Gob-side entry retaining method with cooperation of hydraulic fracturing and dense drill hole blocking roof cutting

Abstract

The invention discloses a gob-side entry retaining method with cooperation of hydraulic fracturing and dense drill hole blocking roof cutting, and relates to the technical field of coal mine roadway supporting and mining. Effective control over a hydraulic fracture expansion path is achieved, and cracks are prevented from invading an anchor cable anchoring area. A guide blocking structure and a pressure relief fracturing structure are designed and constructed above a top plate of a roadway of the reserved roadway along the trend of the working face; the pressure relief fracturing structure comprises a row of fracturing drill holes, the guide blocking structure comprises a row of dense drill holes, and the dense drill holes are located between the row of fracturing drill holes and a top plate anchoring area of the reserved roadway roadway. The roof fracture position is effectively and controllably limited to the preset position, smooth gob-side entry retaining is guaranteed, and good engineering applicability and economic benefits are achieved.

Description

Technical Field

[0001] This invention relates to the field of coal mine roadway support and mining technology, and in particular to a method for cutting the roof and leaving the roadway in scenarios where the anchor cable anchoring zone is located in a pre-hydraulic fracturing hard rock layer. Background Technology

[0002] Gob-side entry technology is a key technology for achieving pillarless continuous mining and improving resource recovery. However, in practical applications, especially under special geological conditions where the anchoring zone is located in hard rock strata, when using hydraulic fracturing to cut the roof for gob-side entry, the propagation path of hydraulic fractures is difficult to control precisely, easily spreading into the anchoring zone, affecting the support effect, and even causing instability of the anchoring zone, threatening roadway safety. With the introduction of dense borehole technology to form a barrier zone to prevent hydraulic fractures from intruding into the anchoring zone, conventional dense borehole designs typically use a hole spacing of 20-30 cm, resulting in a large drilling workload and high cost, especially in hard rock strata where construction is even more difficult, limiting its widespread application.

[0003] Therefore, there is an urgent need for a method that can effectively control the direction of hydraulic fracture propagation, reduce the amount of dense drilling and lower costs, and is suitable for use in anchor cable anchoring zones located in hard rock strata under pre-hydraulic fracturing. Summary of the Invention

[0004] To address the above problems, this invention proposes a method for cutting the roof and leaving a roadway along the goaf using hydraulic fracturing combined with dense drilling to block the roof cut. This method is suitable for roof cut and roadway leaving operations in scenarios where the anchor cable anchorage zone is located in hard rock strata that have undergone pre-hydraulic fracturing. This allows for effective control of the hydraulic fracture propagation path, preventing fractures from intruding into the anchor cable anchorage zone, and improving the stability of the surrounding rock and mining efficiency.

[0005] The technical solution of the present invention is as follows: above the top plate of the reserved roadway 2, along the working face, a guide barrier structure and a pressure relief and cracking structure are designed and constructed. The pressure relief and fracturing structure includes a row of fracturing boreholes 8, the axis of which points to the side and above the pre-mining area 4 of the working face, and the terminal penetrates the roof rock layer 5 of the roadway and terminates in the pre-fracturing target area within the hard rock layer 6. The guiding barrier structure includes a row of densely drilled holes 3, which are located between a row of fracturing holes 8 and the roof anchoring area 1 of the reserved roadway 2. The axis of the densely drilled holes 3 also points to the side above the pre-mined goaf area 4 of the working face, and its length also penetrates the roof rock layer 5 of the roadway and extends into the hard rock layer 6. The elevation angle of the fracturing borehole 8 β The elevation angle is smaller than that of the densely drilled holes 3. αThis allows the fracturing borehole 8 to be separated from the anchoring zone 1 and the hydraulically generated crack 7 by a row of fracturing boreholes 8, forming a barrier zone.

[0006] The guiding barrier structure and the pressure-relieving fracturing structure can be spatially staggered to enhance the control over fracture propagation. Both boreholes terminate at the top of the hard rock stratum 6. The borehole parameters can be optimized according to the following engineering requirements to achieve different control objectives.

[0007] Center distance between adjacent fracturing boreholes 8 L p Based on the different requirements of the project regarding the continuity and isolation strength of the main crack, the design can be carried out by selecting any of the following control conditions: Scenario 1: When the priority requirement is to form a continuous, through-cut seam, the primary engineering objective is to create a continuous, through-cut seam, ensuring that the center-to-center distance between adjacent fracturing boreholes 8 is [missing information]. L p The value of satisfies L p <2 R p , R p The radius of influence of a single fracturing borehole is defined to ensure that the fracture influence zones of adjacent fracturing boreholes overlap, which is conducive to the formation of a continuous fracture. Furthermore, the vertical distance between the fracturing borehole 8 and the plane containing the row of closely spaced boreholes 3 is defined. L k satisfy L k > L p - R p + a , a As a design margin, a range of 0.5~0.8m is adopted; Scenario 2: When the priority is to maximize the directional barrier structure, the primary engineering objective is to maximize the directional barrier structure and prevent any cracks from entering the anchorage zone. The center-to-center distance between adjacent fracturing boreholes 8 should be [missing information]. L p The value of satisfies L p >2 R p , R p The radius of influence of a single fracturing borehole, and the vertical distance between the fracturing location of fracturing borehole 8 and the plane containing a row of closely spaced boreholes 3. L k satisfy L k < L p -R p - a This design increases the spacing between adjacent fracturing holes, reducing mutual interference between fractures. At the same time, by ensuring that the distance between the fracturing hole and the densely packed holes is less than the difference between the influence radius and the spacing, it ensures that the high stress field generated by fracturing can effectively act on the densely packed hole group, thereby forming a high-strength, continuous stress barrier and enhancing the isolation effect.

[0008] The angle between the axis of the densely drilled borehole 3 and the vertical line α The angle is 15° to 25°. The dense boreholes 3 can be empty holes or ordinary boreholes without fracturing operations. The dense boreholes 3 serve as a barrier to the propagation of the hydraulic fractures 7. They are used to induce the hydraulic fractures 7 to deflect, become blunt, or terminate when they propagate to this area through the stress interference effect between the holes. This method maintains the ability to control the propagation of the fractures.

[0009] The dense boreholes 3 and the fracturing boreholes 8 can be arranged in a reasonable order of construction. In order to ensure the effectiveness of hydraulic fracturing operations, the dense boreholes 3 must be completed before hydraulic fracturing operations are carried out, so that the dense boreholes 3 have formed an effective barrier structure against the propagation of hydraulic fractures.

[0010] Follow these steps for construction: S1: Conduct a detailed geological survey and mechanical parameter determination of the reserved tunnel 2 and its overlying hard rock layer 6, determine the stratum, thickness and mechanical parameters of the overlying hard rock layer 6 of the reserved tunnel 2, including key information such as strength and crack propagation trend, and determine the range of the tunnel roof anchorage zone 1, so as to provide a scientific basis for subsequent drilling layout and fracturing scheme. S2: Based on the geomechanical assessment results of step S1, a guide barrier structure and a pressure relief and fracturing structure are designed and constructed above the roof of the reserved roadway 2 and along the working face, and all the dense boreholes 3 and fracturing boreholes 8 are constructed. S3. Hydraulic fracturing operation is carried out in the fracturing borehole 8. The guiding and blocking structure guides and blocks the propagation of hydraulic fractures, causing the hydraulic main fracture 7 generated by fracturing to expand and connect with adjacent boreholes, forming the required pressure relief channel, while preventing the main fracture from intruding into the anchor cable anchoring zone 1; the hydraulic fracturing operation is carried out in a stable area outside the advanced mining influence zone of the working face. S4. During and after fracturing, borehole inspection was used to monitor the development morphology of fracture 7, verify the isolation effect of the guide barrier structure and the propagation path and connectivity of the main fracture, evaluate the propagation path of the main fracture and its connectivity with adjacent boreholes, and thus optimize and adjust subsequent construction parameters.

[0011] In this invention, the densely drilled boreholes are used directly for roof cutting and also serve as a barrier structure between the hydraulic fracturing boreholes and the anchoring zone of the anchor cables, limiting the extension of hydraulic fractures into the anchoring zone and thus effectively protecting the stability of the anchoring zone. The hydraulic fracturing boreholes, by controlling their elevation angle and spacing, guide the fractures to extend towards the goaf while also connecting adjacent hydraulic fracturing boreholes or the densely drilled boreholes. This method leverages the wide fracturing range of hydraulic fracturing technology while avoiding damage to the anchoring zone of the roadway roof anchor cables. It also effectively reduces the spacing of the densely drilled boreholes, decreasing the amount of drilling work, thereby effectively and controllably limiting the roof fracture location to a predetermined position, ensuring the smooth progress of goaf-side roadway retention. It has good engineering applicability and economic benefits.

[0012] This invention achieves the following beneficial effects through a synergistic control design of dense drilling and hydraulic fracturing: 1. By using dense drilling to guide the cracks to extend into the goaf, and inducing the cracks to deflect, blunt or terminate when necessary, the cracks are prevented from intruding into the anchor cable anchorage zone, thus improving the support effect and the safety of the surrounding rock of the roadway. Second, under the premise of prioritizing the isolation strength, the density of dense drilling can be reasonably reduced, thereby effectively reducing the amount of construction work and costs, and has broad application prospects. Attached Figure Description

[0013] Figure 1 This is a schematic diagram illustrating an application scenario of the method of the present invention.

[0014] Figure 2 This is a diagram showing the spatial arrangement of dense drilling and fracturing drilling in the method of this invention.

[0015] In the diagram: 1-Anchoring zone; 2-Leaving roadway; 3-Dense boreholes; 4-Pre-mining goaf area of ​​the working face; 5-Roof strata of the roadway; 6-Hard rock strata; 7-Hydraulic fracture; 8-Fracturing borehole. Detailed Implementation

[0016] To clearly illustrate the technical features of this patent, the following detailed description is provided through specific embodiments and in conjunction with the accompanying drawings.

[0017] The No. 2 coal seam being mined in a coal mine in central China has an average thickness of 4.45m and a burial depth of approximately 450m. It is being mined using fully mechanized mining methods, with the entire height being mined in a single pass. The working face is planned to employ goaf retention technology, preserving a section of the transport roadway as the return airway for the next working face. The immediate roof of the coal seam is 3.32m thick sandy mudstone, while the main roof is approximately 7.55m thick medium-grained sandstone with a uniaxial compressive strength as high as 45MPa. The anchoring zone for the roadway roof is located within the aforementioned medium-grained sandstone.

[0018] The method for leaving a roadway along the goaf using hydraulic fracturing combined with dense drilling to block roof cutting, as proposed in this invention, comprises the following steps: S1: Through the collection of supplementary geological data, downhole core drilling, and indoor rock mechanics tests, the mechanical parameters of fine sandstone in hard rock strata were obtained: compressive strength 45 MPa, tensile strength 3.5 MPa, elastic modulus 28 GPa, and Poisson's ratio 0.22. Using FLAC3D numerical simulation software, a geomechanical model was constructed to simulate the stress distribution in the tunnel and the roof fracture, determining the range of the anchorage zone (2.0-6.0 m above the roof) and providing a basis for borehole parameter design.

[0019] S2: Based on the assessment results of S1, a row of dense boreholes and a row of fracturing boreholes were drilled above the roadway roof along the working face direction: The angle between the axis of the dense borehole and the vertical line is 90°- α =15°, diameter 48mm, borehole center spacing l =0.4m. The borehole depth penetrated the immediate top and entered the top of the medium-grained sandstone. This row of boreholes will not be used for further drilling and will remain as empty holes.

[0020] The angle between the axis of the fracturing borehole and the vertical line is 90°- β =30°, diameter 48mm. The calculated radius of influence of a single fracturing borehole. R p Approximately 2.5m. To form an effective pressure relief cut, geometric control condition Scenario 1 is selected: ensuring the center distance of the fracturing boreholes. L p satisfy L p <2 R p .therefore, L p The design margin is set at 4.5m (less than 5m). In this example, the design margin is... a Take 0.5m, L k ≈3 m, so that the vertical distance between the fracturing location of the fracturing borehole 8 and the plane containing the row of closely spaced boreholes 3 is approximately 3 m. L k That is, draw a perpendicular line segment from the fracturing point to the plane containing a row of closely spaced boreholes, and the length of this perpendicular line segment is... L k >4.5m-2.5m+0.5m=2.5m.

[0021] S3: After drilling is completed, hydraulic fracturing operations are carried out in a stable area outside the advance mining influence zone of the working face. A pump station with a rated pressure of 40 MPa is used, with the pump pressure stabilized at 32 MPa and the water injection flow rate at 0.25 m³ / min. The fracturing location is 1.0 m from the bottom of the borehole. High-pressure water initiates fracturing in the hard rock strata and forms the main fracture. When the fracture tip extends to the densely drilled area, it deflects, becomes blunt, or terminates, thereby preventing the fracture from intruding into the anchoring zone. The interconnection of fractures between adjacent fracturing boreholes forms a pressure relief fracture zone.

[0022] S4: Post-fracturing inspection showed that all fracturing boreholes had longitudinal cracks pointing towards the goaf and were interconnected; some densely packed boreholes had transverse cracks in their walls, but all cracks were blocked below the lower boundary of the anchoring zone, and the barrier was effective.

[0023] There are many specific ways to implement this invention. The above description is only a preferred embodiment of this invention. It should be noted that for those skilled in the art, several improvements can be made without departing from the principle of this invention, and these improvements should also be considered within the scope of protection of this invention.

Claims

1. A method for preventing and retaining roadways along the goaf by hydraulic fracturing combined with dense drilling to block roof cutting, characterized in that, Above the top slab of the reserved roadway (2), along the working face, a guide barrier structure and a pressure relief and cracking structure are designed and constructed; The pressure relief and fracturing structure includes a row of fracturing boreholes (8), the axis of which points to the side and above the pre-mining area (4) of the working face, and the terminal penetrates the roof rock layer (5) of the roadway and terminates in the pre-fracturing target area in the hard rock layer (6). The guiding barrier structure includes a row of dense boreholes (3), which are located between a row of fracturing boreholes (8) and the roof anchorage area (1) of the reserved roadway (2). The axis of the dense boreholes (3) also points to the side above the pre-mining area (4) of the working face, and its length also penetrates the roof rock layer (5) of the roadway and extends into the hard rock layer (6). The elevation angle of the fracturing borehole (8) β smaller than the elevation angle of the dense borehole (3) α This allows the hydraulically fractured borehole (8) to separate the anchorage zone (1) from the hydraulically fractured crack (7) through a row of hydraulically fractured boreholes (8), forming a barrier zone.

2. The method for leaving a roadway along the goaf by hydraulic fracturing combined with dense drilling to block the roof cutting, as described in claim 1, is characterized in that... Center distance between adjacent fracturing boreholes (8) L p Based on the different requirements of the project regarding the continuity and isolation strength of the main crack, the design can be carried out by selecting any of the following control conditions: Scenario 1: When the priority requirement is to form a continuous, through-cut, the primary engineering objective is to form a continuous, through-cut, such that the center-to-center distance between adjacent fracturing boreholes (8) is minimized. L p The value of satisfies L p < 2 R p , R p The radius of influence of a single fracturing borehole is used to ensure that the fracture influence zones of adjacent fracturing boreholes overlap, which is conducive to the formation of a continuous fracture. The vertical distance between the fracturing borehole (8) and the plane containing a row of densely packed boreholes (3) is also considered. L k satisfy L k > L p - R p + a , a As a design margin, a range of 0.5~0.8m is adopted; Scenario 2: When the priority requirement is to maximize the directional barrier structure, the primary engineering objective is to maximize the directional barrier structure and prevent any cracks from entering the anchorage zone. The center-to-center distance between adjacent fracturing boreholes (8) should be minimized. L p The value of satisfies L p > 2 R p , R p The radius of influence of a single fracturing borehole, and the vertical distance between the fracturing borehole (8) and the plane containing a row of closely spaced boreholes (3). L k satisfy L k < L p - R p - a .

3. The method for leaving a roadway along the goaf by hydraulic fracturing combined with dense drilling to block the roof cutting, as described in claim 2, is characterized in that... The angle between the axis of the densely drilled boreholes (3) and the vertical line α The angle is 15° to 25°, and the dense borehole (3) can be an empty hole or a normal borehole without fracturing operation.

4. The method for leaving a roadway along the goaf by hydraulic fracturing combined with dense drilling to block the roof cutting, as described in claim 1, is characterized in that... Follow these steps for construction: S1: Determine the stratigraphic position, thickness and mechanical parameters of the overlying hard rock layer (6) of the reserved tunnel (2), and determine the range of the tunnel roof anchorage zone (1); S2: Based on the geomechanical assessment results of step S1, a guide barrier structure and a pressure relief and fracturing structure are designed and constructed above the roof of the reserved roadway (2) along the working face, and all the dense boreholes (3) and fracturing boreholes (8) are constructed. S3. Hydraulic fracturing operation is carried out in the fracturing borehole (8). The guiding and blocking structure is used to guide and block the expansion of hydraulic fractures, so that the hydraulic main fracture (7) generated by fracturing expands and connects with adjacent boreholes to form the required pressure relief channel, while preventing the main fracture from intruding into the anchor cable anchorage area (1). The hydraulic fracturing operation is carried out in a stable area outside the advance mining influence area of ​​the working face. S4. During and after fracturing, the development morphology of the crack (7) is monitored by borehole inspection to verify the isolation effect of the guide barrier structure and the propagation path and connectivity of the main crack, and to evaluate the propagation path of the main crack and its connectivity with adjacent boreholes, so as to optimize and adjust the subsequent construction parameters.

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