A method for protecting crossheading and main roadway by means of handle type working face partition double sequence fracturing

Abstract

The application discloses a method for protecting connecting roadways and large roadways by means of a handle type working face partitioned double-order fracturing, and belongs to the technical field of mine surrounding rock control. Target fracturing layer position and influence boundary are determined according to roof stratum structure, working face width and narrow conversion position and spatial relationship of connecting roadways and large roadways. In the working face wide face mining stage, when the distance between the working face mining line and the connecting roadway reaches the first advanced starting distance, the first fracturing borehole is arranged; in the working face narrow face mining stage, when the distance between the working face mining line and the large roadway reaches the second advanced starting distance, the inner side fan-shaped borehole and the outer side fracturing borehole facing the outside of the working face are arranged, the inner side and outer side fracturing boreholes are subjected to the second hydraulic fracturing, and a three-dimensional fracture network is formed. The boreholes with insufficient fracture closure, extension or non-connection are subjected to re-pressurization, and the two-stage fracturing effects are verified. The application can weaken the mining stress transmission in stages, reduces the deformation of the connecting roadway and the large roadway surrounding rock, and improves the roadway stability and the coal recovery rate.

Description

Technical Field

[0001] This invention relates to the field of mineral mining technology, specifically to a method for protecting connecting roadways and main roadways by dual-sequence fracturing in a knife-handle-type working face. Background Technology

[0002] With the continuous increase in coal mining intensity and recovery efficiency, the problem of surrounding rock stability control under complex geological conditions is becoming increasingly prominent. The knife-handle working face, as an irregular working face form with varying width along the advancing direction, exhibits significant spatial non-uniformity in the distribution of surrounding rock stress, overburden movement patterns, and stress transmission paths during mining due to abrupt changes in face width. Particularly in the transition area between the wide and narrow faces, the change in face geometry intensifies local stress concentration, leading to severe surrounding rock stability problems in adjacent connecting roadways and main roadways of the mining area.

[0003] During the longwall mining process of a knife-handle-shaped longwall face, the connecting roadway is usually located in the transition zone between the wide and narrow faces. When the wide face mining approaches the connecting roadway, the connecting roadway is first affected by the combined pressure of the working face's advance support and the lateral support pressure of the wide face, causing the surrounding rock stress to rise rapidly. This can easily lead to problems such as convergence of the sidewalls, roof subsidence, and failure of the support structure. Because the connecting roadway is close to the working face, it is mainly controlled by the mining stress transmission path, and it is necessary to reduce or weaken the transmission of the mining stress from the working face to the connecting roadway area.

[0004] Unlike connecting roadways, main roadways are typically laid out over a long distance along the mining area and are adjacent to narrow-face mining areas. During narrow-face mining and the final mining stages, the roof of the goaf may form a lateral cantilever rock beam structure with a certain span. This structure continuously transmits bending moments, shear forces, and lateral loads to the main roadway area during periodic movement and fracture, causing the main roadway to be subject to long-term mining disturbances. Therefore, the key to controlling the stability of the surrounding rock in the main roadway lies in weakening the continuous bearing capacity of the lateral cantilever rock beams and reducing the loads they transmit to the main roadway.

[0005] Currently, common techniques for controlling the surrounding rock in mining roadways include coal pillar retention, reinforced support, grouting, and hydraulic fracturing for pressure relief. Among these, hydraulic fracturing utilizes high-pressure water to act on the rock mass, causing fractures to initiate, expand, and form a fracture network. By disrupting the integrity of hard rock strata and altering stress transmission paths, it weakens the roof and releases mining pressure. This technology has shown good application results in controlling hard roofs, relieving roadway pressure, and reducing mine pressure manifestation.

[0006] However, existing hydraulic fracturing and depressurization technologies are mostly designed for single areas or single protected objects, and fracturing is usually carried out at fixed locations and times, without fully considering the spatiotemporal differences in the impact of mining activities in different areas during the transition between wide and narrow face working faces. On the one hand, connecting roadways are mainly affected by the pre-mining stress and lateral support pressure during the wide-face mining stage, and their control focus is on weakening the stress transmission path; on the other hand, main roadways are mainly affected by the force transmission effect of lateral cantilever rock beams during the narrow-face mining stage, and their control focus is on disrupting the continuous load-bearing structure of the roof. Due to the significant differences in the stress mechanisms and impact stages of the two, it is difficult to simultaneously meet the long-term stability control requirements of connecting roadways and main roadways by using a single fracturing area, a single fracturing timing, or a single fracture control method.

[0007] Therefore, given the complex mining stress evolution characteristics caused by the wide-narrow transition of the knife-handle working face, how to implement targeted fracturing control in stages and regions based on the different main deformation mechanisms of connecting roadways and main roadways, in order to reduce mining stress concentration and improve the stability of the surrounding rock of the roadway, has become an urgent technical problem to be solved. Summary of the Invention

[0008] The purpose of this invention is to provide a method for protecting connecting roadways and main roadways by partitioning and dual-sequence fracturing in a knife-handle-type working face, so as to solve the above-mentioned problems existing in the prior art.

[0009] The concept of this invention is as follows: Based on the difference in the main controlling stress between the connecting roadway and the main roadway during the longwall mining process, the connecting roadway side is mainly controlled by advanced mining stress, while the main roadway side is mainly controlled by lateral cantilever rock beam force transmission. This serves as the basis for zoned fracturing, forming a continuous directional pre-fracture barrier and a three-dimensional fracture network, respectively, to achieve differentiated stress relief protection for the connecting roadway and the main roadway. By implementing two directional hydraulic fracturing operations in different regions and at different times, the two directional hydraulic fracturing operations correspond to different stress evolution stages in the "wide-then-narrow" longwall mining process. First, in the wide face stage, the stress transmission path around the connecting roadway is cut off to protect the connecting roadway. Then, in the narrow face stage, the roof is broken and the mining stress around the main roadway is transferred to protect the main roadway. This solves the problems of high stress concentration, deformation and failure, and support difficulties in the roadway caused by sudden changes in the working face width, achieving full-cycle stable control of the connecting roadway and the main roadway. At the same time, it reduces the amount of coal pillars left, reduces the amount of roadway repair work, and improves coal recovery rate and mine safety.

[0010] To achieve the above-mentioned invention objectives and concepts, the technical solution adopted by this invention is: a method for protecting connecting roadways and main roadways by partitioning a knife-handle type working face with dual-sequence fracturing, wherein the knife-handle type working face sequentially includes a wide working face and a narrow working face along the advancing direction, and a connecting roadway is provided in the transition area between the wide working face and the narrow working face, and the narrow working face is adjacent to the main roadway of the mining area, including the following steps: Step S1, delineate the target fracturing layer on the connecting roadway side and the target fracturing layer on the main roadway side according to the roof rock strata structure, and determine the mining influence boundary of the connecting roadway and the mining influence boundary of the main roadway according to the range of mining stress propagation, overburden structure characteristics and the range of deformation influence of the protected roadway, and check and correct the influence boundary through surrounding rock displacement, roof delamination, borehole stress or microseismic monitoring data.

[0011] Step S2: During the wide face mining stage, combine the mining parameters of the wide face with the entire construction cycle of the continuous pre-fracture barrier to determine the advance start distance of the first hydraulic fracturing; construct a fracturing borehole group between the connecting roadway and the wide face, control the fracturing section of all boreholes to enter the target fracturing layer on the connecting roadway side, and distribute the final boreholes at intervals along the dip of the working face.

[0012] Step S3: When the distance between the working face mining line and the connecting roadway reaches the advance start distance, the first hydraulic fracturing is carried out on the target rock strata to form a continuous directional pre-fracturing barrier, which weakens or blocks the influence of the wide face mining stress on the connecting roadway.

[0013] Step S4: In the narrow face mining stage after the first hydraulic fracturing, based on the narrow face mining parameters and the mechanical properties of the lateral cantilever rock beam, determine the advance start distance for the second hydraulic fracturing. Construct at least two staggered fracturing borehole groups between the narrow face and the main roadway, control the fracturing section of all boreholes to enter the target fracturing layer on the side of the main roadway, and distribute the final boreholes at intervals along the dip of the working face.

[0014] Step S5: When the distance between the working face retreat line and the main roadway reaches the advance start distance, initiate the second hydraulic fracturing to form a three-dimensional fracture network in the target rock strata.

[0015] Step S6: After each hydraulic fracturing operation, combine the fracturing operation response data, fracture detection results, and stress and displacement monitoring data of the protected roadway to comprehensively evaluate the fracture development and connectivity effect and the roadway protection effect; implement supplementary re-fracturing for areas that have not reached the preset control indicators.

[0016] As a further improvement of the present invention, step S1 specifically includes: step S11, based on the critical layer theory, identifying the key hard and thick rock layers in the rock strata above the coal seam whose fracture can control the movement of overburden or the transmission of mining stress, determining the key hard and thick rock layers near the connecting roadway and bearing the lateral support pressure transmission function of the working face as the target fracturing layer on the connecting roadway side, and determining the key hard and thick rock layers near the main roadway and forming a lateral cantilever force transmission structure as the target fracturing layer on the main roadway side.

[0017] Step S12: Determine the influence boundaries of the connecting roadway and the main roadway based on the influence range of the advance support pressure, the ultimate span of the lateral cantilever rock beam, or the results of numerical simulation, and verify them through the results of surrounding rock displacement, roof delamination, borehole stress, or microseismic monitoring.

[0018] As a further improvement of the present invention, step S2 specifically includes the following steps: Step S21, determining the advance start distance D1 for the first hydraulic fracturing: , m , In the formula, A represents the range of the boundary affected by the mining in the connecting roadway, determined by the significant influence range of the mining advance support pressure, in meters (m); B represents the comprehensive safety margin for fracturing construction required to form a continuous directional pre-fracture barrier, in meters (m); k is the mining stress influence coefficient, taken as 0.30–0.45; H m V is the average burial depth of the coal seam, in meters; v1 is the average advance speed of the working face during the wide-face mining stage, in meters per day; T is the comprehensive construction time for all drilling, sealing, fracturing, and pressure stabilization observation required to form the first continuous directional pre-fracture barrier, in days; C is the overlap length of adjacent fracturing boreholes, in meters, used to ensure that the fractures formed by adjacent fracturing boreholes overlap with each other within the target fracturing layer on the connecting roadway side and form a continuous directional pre-fracture barrier.

[0019] Step S22: Drill a set of fan-shaped fracturing boreholes from both the working face transport roadway and the working face track roadway towards the inner side of the working face width. The two sets of fan-shaped fracturing boreholes are located on the same straight line and are symmetrically arranged with respect to the working face centerline. The borehole azimuth angle is determined based on the design coordinates of each final borehole point in the plane, and the inclination angle θ is... n Used to control the vertical entry of the borehole into the corresponding target fracturing layer; the inclination angle θ of the fracturing borehole. n Determined according to the following formula: In the formula, H1 is the vertical distance from the bottom of the roadway where the fracturing borehole is located to the center of the fracturing section of the target fracturing layer on the connecting roadway side, in meters; L0 is the horizontal projection distance from the first end point to the center point of the borehole, in meters; ΔL is the horizontal projection distance between adjacent end points, in meters; n is the borehole number, n=1, 2, 3, ...

[0020] As a further improvement of the present invention, step S3 specifically includes the following steps: Step S31, for the first group of fan-shaped fracturing boreholes arranged in the working face track roadway, they are divided into staggered odd-numbered borehole groups and even-numbered borehole groups according to the borehole number. First, fracturing is performed on the odd-numbered borehole groups, and then fracturing is performed on the even-numbered borehole groups. After all the fracturing boreholes in the roadway have been fracturing, the fracturing boreholes in the working face transport roadway are then subjected to staggered fracturing in the same odd-even grouping method.

[0021] Step S32: For a single borehole, perform segmented retreat fracturing sequentially on multiple fracturing sections falling into the target fracturing layer on the connecting roadway side along the borehole depth direction to form a continuous strip-shaped pre-fracturing fracture.

[0022] Step S33: When the working face track roadway is not suitable for setting up fracturing boreholes due to working space, surrounding rock stability, drilling trajectory accuracy, or production system avoidance conditions, supplementary boreholes are drilled from the connecting roadway to the target fracturing layer on the connecting roadway side, and the cracks formed by the supplementary boreholes are connected or overlapped with the continuous directional pre-splitting barrier.

[0023] As a further improvement of the present invention, step S4 specifically includes the following steps: Step S41, determining the advance start distance D2 for the second hydraulic fracturing: ; In the formula, L c The extent of the lateral mining impact boundary in the main roadway, in meters (m) and lengths (L). c The ultimate span of the lateral cantilever rock beam in the goaf is taken; v2 is the average advance speed of the working face during the narrow-face mining stage, in m / d; t is the comprehensive construction time required for all drilling, sealing, staged fracturing, and pressure stabilization monitoring to form the second three-dimensional fracture network, in days; S f The reserved width (in meters) for the second hydraulic fracturing fracture network coverage is used to compensate for errors in determining the fracture propagation range between the inner fan-shaped boreholes and the outer fracturing boreholes, the overlap between borehole groups, and the boundary of the lateral cantilever rock beam force transmission area. This ensures that the fracture network formed by the second hydraulic fracturing can cover the lateral cantilever rock beam force transmission area on the main roadway side, and is typically 5–20 meters wide. s σ is the safety factor for the ultimate span of the cantilever rock beam, taken as 1.1 to 1.3; h is the thickness of the target hard rock layer, in meters; σ t q represents the tensile strength of the rock stratum, in MPa; q represents the uniformly distributed load strength calculated based on a rock beam of unit width, in MPa.

[0024] Step S42: Symmetrical fan-shaped boreholes are arranged towards the inner side of the narrow face of both the working face transport roadway and the working face track roadway. Simultaneously, outer fracturing boreholes are arranged towards the outer side of the track roadway on the side of the working face where the width of the working face changes abruptly. The termination point of the outer fracturing boreholes is located outside the narrow face of the working face in planar projection and maintains a preset safe distance from the main roadway, ensuring that the fracturing sections of both the inner fan-shaped boreholes and the outer fracturing boreholes fall into the target fracturing layer on the main roadway side. At least two fracturing borehole groups are arranged at intervals along the working face advance direction or along the main roadway extension direction, with staggered spacing between adjacent fracturing borehole groups. The borehole azimuth is determined based on the design coordinates of each termination point in the plane, and the dip angle θ... n Used to control the vertical entry of the borehole into the corresponding target fracturing layer; the borehole inclination angle θ n Determined according to the following formula: In the formula, H2 is the vertical distance from the bottom of the roadway where the fracturing borehole opening point is located to the center of the corresponding fracturing section of the target fracturing layer on the side of the main roadway, in meters; L0 is the horizontal projection distance from the first end point to the center point of the opening, ΔL is the horizontal projection distance between adjacent end points, in meters; n is the borehole number, n=1, 2, 3, ...

[0025] As a further improvement of the present invention, the offset between two adjacent fracturing boreholes is 3 to 8 m.

[0026] As a further improvement of the present invention, step S5 specifically includes: firstly, performing segmented fracturing on the fan-shaped borehole group drilled in the transport roadway on the other side of the working face abruptly changing the working face towards the target rock stratum on the narrow side of the working face; then, in the track roadway on the side of the working face abruptly changing the working face width, firstly, performing segmented fracturing on the inward fan-shaped borehole group drilled towards the inner side of the working face, forming a continuous horizontal main fracture zone along the dip of the working face within the target stratum; finally, performing segmented fracturing on the target rock stratum drilled in the track roadway towards the outer side of the narrow side of the working face; within the same fracturing borehole group, sequential fracturing is performed in the direction from the goaf side to the solid coal side.

[0027] The same fracturing process was performed on another parallel fracturing borehole.

[0028] As a further improvement of the present invention, step S6 specifically includes the following steps: Step S61, based on the fracturing pressure response, injection absorption amount, borehole inspection results, spatial distribution of microseismic events, and displacement or stress monitoring results of the protected roadway, determine the fracturing boreholes that are closed, have insufficient extension, or are not connected to adjacent fractures.

[0029] Step S62: Only the identified defective fracturing boreholes are re-pressurized to allow fracturing fluid to re-enter the original fractures and activate the surrounding natural and secondary fractures.

[0030] Step S63: The re-pressurization after the first hydraulic fracturing is completed during the wide face mining stage, and the re-pressurization after the second hydraulic fracturing is completed during the narrow face mining stage.

[0031] As a further improvement of the present invention, it also includes step S7, performing a phased fracturing qualification determination; specifically including the following steps: step S71, after the first hydraulic fracturing, monitoring the approach of the two sides, the stress of the surrounding rock, and the fracture connectivity within the target fracturing layer on the side of the connecting roadway in the protected section; when the approach rate of the two sides of the connecting roadway meets the preset displacement control index, and at least one of the stress relief degree or the length of the continuous fracture meets the corresponding preset index, the first hydraulic fracturing is determined to be qualified.

[0032] Step S72: After the second hydraulic fracturing, monitor the approach of the roof and floor plates, the lateral stress of the main roadway, and the crack penetration at the root of the lateral cantilever rock beam in the protected section of the main roadway. When the approach rate of the roof and floor plates of the main roadway meets the preset displacement control index, and at least one of the stress relief degree or crack penetration length meets the corresponding preset index, the second hydraulic fracturing is deemed qualified.

[0033] Step S73: If the corresponding judgment conditions are not met at any stage, supplementary fracturing or re-fracturing shall be carried out on the area that has not met the requirements for crack propagation or connection.

[0034] Compared with the prior art, the present invention has the following beneficial effects: 1. The present invention addresses the change in the stress transmission path caused by the transition from the wide face to the narrow face along the advancing direction of the knife-handle working face. It divides the protection of the connecting roadway and the main roadway into two interconnected control stages. The first hydraulic fracturing is implemented in the wide face mining stage, and the second hydraulic fracturing is implemented in the narrow face mining stage. This makes the fracturing area, fracturing timing and protection object match the stress evolution characteristics of different mining stages of the working face. It overcomes the problem that conventional single-area, single-stage pressure relief methods cannot take into account the stability of the connecting roadway and the main roadway, and realizes differentiated protection for the entire mining process of the knife-handle working face.

[0035] 2. Based on the roof rock structure, the location of the transition between wide and narrow face sections, and the spatial relationship between connecting roadways and main roadways, this invention determines the target fracturing layers on the connecting roadway side and the target fracturing layers on the main roadway side, respectively. It also combines empirical calculations with surrounding rock displacement, roof delamination, borehole stress, or microseismic monitoring results to verify the corresponding influence boundaries. This improves the accuracy of identifying key hard and thick rock layers and determining the timing of fracturing initiation, avoiding premature fracturing leading to crack failure or premature fracturing leading to the protected roadway entering a high-stress state.

[0036] 3. In the wide face mining stage of the present invention, before the working face reaches the boundary of the connecting roadway, a continuous directional pre-fracture barrier extending along the dip of the working face is formed between the connecting roadway and the wide face by using fan-shaped boreholes and alternating fracturing of adjacent boreholes. This can reduce the stress shadow effect between adjacent fracturing holes, promote the interconnection of fractures in each borehole, destroy the continuous bearing structure of key hard and thick rock layers on the side of the connecting roadway, weaken the path of lateral support pressure from the wide face of the working face to the connecting roadway, thereby reducing the stress concentration and deformation damage of the surrounding rock of the connecting roadway.

[0037] 4. In the narrow-face mining stage of the working face, this invention arranges inner fan-shaped boreholes towards the inner side of the working face through the working face transport roadway and the working face track roadway, and adds outer fracturing boreholes from the roadway on the side of the working face where the width of the working face changes abruptly towards the main roadway on the outer side of the working face. All boreholes end within the same height range of the target fracturing layer on the main roadway side. The invention adopts a sequential fracturing method of first the inner side and then the outer side, using the induced stress field formed after the inner side fractures open to guide the outer side fractures to intersect and overlap with the inner side fractures in the target layer. Through the circulating hydraulic disturbance of the segmented fracturing and the shear displacement at the fracture intersection ends, the bedding and natural fractures of the upper and lower interfaces of the target layer are activated, thereby forming a three-dimensional fracture network covering the lateral cantilever force transmission area. This can effectively destroy the integrity of the cantilever rock beams of the roof on the main roadway side, shorten their effective force transmission span, and weaken the lateral load and dynamic pressure disturbance generated by narrow-face mining on the main roadway.

[0038] 5. This invention distinguishes the controlling stresses on the main roadway and connecting roadways: the connecting roadway side is mainly affected by the pre-mining stress and the lateral support pressure of the working face, while the main roadway side is mainly affected by the force transmission of the cantilever rock beams in the goaf. Therefore, the first fracturing mainly forms a continuous directional pre-fracturing barrier through segmented retreat fracturing and staggered inter-hole fracturing, focusing on cutting off the pre-stress transmission path on the connecting roadway side; the second fracturing, through the combined action of inner and outer fracturing boreholes and multiple fracturing borehole groups staggered along the advance direction, focuses on destroying the integrity of the cantilever rock beams and the long-term force transmission structure on the main roadway side. This avoids the problem of insufficient targeting caused by using the same fracturing method to treat both connecting roadways and main roadways simultaneously, and improves the stress relief accuracy and protection effect for different protected objects.

[0039] 6. After the two-stage fracturing is completed, this invention identifies boreholes with closed fractures, insufficient extension, or no connection with adjacent fractures based on the fracturing pressure response, injection absorption, borehole inspection, spatial distribution of microseismic events, and displacement or stress monitoring results of the protected roadway. Re-fracturing or supplementary fracturing is only performed on the corresponding boreholes, which can reactivate the original fractures and surrounding natural fissures, improve the fracture extension range and connectivity, and avoid repeated construction on boreholes that have met the fracturing requirements. This helps to reduce fracturing water consumption, construction workload, and project costs.

[0040] 7. This invention monitors the surrounding rock movement, stress relief, and crack connectivity or penetration in the protected sections of connecting roadways and main roadways, respectively. It evaluates the fracturing effect by combining displacement control indicators with stress relief indicators and crack development indicators. When any stage fails to meet the preset judgment conditions, supplementary fracturing or re-fracturing can be carried out in the substandard areas. This forms a closed-loop control of "fracturing construction - effect monitoring - standard judgment - reinforcement treatment", which improves the detectability, verifiability, and construction reliability of fracturing protection effect, thereby reducing the amount of roadway repair and reinforcement support work, and creating favorable conditions for appropriately reducing the retention of protective coal pillars and improving the coal resource recovery rate. Attached Figure Description

[0041] Figure 1 This is a plan view of the drilling layout according to an embodiment of the present invention.

[0042] Figure 2 This is a cross-sectional view of the first hydraulic fracturing borehole layout in an embodiment of the present invention.

[0043] Figure 3 This is a cross-sectional view of the second hydraulic fracturing borehole layout in an embodiment of the present invention.

[0044] Figure 4 This is a plan view of the layout of supplementary boreholes in the connecting tunnel according to an embodiment of the present invention.

[0045] Figure 5 This is a cross-sectional view of the supplementary borehole layout in the connecting tunnel according to an embodiment of the present invention.

[0046] In the diagram, 1. Key hard and thick rock strata; 2. Connecting roadway; 3. Wide face of working face; 4. Main roadway; 5. Narrow face of working face; 6. Working face transport roadway; 7. Working face track roadway; 8. Fan-shaped fracturing borehole; 9. Supplementary borehole; 10. Stop mining line; 11. Outer fracturing borehole. Detailed Implementation

[0047] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the described embodiments of the present invention are within the scope of protection of the present invention.

[0048] like Figures 1 to 5As shown, this invention provides a method for protecting connecting roadways and main roadways in a dual-sequence fracturing process along a knife-handle-type working face. The knife-handle-type working face sequentially includes a wide working face 3 and a narrow working face 5 along the advancing direction. A connecting roadway 2 is provided in the transition area between the wide working face 3 and the narrow working face 5. The narrow working face 5 is adjacent to the main roadway 4 of the mining area. A working face transport roadway 6 and a working face track roadway 7 are respectively arranged on both sides of the working face. The final mining position of the narrow working face 5 corresponds to the stop line 10. The knife-handle-type working face refers to a non-uniform width working face with varying width sections along the advancing direction. Under fully mechanized mining conditions, the width of the narrow working face 5 is typically 60–120 m, preferably 80–100 m; the width of the wide working face 3 is typically 180–260 m, preferably 200–240 m. The width-to-narrow ratio is typically not less than 1.4, preferably 1.5–3.5, but this width-to-narrow ratio is a preferred applicable condition and not a necessary limitation for implementing this invention.

[0049] Step S1: Determine the target fracturing layers on the connecting roadway side and the main roadway side based on the roof rock strata structure. Determine the mining influence boundaries of the connecting roadway and the main roadway based on the range of mining stress propagation, overlying rock structure characteristics, and the range of deformation of the protected roadway. Verify and correct the influence boundaries using surrounding rock displacement, roof delamination, borehole stress, or microseismic monitoring data.

[0050] Step S11: Identify key hard and thick rock layers.

[0051] Collect geological columnar sections of the working face, borehole data, well logging data, core test data, mining engineering plan, and roadway support data. Conduct supplementary geological exploration or core sampling in the wide-narrow transition area, the area adjacent to connecting roadway 2, and the area adjacent to main roadway 4. Based on the key layer theory, identify the key hard and thick rock layer 1 from the strata above the coal seam whose fracture can control overburden movement or mining stress transmission.

[0052] The key hard and thick rock stratum 1 is not determined solely by lithology or thickness, but rather by a comprehensive consideration of its thickness, tensile strength, elastic modulus, burial depth, stratigraphic position, and its control over the movement of overlying strata and the transmission of mining stress. Based on the roof strata structure, the location of working face width changes, and the spatial relationship between connecting roadway 2 and main roadway 4, the target fracturing strata on the connecting roadway side, the target fracturing strata on the main roadway side, the influence boundary of the connecting roadway, and the influence boundary of the main roadway are determined. The location of the width transition is jointly determined by the layout of the mining area roadways, the location of auxiliary cut-outs or connecting chambers, equipment docking conditions, and roof integrity, preferentially avoiding fault fracture zones, areas of intense periodic pressure from the old roof, and areas of roof fracture. The key hard and thick rock stratum 1, located near connecting roadway 2 and bearing the lateral support pressure transmission function of working face width 3, is identified as the target fracturing stratum on the connecting roadway side; the key hard and thick rock stratum 1, located near main roadway 4 and forming a lateral cantilever rock beam force transmission structure, is identified as the target fracturing stratum on the main roadway side. The target fracturing layer is preferably located within 10 times the mining height above the coal seam. The fracturing section is usually set in the middle and upper part of the key hard and thick rock layer 1, and the length of a single fracturing section is usually about 20m.

[0053] The first and second hydraulic fracturing operations can be performed on different spatial locations of the same key, hard, thick rock stratum 1, or they can be performed on different controlling key strata. The first hydraulic fracturing operation is preferentially performed on the target stratum closer to the connecting roadway 2 to form a continuous directional pre-fracture barrier; the second hydraulic fracturing operation is preferentially performed on the target stratum closer to the main roadway 4 and with lateral cantilever rock beam force transmission to form a three-dimensional fracture network covering the cantilever force transmission area.

[0054] Step S11 allows for the limitation of fracturing targets to rock strata that have substantial control over overburden movement and mining stress transmission, thus avoiding ineffective fracturing of general weak rock strata or uncontrolled rock strata.

[0055] Step S12: Determine the influence boundaries of the connecting roadway and the main roadway based on the influence range of the advance support pressure, the ultimate span of the lateral cantilever rock beam, or the results of numerical simulation, and verify them through the results of surrounding rock displacement, roof delamination, borehole stress, or microseismic monitoring.

[0056] The boundary of influence of mining in the connecting roadway refers to the spatial boundary where the advance support pressure and lateral support pressure of the working face have a significant impact on the surrounding rock of the connecting roadway during the wide-face mining process. It is determined based on the range of the peak area of ​​the advance support pressure of the working face, the range of influence of the lateral support pressure, and the results of numerical simulation. The boundary of influence of mining in the main roadway refers to the spatial boundary where the stress and deformation of the surrounding rock of the main roadway are significantly increased due to the force transmission effect of the lateral cantilever rock beam during the narrow-face mining process. It is determined based on the calculation results of the ultimate span of the lateral cantilever rock beam, the range of roof movement, and the area of ​​concentrated mining stress.

[0057] Monitoring sections were arranged along the roadway strike in the fracturing-affected zone, transition zone, and unfractured control zone to observe the approach of the sidewalls, roof and floor, roof delamination, borehole stress, and microseismic activity in connecting roadway 2, main roadway 4, corner coal pillars, and adjacent roadways. If continuous deformation, increased stress, or enhanced microseismic activity occurred in the vicinity of connecting roadway 2 or main roadway 4 before the working face mining line reached the affected boundary, the affected boundary was revised based on the field monitoring results.

[0058] It should be noted that the mining disturbances experienced by Connecting Roadway 2 and Main Roadway 4 are not caused by a single stress, but are the result of the combined effects of advance mining stress, lateral support pressure, and force transmission from the lateral cantilever rock beam. However, due to their different spatial locations and the different timing of the disturbances, their primary sources of stress are different. Connecting Roadway 2 is located in the transition zone between the wide face 3 and the narrow face 5 of the working face, and its affected length along the advancement direction is relatively short. When the mining of the wide face 3 approaches Connecting Roadway 2, Connecting Roadway 2 is first affected by the concentration of advance mining stress ahead of the working face and the superposition of lateral support pressure from the wide face 3. Therefore, the first fracturing operation mainly aims to weaken the transmission of advance mining stress and form a continuous directional pre-fracture barrier. Main roadway 4 is usually arranged along a long distance in the mining area and is located near or behind the narrow face 5 mining and stop line 10. After the narrow face 5 mining and the end mining, the roof of the goaf forms a lateral cantilever rock beam supported by solid coal or stable rock mass. This cantilever rock beam can continuously transmit bending moment, shear force and lateral load to main roadway 4. Therefore, the second fracturing is mainly to destroy the integrity of the lateral cantilever rock beam, shorten its effective span and weaken its force transmission effect on main roadway 4.

[0059] Step S2: Determine the advance start distance for the first hydraulic fracturing and set up a fracturing borehole group.

[0060] During the mining phase of the wide face 3, the advance distance for the first hydraulic fracturing is determined by combining the average advance speed of the wide face 3, the influence range of the mining advance support pressure, and the total construction time required to form a continuous directional pre-fracture barrier. At the same time, fracturing boreholes are designed in a fan-shaped arrangement between the connecting roadway 2 and the wide face 3, so that the fracturing section of each borehole enters the target fracturing layer on the connecting roadway side, and the end point of the borehole is distributed at intervals along the dip of the working face.

[0061] Step S21: Determine the advance start distance D1 for the first hydraulic fracturing: , m , In the formula, A represents the range of the boundary affected by the mining in the connecting roadway, determined by the significant influence range of the mining advance support pressure, in meters (m); B represents the comprehensive safety margin for fracturing construction required to form a continuous directional pre-splitting barrier, in meters (m); k is the mining stress influence coefficient, taken as 0.30–0.45, with 0.40–0.45 preferred for hard and intact roof and 0.30–0.40 preferred for medium-hard roof; H m Va represents the average burial depth of the coal seam, in meters (m); V1 represents the average advance speed of the working face during the wide-face mining stage, in meters per day (m / d); T represents the total construction time required for all borehole construction, sealing, fracturing, and pressure stabilization monitoring to form the first continuous directional pre-fracture barrier, in days (d); C represents the overlap length of fractures in adjacent fracturing boreholes, in meters (m), used to ensure that the fractures formed by adjacent fracturing boreholes overlap within the target fracturing layer on the connecting roadway side and form a continuous directional pre-fracture barrier, and is taken as 5–15 m. T uses the total construction time required to form the entire first continuous directional pre-fracture barrier, rather than the construction time of a single borehole, to ensure that the calculated starting distance can cover the time required for all borehole construction and stable fracture propagation.

[0062] Along the working face advance direction, when the working face retreat line has not yet reached the boundary of influence of the connecting roadway and the horizontal projection distance between the two reaches D1, the first hydraulic fracturing is initiated and completed before the working face retreat line reaches the boundary of influence of the connecting roadway.

[0063] By simultaneously incorporating the advance impact range of mining, the working face advance distance during construction, and the crack connection margin into D1, the continuous directional pre-splitting barrier can be formed before the connecting roadway 2 enters the significant mining impact zone, preventing the connecting roadway 2 from bearing high stress prematurely due to late initiation.

[0064] Step S22: Drill a set of fan-shaped fracturing boreholes from the working face transport roadway 6 and the working face track roadway 7 towards the inner side of the wide face. The two sets of fan-shaped fracturing boreholes are located on the same straight line and are symmetrically arranged with respect to the center line of the working face. The borehole azimuth angle is determined according to the design coordinates of each end point in the plane.

[0065] like Figure 1 and Figure 2 As shown, with the centerline of the working face as the symmetrical reference, fracturing boreholes are arranged in a fan shape from the working face transport roadway 6 and the working face track roadway 7 towards the inner side of the working face. These fracturing boreholes can be fan-shaped fracturing boreholes 8 as shown in the diagram. The inclination angle θ of the nth first fracturing borehole... n Determined according to the following formula: In the formula, H 11L0 is the vertical distance from the bottom of the roadway where the fracturing borehole is located to the center of the fracturing section of the target fracturing layer on the connecting roadway side; L0 is the horizontal projection distance from the first end point of the borehole to the center point of the borehole, usually taken as 10m; ΔL is the horizontal projection distance between adjacent end points of the borehole, usually taken as 20m; n is the borehole number, n=1, 2, 3, ...

[0066] The symmetry reference is defined as the centerline of the working face. H1 is measured from the floor of the roadway where the borehole is located to the center of the target fracturing section; L0 and ΔL are both horizontal projection distances. During actual construction, corrections are made according to the undulation of the target layer, the drilling rig posture, and the roadway cross-section to control the borehole azimuth deviation within ±3° and the final borehole deviation within ±5m.

[0067] The fracturing sections of each fracturing borehole are directed to the target fracturing layer on the connecting roadway side, and the final borehole points are distributed at intervals along this target fracturing layer. The continuous directional pre-fracture barrier is preferably located near the transition interface between the wide face 3 and the narrow face 5 of the working face, extending diagonally along the roof of the connecting roadway 2 on the goaf side of the wide face 3. Its continuous length covers the difference in diagonal length between the wide face 3 and the narrow face 5, and extends 5–10 m to both ends.

[0068] The barrier center can maintain a horizontal distance of 5-6m from the narrow face coal wall, and construction should be completed 5-8m before the corresponding stop mining position on the wide face; the vertical roof cutting height is preferably 8-10 times the mining height, so as to penetrate the basic roof and the key layer above it that will cooperate in breaking through. The above parameters are used to illustrate the preferred implementation method and can be adjusted according to the actual roadway layout and target layer conditions.

[0069] Step S3: Initiate and implement the first hydraulic fracturing to form a continuous directional pre-fracturing barrier.

[0070] When the horizontal projection distance between the working face's recovery line and the influence boundary of the connecting roadway reaches D1 but has not yet entered the influence boundary, the first hydraulic fracturing is initiated; the fracturing boreholes are divided into two staggered groups and fracturing is performed sequentially to connect adjacent fractures, forming a continuous directional pre-fracture barrier extending along the working face's dip before the working face's recovery line reaches the influence boundary of the connecting roadway.

[0071] The first hydraulic fracturing operation preferably employs a combination of "inter-hole staggered fracturing + in-hole segmented retreat fracturing." For a single fracturing borehole, fracturing is first carried out in the target fracturing segment corresponding to the final borehole position, allowing fractures to form preferentially in the deep layers of the target critical strata. Once the final fracturing segment reaches the design pressure response, injection volume, water production from adjacent boreholes, or microseismic response requirements, the borehole is retreated towards the borehole opening to the next fracturing segment for continued fracturing. By sequentially retreating fracturing through multiple fracturing segments, localized fractures can be avoided only near the final borehole, ensuring continuous propagation of fractures along the target strata within the same borehole control area. Combining staggered grouping fracturing of adjacent boreholes allows the fractures formed in each borehole to overlap in the plane, thus forming a continuous directional pre-fracture barrier and reducing the impact of the advanced mining stress and lateral support pressure generated during the wide face 3 mining on the connecting roadway 2.

[0072] Step S31: For the first group of fan-shaped fracturing boreholes arranged in the working face track roadway, divide them into staggered odd-numbered and even-numbered borehole groups according to their borehole numbers. First, fracturing is performed on the odd-numbered borehole groups, followed by fracturing on the even-numbered borehole groups. After all fracturing boreholes in this roadway are completed, fracturing is then performed on the fracturing boreholes in the working face transport roadway 6 using the same odd-even grouping method. During fracturing, the target fracturing section is sealed with a packer, and high-pressure water is injected into the sealed section. The preferred single-stage fracturing time for thick, hard sandstone layers is 25–30 minutes, and for thick, hard mudstone layers, it is 10–15 minutes. Alternating fracturing between adjacent boreholes can reduce the stress shadowing effect between adjacent fractures, reduce fracture deflection or mutual inhibition, and allow fractures to continuously expand and connect along the target direction.

[0073] Step S32: For a single borehole, perform segmented retreat fracturing sequentially along the borehole depth direction on multiple fracturing sections falling into the target fracturing layer on the connecting roadway side, forming a continuous band of pre-fracturing fractures. The injection of the corresponding fracturing section can be stopped if any of the following occurs: significant water seepage from adjacent boreholes; continuous water spraying and seepage at surrounding anchor bolts or cables for 5–7 minutes; construction pressure drops below 5 MPa; or a significant pressure drop occurs during injection, indicating that the fracturing section has connected with higher-level fractures or adjacent fractures.

[0074] Step S33: When the working face track roadway 7 is not suitable for setting up fracturing boreholes, supplementary boreholes 9 are drilled from the connecting roadway 2 to the target fracturing layer on the connecting roadway side.

[0075] like Figure 4 and Figure 5 As shown, when the working face track roadway 7 is not suitable for setting up fracturing boreholes due to working space, surrounding rock stability, drilling trajectory accuracy, or production system avoidance conditions, supplementary boreholes 9 are drilled from the connecting roadway 2 to the target fracturing layer on the connecting roadway side, and the cracks formed by the supplementary boreholes 9 are connected or overlapped with the continuous directional pre-splitting barrier.

[0076] The unsuitable conditions for construction include: an effective working width of less than 3.0m or a height of less than 2.5m; a roadway convergence rate exceeding 3mm / d for 48 consecutive hours; anchor bolts or cables being subjected to stress exceeding 80% of their design bearing capacity; a borehole azimuth deviation expected to exceed ±3° or a final borehole deviation expected to exceed ±5m; and any one or more of the following: conflicts between fracturing operations and production systems such as transportation and ventilation.

[0077] The angle between the axis of the supplementary borehole 9 and the sidewall of the connecting roadway is 80° to 100°, preferably 90°. The final borehole enters the target fracturing layer on the side of the connecting roadway, and the crack formed by the supplementary borehole 9 overlaps with the original pre-fracture barrier on the plane projection by 5 to 10 meters. This supplementary method can maintain the continuity of the first pre-fracture barrier and prevent local pressure leakage from forming new stress transmission channels.

[0078] Step S4: Determine the advance start distance for the second hydraulic fracturing and construct two types of fracturing borehole systems.

[0079] After the first hydraulic fracturing is completed, the working face continues to advance from the wide face 3 to the narrow face 5. During the mining stage of the narrow face 5, the advance distance for the second hydraulic fracturing is determined based on the average advance speed of the narrow face 5 and the mechanical properties of the lateral cantilever rock beam. Simultaneously, inner fan-shaped boreholes are constructed from the roadway towards the inside of the working face, and outer fracturing boreholes 11 are constructed from the working face track roadway 7 on the side of the abrupt change in working face width towards the main roadway 4.

[0080] Step S41: Determine the advance start distance D2 for the second hydraulic fracturing: ; In the formula, L c The extent of the lateral mining impact boundary in the main roadway, in meters (m) and lengths (L). c The ultimate span of the lateral cantilever rock beam in the goaf is taken; v2 is the average advance speed of the working face during the narrow-face mining stage, in m / d; t is the comprehensive construction time required for all drilling, sealing, staged fracturing, and pressure stabilization monitoring to form the second three-dimensional fracture network, in days; S f The reserved width (in meters) for the second hydraulic fracturing fracture network coverage is used to compensate for errors in determining the fracture expansion range between the inner fan-shaped borehole and the outer fracturing borehole 11, the cross-over between borehole groups, and the boundary of the lateral cantilever rock beam force transmission area (the lateral cantilever rock beam force transmission area refers to the rock beam area in the adjacent roof of the main roadway 4 after the narrow face 5 is mined, which is supported by solid coal or stable rock mass, extends towards the goaf side, and transmits bending moment and lateral load to the main roadway 4). This ensures that the fracture network formed by the second hydraulic fracturing can cover the lateral cantilever force transmission area on the main roadway side, with a width of 5–20 meters, preferably 10–15 meters. s σ is the safety factor for the ultimate span of the cantilever rock beam, taken as 1.1 to 1.3; h is the thickness of the target hard rock layer, in meters; σt q represents the tensile strength of the rock stratum, in MPa; q represents the uniformly distributed load strength calculated based on a rock beam of unit width, in MPa.

[0081] When the target layer is hard sandstone or a hard rock layer that can be approximated by beam-type fracture analysis, L c Based on the thickness h and tensile strength σ of the target hard rock layer t And determine σ by calculating the uniformly distributed load strength q based on the unit width of the rock beam. t The unit weight and load parameters of the rock strata were obtained from the indoor core test, well logging interpretation, geological column data and the load of the overlying rock strata, and a safety factor of 1.1 to 1.3 was introduced.

[0082] When the target layer is mudstone, sand-mud interbedded layer, or a composite key layer, L is calculated using equivalent thickness, equivalent tensile strength, and equivalent uniformly distributed load, or determined through numerical simulation or similar material tests. c .

[0083] Along the working face advance direction, when the working face retreat line has not yet reached the influence boundary of the main roadway and the horizontal projection distance between the two reaches D2, the second hydraulic fracturing is initiated and completed before the working face retreat line reaches the influence boundary of the main roadway.

[0084] The second hydraulic fracturing must occur during the narrow face 5 mining stage after the first hydraulic fracturing is completed. The order of the two is fixed as first fracturing and then second fracturing, so as to match the stress state of connecting roadway 2 and main roadway 4 at different mining stages.

[0085] Step S42: Symmetrical fan-shaped boreholes are arranged inwards towards the narrow face 5 of both the working face transport roadway 6 and the working face track roadway 7. Simultaneously, a set of fracturing boreholes is drilled outwards from the track roadway on the side of the working face where the width of the working face changes abruptly. The vertical position of the far end of the borehole is at or outside the edge of the wide face working area. The boreholes drilled in the two roadsway form a row of fracturing boreholes. Two fracturing boreholes are drilled between the narrow face 5 of the working face and the main roadway. The second hydraulic fracturing preferably involves setting at least two fracturing borehole groups on the plane. After the first row of fracturing borehole groups completes fracturing, the second row of fracturing borehole groups is arranged at a staggered distance along the working face advancement direction or along the extension direction of the main roadway 4. The staggered distance is preferably 5m, but can also be adjusted to 3-8m according to the target rock layer thickness, fracture propagation radius, and the protected area of ​​the main roadway 4. This arrangement can compensate for the limited coverage of cracks in a single borehole group, so that the protected areas on the four sides of the main roadway form a continuous weakened zone along the strike or advance direction, and together with the cracks formed by the segmented fracturing of the inner and outer fracturing boreholes, they form a three-dimensional crack network.

[0086] like Figure 1 and Figure 3As shown, with the centerline of the working face as the symmetrical reference, inner fan-shaped fracturing boreholes 8 are arranged from the working face transport roadway 6 and the working face track roadway 7 towards the inner side of the working face. The borehole azimuth angle is determined according to the design coordinates of each final borehole point in the plane, and the inclination angle θ n Used to control the vertical entry of the borehole into the corresponding target fracturing layer. The inclination angle θ of the nth inner fan-shaped borehole. n Determined according to the following formula: In the formula, H2 is the vertical distance from the bottom of the roadway where the fracturing borehole is located to the center of the corresponding fracturing section of the target fracturing layer on the side of the main roadway; L0 is the horizontal projection distance from the first end point to the center point of the borehole; ΔL is the horizontal projection distance between adjacent end points; n is the borehole number, n=1, 2, 3, ...

[0087] The inner fan-shaped boreholes in the working face transport roadway 6 and the working face track roadway 7 are symmetrical with respect to the working face centerline. Their main function is to form opposing fractures from both sides of the working face into the critical layer above the narrow face 5. The symmetry is mainly reflected in the fact that the boreholes in the working face transport roadway 6 and the working face track roadway 7 are symmetrical with respect to the working face centerline. The difference is mainly reflected in the fact that the track roadway 7 also has an outer fracturing borehole 11, which is different from the inner fan-shaped boreholes in terms of orientation, final hole position, and control area.

[0088] The second fracturing control zone is preferably located near the stop line 10 of the narrow face 5 of the working face, and is located in the roof of the stop line 10 on the side of the working face closer to the working face, preferably 5 to 8 m away from the final coal wall of the narrow face.

[0089] Step S5: Initiate and implement the second hydraulic fracturing to form a three-dimensional fracture network.

[0090] When the horizontal projection distance between the working face's retreat line and the main roadway's influence boundary reaches D2 and has not yet entered the influence boundary, the second hydraulic fracturing is initiated; sequential and segmented fracturing is carried out in the order of first the inner fan-shaped fracturing borehole 8, and then the outer borehole group.

[0091] Specifically, segmented fracturing is carried out on the inner hole group consisting of multiple inner fan-shaped fracturing boreholes to form a continuous inner horizontal main fracture zone extending along the working face within the target fracturing layer.

[0092] The inner fan-shaped boreholes are divided into a first inner borehole group and a second inner borehole group according to an alternating principle. First, fracturing is performed on the first inner borehole group. After the pump shutdown pressure has stabilized and the frequency of micro-vibration events has decreased, fracturing is performed on the second inner borehole group. The induced stress field generated after the opening of the first-fracturing fracture guides the subsequent fracturing to extend towards and connect with the end of the first-fracturing fracture, forming a continuous inner horizontal main fracture zone extending along the dip of the working face. This fracture zone preferentially cuts through the high bending moment and high tensile stress region at the root of the lateral cantilever rock beam, weakening the continuous connection between the rock beam and the solid coal or stable rock mass.

[0093] Then, segmented fracturing was carried out on the outer borehole group to form an outer horizontal main fracture zone extending towards the main roadway within the target fracturing layer.

[0094] After all the inner boreholes are fracturing, the boreholes formed by the outer fracturing borehole 11 are subjected to segmented fracturing to form an outer horizontal main fracture zone extending from the working face track roadway to the main roadway. During the fracturing process, the stress shadow zone formed by the inner horizontal main fracture zone after tensioning is used to induce the outer fracture to deflect towards this low stress zone during expansion, so that the outer fracture and the inner fracture form a spatial intersection and overlap within the target layer.

[0095] The second fracturing operation prioritizes the inner borehole group, followed by the outer borehole group. Within the same borehole group, fracturing can be gradually performed from the goaf side to the solid coal side to reduce the risk of localized high stress during subsequent drilling operations.

[0096] Through the cross-expansion of fractures in the inner fan-shaped fracturing borehole 8 and the outer fracturing borehole 11, as well as the hydraulic disturbance caused by segmented fracturing, a three-dimensional fracture network is formed that connects the horizontal main fracture cross network with the vertical secondary fracture zone.

[0097] By leveraging the cross-expansion of fractures in the inner fan-shaped fracturing borehole 8 and the outer fracturing borehole 11, and utilizing the circulating hydraulic disturbance generated by multiple pump restarts during the segmented fracturing process, as well as the shear displacement effect at the intersection of the inner and outer fractures, the primary bedding and natural fractures at the upper and lower interfaces of the target fracturing layer are activated, causing horizontal fractures to branch and expand vertically, forming a vertical secondary fracture zone that penetrates the bedding plane. This vertical secondary fracture zone intersects and connects with the inner and outer horizontal main fracture zones, together forming a three-dimensional fracture network covering the lateral cantilever force transmission area.

[0098] The three-dimensional fracture network is a combination of planar and mesh-like fracture structures, which differs from the linear or banded continuous pre-fracture barriers formed by the first hydraulic fracturing. This three-dimensional fracture network can shorten the effective span of the lateral cantilever rock beam, reduce its root bending moment and tensile stress, and disperse the mining load that may be transmitted to the main roadway 4 to the goaf side or deep rock mass, thereby reducing the dynamic pressure disturbance to the main roadway 4 during the narrow face 5 mining and final mining stages.

[0099] Step S6: Evaluate the fracturing effect and re-fracturing the defective fracturing borehole.

[0100] After each hydraulic fracturing operation, the fracturing operation response data, fracture detection results, and stress and displacement monitoring data of the protected roadway are combined to comprehensively evaluate the fracture development and connectivity effect and the roadway protection effect; for defective fracturing boreholes with insufficient fracture closure, insufficient extension, or failure to connect with adjacent fractures, re-fracturing is carried out as needed.

[0101] Step S61: Based on the fracturing operation response, crack detection and protected roadway monitoring results, determine the defective fracturing borehole.

[0102] After steps S2 and S3 are completed, based on the fracturing pressure response, injection absorption, borehole inspection results, spatial distribution of microseismic events, and displacement or stress monitoring results of the protected roadway, fracturing boreholes that are closed, have insufficient extension, or are not connected to adjacent fractures are identified.

[0103] Specifically, the following can be used to determine the cause: the pressure recovers quickly after the pump stops; borehole inspection shows that the crack opening width or continuous length is insufficient; the number of micro-vibration events or the spatial expansion range is less than the design range; the water absorption rate is reduced; and the approach rate or stress relief rate of the protected roadway does not meet the preset standard.

[0104] Step S62: Re-pressure is applied only to the identified defective fracturing boreholes.

[0105] Only confirmed defective fracturing boreholes are repressurized to allow fracturing fluid to re-enter the original fracture and activate surrounding natural and secondary fractures. The repressurization pressure is 70%–95% of the initial fracturing pressure, and the injection flow rate is 50%–100% of the initial fracturing flow rate. Each repressurization stage lasts 15–30 minutes, and each borehole is repressurized 1–3 times.

[0106] Repressurization should be stopped when any of the following occurs: pressure stabilizes, repressurization duration reaches the design value, water emerges from adjacent observation wells, or signs of crack penetration appear. Repressurization should only be performed on boreholes that do not meet the requirements to avoid repeated drilling of boreholes that have met the requirements, thereby reducing water consumption and ineffective construction time.

[0107] Step S63: Control the timing of repressurization after the first hydraulic fracturing and the second hydraulic fracturing, respectively.

[0108] The re-fracturing following the first hydraulic fracturing was completed during the third longwall mining stage on the wide face, and the re-fracturing following the second hydraulic fracturing was completed during the fifth longwall mining stage on the narrow face. The re-fracturing following the first fracturing was used to ensure that the continuous directional pre-fracture barrier on the connecting roadway side achieved the designed continuity before the working face entered the connecting roadway influence zone; the re-fracturing following the second fracturing was used to ensure that the three-dimensional fracture network on the main roadway side achieved the designed coverage range before the working face entered the main roadway influence zone.

[0109] Step S7: Conduct phased fracturing qualification assessment and implement reinforcement for those that do not meet the standards.

[0110] Displacement, stress, and crack development were monitored in the protected sections of Lianxiang 2 and Daxiang 4, respectively. The first and second hydraulic fracturing were evaluated according to the corresponding judgment criteria. If the judgment criteria were not met at any stage, supplementary fracturing or re-fracturing was carried out in the non-compliant area, and monitoring and judgment were carried out again.

[0111] Step S71: Monitor and determine the effect of the first hydraulic fracturing.

[0112] After the first hydraulic fracturing, the movement of the two sidewalls, the stress of the surrounding rock, and the connectivity of fractures within the target fracturing layer on the side of the connecting roadway were monitored in the protected section of the connecting roadway 2. Monitoring sections were arranged along the roadway direction in the fracturing-affected zone, transition zone, and unfractured control zone, with 3 to 5 sections preferably set up. Displacement measuring points for the left sidewall, right sidewall, roof, and floor were set up in each section.

[0113] Continuous monitoring should be conducted for 3 days before fracturing (days are considered days); the stable displacement value in the last 24 hours before fracturing or the average stable value over 3 consecutive days should be used as the benchmark; continuous monitoring should be conducted for no less than 7 days after fracturing, and can be extended to the stable stage after the working face passes through the influence zone of the connecting roadway. A set of surface displacement monitoring stations can be set up every 30-50m within connecting roadway 2, with denser arrangements in geologically abnormal sections and areas of expected maximum deformation, with no fewer than 5 sets of stations.

[0114] The approach rate of the surrounding rock is calculated using the following formula: In the formula, v i Δu is the approach rate of the i-th measuring point, in mm / d; i Δt represents the displacement increment within the corresponding monitoring time; Δt is the time interval. When evaluating the overall effectiveness of the protected area, the maximum approach rate among all control monitoring points is taken as the control value.

[0115] Stress monitoring holes are arranged along the sidewall of connecting roadway 2 near the working face towards the critical layer. Each protected object has no fewer than three stress monitoring points, and at least one set of unfractured control monitoring points is also provided. The vertical stress relief rate is calculated using the following formula: In the formula, η is the vertical stress relief rate; σ0 is the stable vertical stress at the same measuring point before fracturing; and σ1 is the stable vertical stress at the same measuring point after fracturing.

[0116] Fracture observation wells are installed between adjacent fracturing wells, within the target critical layer, and along the fracture propagation direction. The continuous length of the fracture is determined by borehole inspection, spatial distribution of microseismic events, fracturing pressure response, and connectivity between adjacent wells. When two adjacent observation wells both observe clearly opened fracturing fractures or connectivity responses within the same target rock layer depth, the fracture segment between them is considered to be connected.

[0117] When the approach rate of the two sides of the connecting tunnel meets the preset displacement control index, and at least one of the stress relief degree or continuous fracture length meets the corresponding preset index, the first hydraulic fracturing is deemed qualified. Preferably, the approach rate of the two sides of the connecting tunnel not exceeding 1 mm / d is a mandatory index, and at the same time, at least one of the following requirements must be met: the vertical stress relief rate not less than 20% or the continuous fracture length in the key layer not less than 80% of the total designed fracturing section length.

[0118] Step S72: Monitor and determine the effect of the second hydraulic fracturing.

[0119] Following the second hydraulic fracturing, monitoring was conducted in the protected section of Main Roadway 4 to assess the approach of the roof and floor plates, lateral stress within the main roadway, and crack penetration at the root of the lateral cantilever beam. Monitoring sections were arranged in the mining-affected section of Main Roadway 4, before and after the affected boundary, and in the unfractured control section. The roof and floor plate approach rate was preferably evaluated based on continuous 72-hour monitoring results.

[0120] Stress monitoring points on the main roadway sides were set up in key strata, roadway sides, and stress concentration areas of the roof within the fracturing zone, and were simultaneously deployed with those in the unfracturing control area. The crack penetration length at the root of the lateral cantilever rock beam was determined by a combination of borehole inspection, microseismic positioning, and the connection response of adjacent fracturing boreholes.

[0121] The second hydraulic fracturing is deemed successful when the approach rate of the roof and floor of the main roadway meets the preset displacement control index, and at least one of the stress relief degree or crack penetration length meets the corresponding preset index. Preferably, the approach rate of the roof and floor of the main roadway is required to be no greater than 0.5 mm / d after continuous 72 hours of monitoring, and at least one of the following must be met: the vertical stress relief rate of the main roadway side is no less than 15%, or the crack penetration length at the root of the lateral cantilever rock beam is no less than 15 m.

[0122] Step S73: Perform supplementary fracturing or re-fracturing on areas that do not meet the standards.

[0123] If any stage fails to meet the corresponding judgment conditions, the area that does not meet the requirements for crack propagation or connectivity is determined based on the location of abnormal displacement, stress residual concentration area, crack interruption location, and microseismic event blank area. Supplementary fracturing is then performed in this area, or the corresponding borehole is subjected to re-fracturing as described in step S62. After the reinforcement treatment is completed, monitoring and judgment are performed again according to step S71 or step S72. Through steps S6 to S7, a closed-loop control process is formed, encompassing fracturing construction, effect monitoring, compliance judgment, and reinforcement for non-compliance, making the effects of the two fracturing operations measurable, measurable, and correctable.

[0124] The embodiments of the present invention have been described in detail above with reference to the accompanying drawings. However, the present invention is not limited thereto. Various changes that can be made within the scope of knowledge possessed by those skilled in the art without departing from the spirit of the present invention are within the protection scope of the claims of the present invention.

Claims

1. A method for protecting connecting roadways and main roadways in a dual-sequence fracturing operation of a knife-handle-type working face, wherein the knife-handle-type working face sequentially includes a wide working face and a narrow working face along the advancing direction, a connecting roadway is provided in the transition area between the wide working face and the narrow working face, and the narrow working face is adjacent to the main roadway of the mining area, characterized in that, Includes the following steps: Step S1: Determine the target fracturing layers on the connecting roadway side and the main roadway side based on the roof rock strata structure, and determine the mining influence boundary of the connecting roadway and the mining influence boundary of the main roadway based on the range of mining stress propagation, overburden structure characteristics and the range of deformation influence of the protected roadway. The influence boundary is checked and corrected by the surrounding rock displacement, roof delamination, borehole stress or microseismic monitoring data. Step S2: During the wide face mining stage, combine the mining parameters of the wide face with the entire construction cycle of the continuous pre-fracture barrier to determine the advance start distance of the first hydraulic fracturing; construct a fracturing borehole group between the connecting roadway and the wide face, control the fracturing section of all boreholes to enter the target fracturing layer on the connecting roadway side, and distribute the final boreholes at intervals along the dip of the working face. Step S3: When the distance between the working face mining line and the connecting roadway reaches the advance start distance, the first hydraulic fracturing is carried out on the target rock strata to form a continuous directional pre-fracturing barrier, which weakens or blocks the influence of the wide face mining stress on the connecting roadway. Step S4: In the narrow face mining stage after the first hydraulic fracturing is completed, the advance start distance of the second hydraulic fracturing is determined by combining the narrow face mining parameters and the mechanical properties of the lateral cantilever rock beam. At least two staggered fracturing borehole groups are constructed between the narrow face of the working face and the main roadway. The fracturing sections of all boreholes are controlled to enter the target fracturing layer on the side of the main roadway, and the final boreholes are distributed at intervals along the dip of the working face. Step S5: When the distance between the working face retreat line and the main roadway reaches the advance start distance, initiate the second hydraulic fracturing to form a three-dimensional fracture network in the target rock strata. Step S6: After each hydraulic fracturing operation, combine the fracturing operation response data, fracture detection results, and stress and displacement monitoring data of the protected roadway to comprehensively evaluate the fracture development and connectivity effect and the roadway protection effect; implement supplementary re-fracturing for areas that have not met the preset control indicators. Step S2 specifically includes the following steps: Step S21: Determine the advance start distance D1 for the first hydraulic fracturing: ， m ， ， In the formula, A represents the range of the boundary affected by mining in the connecting roadway, determined by the significant influence range of the mining advance support pressure, in meters (m); B represents the comprehensive safety margin for fracturing construction required to form a continuous directional pre-fracture barrier, in meters (m); k is the mining stress influence coefficient, taken as 0.30–0.45; H m V1 represents the average burial depth of the coal seam in meters; V2 represents the average advance speed of the working face during the wide-face mining stage in meters per day; T represents the comprehensive construction time required for all drilling, sealing, fracturing, and pressure stabilization observations to form the first continuous directional pre-fracture barrier in days; C represents the overlap length of fractures in adjacent fracturing boreholes in meters, used to ensure that the fractures formed by adjacent fracturing boreholes overlap with each other within the target fracturing layer on the connecting roadway side and form a continuous directional pre-fracture barrier. Step S22: Drill a set of fan-shaped fracturing boreholes from both the working face transport roadway and the working face track roadway towards the inner side of the working face width. The two sets of fan-shaped fracturing boreholes are located on the same straight line and are symmetrically arranged with respect to the working face centerline. The borehole azimuth angle is determined based on the design coordinates of each final borehole point in the plane, and the inclination angle θ is... n Used to control the vertical entry of the borehole into the corresponding target fracturing layer; the inclination angle θ of the fracturing borehole. n Determined according to the following formula: ； In the formula, H1 is the vertical distance from the bottom of the roadway where the fracturing borehole is located to the center of the fracturing section of the target fracturing layer on the connecting roadway side, in meters; L0 is the horizontal projection distance from the first hole termination point to the center point of the hole, in meters. ΔL is the horizontal projected distance between adjacent end points, in meters; n is the borehole number, n=1, 2, 3, ...

2. The method for protecting connecting roadways and main roadways by dual-sequence fracturing in a knife-handle-type working face according to claim 1, characterized in that, Step S1 specifically includes: Step S11: Based on the critical layer theory, identify the key hard and thick rock layers in the rock strata above the coal seam whose fracture can control the movement of the overburden or the transmission of mining stress. The key hard and thick rock layers that are close to the connecting roadway and bear the lateral support pressure transmission of the working face are identified as the target fracturing layers on the connecting roadway side. The key hard and thick rock layers that are close to the main roadway and form a lateral cantilever force transmission structure are identified as the target fracturing layers on the main roadway side. Step S12: Determine the influence boundaries of the connecting roadway and the main roadway based on the influence range of the advance support pressure, the ultimate span of the lateral cantilever rock beam, or the results of numerical simulation, and verify them through the results of surrounding rock displacement, roof delamination, borehole stress, or microseismic monitoring.

3. The method for protecting connecting roadways and main roadways by dual-sequence fracturing in a knife-handle-type working face according to claim 1, characterized in that, Step S3 specifically includes the following steps: Step S31: For the first group of fan-shaped fracturing boreholes arranged in the track roadway of the working face, divide them into staggered odd-numbered borehole groups and even-numbered borehole groups according to the borehole number. First, perform fracturing on the odd-numbered borehole groups, and then perform fracturing on the even-numbered borehole groups. After all the fracturing boreholes in the roadway have been fracturing, perform staggered fracturing on the fracturing boreholes in the transport roadway of the working face according to the same odd-even grouping method. Step S32: For a single borehole, perform segmented retreat fracturing sequentially on multiple fracturing sections that fall into the target fracturing layer on the connecting roadway side along the borehole depth direction to form a continuous strip-shaped pre-fracturing fracture. Step S33: When the working face track roadway is not suitable for setting up fracturing boreholes due to working space, surrounding rock stability, drilling trajectory accuracy, or production system avoidance conditions, supplementary boreholes are drilled from the connecting roadway to the target fracturing layer on the connecting roadway side, and the cracks formed by the supplementary boreholes are connected or overlapped with the continuous directional pre-splitting barrier.

4. The method for protecting connecting roadways and main roadways by dual-sequence fracturing in a knife-handle-type working face according to claim 1, characterized in that, Step S4 specifically includes the following steps: Step S41: Determine the advance start distance D2 for the second hydraulic fracturing: ； ； In the formula, L c The extent of the lateral mining impact boundary in the main roadway, in meters (m) and lengths (L). c The ultimate span of the lateral cantilever rock beam in the goaf is taken; v2 is the average advance speed of the working face during the narrow-face mining stage, in m / d; t is the comprehensive construction time required for all drilling, sealing, staged fracturing, and pressure stabilization monitoring to form the second three-dimensional fracture network, in days; S f The reserved width (in meters) for the second hydraulic fracturing fracture network coverage is used to compensate for errors in determining the fracture propagation range between the inner fan-shaped boreholes and the outer fracturing boreholes, the overlap between borehole groups, and the boundary of the lateral cantilever rock beam force transmission area. This ensures that the fracture network formed by the second hydraulic fracturing can cover the lateral cantilever rock beam force transmission area on the main roadway side, and is typically 5–20 meters wide. s σ is the safety factor for the ultimate span of the cantilever rock beam, taken as 1.1 to 1.3; h is the thickness of the target hard rock layer, in meters; σ t q represents the tensile strength of the rock stratum, in MPa; q represents the uniformly distributed load strength calculated based on a rock beam of unit width, in MPa. Step S42: Symmetrical fan-shaped boreholes are arranged in the working face transport roadway and the working face track roadway towards the inner side of the narrow face. Simultaneously, outer fracturing boreholes are arranged in the track roadway located on the side of the working face where the width abruptly changes, towards the outer side. The termination point of the outer fracturing boreholes is located outside the narrow face of the working face in planar projection and maintains a preset safe distance from the main roadway, ensuring that the fracturing sections of both the inner fan-shaped boreholes and the outer fracturing boreholes fall into the target fracturing layer on the main roadway side. At least two fracturing borehole groups are arranged at intervals along the working face advance direction or along the main roadway extension direction, with staggered spacing between adjacent fracturing borehole groups. The borehole azimuth is determined based on the design coordinates of each termination point in the plane, and the dip angle θ... n Used to control the vertical entry of the borehole into the corresponding target fracturing layer; the borehole inclination angle θ n Determined according to the following formula: ； In the formula, H2 is the vertical distance from the bottom of the roadway where the fracturing borehole is located to the center of the corresponding fracturing section of the target fracturing layer on the side of the main roadway, in meters; L0 is the horizontal projection distance from the first end point to the center point of the borehole, ΔL is the horizontal projection distance between adjacent end points, in meters; n is the borehole number, n=1, 2, 3, ...

5. The method for protecting connecting roadways and main roadways by dual-sequence fracturing in a knife-handle-type working face according to claim 4, characterized in that, The offset between two adjacent fracturing boreholes is 3 to 8 meters.

6. The method for protecting connecting roadways and main roadways by dual-sequence fracturing in a knife-handle-type working face according to claim 1, characterized in that, Step S5 specifically includes: First, segmented fracturing is carried out on the target rock strata drilled in the transport roadway on the other side of the working face abruptly changing the working face width, towards the inner side of the narrow working face. Then, on the track roadway on the side of the working face abruptly changing the width, segmented fracturing is carried out on the inward-facing fan-shaped borehole group drilled towards the inner side of the working face, forming a continuous horizontal main fracture zone along the dip of the working face within the target stratum. Finally, segmented fracturing is carried out on the target rock strata drilled in the track roadway towards the outer side of the narrow working face. Within the same fracturing borehole group, sequential fracturing is carried out in the direction from the goaf side to the solid coal side. The same fracturing process was performed on another parallel fracturing borehole.

7. The method for protecting connecting roadways and main roadways by dual-sequence fracturing in a knife-handle-type working face according to claim 1, characterized in that, Step S6 specifically includes the following steps: Step S61: Based on the fracturing pressure response, injection absorption volume, borehole inspection results, spatial distribution of microseismic events, and displacement or stress monitoring results of the protected roadway, determine the fracturing boreholes that are closed, have insufficient extension, or are not connected to adjacent fractures. Step S62: Only the identified defective fracturing boreholes are re-pressurized to allow fracturing fluid to re-enter the original fractures and activate the surrounding natural and secondary fractures. Step S63: The re-pressurization after the first hydraulic fracturing is completed during the wide face mining stage, and the re-pressurization after the second hydraulic fracturing is completed during the narrow face mining stage.

8. The method for protecting connecting roadways and main roadways by dual-sequence fracturing in a knife-handle-type working face according to claim 1, characterized in that, It also includes step S7, determining the success of staged fracturing; specifically, it includes the following steps: Step S71: After the first hydraulic fracturing, monitor the approach of the two sides, the stress of the surrounding rock, and the fracture connectivity within the target fracturing layer on the side of the connecting roadway in the protected section; when the approach rate of the two sides of the connecting roadway meets the preset displacement control index, and at least one of the stress relief degree or the length of the continuous fracture meets the corresponding preset index, the first hydraulic fracturing is deemed qualified. Step S72: After the second hydraulic fracturing, monitor the approach of the roof and floor plates, the lateral stress of the main roadway, and the crack penetration at the root of the lateral cantilever rock beam in the protected section of the main roadway. When the approach rate of the roof and floor plates of the main roadway meets the preset displacement control index, and at least one of the stress relief degree or crack penetration length meets the corresponding preset index, the second hydraulic fracturing is deemed qualified. Step S73: If the corresponding judgment conditions are not met at any stage, supplementary fracturing or re-fracturing shall be carried out on the area that has not met the requirements for crack propagation or connection.

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