilling rig to drill holes in the hard top plate, and stop drilling after drilling into the section of hard rock that requires fracturing.

[0056] S4.2 Advance the borehole packer and water injection pipe to the hydraulic fracturing section;

[0057] S4.3. Use a manual pump to pressurize the packer, causing the rubber sleeve to expand and achieve the purpose of sealing the hole;

[0058] S4.4 Before construction, the connection of the hydraulic fracturing high-pressure pump and high-pressure pipeline must be checked. After the check is completed, connect the packer to the hose and connect the water injection steel pipe to be inserted into the borehole. After the water injection steel pipe is completely inserted into the borehole, the manual pump pressure is increased to 10-12MPa to make the packer contact and fix it with the borehole wall, and the water injection steel pipe is fixed by locking the borehole opening.

[0059] S4.5 Construction Drilling: Starting from the position of the advance support in the two roadways, a set of boreholes, namely hole H and hole L, are arranged at every other borehole spacing. The boreholes are arranged in a three-flower pattern. Borehole H is drilled vertically upward against the coal pillar side, with the opening position 0.4-0.6m away from the coal side, and the depth is determined according to the thickness of the hard roof. Borehole L is drilled vertically upward against the mining side, with the opening position 0.4-0.6m away from the coal side, and the depth is determined according to the thickness of the hard roof. Avoid the support position of the advance support and use the drilling rig to drill vertically above the roof of the advance section of the working face.

[0060] S4.6 Fracturing Drilling: Fracturing is performed once every 2-3 sections of seamless steel pipe are withdrawn. The number of fracturing operations for each drill hole is determined according to the length of the drill hole. The pressure holding and water injection time for each operation shall not be less than 25 minutes.

[0061] S4.7 Start the pump and slowly increase the pressure, and record the maximum pump pressure. Maintain the pressure for 25-30 minutes, and record the pressure changes of the pump and the on-site conditions during the fracturing process.

[0062] S4.8 After fracturing for 25-30 minutes, slowly depressurize. After the pressure is completely released, stop the water pump and disconnect the high-pressure pipeline from the water injection steel pipe. After the water flow in the water injection steel pipe decreases, depressurize the manual pump. After water comes out of the borehole, the water injection steel pipe can be disassembled.

[0063] S4.9 After disassembling the water injection steel pipe and retracting the packer to the designed distance, repeat steps S4.6-S4.8 for subsequent fracturing. After fracturing is completed, disassemble the water injection steel pipe section by section until the packer is removed and restore the support to its original state.

[0064] The present invention also provides a hydraulic fracturing construction system for the end of a coal mining face, comprising a high-pressure water system, a pressure sensor, a three-way valve, a pressure relief valve, a high-pressure water steel pipe, and a packer connected in sequence. The three ends of the three-way valve are respectively connected to the pressure sensor, the pressure relief valve, and the top of the high-pressure water steel pipe. The packer includes a top packer, a bottom packer, and a connecting packer located between the top packer and the bottom packer.

[0065] Preferably, the high-pressure water system includes a water tank, a gas-liquid booster pump, a drain pipe, and a high-pressure pump connected in sequence.

[0066] The beneficial effects of this invention are as follows: This invention uses hydraulic fracturing to inject high-pressure fluid into the target rock strata, thereby generating cracks or restarting natural cracks, modifying the rock strata structure, forming a crack network system, achieving the purpose of weakening the hard roof at the end and causing it to collapse in time, reducing the pressure strength of the roof and ensuring the safety of the working face. Attached Figure Description

[0067] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0068] Figure 1 This is a schematic diagram of rock strata distribution and borehole layout provided in an embodiment of the present invention;

[0069] Figure 2 This is a statistical chart showing the results of the hydraulic fracturing fluid injection process in an embodiment of the present invention;

[0070] Figure 3 These are crack propagation diagrams at different water injection rates in embodiments of the present invention;

[0071] Figure 4 This is a graph showing the variation trend of the number of fractures and Ks under different water injection rates in an embodiment of the present invention;

[0072] Figure 5 This is a schematic diagram of the drilling and fracturing section arrangement in an embodiment of the present invention;

[0073] Figure 6 This is a schematic diagram of the structure of a hydraulic fracturing construction system for the end of a coal mining face, as described in an embodiment of the present invention.

[0074] Explanation of reference numerals in the attached diagram: 1. High-pressure water system; 2. Pressure sensor; 3. Three-way valve; 4. Pressure relief valve; 5. High-pressure water steel pipe; 6. Top packer; 7. Bottom packer; 8. Connecting packer. Detailed Implementation

[0075] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0076] Example

[0077] Taking a coal mine working face as an example, the working face suffers from a suspended roof problem due to the difficulty in timely fracturing and collapse of the overlying limestone roof. This coal mine mainly mines the 9+10 coal seam, a single-seam mining operation. All mine roadways are coal seams, supported by anchor beams and cable nets. The 9+10 coal seam is 5.1m thick, with a relatively hard K2 limestone roof. The coal seam floor elevation is 1080-1180m, with an average thickness of 5.1m (including 2-3 layers of interbedded rock, excluding interbedded rock, 4.7m). The coal seam generally dips at 2-16°, with an average dip of 7°.

[0078] The following describes a hydraulic fracturing method for the overhead roof of a coal face proposed for this mine. The method specifically includes the following steps:

[0079] S1. The uniaxial compressive strength, tensile strength, and shear strength parameters of the coal and rock samples from the mine were measured as follows: uniaxial compressive strength was 48.81 MPa, uniaxial tensile strength was 9.27 MPa, cohesion was 15.35 MPa, and internal friction angle was 36.7°. Compared with the Protodyakonov coefficient (f) of the rock, the strength of the limestone roof of the working face of this coal mine is between V and IVa, which belongs to medium-firm to relatively firm rock.

[0080] S2.1 Based on the actual situation of the roof strata of the 9+10# coal seam in the return airway of the coal mine working face, a schematic diagram of the strata distribution and borehole layout was first drawn, as follows: Figure 1 As shown, the thickness of the 9+10# coal seam in this working face is 4.6-5.3m, with an average of 5.0m. The overlying strata are mudstone and limestone. According to the columnar section of the coal seam in this working face, a 19m section is taken above the coal seam. The lithology and height of the overlying strata are as follows: limestone 3.8m, mudstone 1.2m, limestone 7.6m, mudstone 3.0m, and limestone 3.4m. A long borehole is drilled at each end of the roof of the return airway. The borehole closer to the working face is named L-hole, and the borehole closer to the coal pillar is named S-hole. Each borehole is divided into 4 sections for fracturing, with a total of 8 fracturing sections. The fracturing sections of the L-hole are named L1, L2, L3, and L4 from top to bottom, and the fracturing sections of the S-hole are named S1, S2, S3, and S4 from top to bottom.

[0081] After the model is established, four fracturing sections of the L-hole can be selected to simulate the water injection process. A simulation scheme is designed for these four fracturing sections, with the water injection rate set to 2 × 10⁻⁶. -3 The fracturing rate is m3 / s (120L / min), the horizontal stress is 8.8MPa, the vertical stress is 7.3MPa, and the designed fracturing sequence is to inject water sequentially from top to bottom in the four fracturing sections (L1, L2, L3, L4) of hole L.

[0082] After fracturing was completed in each section of the L-hole, the simulation results were statistically analyzed. The statistical results are shown in [the table below]. Figure 2 The blue area in the diagram represents the permeability range after water injection in each fracturing stage. All fractures are generated within the affected area. It can be seen that the number of fracturing operations is positively correlated with fracture propagation; the more water injections, the more fractures are generated, the longer the fractures extend, and the larger the permeability range.

[0083] (2) Permeability range ratio and number of fractures:

[0084] Introducing the parameter "permeability ratio", denoted as Ks, it is defined as the ratio of the area of ​​influence after water injection in each fracturing section to the total area of ​​the model. The formula for the permeability ratio is:

[0085]

[0086] In the formula, S1 is the area affected after water injection in the fracturing section, and S is the total area of ​​the model.

[0087] according to Figure 2 The area of ​​the blue (shaded) region in the simulation results is measured, and then divided by the total area of ​​the model to obtain the permeability ratio of each fracturing section after water injection. The value of Ks for each fracturing section is calculated, and the results are shown in Table 1.

[0088] Table 1. Ks values ​​after water injection in the fracturing section.

[0089] L1 L2 L3 L4 Ks 0.0703 0.1172 0.1525 0.2734

[0090] Ks is a good indicator of the effectiveness of hydraulic fracturing, and the magnitude of Ks allows for a direct observation of the area of ​​influence. Table 1 shows that as high-pressure water is continuously injected, the larger the Ks value, the wider the area of ​​influence. The areas of influence of different fracturing sections will overlap and connect, eventually forming a large area of ​​weakened roof from top to bottom.

[0091] Table 1 summarizes the number of fractures generated in the model after water injection in each fracturing stage, including tensile fractures, shear fractures, and total fractures. The number of fractures increases with the number of water injection cycles. The number of fractures can also reflect the fracturing effect. If the number of fractures is low, it indicates a poor fracturing effect, possibly due to mismatched injection parameters. If too many fractures are generated, the fracturing effect is also unsatisfactory, potentially leading to roof collapse and excessively severe fracturing.

[0092] Table 2 Number of fractures after water injection in the fracturing section

[0093] L1 L2 L3 L4 Number of tensile cracks 12 7 26 92 Number of shear cracks 7 3 8 26 Total number of cracks 19 10 34 118

[0094] S2.3 Analysis of the sensitivity factors of segmented hydraulic fracturing: A segmented hydraulic fracturing numerical model was constructed, and simulation experiments were conducted on four sensitivity factors: injection rate, stress difference, fracturing sequence, and borehole inclination angle. The influence of each sensitivity factor on the fracture propagation law of hydraulic fracturing was analyzed. For different sensitivity factors, a single-variable method was used for simulation experiments, and finally, the fracture propagation law under the influence of different factors was obtained.

[0095] (1) Water injection rate:

[0096] The water injection rate directly affects the pressure, which in turn affects fracture initiation and propagation, significantly impacting the fracturing effect. To investigate the influence of the water injection rate on fracture propagation, four different water injection rates were selected for numerical simulation. The parameters for each group are shown in Table 3, and the simulation results were analyzed.

[0097] Table 3 Numerical simulation schemes for different water injection rates

[0098]

[0099] Figure 3The fracture propagation diagrams at different water injection rates show the fracture propagation under different water injection rates. When the water injection rate is low, each fracturing segment produces shorter fractures, and some fracturing segments may not even produce fractures. The higher the water injection rate, the more fractures are generated, and the longer the fractures become. Comparing the four sets of simulation results, when the water injection rate reaches 2.0 × 10⁻⁶, the fracture propagation rate increases significantly. -3 At a flow rate of m³ / s (120 L / min), the effects of hydraulic fracturing are interconnected. When the injection rate reaches 2.5 × 10⁻⁶ m³ / s (120 L / min), the effects are continuous. -3 When the injection rate is m3 / s (150 L / min), the angle of the crack deflects during its extension, no longer along the direction of the maximum principal stress. This indicates that the injection rate is too high at this point, which will affect the propagation of the main crack.

[0100] Therefore, in order to ensure the reasonable extension and expansion of cracks, the water injection rate cannot be increased indiscriminately, but needs to be determined in combination with the on-site pump discharge rate and construction process.

[0101] As can be seen from Table 4, the higher the injection rate, the larger the Ks value. This is because, within the same time period, a higher injection rate results in a larger volume of water injected, and thus a wider range of influence.

[0102] Table 4. Number of fractures and Ks under different water injection rates

[0103]

[0104] For the data in Table 4, the trends of the four parameters with water injection rate are as follows: Figure 4 As shown, the higher the water injection rate, the larger the Ks value, and the greater the increase in Ks.

[0105] (2) Principal stress difference:

[0106] To investigate the effect of principal stress difference on crack propagation, four sets of simulation experiments were set up. A fixed confining pressure of 8.8 MPa was applied to the horizontal direction of the model, and the vertical confining pressure of the model was changed. The stress differences of the four sets of simulation experiments were 3.5 MPa, 2.5 MPa, 1.5 MPa and 0.5 MPa, respectively.

[0107] Similarly, the penetration range ratio Ks and the number of cracks for these four sets of simulation processes were statistically analyzed, as shown in Table 5. The data in the table shows that when the stress difference is 0.5 MPa, the value of Ks is 0.2336, and when the stress difference is 3.5 MPa, the value of Ks reaches 0.3285, a difference of 0.0949. The number of cracks generated is also greater for tensile cracks than for shear cracks.

[0108] Table 5. Number of cracks and Ks under different stress differences

[0109] Stress difference Number of shear cracks Number of tensile cracks Total number of cracks Ks 3.5MPa 56 146 202 0.3285 2.5MPa 34 124 158 0.2734 1.5MPa 29 72 101 0.2499 0.5MPa 31 71 102 0.2336

[0110] (3) Fracturing sequence:

[0111] After the basic conditions for hydraulic fracturing are determined, the selection of the fracturing sequence is also a key factor influencing the fracturing effect. For example... Figure 5 As shown, in the modeling process of simulating the segmented hydraulic fracturing of the roof of the return airway, two boreholes were set up, and four fracturing sections were set up in each borehole, for a total of eight fracturing sections: L1, L2, L3, L4 of borehole L and S1, S2, S3, S4 of borehole S.

[0112] Table 6 Simulation Schemes for Different Fracturing Sequences

[0113]

[0114] Similarly, the permeability ratio Ks of these three sets of simulation experiments were statistically analyzed, as were the number of cracks generated in the three sets of simulations. The statistical results are shown in Table 7.

[0115] Table 7 Number of fractures under different fracturing sequences

[0116] Fracturing sequence Number of shear cracks Number of tensile cracks Total number of cracks Ks L1, L2, L3, L4 34 124 158 0.2734 L1, L2, L3, L4 30 108 138 0.2556 L1, L2, L3, L4 15 38 53 0.1916

[0117] As can be seen from the data in Table 7, when the fracturing sequence is L1, L2, L3, L4, the total number of fractures is 158, which is the largest number of fractures and the largest area of ​​influence. When the fracturing sequence is L4, L3, L2, L1, the number of fractures and the area of ​​influence are the smallest.

[0118] (4) Drilling inclination angle:

[0119] As shown in the borehole layout diagram, borehole S is vertically upward, while borehole L has a certain inclination angle. The smaller the inclination angle, the farther apart the two boreholes are, which will have a certain impact on the fracturing effect. In order to explore the optimal angle for the fracturing effect of borehole L, four sets of simulation experiments were designed to explore the fracturing effect when the inclination angle of borehole L is 55º, 65º, 75º, and 85º.

[0120] The impact range of fracturing increases as the borehole inclination angle (L) decreases, but the number of fractures is highest when the borehole inclination angle is 75°. Considering the impact range, the number of fractures, and the correlation between fractures, fracturing achieves the optimal effect when the borehole inclination angle (L) is ≥75°.

[0121] Furthermore, step S3 specifically includes:

[0122] S3.1 Determine the fracturing water pressure. To increase the number of fractures during hydraulic fracturing, a 9304 on-site emulsion pump station or other standalone emulsion pump should be used. The pump flow rate Q is determined to be approximately 120 L / min, with a maximum not exceeding 150 L / min. The theoretical hydraulic fracturing pressure is calculated using the following formula:

[0123] P1=1.3(3σ3-σ1+R t );

[0124] Where: P1 -- estimated fracturing pressure required for direct hydraulic fracturing, MPa;

[0125] σ1 -- Maximum principal stress at the fracturing point, MPa;

[0126] σ3 -- Minimum principal stress at the fracturing point, MPa;

[0127] R t --Tensile strength of the rock strata above the fracturing point, MPa;

[0128] The coal seam floor elevation is 1080-1180m, the corresponding ground elevation of the working face is 1344-1432m, and the coal seam burial depth is 250-270m. Based on a maximum burial depth of 270m, the vertical stress is:

[0129] σ² = 0.027 × 270 = 7.3 MPa;

[0130] The minimum principal stress σ3 is calculated based on a lateral compression coefficient of 0.8. Therefore, the minimum principal stress σ3 is:

[0131] σ3 = 7.3 × 0.8 = 5.8 MPa;

[0132] If the maximum principal stress σ1 is calculated as 1.2 times the intermediate principal stress, then the minimum principal stress σ1 is:

[0133] σ1 = 7.3 × 1.2 = 8.8 MPa;

[0134] Based on the above formulas, σ3 = 5.8 MPa, σ1 = 8.8 MPa, and the laboratory test results from on-site sampling, the uniaxial compressive strength is 48.81 MPa, and the tensile strength R... t The pressure is 9.27 MPa, therefore the required start-up pressure for hydraulic fracturing is:

[0135] P1=1.3(3σ3-σ1+R t =23.2 MPa;

[0136] Therefore, the initiation water pressure for hydraulic fracturing is rounded to 24 MPa.

[0137] S3.2 Determine the fracturing time. The formula for calculating the fracturing time is as follows:

[0138]

[0139] In the formula: α -- radius of the crack, taken as 10m;

[0140] B -- The width of the crack, taken as 2cm;

[0141] H -- Crack height, taken as 4.5m;

[0142] n -- the number of cracks, taken as 3;

[0143] Q -- flow rate of the fracturing device, 120L / min. Substituting into the formula, the calculated result is 23.6min. Therefore, the fracturing time for each stage should not be less than 24min, and can be taken as 25-30min.

[0144] S3.3 Determine the borehole spacing based on the elliptical shape of the hydraulic fracturing fracture and the displacement field relationship of the crack under plane stress in linear elastic fracture mechanics:

[0145]

[0146] In the formula, σ is the hydraulic pressure inside the crack; E is the elastic modulus of the coal and rock; and a is the length of the crack.

[0147] The fracturing fracture can be considered as the synthesis of many consecutive Type I through cracks arranged along the fracture length. Applying Equation 4 to any crack yields the fracture width equation:

[0148]

[0149] Then, by the principle of volume conservation, we can obtain:

[0150]

[0151] In the formula, H is the seam height in the Z direction, which should be a variable. Considering that this model would have no solution if variable analysis were performed, we assume that the seam heights are equal, and a simplified analysis yields:

[0152]

[0153] Replacing σ(x) with the average value σ and rearranging, we can obtain the crack radius as:

[0154]

[0155] Taking Q = 13 m³ / h, T = 0.5 h (30 min), E = 5 GPa, σ = 32 MPa, and H = 4.5 m, substituting these parameters into the above formula, we obtain the borehole spacing d as:

[0156]

[0157] In the formula, a is the radius of the hydraulic fracture; k is the fracture superposition coefficient, which is taken as 0.7. To ensure the quality of hydraulic fracturing, the spacing between boreholes on one side is set to 6m.

[0158] In summary, the key parameters for hydraulic fracturing at the end of the coal mine working face are listed in Table 8:

[0159] Table 8 Key Parameters for Hydraulic Fracturing at the End of Coal Mine Working Face

[0160] parameter symbol unit numerical values Remark Hydraulic fracturing borehole height L m 13 Hydraulic fracturing borehole diameter Φ mm 50 Pump flow rate Q L / min 120 120-150 Rupture water pressure <![CDATA[P1]]> MPa >21 Single-hole fracturing time per segment T min 25-30 Drilling spacing d m 6 Three-flower arrangement Hydraulic fracturing stages part 4 Should we withdraw anchor? anchoring Drilling timing More than 20m ahead Fracturing timing Advanced stent Following the working face

[0161] Reference Figure 6 A hydraulic fracturing construction system for the end of a coal mining face includes a high-pressure water system 1, a pressure sensor 2, a three-way valve 3, a pressure relief valve 4, a high-pressure water steel pipe 5, and a packer connected in sequence. The packer includes a top packer 6, a bottom packer 7, and a connecting packer 8 located between the top packer 6 and the bottom packer 7. The high-pressure water system 1 includes a water tank, a gas-liquid booster pump, a drain pipe, and a high-pressure pump connected in sequence.

[0162] Step S4 includes:

[0163] S4.1. Use a drilling rig to drill holes in the hard top plate, and stop drilling after drilling into the section of hard rock that requires fracturing.

[0164] S4.2 Advance the borehole packer and high-pressure water pipe to the hydraulic fracturing section;

[0165] S4.3. Use a manual pump to pressurize the packer, causing the rubber sleeve to expand and achieve the purpose of sealing the hole;

[0166] S4.4 Construction Drilling: Starting from the position of the advance support in the two roadways, a set of boreholes, namely hole H and hole L, are arranged at every other borehole spacing. The boreholes are arranged in a three-flower pattern. Borehole H is drilled vertically upward against the coal pillar side, with the opening position 0.4-0.6m away from the coal side and the depth of 13m. Borehole L is drilled vertically upward against the mining side, with the opening position 0.4-0.6m away from the coal side and the depth of 12m. Avoiding the support position of the advance support, a pneumatic anchor drilling rig is used to drill vertically above the roof of the advance section of the working face.

[0167] S4.5 Fracturing Drilling: Fracturing is performed once every two seamless steel pipe sections are withdrawn, with four fracturing points per drill hole, and the pressure holding and water injection time for each time shall not be less than 25 minutes;

[0168] S4.6 Before construction, the high-pressure water system and high-pressure water steel pipe connection of hydraulic fracturing should be checked. After the check is completed, the packer is connected to the hose and the high-pressure water steel pipe is inserted into the borehole. After the high-pressure water steel pipe is completely inserted into the borehole, the manual pump pressure is increased to 10-12MPa to make the packer contact and fix it with the borehole wall. The high-pressure water steel pipe is then locked and fixed using the borehole opening.

[0169] S4.7 Start the pump and slowly increase the pressure, and record the maximum pump pressure. Maintain the pressure for 25-30 minutes, and record the pressure changes of the pump and the on-site conditions during the fracturing process.

[0170] S4.8 After fracturing for 25-30 minutes, slowly depressurize. After the pressure is completely released, stop the water pump and separate the high-pressure water system from the high-pressure water steel pipe. After the water flow in the high-pressure water steel pipe decreases, depressurize the manual pump. After water comes out of the borehole, the high-pressure water steel pipe can be disassembled.

[0171] S4.9 After disassembling the high-pressure water steel pipe and retracting the packer to the designed distance, repeat steps S4.6-S4.8 for subsequent fracturing. After fracturing is completed, disassemble the high-pressure water steel pipe section by section until the packer is removed and restore the support to its original state.

[0172] Obviously, those skilled in the art can make various modifications and variations to this invention without departing from its spirit and scope. Therefore, if these modifications and variations fall within the scope of the claims of this invention and their equivalents, this invention also intends to include these modifications and variations.

Claims

1. A method for hydraulic fracturing at the end of a coal mining face, characterized in that, Includes the following steps: S1. Test the physical and mechanical properties of the roof of the coal mine working face, determine the rock firmness, and assess the area of ​​the overhang at the end. S2. Through theoretical analysis and numerical calculations, this study explores the effects of several influencing factors—water injection rate, principal stress difference, fracturing sequence, and borehole inclination angle—on the weakening of fracturing damage in hard roofs, providing a reference for the on-site design of hydraulic fracturing operations. The specific steps include: S2.1 Establish a segmented hydraulic fracturing numerical model; S2.2 Analyze the high-pressure fluid injection process of segmented hydraulic fracturing, and observe the propagation law of each fracture segment, the seepage range after injection, and the changes in stress cloud diagram; S2.3 Analyze the sensitivity factors of segmented hydraulic fracturing, that is, construct a segmented hydraulic fracturing numerical model, conduct simulation experiments on four sensitivity factors: water injection rate, stress difference, fracturing sequence, and borehole inclination angle, analyze the influence of each sensitivity factor on the fracture propagation law of hydraulic fracturing, and conduct simulation experiments using the single variable method for different sensitivity factors, and finally obtain the fracture propagation law under the influence of different factors. S3. Determine the hydraulic fracturing construction parameters for the suspended ceiling at the working face; specifically including: S3.1 Determine the fracturing water pressure. The theoretical pressure for hydraulic fracturing is calculated using the following formula: ； Where: P1 -- estimated fracturing pressure required for direct hydraulic fracturing, MPa; σ1 -- Maximum principal stress at the fracturing point, MPa; σ3 -- Minimum principal stress at the fracturing point, MPa; Rt -- Tensile strength of the rock strata above the fracturing point, MPa; S3.2 Determine the fracturing time. The formula for calculating the fracturing time is as follows: ； In the formula: α -- radius of the crack; B -- the width of the crack; H -- Crack height; n -- the number of cracks; Q -- Flow rate of the fracturing unit; S3.3 Determine the borehole spacing based on the elliptical shape of the hydraulic fracturing fracture and the displacement field relationship of the crack under plane stress in linear elastic fracture mechanics: ； In the formula, σ is the hydraulic pressure inside the crack; E is the elastic modulus of the coal and rock; and a is the length of the crack. Treating a hydraulic fracturing fracture as the synthesis of many consecutive Type I through cracks along its length, applying the above formula to any crack yields the fracture width equation: ； Then, by the principle of volume conservation, we can obtain: ； In the formula, H is the seam height in the Z direction, which should be a variable. Considering that this model would have no solution if variable analysis were performed, we assume that the seam heights are equal, and a simplified analysis yields: ； Replacing σ(x) with the average value σ and rearranging, we can obtain the crack radius as: ； enter Q , T , E , σ , H , to obtain the drilling spacing d for: ； In the formula, a is the radius of the hydraulic crack; k is the crack superposition coefficient; S4. Carry out hydraulic fracturing construction according to the determined construction parameters.

2. The hydraulic fracturing construction method for the end of a coal mining face as described in claim 1, characterized in that, The mechanical properties described in step S1 include uniaxial compressive strength, tensile strength, and shear strength.

3. The hydraulic fracturing construction method for the end of a coal mining face as described in claim 1, characterized in that, Step S2.1 includes the following steps: S2.1.

1. Based on the rock strata distribution and borehole layout diagram, a numerical calculation model was established using discrete element software; S2.1.2 Determine the parameters and boundary conditions of the model, group and assign parameters to different lithologies, and assign different mechanical parameters to simulate the propagation of cracks in different rock masses.

4. The hydraulic fracturing construction method for the end of a coal mining face as described in claim 1, characterized in that, Step S2.2 includes the following steps: S2.2.1 After the model is established, four fracturing sections of hole L are selected to simulate the water injection process. After each section of hole L is fracturing, the simulation results are statistically analyzed. S2.2.2 Determining the Permeability Ratio and Number of Fractures: Introducing the parameter "Permeability Ratio," denoted as Ks, defined as the ratio of the area of ​​influence after water injection in each fracturing section to the total area of ​​the model. The formula for the permeability ratio is: ； In the formula, S1 is the area of ​​influence after water injection in the fracturing section, and S is the total area of ​​the model; Measure the area of ​​the blue region in the simulation results, and then divide it by the total area of ​​the numerical model to obtain the permeability ratio of each fracturing section after water injection. The number of fractures generated in the model after water injection in each fracturing section was counted, including tensile fractures, shear fractures, and total fractures.

5. The hydraulic fracturing construction method for the end of a coal mining face as described in claim 1, characterized in that, Step S4 includes: S4.

1. Use a drilling rig to drill holes in the hard top plate, and stop drilling after drilling into the section of hard rock that requires fracturing. S4.2 Advance the borehole packer and water injection pipe to the hydraulic fracturing section; S4.

3. Use a manual pump to pressurize the packer, causing the rubber sleeve to expand and achieve the purpose of sealing the hole; S4.4 Before construction, the connection of the hydraulic fracturing high-pressure pump and high-pressure pipeline must be checked. After the check is completed, connect the packer to the hose and connect the water injection steel pipe to be inserted into the borehole. After the water injection steel pipe is completely inserted into the borehole, the manual pump pressure is increased to 10-12 MPa to make the packer contact and fix it with the borehole wall, and the water injection steel pipe is fixed by locking the borehole opening. S4.5 Construction Drilling: Starting from the position of the advance support in the two roadways, a set of boreholes, namely hole H and hole L, are arranged at every other borehole spacing. The boreholes are arranged in a three-flower pattern. Borehole H is drilled vertically upward against the coal pillar side, with the opening position 0.4-0.6m away from the coal side, and the depth is determined according to the thickness of the hard roof. Borehole L is drilled vertically upward against the mining side, with the opening position 0.4-0.6m away from the coal side, and the depth is determined according to the thickness of the hard roof. Avoid the support position of the advance support and use the drilling rig to drill vertically above the roof of the advance section of the working face. S4.6 Fracturing Drilling: Fracturing is performed once every 2-3 sections of seamless steel pipe are withdrawn. The number of fracturing operations for each drill hole is determined according to the length of the drill hole. The pressure holding and water injection time for each operation shall not be less than 25 minutes. S4.7 Start the pump and slowly increase the pressure, and record the maximum pump pressure. Maintain the pressure for 25-30 minutes, and record the pressure changes of the pump and the on-site conditions during the fracturing process. S4.8 After fracturing for 25-30 minutes, slowly depressurize. After the pressure is completely released, stop the water pump and disconnect the high-pressure pipeline from the water injection steel pipe. After the water flow in the water injection steel pipe decreases, depressurize the manual pump. After water comes out of the borehole, the water injection steel pipe can be disassembled. S4.9 After disassembling the water injection steel pipe and retracting the packer to the designed distance, repeat steps S4.6-S4.8 for subsequent fracturing. After fracturing is completed, disassemble the water injection steel pipe section by section until the packer is removed and restore the support to its original state.

6. A hydraulic fracturing construction system for the end of a coal mining face, characterized in that, The method employs the hydraulic fracturing construction method for the end of a coal mining face as described in any one of claims 1-5, comprising a high-pressure water system, a pressure sensor, a three-way valve, a pressure relief valve, a high-pressure water steel pipe, and a packer connected in sequence. The three ends of the three-way valve are respectively connected to the top of the pressure sensor, the pressure relief valve, and the high-pressure water steel pipe. The packer includes a top packer, a bottom packer, and a connecting packer located between the top packer and the bottom packer.

7. The hydraulic fracturing construction system equipment for the end of a coal mining face as described in claim 6, characterized in that, The high-pressure water system includes a water tank, a gas-liquid booster pump, a drain pipe, and a high-pressure pump connected in sequence.

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