Hydraulic fracturing method and system equipment for end suspended roof of coal face

Abstract

The hydraulic fracturing construction method comprises the following steps that S1, the physical and mechanical properties of a coal mine working face roof are tested, the firmness degree of roof rock is determined, and the area of the end suspended roof is evaluated; s2, through theoretical analysis and numerical calculation, the influence of several different influence factors such as the water injection rate, the main stress difference, the fracturing sequence and the drilling inclination angle on hard roof fracturing damage weakening is explored, and reference is provided for hydraulic fracturing site construction design; s3, hydraulic fracturing construction parameters of a working face end suspended roof are determined; and S4, hydraulic fracturing construction is conducted according to the determined construction parameters. According to the method, a hydraulic fracturing method is used, high-pressure fluid is injected into a target hard rock stratum, so that a crack is generated or a natural crack is restarted, a rock stratum structure is transformed, a crack network system is formed, the purposes of weakening an end hard top plate and timely collapsing are achieved, the weighting strength of the top plate is reduced, and the safety of a working face is guaranteed.

Description

Technical Field

[0001] The invention relates to the technical field of coal mining, and in particular to a method and system equipment for hydraulic fracturing at the end of a coal mining working face. Background Art

[0002] Hard roof is a thick, stable, hard rock layer such as limestone, sandstone and conglomerate that exists directly above the coal seam. Hard roof has the characteristics of large thickness, strong integrity, high strength, dense structure, undeveloped joints and fissures, and strong bearing capacity. The Proctor coefficient of hard roof rock is generally 5-8, and some even exceed 10. In my country's coal mine conditions, coal seams with hard roof account for about 30%, distributed in more than 50% of the mining areas in the country. More than 40% of the working face roofs are hard and difficult to collapse with strong pressure, especially the hard roof working faces with thin direct roofs are more widely distributed.

[0003] During the mining process of coal mine working face, the hard and thick roof at the end or side of the tunnel is often not easy to collapse, thus forming a large area of ​​suspended roof. The hard and thick roof is the main cause of strong mining pressure and tunnel deformation. If the problem of suspended roof is not solved, it is easy to cause safety hazards. The sudden break of the suspended roof will cause a large area of ​​sudden pressure on the roof. Stress disturbance is easy to cause accidents of impact mine pressure, which will cause losses in safety and economy. Summary of the invention

[0004] In view of the above-mentioned technical deficiencies, the purpose of the present invention is to provide a method and system equipment for hydraulic fracturing at the end of a coal mining working face.

[0005] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0006] The present invention provides a hydraulic fracturing method for a coal mining working face end suspended top hydraulic fracturing construction method, which is 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 evaluate the area of ​​the end hanging roof;

[0008] S2. Through theoretical analysis and numerical calculation, we explored the influence of water injection rate, principal stress difference, fracturing sequence, and drilling inclination on the weakening of hard roof fracture damage, and provided a design basis for the on-site construction design of hydraulic fracturing;

[0009] S3, determining the construction parameters of the suspended top hydraulic fracturing at the 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 numerical model for staged hydraulic fracturing;

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

[0015] S2.3. Analyze the sensitivity factors of staged hydraulic fracturing, that is, build a numerical model of staged hydraulic fracturing, conduct simulation experiments on the four sensitivity factors of water injection rate, stress difference, fracturing sequence, and drilling inclination, analyze the influence of each sensitivity factor on hydraulic fracturing on the law of crack propagation, and use the single variable method to conduct simulation experiments for different sensitivity factors, and finally obtain the law of crack propagation under the influence of different factors.

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

[0017] S2.1.1. According to the rock layer distribution and drilling layout diagram, a numerical calculation model is established using discrete element software;

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

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

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

[0021] S2.2.2. Determine the permeability ratio and the number of fractures: Introduce the parameter "permeability ratio", denoted as Ks, which is defined as the ratio of the affected area after water injection in each fracturing stage to the total area of ​​the model. The definition formula of the permeability ratio is:

[0022]

[0023] Where S1 is the affected area after water injection in the fracturing stage, and S is the total area of ​​the model;

[0024] The permeability range ratio after water injection of each fracturing stage can be obtained by measuring the area of ​​the blue area in the simulation results and dividing it by the total area of ​​the numerical model;

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

[0026] Preferably, step S3 specifically includes:

[0027] S3.1. Determine the fracture water pressure. The hydraulic fracturing theoretical pressure is calculated according to the following formula:

[0028] P 1 =1.3(3σ 3 -σ 1 +R t );

[0029] Where: P1--estimated value of the fracture 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 top rock formation at the fracturing point, MPa;

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

[0034]

[0035] Where: α--the 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 fracturing device;

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

[0041]

[0042] In the formula, σ is the fluid pressure in the fracture; E is the elastic modulus of coal rock; a is the length of the fracture;

[0043] The hydraulic fracture is considered as the synthesis of many I-type through-cracks arranged continuously in the fracture length direction. Applying formula 4 to any crack can obtain the fracture width equation:

[0044]

[0045] According to the volume conservation principle, we can get:

[0046]

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

[0048]

[0049] Replace σ(x) with the average value σ and the crack radius can be obtained after sorting:

[0050]

[0051] Input Q, T, E, σ, H, and get the drilling spacing d:

[0052]

[0053] Where a is the radius of the hydraulic fracture and k is the fracture superposition coefficient.

[0054] Preferably, step S4 comprises:

[0055] S4.1. Use a drilling rig to drill holes in the hard roof, and stop drilling after drilling to the hard rock layer that needs fracturing;

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

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

[0058] S4.4. Before construction, check the connection of the hydraulic fracturing high-pressure pump and high-pressure pipeline. After the inspection, connect the packer to the hose and connect the water injection pipe to penetrate the borehole. After the water injection pipe is fully penetrated into the borehole, pump the pressure of the manual pump to 10-12MPa, make the packer contact and fix with the hole wall, and use the hole mouth to lock and fix the water injection pipe;

[0059] S4.5, Construction drilling: Starting from the position of the advance support of the two lanes, a group of holes are arranged at every other drilling distance, namely hole H and hole L, and the holes are arranged in a three-flower arrangement. The drilling hole H is constructed vertically upward close to the coal pillar, and the hole opening position is 0.4-0.6m away from the coal side. The depth is determined according to the thickness of the hard roof; the drilling hole L is constructed vertically upward close to the mining side, and the hole opening position is 0.4-0.6m away from the coal side. The depth is determined according to the thickness of the hard roof, avoiding the support position of the advance support, and using a drilling rig to construct 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 staged fracturing of each borehole is determined according to the length of the borehole. The pressure maintenance and water injection time for each time shall not be less than 25 minutes;

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

[0062] S4.8. After fracturing for 25-30 minutes, slowly release the pressure. After the pressure is completely released, stop the water pump and separate the high-pressure pipeline from the water injection steel pipe. Wait for the water flow in the water injection steel pipe to become smaller before releasing the pressure with the manual pump. Wait for water to come out of the borehole before disassembling the water injection steel pipe.

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

[0064] The present invention also provides a suspended top hydraulic fracturing construction system equipment at the end of a coal mining working 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, wherein 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, and the packer comprises 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 comprises a water tank, a gas-liquid booster pump, a drain pipe and a high-pressure pump which are connected in sequence.

[0066] The beneficial effects of the present invention are as follows: the present invention uses a hydraulic fracturing method to inject high-pressure fluid into the target rock formation, thereby generating cracks or restarting natural cracks, transforming the rock formation structure, and forming a crack network system, thereby achieving the purpose of weakening the hard top plate at the end and causing it to collapse in time, reducing the compressive strength of the top plate, and ensuring the safety of the working face. BRIEF DESCRIPTION OF THE DRAWINGS

[0067] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0068] Figure 1 A schematic diagram of rock formation distribution and drilling arrangement provided in an embodiment of the present invention; Figure 2 It is a statistical diagram of the results of the hydraulic fracturing fluid injection process in an embodiment of the present invention;

[0069] Figure 3 It is a fracture expansion diagram of different water injection rates in an embodiment of the present invention;

[0070] Figure 4 This is a graph showing the variation trend of the number of cracks and Ks at different water injection rates in an embodiment of the present invention;

[0071] Figure 5 It is a schematic diagram of the arrangement of drilling holes and fracturing stages in an embodiment of the present invention;

[0072] Figure 6 It is a structural schematic diagram of a suspended top hydraulic fracturing construction system equipment at the end of a coal mining working face in an embodiment of the present invention.

[0073] Explanation of the accompanying drawings: 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 DESCRIPTION

[0074] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0075] Example

[0076] Take a coal mine working face as an example. The working face of the coal mine is the problem of end hanging roof caused by the difficulty of timely breaking and collapsing of the limestone roof of the overlying rock layer. The coal mine mainly mines 9+10 coal, which belongs to single coal seam mining. The mine tunnels are all coal tunnels, supported by anchor beams and mesh cables. The thickness of 9+10 coal is 5.1m, and the roof of the coal seam is relatively hard K2 limestone. The coal seam bottom plate has an elevation of 1080-1180m, an average coal thickness of 5.1m (including 2-3 layers of interlayers, 4.7m without interlayers), and the coal seam generally has a dip of 2-16°, with an average dip of 7°.

[0077] The following is a description of a method for hydraulic fracturing at the end of a coal mining face proposed for the mine, which specifically includes the following steps:

[0078] S1. The uniaxial compressive strength, tensile strength and shear strength parameters of the coal and rock samples of the mine are as follows: uniaxial compressive strength is 48.81MPa, uniaxial tensile strength is 9.27MPa, cohesion is 15.35MPa, internal friction angle is 36.7°. Compared with the Proctor coefficient (f) of the rock, the strength of the limestone on the roof of the coal mine working face is between V-IVa, which belongs to medium-strong to relatively strong rock;

[0079] S2.1. According to the actual situation of the top rock layer of the 9+10# coal seam in the return air lane of the coal mine working face, the rock layer distribution and drilling arrangement schematic diagram were first drawn, as shown in the figure. Figure 1 As shown, the thickness of the 9+10# coal seam of the working face is 4.6-5.3m, with an average of 5.0m. The overlying rock layers are mudstone and limestone. According to the coal seam columnar diagram of the working face of the coal mine, 19m above the coal seam, the lithology and height of the overlying rock layers are: limestone 3.8m, mudstone 1.2m, limestone 7.6m, mudstone 3.0m, limestone 3.4m. A long borehole is drilled at each end of the top plate of the return air lane. The borehole close to the working face is named L hole, and the borehole close to the coal pillar is named S hole. Each borehole is divided into 4 sections for fracturing. A total of 8 fracturing sections are set. 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.

[0080] After the model is established, the four fracturing stages of the L hole can be selected to simulate the water injection process. A set of simulation schemes is designed for these four fracturing stages, and the water injection rate is set to 2×10 -3 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 in the four fracturing sections (L1, L2, L3, L4) of the L hole from top to bottom.

[0081] After the fracturing is completed in each section of the L hole, the simulation results are statistically analyzed. Figure 2 The blue area in the figure is the permeability range after water injection in each fracturing stage. All cracks are generated in the affected area. It can be seen that the number of fracturing times and the expansion of cracks are positively correlated. The more water injection times, the more cracks are generated, the longer the crack expansion, and the larger the permeability range.

[0082] (2) Permeability ratio and number of cracks:

[0083] The parameter "permeability range ratio" is introduced, denoted as Ks, which is defined as the ratio of the affected area after water injection in each fracturing stage to the total area of ​​the model. The definition formula of the permeability range ratio is:

[0084]

[0085] Where S1 is the affected area after water injection in the fracturing stage, and S is the total area of ​​the model.

[0086] according to Figure 2 , measure the area of ​​the blue (shaded) area in the simulation results, and then divide it by the total area of ​​the model to get the permeability range ratio of each fracturing stage after water injection. The value of Ks of the fracturing stage is calculated, and the results are shown in Table 1.

[0087] Table 1 Ks value after water injection in the fracturing stage

[0088] L1 L2 L3 L4 Ks 0.0703 0.1172 0.1525 0.2734

[0089] Ks is a good reflection of the hydraulic fracturing effect. The size of the impact range can be intuitively observed through the size of Ks. Table 1 shows that with the continuous injection of high-pressure water, the larger the Ks value, the wider the impact range, and the impact ranges of different fracturing sections will cross and penetrate, and finally form a large area of ​​roof weakening from top to bottom.

[0090] 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 increase in water injection times. The number of fractures can also reflect the fracturing effect from the side. If the number of fractures is small, it proves that the fracturing effect is poor, which may be due to the mismatch of injection parameters. If the number of fractures is too large, the fracturing effect is not ideal, which may cause the roof to collapse directly and cause excessive fracturing.

[0091] Table 2 Number of fractures after water injection in the fracturing stage

[0092] 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

[0093] S2.3, analyze the sensitivity factors of staged hydraulic fracturing, build a staged hydraulic fracturing numerical model, and conduct simulation experiments on four sensitivity factors: water injection rate, stress difference, fracturing sequence, and drilling inclination. To analyze the influence of each sensitivity factor on the law of crack expansion caused by hydraulic fracturing. For different sensitivity factors, the single variable method is used to conduct simulation experiments, and finally the law of crack expansion under the influence of different factors is obtained.

[0094] (1) Water injection rate:

[0095] The water injection rate directly affects the pressure, which in turn affects the initiation and expansion of cracks, and has a significant impact on the effect of fracturing. In order to explore the effect of water injection rate on crack expansion, four groups of different water injection rates were selected for numerical simulation. The parameters of each group are shown in Table 3, and the simulation results are analyzed.

[0096] Table 3 Numerical simulation schemes for different water injection rates

[0097]

[0098] Figure 3The crack expansion diagrams at different water injection rates are the crack expansion diagrams at different water injection rates. When the water injection rate is low, the cracks produced in each fracturing section are shorter, and some fracturing sections do not even produce cracks. The higher the water injection rate, the more cracks are produced and the longer the cracks are. Comparing the four groups of simulation results, when the water injection rate reaches 2.0×10 -3 m3 / s (120L / min), the hydraulic fracturing influence range is interconnected. When the water injection rate reaches 2.5×10 -3 When the injection rate was 150 L / min (m3 / s), the crack angle deviated during the extension process and was no longer along the direction of the maximum principal stress, indicating that the injection rate was too high and the expansion of the main crack would be affected.

[0099] Therefore, in order to ensure the rationality of crack extension and expansion, the water injection rate cannot be increased blindly, but needs to be determined comprehensively based on the on-site pump displacement and construction technology.

[0100] It can be seen from Table 4 that the higher the injection rate, the larger the Ks value. This is because in the same time, the higher the injection rate, the more water is injected and the wider the impact range.

[0101] Table 4 Number of fractures and Ks at different water injection rates

[0102]

[0103] For the data in Table 4, the changing trends of the four parameters with water injection rate are as follows: Figure 4 As shown, it can be seen that the higher the water injection rate, the larger the Ks value, and the increase in Ks also becomes larger.

[0104] (2) Principal stress difference:

[0105] In order to study the influence of principal stress difference on crack propagation, four groups of simulation experiments were set up. A fixed confining pressure of 8.8 MPa was applied to the model in the horizontal direction, and the vertical confining pressure of the model was changed. The stress differences of the four groups of simulation experiments were 3.5 MPa, 2.5 MPa, 1.5 MPa, and 0.5 MPa, respectively.

[0106] The permeability range ratio Ks and the number of cracks of the four simulation processes are also statistically analyzed, as shown in Table 5. From the data in the table, it can be seen that when the stress difference is 0.5MPa, the value of Ks is 0.2336, and when the stress difference is 3.5MPa, the value of Ks reaches 0.3285, with a difference of 0.0949 between the two. The number of cracks generated is also that the number of tensile cracks is greater than the number of shear cracks.

[0107] Table 5 Number of cracks and Ks under different stress differences

[0108] 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

[0109] (3) Fracturing sequence:

[0110] After the basic conditions of hydraulic fracturing are determined, how to choose the fracturing sequence is also a key factor in determining the fracturing effect. Figure 5 As shown, in the process of modeling the segmented hydraulic fracturing of the return air tunnel roof, two boreholes were set, and four fracturing sections were set in each borehole, for a total of eight fracturing sections: L1, L2, L3, L4 of the L borehole and S1, S2, S3, S4 of the S borehole.

[0111] Table 6 Simulation schemes of different fracturing sequences

[0112]

[0113] Similarly, the permeability range ratio Ks of the three groups of simulation experiments was statistically analyzed, and the number of cracks generated by the three groups of simulations was also statistically analyzed. The statistical results are shown in Table 7.

[0114] Table 7 Number of fractures under different fracturing sequences

[0115] 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

[0116] From the data in Table 7, it can be seen that when the fracturing order is L1, L2, L3, L4, the total number of cracks is 158, the number of cracks produced is the largest, and the impact range is also the largest, while when the fracturing order is L4, L3, L2, L1, the number of cracks produced and the impact range are the smallest.

[0117] (4) Drilling inclination:

[0118] It can be seen from the drilling arrangement diagram that borehole S is vertically upward, while borehole L has a certain inclination angle. The smaller the inclination angle, the farther the two boreholes are, which will have a certain impact on the fracturing effect. In order to explore the optimal angle of the fracturing effect of the L borehole, four groups of simulation experiments were designed to explore the fracturing effect when the inclination angle of the L hole is 55º, 65º, 75º, and 85º.

[0119] The influence range of fracturing increases with the decrease of the inclination angle of L borehole, but the number of cracks is the largest when the inclination angle is 75°. Taking into account the influence range, the number of cracks, and the correlation between cracks, fracturing achieves the best effect when the inclination angle of L borehole is ≥75°.

[0120] Furthermore, step S3 specifically includes:

[0121] S3.1. Determine the fracturing water pressure. In order to increase the number of fractures during hydraulic fracturing, a 9304 on-site emulsion pump station or other separate emulsion pumps are required. The pump flow rate Q is determined to be about 120L / min, and the maximum does not exceed 150L / min. The hydraulic fracturing theoretical pressure is calculated according to the following formula:

[0122] P 1 =1.3(3σ 3 -σ 1 +R t );

[0123] Where: P1--estimated value of the fracture pressure required for direct hydraulic fracturing, MPa;

[0124] σ 1 --Maximum principal stress at the fracturing point, MPa;

[0125] σ 3 --Minimum principal stress at the fracturing point, MPa;

[0126] R t --Tensile strength of the top rock formation at the fracturing point, MPa;

[0127] The coal seam floor has an elevation of 1080-1180m, the working surface corresponds to a ground elevation of 1344-1432m, and the coal seam is buried at a depth of 250-270m. According to the maximum burial depth of 270m, the vertical stress is:

[0128] σ 2 =0.027×270=7.3MPa;

[0129] Minimum principal stress σ 3 According to the lateral pressure coefficient of 0.8, the minimum principal stress σ 3 for:

[0130] σ 3 =7.3×0.8=5.8MPa;

[0131] Maximum principal stressσ 1 Calculated as 1.2 times the intermediate principal stress, the minimum principal stress σ 1 for:

[0132] σ 1 =7.3×1.2=8.8MPa;

[0133] According to the above formula, σ 3 =5.8MPa,σ 1 =8.8MPa. According to the on-site sampling laboratory test results, the uniaxial compressive strength is 48.81MPa and the tensile strength R t is 9.27MPa, and the starting pressure required for hydraulic fracturing is:

[0134] P 1 =1.3(3σ 3 -σ 1 +R t )=23.2MPa;

[0135] Therefore, the hydraulic fracturing water pressure is rounded to 24 MPa.

[0136] S3.2. Determine the fracturing time. The calculation formula for fracturing time is as follows:

[0137]

[0138] Where: α--the radius of the crack, taken as 10m;

[0139] B--the width of the crack, take 2cm;

[0140] H--crack height, take 4.5m;

[0141] n--the number of cracks, take 3;

[0142] Q--Flow rate of the fracturing device, 120L / min, substituting into the formula, the calculated result is 23.6min, so the fracturing time for each stage is not less than 24min, and can be 25-30min;

[0143] S3.3. Determine the borehole spacing based on the hydraulic fracturing fracture height ellipse shape and the displacement field relationship of the linear elastic fracture mechanics crack under plane stress state:

[0144]

[0145] In the formula, σ is the fluid pressure in the fracture; E is the elastic modulus of coal rock; a is the length of the fracture;

[0146] The hydraulic fracture is considered as the synthesis of many I-type through-cracks arranged continuously in the fracture length direction. Applying formula 4 to any crack can obtain the fracture width equation:

[0147]

[0148] According to the volume conservation principle, we can get:

[0149]

[0150] In the formula, H is the seam height in the Z direction, which should be a variable. Considering that this model will have no solution if a variable analysis is performed, the seam heights are considered to be equal, and a simplified analysis is performed to obtain:

[0151]

[0152] Replace σ(x) with the average value σ and the crack radius can be obtained after sorting:

[0153]

[0154] Take Q = 13m3 / h, T = 0.5h (30min), E = 5GPa, σ = 32MPa, H = 4.5m, and substitute the parameters into the above formula to obtain the drilling spacing d:

[0155]

[0156] 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 fracturing, the single-side drilling spacing is set to 6m.

[0157] In summary, the key parameters of the suspended top hydraulic fracturing at the end of the coal mine working face are shown in Table 8:

[0158] Table 8 Key parameters of hydraulic fracturing at the end of coal mine working face

[0159] parameter symbol unit Numeric Remark Hydraulic fracturing drilling height L m 13 Hydraulic fracturing drilling hole diameter Φ mm 50 Pump flow Q L / min 120 120-150 Fracture water pressure <![CDATA[P 1 ]]> MPa >21 Single hole fracturing time per section T min 25-30 Drilling spacing d m 6 Three flower arrangement Hydraulic fracturing stages part 4 Whether to withdraw anchor Anchoring Drilling time More than 20m ahead Fracturing timing In front bracket Close to the working surface

[0160] Reference Figure 6 A coal mining face end suspended top hydraulic fracturing construction system equipment, including 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 including 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 drainage pipe and a high-pressure pump connected in sequence.

[0161] Step S4 includes:

[0162] S4.1. Use a drilling rig to drill holes in the hard roof, and stop drilling after drilling to the hard rock layer that needs fracturing;

[0163] S4.2, push the borehole packer and high-pressure water steel pipe to the hydraulic fracturing section;

[0164] S4.3. Use a manual pump to pressurize the packer to expand the rubber tube to achieve the purpose of sealing the hole;

[0165] S4.4, Construction drilling: Starting from the position of the advance support of the two lanes, a group of holes are arranged at every other drilling distance, namely hole H and hole L, and the holes are arranged in a three-flower arrangement. The drilling hole H is constructed vertically upward close to the coal pillar, and the hole opening position is 0.4-0.6m away from the coal wall and the depth is 13m; the drilling hole L is constructed vertically upward close to the mining wall, and the hole opening position is 0.4-0.6m away from the coal wall and the depth is 12m, avoiding the support position of the advance support, and using a pneumatic anchor drill to construct vertically above the top plate of the advance section of the working face;

[0166] S4.5, Fracturing drilling: Fracturing is performed once every two sections of seamless steel pipes are withdrawn, with four fracturing points in each drilling hole, and the pressure-maintaining and water-injection time for each time shall not be less than 25 minutes;

[0167] S4.6. Before construction, check the hydraulic fracturing high-pressure water system and the connection of the high-pressure water steel pipe. After the inspection, connect the packer to the hose and connect the high-pressure water steel pipe to penetrate the borehole. After the high-pressure water steel pipe is fully penetrated into the borehole, pump the pressure of the manual pump to 10-12MPa to make the packer contact and fix with the hole wall, and use the hole mouth to lock and fix the high-pressure water steel pipe;

[0168] S4.7. Start the pump and slowly increase the pressure, and record the maximum pressure of the water pump. Maintain the pressure for 25-30 minutes, and record the pressure changes of the water pump during the fracturing process and the on-site situation.

[0169] S4.8. After fracturing for 25-30 minutes, slowly release the pressure. After the pressure is completely released, stop the water pump and separate the high-pressure water system from the high-pressure water steel pipe. Wait for the water flow in the high-pressure water steel pipe to become smaller before releasing the pressure with the manual pump. Wait for water to come out of the borehole before disassembling the high-pressure water steel pipe.

[0170] S4.9. After disassembling the high-pressure water steel pipe and allowing the packer to retreat to the designed distance, repeat steps S4.6-S4.8 for subsequent fracturing. After the fracturing is completed, disassemble the high-pressure water steel pipe section by section until the packer is removed and the support is restored to its state.

[0171] Obviously, those skilled in the art can make various changes and modifications to the present invention without departing from the spirit and scope of the present invention. Thus, if these modifications and variations of the present invention fall within the scope of the claims of the present invention and their equivalents, the present invention is also intended to include these modifications and variations.

Claims

1. A method for hydraulic fracturing at the end of a coal mining face, characterized in that: The following steps are involved: S1. Test the physical and mechanical properties of the roof of the coal mine working face to determine the rock solidity and evaluate the area of ​​the end hanging roof; S2. Through theoretical analysis and numerical calculation, we explored the influence of different influencing factors such as water injection rate, principal stress difference, fracturing sequence, and drilling inclination on the weakening of fracture damage in hard roof, and provided reference for the on-site construction design of hydraulic fracturing; S3, determining the construction parameters of the suspended top hydraulic fracturing at the end of the working face; S4. Carry out hydraulic fracturing construction according to the determined construction parameters.

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

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

1. Establish a numerical model for staged hydraulic fracturing; S2.

2. Analyze the high-pressure fluid injection process of staged hydraulic fracturing, and observe the expansion law of each fracture, the scope of seepage after injection, and the changes in stress cloud map; S2.

3. Analyze the sensitivity factors of staged hydraulic fracturing, that is, build a numerical model of staged hydraulic fracturing, conduct simulation experiments on the four sensitivity factors of water injection rate, stress difference, fracturing sequence, and drilling inclination, analyze the influence of each sensitivity factor on hydraulic fracturing on the law of crack propagation, and use the single variable method to conduct simulation experiments for different sensitivity factors, and finally obtain the law of crack propagation under the influence of different factors.

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

1. According to the rock layer distribution and drilling layout diagram, a numerical calculation model is established using discrete element software; S2.1.

2. Determine the parameters and boundary conditions of the model, group and assign parameters for different rock types, and assign different mechanical parameters to simulate the expansion of cracks in different rock masses.

5. The method for hydraulic fracturing at the end of a coal mining face as claimed in claim 3, characterized in that: Step S2.2 includes the following steps: S2.2.

1. After the model is established, four fracturing sections of the L hole are selected to simulate the water injection process. After each section of the L hole is fractured, the simulation results are statistically analyzed. S2.2.

2. Determine the permeability ratio and the number of fractures: Introduce the parameter "permeability ratio", denoted as Ks, which is defined as the ratio of the affected area after water injection of each fracturing stage to the total area of ​​the model. The definition formula of the permeability ratio is: Where S1 is the affected area after water injection in the fracturing stage, and S is the total area of ​​the model; The permeability range ratio after water injection of each fracturing stage can be obtained by measuring the area of ​​the blue area in the simulation results and dividing it by the total area of ​​the numerical model; The number of fractures generated in the model after water injection is completed in each fracturing stage is counted, including tensile fractures, shear fractures and total fractures.

6. The method for hydraulic fracturing at the end of a coal mining face as claimed in claim 1, characterized in that: Step S3 specifically includes: S3.

1. Determine the fracture water pressure. The hydraulic fracturing theoretical pressure is calculated according to the following formula: P1=1.3(3σ3-σ1+R t ); Where: P1--estimated value of the fracture 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 top rock formation at the fracturing point, MPa; S3.

2. Determine the fracturing time. The calculation formula for fracturing time is as follows: Where: α--the radius of the crack; B--the width of the crack; H--crack height; n – the number of cracks; Q--Flow rate of fracturing device; S3.

3. Determine the borehole spacing based on the hydraulic fracturing fracture height ellipse shape and the displacement field relationship of the linear elastic fracture mechanics crack under plane stress state: In the formula, σ is the fluid pressure in the fracture; E is the elastic modulus of coal rock; a is the length of the fracture; The hydraulic fracture is considered as the synthesis of many I-type through-cracks arranged continuously in the fracture length direction. Applying formula 4 to any crack can obtain the fracture width equation: According to the volume conservation principle, we can get: In the formula, H is the seam height in the Z direction, which should be a variable. Considering that this model will have no solution if a variable analysis is performed, the seam heights are considered to be equal, and a simplified analysis is performed to obtain: Replace σ(x) with the average value σ and the crack radius can be obtained after sorting: Input Q, T, E, σ, H, and get the drilling spacing d: Where a is the radius of the hydraulic fracture and k is the fracture superposition coefficient.

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

1. Use a drilling rig to drill holes in the hard roof, and stop drilling after drilling to the hard rock layer that needs fracturing; S4.2, advancing the borehole packer and water injection pipe to the hydraulic fracturing section; S4.

3. Use a manual pump to pressurize the packer to expand the rubber tube to achieve the purpose of sealing the hole; S4.

4. Before construction, check the connection of the hydraulic fracturing high-pressure pump and high-pressure pipeline. After the inspection, connect the packer to the hose and connect the water injection pipe to penetrate the borehole. After the water injection pipe is fully penetrated into the borehole, pump the pressure of the manual pump to 10-12MPa, make the packer contact and fix with the hole wall, and use the hole mouth to lock and fix the water injection pipe; S4.5, Construction drilling: Starting from the position of the advance support of the two lanes, a group of holes are arranged at every other drilling distance, namely hole H and hole L, and the holes are arranged in a three-flower arrangement. The drilling hole H is constructed vertically upward close to the coal pillar, and the hole opening position is 0.4-0.6m away from the coal side. The depth is determined according to the thickness of the hard roof; the drilling hole L is constructed vertically upward close to the mining side, and the hole opening position is 0.4-0.6m away from the coal side. The depth is determined according to the thickness of the hard roof, avoiding the support position of the advance support, and using a drilling rig to construct 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 staged fracturing of each borehole is determined according to the length of the borehole. The pressure maintenance and water injection time for each time shall not be less than 25 minutes; S4.

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

8. After fracturing for 25-30 minutes, slowly release the pressure. After the pressure is completely released, stop the water pump and separate the high-pressure pipeline from the water injection steel pipe. Wait for the water flow in the water injection steel pipe to become smaller before releasing the pressure with the manual pump. Wait for water to come out of the borehole before disassembling the water injection steel pipe. S4.9, after disassembling the water injection steel pipe and allowing the packer to retreat to the designed distance, repeat steps S4.6-S4.8 for subsequent fracturing. After the fracturing is completed, disassemble the water injection steel pipe section by section until the packer is removed and restore the state of the support.

8. A coal mining face end suspended top hydraulic fracturing construction system equipment, characterized in that: It includes 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 that are 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.

9. The coal mining face end suspended top hydraulic fracturing construction system equipment according to claim 8, characterized in that: The high-pressure water system comprises a water tank, a gas-liquid booster pump, a drain pipe and a high-pressure pump which are connected in sequence.

Images