A method for multiple complementary hydraulic fracturing of top coal caving in a coal mining face

Abstract

The present invention belongs to the technical field of residual coal recovery and top coal caving, and specifically relates to a method for multi-complementary hydraulic fracturing and top coal caving in the mining face of residual coal recovery from the remaining incomplete coal seam; boreholes are arranged according to the broken conditions of the two sides of the gateways in the mining face. (a) At the places where boreholes can be formed on the two sides of the gateways in the mining face, "small-angle" hydraulic fracturing boreholes are arranged in the coal seam. (b) At the places where boreholes can be formed on the two sides of the gateways in the mining face, "small-angle and near-coal-seam" hydraulic fracturing boreholes are arranged in the coal seam and the roof. (c) At the places where boreholes cannot be formed on the two sides of the gateways in the mining face, "small-angle and near-coal-seam" hydraulic fracturing boreholes are arranged in the coal seam and the roof. (d) At the places where boreholes cannot be formed on the two sides of the gateways in the mining face and hydraulic fracturing cannot be implemented, "small-angle" dense boreholes are arranged in the coal seam. The four arrangements of fracturing boreholes complement each other to achieve a comprehensive layout of the fracturing boreholes in the gateways, forming a complete "system" for the layout of fracturing boreholes, thereby improving the caving performance of the top coal.

Description

Technical Field

[0001] The present invention belongs to the technical field of residual coal recovery and top coal caving, and particularly relates to a method for multiple complementary hydraulic fracturing and top coal caving in a mining face for residual coal recovery of a remaining incomplete coal seam. Background Art

[0002] The key to the success of fully mechanized top coal caving mining is that the top coal can be smoothly caved to achieve the designed top coal recovery rate. Generally, during fully mechanized top coal caving mining, the top coal is fragmented in advance of the working face under the action of roof pressure, and then moves to the coal discharge opening as the working face advances. Under the repeated support of the hydraulic support and the swinging of the tail beam, it is smoothly discharged. The present invention aims at a coal seam with a large thickness, a hard and thick roof, and an incomplete remaining coal seam at the top after initial mining, and there are the following three top coal layout situations: ① Part of the coal seam automatically collapses during initial mining, forming a partial goaf; ② Due to obvious roof strata pressure manifestation, large fissures are generated between some coal seams; ③ The roof strata pressure manifestation is not obvious, and the coal seam integrity is good.

[0003] Hydraulic fracturing is to use a high-pressure pump to cut grooves and inject water, and make full use of the high pressure generated during water injection to generate several cracks at the end of the cut seam. At the same time, it continuously expands and extends in the rock formation, significantly increasing the rock formation cracks, dividing the roof rock formation into several layers, destroying the integrity of the roof, and reducing the overall strength. Conducting hydraulic fracturing in the coal body can increase the fissures of the coal body while increasing the moisture content of the coal body. The compressive strength and elastic modulus of the coal body decrease with the increase of the moisture content. After the coal body contains water, it becomes loose and soft, and the strength decreases. After containing water, the stress-strain curve of the coal body becomes relatively flat, the stress of the coal body decreases, and the strain increases, widening the pressure relief zone in front of the working face, reducing the possibility of sudden instability and failure of the coal body, and reducing the outburst risk.

[0004] By performing advanced hydraulic fracturing on the top coal and the roof, the overall stability of the basic roof is destroyed, the rock mass strength is reduced, the caving property of the top coal is improved, and at the same time, the threats and hazards caused by large-area caving are avoided, so that the mining of the working face can be completed safely and smoothly, increasing the coal recovery rate and reducing the loss of coal resources.

[0005] In view of the deficiencies of the above background art, the present invention provides a method for multiple complementary hydraulic fracturing and top coal caving in a mining face for residual coal recovery of a remaining incomplete coal seam. Summary of the Invention

[0006] The present invention provides a method for multiple complementary hydraulic fracturing and top coal caving in a mining face for residual coal recovery of a remaining incomplete coal seam, which is used to solve the problems that the caved top coal blocks are large or do not cave and cannot be smoothly discharged in the working face, thereby increasing the coal recovery rate and reducing the loss of coal resources.

[0007] The present invention adopts the following technical solutions: A method for multiple complementary hydraulic fracturing and top coal caving in a mining face includes the following steps

[0008] S1: Master the lithology, joint fissures, thickness variation of each rock stratum in the coal mining face and the situation of the roof rock stratum;

[0009] S2: Determine the top coal fracturing parameters in the coal mining face;

[0010] S3: Arrange boreholes according to the broken conditions of the two sides of the gateways in the coal mining face, and adopt one or more of the following arrangement methods;

[0011] (a) At the places where boreholes can be drilled on the two sides of the gateways in the coal mining face, and the tensile strength of the roof ≥ 8 MPa and the compressive strength ≥ 80 Mpa, arrange "small-angle" hydraulic fracturing boreholes in the coal seam;

[0012] (b) At the places where boreholes can be drilled on the two sides of the gateways in the coal mining face, and the tensile strength of the roof < 8 MPa and the compressive strength < 80 Mpa, arrange "small-angle, near-coal-seam" hydraulic fracturing boreholes in the coal seam and the roof;

[0013] (c) At the places where boreholes cannot be drilled on the two sides of the gateways in the coal mining face, arrange "small-angle, near-coal-seam" hydraulic fracturing boreholes in the coal seam and the roof;

[0014] (d) At the places where boreholes cannot be drilled and hydraulic fracturing cannot be implemented on the two sides of the gateways in the coal mining face, arrange "small-angle" dense boreholes in the coal seam;

[0015] S4: Seal the fracturing boreholes in the coal mining face;

[0016] S5: Conduct high-pressure hydraulic fracturing on the top coal in the coal mining face;

[0017] S6: Analyze the hydraulic fracturing effect in the coal mining face.

[0018] Further, the specific arrangement method (a) in step S3 is: arrange "small-angle" hydraulic fracturing boreholes in the gateways on both sides of the coal mining face, drill obliquely from the gateway to the coal seam above the working face, that is, drill holes at a certain distance from the coal wall of the working face on the roof of the gateway, the drilling elevation angle is 0 - 10°, the boreholes are always arranged in the coal seam until the "small-angle" hydraulic fracturing boreholes are drilled, and then only the coal seam is fractured.

[0019] Further, the specific arrangement method (b) in step S3 is: arrange "small-angle, near-coal-seam" hydraulic fracturing boreholes in the gateways on both sides of the coal mining face, drill obliquely from the gateway to the coal seam and the roof above the working face, that is, drill holes at a certain distance from the coal wall of the working face on the roof of the gateway, the drilling elevation angle is 0 - 10°, the boreholes enter the roof rock stratum after passing through a section of the coal seam, and the maximum vertical distance between the boreholes in the roof rock stratum and the coal seam does not exceed 10 m until the "small-angle, near-coal-seam" hydraulic fracturing boreholes are drilled, and then the coal seam and the roof rock stratum are fractured.

[0020] Furthermore, the specific arrangement method (c) of step S3 is as follows: "Small-angle, near-seam" hydraulic fracturing holes are arranged in the gateways on both sides of the coal mining face. Drill obliquely from the gateway to the coal seam and roof above the working face, that is, drill holes at a certain distance from the coal wall of the working face on the gateway roof. The drilling elevation angle is 0-10°. After drilling through a section of the coal seam, enter the roof rock stratum. The maximum vertical distance between the hole in the roof rock stratum and the coal seam does not exceed 10 m. Until the "small-angle, near-seam" hydraulic fracturing holes are drilled, only the roof rock stratum is fractured afterwards, and the coal seam section is not fractured.

[0021] Furthermore, the specific arrangement method (d) of step S3 is as follows: "Small-angle" dense holes are arranged in the gateways on both sides of the coal mining face. Drill obliquely from the gateway to the coal seam above the working face, that is, arrange dense holes at a certain distance from the coal wall of the working face on the gateway roof. The drilling elevation angle is 0-10°, and the holes are always arranged in the coal seam. The hole spacing is 0-1 m. Until the "small-angle" dense holes are drilled.

[0022] Furthermore, step S1 is specifically as follows: Use a rock drilling detector to conduct borehole peeping on the surrounding rock of the roadway, explore the geological structure, rock stratum strike, interface, fault, fissure, broken zone, and then master the lithology, joint fissures, thickness change conditions of each rock stratum in the coal mining face and the roof rock stratum before roof cutting and pressure relief.

[0023] Furthermore, the steps for determining the top coal fracturing parameters in step S2 include: According to the geological exploration data of the mine, use a borehole peeping instrument to observe the internal structure of the top coal, and use RFPA2D and FLAC3D numerical simulation software to conduct numerical simulation calculations on the hydraulic fracturing of the top coal in the coal mining face, and obtain the top coal fracturing parameters of the working face.

[0024] Furthermore, the borehole sealing steps in step S4 include: After drilling to the designed depth, withdraw the drill pipe, replace it with a packer, push the packer to the fracturing position, and then pressurize through a high-pressure resin thin pipe to make the packer rubber cylinder expand. The sealing pressure is 10-16 MPa. Close the valve of the resin thin pipe. When pressure testing, pressurize to 2-5 MPa to check the sealing condition. After meeting the requirements, the borehole sealing is completed.

[0025] Furthermore, the high-pressure hydraulic fracturing steps in step S5 include: Open the stop valve connected to the high-pressure water injection rod, inject water and pressurize, observe the borehole sealing effect, continuously pressurize for pressure-holding water injection until water gushes out from adjacent boreholes or nearby fissures, and then continue to pressurize. Stop the pump when the water gushing situation does not change. The first fracturing is completed;

[0026] After pressure relief, move the packer downward successively according to the design requirements, and repeat the borehole sealing and high-pressure hydraulic fracturing processes of the fracturing boreholes in the coal mining face until the fracturing of this borehole is completed.

[0027] Further, the analysis of the hydraulic fracturing effect in step S6 is specifically as follows: The borehole that has completed hydraulic fracturing serves as the observation borehole for the next fracturing borehole; during the fracturing process, if water gushes out from the observation borehole, the distance between the fracturing boreholes is increased, and if no water gushes out from the observation borehole, the distance between the fracturing boreholes is decreased;

[0028] By monitoring the approach amount of the roof and floor, the shortening amount of the movable support column, and the support load, the hydraulic fracturing effect is judged.

[0029] Compared with the prior art, the advantages of the present invention are as follows:

[0030] 1. When arranging the fracturing boreholes, the broken (intact) conditions of the two sides of the gateway and the integrity of the top coal are considered, and four borehole arrangement methods are proposed for the borehole formation conditions on the two sides of the gateway. These four schemes form a complete "system" for hydraulic fracturing and top coal caving in the gateway of the mining face. In different situations, a suitable scheme can be selected from the four schemes, and the borehole arrangement method can be changed in a timely manner according to the on-site borehole conditions. The four borehole arrangement methods are complementary and can be used in combination by complementing each other. This "system" has strong flexibility. On the premise that the specific broken conditions of the two sides of the gateway and the on-site borehole operation conditions permit, the borehole arrangement method can be changed in a timely manner according to the on-site borehole fracturing situation to achieve precise fracturing. For example, when part of the coal seam has caved or there are cracks in the coal seam during the borehole drilling process and hydraulic fracturing cannot be carried out, the borehole arrangement method can be changed in a timely manner, and the dense borehole arrangement method can be adopted.

[0031] 2. In the present invention, the four fracturing borehole arrangement methods of (a), (b), (c), and (d) all adopt the "small-angle" borehole arrangement method, that is, the angle of elevation of the fracturing borehole is 0 - 10°. The borehole has a small angle of elevation and a long hole depth, which can precisely fracture the coal seam and roof to be fractured, avoid the phenomenon that the top coal does not collapse after fracturing, effectively improve the fracturing effect, and improve the caving property of the top coal. Generally, the fracturing boreholes are vertical or large-angle boreholes with a short hole depth. The present invention adopts a "small-angle" borehole angle of elevation and a long hole depth, which can penetrate deeper into the roof to be fractured compared with general fracturing boreholes, form a certain scale of hydraulic fractures around the roof where the deep boreholes are located. As the hydraulic fractures expand, a weak surface is formed in the deep coal seam roof, and then the roof is precisely fractured, the stress concentration phenomenon of the roof is effectively alleviated, and the top coal collapses with the roof, improving the caving property of the top coal.

[0032] When the angle of elevation of the fracturing borehole is within the range of 0 - 10°, the angle of elevation of the borehole can be adjusted according to the existing conditions of the on-site coal seam and roof. For example, in the case of partial caving of the top coal or cracks in the top coal, the angle of elevation of the borehole can be appropriately increased. Due to the existence of the caving layer, it has a certain buffering effect on the stress, and the stress concentration phenomenon in the overlying strata is not obvious. Increasing the angle of elevation of the borehole to fracture the roof can also cause the top coal to collapse.

[0033] 3. In the present invention, both the fracturing borehole layout methods (b) and (c) adopt the "near coal seam" layout method, which is formed on the basis of the "small angle" layout method. That is, after the borehole enters the roof through the top coal, due to the small elevation angle of the borehole, the vertical distance between the borehole and the coal seam does not exceed 10 m and is close to the coal seam. The fracturing boreholes are arranged on both sides, and the working face length is generally less than 200 m. The fracturing boreholes can penetrate the working face (i.e., the boreholes in the two gateways are arranged opposite to each other, and the borehole depth is approximately half of the working face length). Therefore, the vertical distance between the borehole and the coal seam is very small compared to the borehole depth of dozens of meters, that is, the ratio of the vertical distance between the borehole and the coal seam to the borehole depth (the sine function sin of the elevation angle) is very small, and at the same time, the elevation angle is also very small. Therefore, it is the "near coal seam" layout method. The "near coal seam" and "small angle" borehole layout methods complement each other.

[0034] 4. Regarding the dense borehole layout method of the fracturing borehole layout method (d) in the present invention, from the perspective of increasing the cracks in the top coal, on the basis of giving full play to the role of increasing cracks and softening the top coal in hydraulic fracturing, appropriately more dense boreholes are drilled to increase the degree of cracks in the top coal. This method is used in places where boreholes cannot be drilled and hydraulic fracturing cannot be implemented on both sides of the gateways, and the top coal is relatively broken and the dense boreholes are always arranged in the coal seam. The action mechanism of the dense boreholes and hydraulic fracturing in weakening the top coal is different. The former is to form a plastic zone around the dense boreholes in the top coal and make them overlap and communicate with each other, and finally form a weakening zone to cause the top coal to collapse. Therefore, drilling dense boreholes in the top coal can also achieve the effect of caving the top coal. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 is the stratigraphic column diagram of the embodiment of the present invention;

[0036] Figure 2 is the layout plan of the fracturing borehole layout method (a) of the embodiment of the present invention;

[0037] Figure 3 is the cross-sectional view of the layout of the fracturing borehole layout method (a) in the haulage gateway;

[0038] Figure 4 is the cross-sectional view of the layout of the fracturing borehole layout method (a) in the return airway;

[0039] Figure 5 is the layout plan of the fracturing borehole layout method (b) of the present invention;

[0040] Figure 6 is the cross-sectional view of the layout of the fracturing borehole layout method (b) in the haulage gateway;

[0041] Figure 7 is the cross-sectional view of the layout of the fracturing borehole layout method (b) in the return airway;

[0042] Figure 8 is the layout plan of the fracturing borehole arrangement mode (c) of the present invention;

[0043] Figure 9 is the sectional view of the transportation gateway layout of the fracturing borehole arrangement mode (c) of the present invention;

[0044] Figure 10 is the sectional view of the return airway layout of the fracturing borehole arrangement mode (c) of the present invention;

[0045] Figure 11 is the layout plan of the fracturing borehole arrangement mode (d) of the present invention;

[0046] Figure 12 is the sectional view of the transportation gateway layout of the fracturing borehole arrangement mode (d) of the present invention;

[0047] Figure 13 is the sectional view of the return airway layout of the fracturing borehole arrangement mode (d) of the present invention;

[0048] Figure 14 is the schematic diagram of the hydraulic fracturing pressure relief borehole of the present invention;

[0049] Figure 15 is the schematic diagram of the hydraulic fracturing pressure relief hole sealing of the present invention;

[0050] Figure 16 is the schematic diagram of the hydraulic fracturing pressure relief of the present invention.

[0051] In the figure: 1 - fully mechanized caving face; 2 - return airway; 3 - protective coal pillar; 4 - transportation gateway; 5 - stop line; 6 - "small angle" hydraulic fracturing borehole; 7 - coarse sandstone; 8 - fine sandstone; 9 - sandy mudstone; 10 - intact coal seam; 11 - "small angle, near coal seam" hydraulic fracturing borehole; 12 - broken coal seam; 13 - "small angle" dense boreholes; 14 - drill rig; 15 - drill pipe; 16 - drill bit; 17 - hydraulic pump; 18 - separation section; 19 - separator; 20 - high-pressure resin hose; 21 - high-pressure water injection pipe; 22 - pressure gauge; 23 - water pressure gauge; 24 - high-pressure pump. Specific Embodiments

[0052] The following specific embodiments illustrate the implementation manners of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification.

[0053] Please refer to Figures 1 to 16It should be noted that the structures, proportions, sizes, etc. shown in the drawings of this specification are only used to cooperate with the content disclosed in the specification for those familiar with this technology to understand and read, and are not used to limit the conditions for the implementation of the present invention. Therefore, they do not have substantial technical significance. Any modification of the structure, change in the proportional relationship, or adjustment of the size, without affecting the effects that the present invention can produce and the purposes that can be achieved, should still fall within the scope covered by the technical content disclosed in the present invention. At the same time, the terms such as "upper", "lower", "left", "right", "middle", and "one" cited in this specification are only for the convenience of clear description and are not used to limit the scope for the implementation of the present invention. The change or adjustment of their relative relationships, without substantial change in the technical content, should also be regarded as the scope for the implementation of the present invention.

[0054] The present invention provides a technical solution: a method for multiple complementary hydraulic fracturing of top coal in a coal mining face, which includes the following steps:

[0055] S1: Master the lithology, joint fissures, thickness change conditions of each rock stratum in the coal mining face and the situation of the roof rock stratum;

[0056] S2: Determine the top coal fracturing parameters in the coal mining face;

[0057] S3: Arrange boreholes according to the broken conditions of the two sides of the gateways in the coal mining face, and adopt one or more of the following arrangement methods;

[0058] (a) At the places where boreholes can be formed on the two sides of the gateways in the coal mining face, and the tensile strength of the roof is ≥8 MPa and the compressive strength is ≥80 Mpa, arrange "small-angle" hydraulic fracturing boreholes in the coal seam;

[0059] (b) At the places where boreholes can be formed on the two sides of the gateways in the coal mining face, and the tensile strength of the roof is <8 MPa and the compressive strength is <80 Mpa, arrange "small-angle, near-coal-seam" hydraulic fracturing boreholes in the coal seam and the roof;

[0060] (c) At the places where boreholes cannot be formed on the two sides of the gateways in the coal mining face, arrange "small-angle, near-coal-seam" hydraulic fracturing boreholes in the coal seam and the roof;

[0061] (d) At the places where boreholes cannot be formed on the two sides of the gateways in the coal mining face and hydraulic fracturing cannot be implemented, arrange "small-angle" dense boreholes in the coal seam;

[0062] The four fracturing borehole arrangement methods can be used complementarily according to the actual situation on site, so as to achieve the full arrangement of the gateway fracturing boreholes;

[0063] S4: Seal the boreholes of the fracturing boreholes in the coal mining face;

[0064] S5: Conduct high-pressure hydraulic fracturing on the top coal in the coal mining face;

[0065] S6: Analyze the hydraulic fracturing effect of the coal mining face.

[0066] As Figure 2 , Figure 3 , Figure 4 shown; the specific layout method (a) of step S3 is: arrange "small-angle" hydraulic fracturing boreholes in the gateways on both sides of the coal mining face, drill obliquely from the gateway to the coal seam above the working face, that is, drill at a certain distance from the coal wall of the working face on the roof of the gateway, with the drilling angle of elevation being 0-10°, and the boreholes are always arranged in the coal seam until the "small-angle" hydraulic fracturing boreholes are completed, and then only the coal seam is fractured.

[0067] As Figure 5 , Figure 6 , Figure 7 shown; the specific layout method (b) of step S3 is: arrange "small-angle, near-coal-seam" hydraulic fracturing boreholes in the gateways on both sides of the coal mining face, drill obliquely from the gateway to the coal seam and the roof above the working face, that is, drill at a certain distance from the coal wall of the working face on the roof of the gateway, with the drilling angle of elevation being 0-10°, the boreholes enter the roof rock formation after passing through a section of the coal seam, and the maximum vertical distance between the boreholes in the roof rock formation and the coal seam does not exceed 10 m, until the "small-angle, near-coal-seam" hydraulic fracturing boreholes are completed, and then the coal seam and the roof rock formation are fractured.

[0068] As Figure 8 , Figure 9 , Figure 10 shown; the specific layout method (c) of step S3 is: arrange "small-angle, near-coal-seam" hydraulic fracturing boreholes in the gateways on both sides of the coal mining face, drill obliquely from the gateway to the coal seam and the roof above the working face, that is, drill at a certain distance from the coal wall of the working face on the roof of the gateway, with the drilling angle of elevation being 0-10°, the boreholes enter the roof rock formation after passing through a section of the coal seam, and the maximum vertical distance between the boreholes in the roof rock formation and the coal seam does not exceed 10 m, until the "small-angle, near-coal-seam" hydraulic fracturing boreholes are completed, and then only the roof rock formation is fractured, and the coal seam section is not fractured.

[0069] As Figure 11 , Figure 12 , Figure 13 shown; the specific layout method (d) of step S3 is: arrange "small-angle" dense boreholes in the gateways on both sides of the coal mining face, drill obliquely from the gateway to the coal seam above the working face, that is, arrange dense boreholes at a certain distance from the coal wall of the working face on the roof of the gateway, with the drilling angle of elevation being 0-10°, and the boreholes are always arranged in the coal seam, and the borehole spacing is 0-1 m until the "small-angle" dense boreholes are completed.

[0070] Step S1 specifically is: Use a borehole detector for rock strata to conduct borehole peeping on the surrounding rock of the roadway, explore the geological structure, rock strata strike, interface, fault, fracture, broken zone, and thereby master the lithology, joint fractures, thickness variation of each rock stratum in the coal mining face and the roof rock stratum situation before roof cutting and pressure relief, providing a practical basis for optimizing the working parameters and analyzing the effect of subsequent roof cutting and pressure relief in the coal mining face.

[0071] The steps for determining the top coal fracturing parameters in Step S2 include: According to the geological exploration data of the mine, use a borehole peeping instrument to observe the internal structure of the top coal, and use RFPA 2D and FLAC3D numerical simulation software to conduct numerical simulation calculations on the hydraulic fracturing of the top coal in the coal mining face, and obtain the top coal fracturing parameters of the working face.

[0072] The steps for borehole sealing in Step S4 include: After drilling to the designed depth, withdraw the drill pipe, replace it with a packer, push the packer to the fracturing position, then pressurize through a high-pressure resin fine pipe to make the packer rubber cylinder expand. The sealing pressure is 10 - 16 MPa. Close the valve of the resin fine pipe, and when pressure testing, pressurize to 2 - 5 MPa to check the sealing condition. After meeting the requirements, the borehole sealing is completed.

[0073] The steps for high-pressure hydraulic fracturing in Step S5 include: Slowly open the stop valve connected to the high-pressure water injection rod, slowly inject water and pressurize, observe the borehole sealing effect. After observing for 1 minute, if water does not flow out of this hole, continue to pressurize for pressure-holding water injection, generally lasting for 30 minutes (depending on the specific situation). If water gushes out from the adjacent borehole or nearby fissures before reaching 30 minutes, it indicates that the fracturing hole has communicated with the adjacent hole or surrounding fissures, then continue to pressurize. If the water gushing situation remains unchanged, the pump can be stopped, and the first fracturing ends. After pressure relief, move the packer downward successively according to the design requirements, and repeat the processes of borehole sealing and high-pressure hydraulic fracturing in the coal mining face until the fracturing of this hole ends.

[0074] The specific analysis of the hydraulic fracturing effect in Step S6 is: The borehole that has completed hydraulic fracturing serves as the observation hole for the next fracturing borehole; during the fracturing process, if water gushes out from the observation borehole, increase the distance between the fracturing boreholes, and if no water gushes out from the observation borehole, decrease the distance between the fracturing boreholes;

[0075] Judge the hydraulic fracturing effect by monitoring the roof-to-floor convergence amount, the shortening amount of the live column of the support, and the support load.

[0076] Take the 3132 fully mechanized caving face of Jingxin Coal Industry as an example (attached drawing) to further illustrate the present invention.

[0077] Specifically, the 3132 working face is a secondary mining face with a length of 150 m and a remaining advancing length of 190 m. The average thickness of the No. 3 coal seam in the 3132 working face is 3.5 m, generally without parting. The coal seam dip angle is 2°, and the coal seam is stable. The cutting height of the shearer in the working face is 2.20 m, the height of top coal caving is 1.3 m, and the mining-to-caving ratio is 1:0.59. The immediate roof of the 3132 working face is mostly sandy mudstone, fine-grained sandstone, and mudstone, locally siltstone. There is often a 0-0.2 m carbonaceous mudstone and gray mudstone false roof under the immediate roof. The main roof is an interlayer of gray fine-grained sandstone and mudstone, mainly composed of quartz, cemented with argillaceous and calcareous materials. The direct floor is mudstone with an average thickness of 2.36 m. The apparent density of the No. 3 coal seam in the working face is 1.45 t / m3, belonging to mirror anthracite with a hardness of 4. The coal seam bedding is distinct, joints are developed, and the structure is generally relatively simple.

[0078] Before the operation of the hydraulic fracturing top coal caving process, it is necessary to first determine the lithology, joint fissures, thickness variation of each rock stratum in the mining face, and the roof rock stratum situation before roof cutting and pressure relief, and conduct numerical simulation calculations on the hydraulic fracturing of the top coal in the mining face. The specific content is as follows:

[0079] (1) Use a rock stratum borehole detector to conduct borehole peeping on the surrounding rock of the roadway, which can detect geological features such as geological structure, rock stratum strike, interface, fault, fissure, and broken zone, so as to master the lithology, joint fissures, thickness variation of each rock stratum in the mining face, and the roof rock stratum situation before roof cutting and pressure relief, providing a practical basis for the optimization of subsequent roof cutting and pressure relief working parameters and effect analysis in the mining face. The rock stratum columnar diagram of the working face is as attached Figure 1 as shown.

[0080] (2) According to the geological exploration data of the mine and the observation results of the internal structure of the top coal by the borehole peeping instrument, use the RFPA and FLAC3D numerical simulation software to conduct numerical simulation calculations on the hydraulic fracturing of the top coal in the mining face, and obtain the fracturing parameters of the top coal in the working face.

[0081] The transportation and return air sublevels of the 3132 working face are arranged along the floor of the No. 3 coal seam, and both adopt trapezoidal sections and are supported by I-beam sheds with backings. Since the two sides of the transportation and return air sublevels of the 3132 working face are relatively broken, rattan, coal blocks, and bricks are used for piling and reinforcement on site. Therefore, according to the fracturing parameters of the top coal obtained by numerical simulation, combined with the actual broken (intact) situation of the two sides of the sublevel and the integrity of the top coal, the following four types of borehole layout schemes are adopted for the hole-forming conditions of the two sides of the sublevel:

[0082] Plan 1: At the relatively intact parts of the two sides of the transportation and return air headings of the 3132 working face, "small-angle" hydraulic fracturing boreholes are arranged on both sides. The "small-angle" hydraulic fracturing boreholes are drilled obliquely from the heading into the coal seam above the working face. The "small-angle" hydraulic fracturing boreholes are arranged at a distance of 1.5 m from the coal wall of the working face in the roof of the heading. The borehole diameter is Φ60 mm, the designed borehole depth is 75.01 m, the borehole elevation angle is 1°, the borehole spacing is 6 m, and the boreholes are always arranged in the coal seam until the "small-angle" hydraulic fracturing boreholes are completed, and then the coal seam is fractured. The layout of the "small-angle" hydraulic fracturing boreholes is as shown in the appendix Figures 2 - 4 as shown.

[0083] Plan 2: At the relatively intact parts of the two sides of the transportation and return air headings of the 3132 working face, "small-angle, near-coal seam" hydraulic fracturing boreholes are arranged on both sides. The "small-angle, near-coal seam" hydraulic fracturing boreholes are drilled obliquely from the heading into the coal seam and roof above the working face. The "small-angle, near-coal seam" hydraulic fracturing boreholes are arranged at a distance of 1.5 m from the coal wall of the working face in the roof of the heading. The borehole diameter is Φ60 mm, the designed borehole depth is 75.8 m, the borehole angle is 8°, the borehole spacing is 6 m, the borehole depth in the coal seam is 9.4 m, and the borehole depth in the roof rock stratum is 66.4 m. The maximum vertical distance between the boreholes in the roof rock stratum and the coal seam is 9.2 m < 10 m. After the "small-angle, near-coal seam" hydraulic fracturing boreholes are completed, the coal seam and roof rock stratum can be fractured, and the area within 5 m is not fractured, and the fracturing length is 70.8 m. The layout of the "small-angle, near-coal seam" hydraulic fracturing boreholes is as shown in the appendix Figures 5 - 7 as shown.

[0084] Plan 3: At the parts where boreholes cannot be formed on the two sides of the transportation and return air headings of the 3132 working face, "small-angle, near-coal seam" hydraulic fracturing boreholes are arranged on both sides. The "small-angle, near-coal seam" hydraulic fracturing boreholes are drilled obliquely from the heading into the coal seam and roof above the working face. The "small-angle, near-coal seam" hydraulic fracturing boreholes are arranged at a distance of 1.5 m from the coal wall of the working face in the roof of the heading. The borehole diameter is Φ60 mm, the designed borehole depth is 75.8 m, the borehole angle is 8°, the borehole spacing is 6 m, the borehole depth in the coal seam is 9.4 m, and the borehole depth in the roof rock stratum is 66.4 m. The maximum vertical distance between the boreholes in the roof rock stratum and the coal seam is 9.2 m < 10 m. After the "small-angle, near-coal seam" hydraulic fracturing boreholes are completed, only the roof rock stratum is fractured, and the coal seam section is not fractured, and the fracturing length is 66.4 m. The layout of the "small-angle, near-coal seam" hydraulic fracturing boreholes is as shown in the appendix Figures 8 - 10 as shown.

[0085] Plan 4: At the two sides of the haulage and return air headings of the 3132 working face where it is impossible to drill holes or impossible to carry out hydraulic fracturing, "small-angle" dense boreholes are arranged on both sides. Oblique "small-angle" dense boreholes are drilled from the heading into the coal seam above the working face. "Small-angle" dense boreholes are arranged at a distance of 1.5 m from the coal wall of the working face in the roof of the heading. The borehole diameter is Φ60 mm, the designed borehole depth is 75.01 m, the borehole elevation angle is 1°, the borehole spacing is 0.5 m, and the boreholes are always arranged in the coal seam until the "small-angle" dense boreholes are completed. The layout of the "small-angle" dense boreholes is as shown in Figures 11 - 13 the appendix.

[0086] According to the actual on-site conditions of the haulage and return air headings of the 3132 working face, the above four plans can be adjusted or used in combination complementarily.

[0087] After the borehole reaches the designed depth, withdraw the drill pipe by 15, replace it with a packer 19, push the separator 19 to the fracturing position manually or mechanically, and then slowly pressurize through the high-pressure resin hose 20 to expand the rubber cylinder of the separator 19. The hole sealing pressure is 10 - 16 MPa to ensure that the connection is sealed well. Close the valve of the high-pressure resin hose 20, and pressurize to 2 - 5 MPa during the pressure test to check the sealing condition. After meeting the requirements, the hole sealing is completed.

[0088] Slowly open the stop valve connected to the high-pressure water injection pipe 21, slowly inject water and pressurize, and observe the hole sealing effect. After observing for 1 min, if water does not flow out of this hole, continue to pressurize for pressure-maintaining water injection, generally for 30 min (depending on the specific situation). If water gushes out from the adjacent borehole or nearby fissures before 30 min, it indicates that the fracturing hole has communicated with the adjacent hole or surrounding fissures. Then continue to pressurize. If the water gushing situation remains unchanged, the pump can be stopped and the first fracturing ends. After pressure relief, move the separator 19 down successively according to the design requirements, and repeat the hole sealing and high-pressure hydraulic fracturing processes of the fracturing boreholes in the longwall face until the fracturing of this hole ends. The schematic diagrams of the hydraulic fracturing boreholes, hole sealing, and high-pressure water fracturing are as shown in Figures 14 - 16 the appendix.

[0089] The steps for analyzing the hydraulic fracturing effect in the longwall face include:

[0090] (1) The boreholes that have completed hydraulic fracturing can be used as observation holes for the next fracturing borehole. During the fracturing process, if water gushes out from the observation borehole, the distance between the fracturing boreholes can be appropriately enlarged; if no water gushes out, the distance between the fracturing boreholes needs to be appropriately reduced;

[0091] (2) Design of the mine pressure monitoring in the working face. Measure and sort out the data of the "three quantities" (i.e., the roof-to-floor convergence, the shortening of the live column of the support, and the support load) to evaluate the hydraulic fracturing effect.

[0092] The above embodiments are only illustrative of the principles and effects of the present invention and are not intended to limit the present invention. Any person familiar with this technology can modify or change the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or changes made by those with ordinary knowledge in the technical field without departing from the spirit and technical idea disclosed by the present invention should still be covered by the claims of the present invention.

Claims

1. A method for multiple complementary hydraulic fracturing and top coal caving in a coal mining face, characterized in that, It includes the following steps: S1: Master the lithology, joint fissures, thickness variation of each rock stratum in the coal mining face and the situation of the roof rock stratum; S2: Determine the fracturing parameters of the top coal in the coal mining face; S3: Arrange boreholes according to the broken conditions of the two sides of the gateways in the coal mining face, and adopt one or more of the following arrangement methods; (a) At the places where boreholes can be formed on the two sides of the gateways in the coal mining face, and the tensile strength of the roof ≥ 8 MPa and the compressive strength ≥ 80 Mpa, arrange "small-angle" hydraulic fracturing boreholes in the coal seam; the drilling angle of elevation is 0 - 10°; (b) At the places where boreholes can be formed on the two sides of the gateways in the coal mining face, and the tensile strength of the roof < 8 MPa and the compressive strength < 80 Mpa, arrange "small-angle, near-coal-seam" hydraulic fracturing boreholes in the coal seam and the roof; the drilling angle of elevation is 0 - 10°, and the maximum vertical distance between the boreholes in the roof rock stratum and the coal seam does not exceed 10 m; (c) At the places where boreholes cannot be formed on the two sides of the gateways in the coal mining face, arrange "small-angle, near-coal-seam" hydraulic fracturing boreholes in the coal seam and the roof; the drilling angle of elevation is 0 - 10°, and the maximum vertical distance between the boreholes in the roof rock stratum and the coal seam does not exceed 10 m; (d) At the places where boreholes cannot be formed on the two sides of the gateways in the coal mining face and hydraulic fracturing cannot be implemented, arrange "small-angle" dense boreholes in the coal seam; the drilling angle of elevation is 0 - 10°; S4: Seal the fracturing boreholes in the coal mining face; S5: Conduct high-pressure hydraulic fracturing on the top coal in the coal mining face; S6: Analyze the hydraulic fracturing effect in the coal mining face.

2. The method for multiple complementary hydraulic fracturing of top coal caving in a coal mining face according to claim 1, characterized in that, The specific arrangement method (a) of step S3 is: Arrange "small-angle" hydraulic fracturing boreholes in the gateways on both sides of the coal mining face, drill obliquely from the gateways to the coal seam above the working face, that is, drill holes at a certain distance from the coal wall of the working face on the roof of the gateway, and the boreholes are always arranged in the coal seam until the "small-angle" hydraulic fracturing boreholes are drilled, and then only the coal seam is fractured.

3. The method for multiple complementary hydraulic fracturing of top coal caving in a mining face according to claim 2, characterized in that, The specific arrangement method (b) of step S3 is: Arrange "small-angle, near-coal-seam" hydraulic fracturing boreholes in the gateways on both sides of the coal mining face, drill obliquely from the gateways to the coal seam and the roof above the working face, that is, drill holes at a certain distance from the coal wall of the working face on the roof of the gateway, the boreholes enter the roof rock stratum after passing through a section of the coal seam until the "small-angle, near-coal-seam" hydraulic fracturing boreholes are drilled, and then the coal seam and the roof rock stratum are fractured.

4. A method for multiple complementary hydraulic fracturing of top coal caving in a coal mining face according to claim 3, characterized in that, The specific arrangement method (c) of step S3 is: Arrange "small-angle, near-coal-seam" hydraulic fracturing boreholes in the gateways on both sides of the coal mining face, drill obliquely from the gateways to the coal seam and the roof above the working face, that is, drill holes at a certain distance from the coal wall of the working face on the roof of the gateway, the boreholes enter the roof rock stratum after passing through a section of the coal seam until the "small-angle, near-coal-seam" hydraulic fracturing boreholes are drilled, and then only the roof rock stratum is fractured, and the coal seam section is not fractured.

5. A method for multiple complementary hydraulic fracturing and top coal caving in a coal mining face according to claim 4, characterized in that, The specific arrangement method (d) of step S3 is: Arrange "small-angle" dense boreholes in the gateways on both sides of the coal mining face, drill obliquely from the gateways to the coal seam above the working face, that is, arrange dense boreholes at a certain distance from the coal wall of the working face on the roof of the gateway, and the boreholes are always arranged in the coal seam, and the drilling spacing is 0 - 1 m until the "small-angle" dense boreholes are drilled.

6. The method for multiple complementary hydraulic fracturing and top coal caving in a coal mining face according to claim 5, characterized in that, Step S1 specifically is: Use a rock stratum borehole detector to conduct borehole peeping on the surrounding rock of the roadway, explore the geological structure, rock stratum trend, interface, fault, fracture, and broken zone, and then master the lithology, joint fractures, thickness change conditions of each rock stratum in the mining face and the roof rock stratum conditions before roof cutting and pressure relief.

7. A method for multiple complementary hydraulic fracturing of top coal caving in a coal mining face according to claim 6, characterized in that, The steps for determining the top coal fracturing parameters in the coal mining face include: observing the internal structure of the top coal using a borehole peephole instrument based on the geological exploration data of the mine, and using RFPA 2D and FLAC 3D numerical simulation software to conduct numerical simulation calculations on the hydraulic fracturing of the top coal in the coal mining face, and obtaining the top coal fracturing parameters of the working face.

8. A method for multiple complementary hydraulic fracturing and top coal caving in a coal mining face according to claim 7, characterized in that The borehole sealing step of Step S4 includes: After drilling to the designed depth, withdraw the drill pipe, replace it with a packer, push the packer to the fracturing position, then pressurize through a high-pressure resin thin pipe to make the packer rubber cylinder expand. The sealing pressure is 10 - 16 MPa. Close the valve of the resin thin pipe. When pressure testing, pressurize to 2 - 5 MPa to check the sealing condition. After meeting the requirements, the borehole sealing is completed.

9. A method for multiple complementary hydraulic fracturing of top coal caving in a coal mining face according to claim 8, characterized in that The high-pressure hydraulic fracturing step of Step S5 includes: Open the stop valve connected to the high-pressure water injection rod, inject water and pressurize, observe the borehole sealing effect, continuously pressurize for pressure maintenance water injection until water gushes out from the adjacent borehole or nearby fractures, then continue to pressurize. Stop the pump when the water gushing situation remains unchanged. The first fracturing ends; After pressure relief, move the packer downward successively according to the design requirements, and repeat the borehole sealing of the fracturing borehole in the mining face and the high-pressure hydraulic fracturing process until the fracturing of this borehole ends.

10. A method for multiple complementary hydraulic fracturing of top coal caving in a coal mining face according to claim 9, characterized in that, The specific analysis of the hydraulic fracturing effect in Step S6 is: The borehole that has completed hydraulic fracturing is used as the observation borehole for the next fracturing borehole; during the fracturing process, if water gushes out from the observation borehole, increase the distance between the fracturing boreholes. If no water gushes out from the observation borehole, decrease the distance between the fracturing boreholes; Judge the hydraulic fracturing effect by monitoring the roof-to-floor convergence amount, the telescopic amount of the live column of the support, and the support load.

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