A method for reducing the first roof pressure step distance of a fully-mechanized caving face and improving top coal recovery rate

Abstract

This application discloses a method for reducing the initial pressure step distance of the roof in a fully mechanized longwall mining face and improving the top coal recovery rate. It belongs to the field of coal mining, and its key technical points are: specifically, in the initial mining stage, a series of horizontal boreholes are drilled along the working face direction into the hard rock strata using a kilometer-level drilling rig in the drilling site. Then, hydraulic fracturing is carried out in the horizontal boreholes to cut off the hard roof, thereby effectively shortening the initial pressure step distance and improving the top coal recovery rate. It includes the following steps: S1, collecting engineering geological data of the cut-off and working face, such as: cut-off tunneling operation procedures and support cross-section diagrams, longwall mining operation procedures, mining engineering plan, and surrounding borehole columnar diagrams; S2, determining the strata that need to be fractured in the roof; S3, designing kilometer-level horizontal borehole and hydraulic fracturing parameters; S4, setting up a drilling site near the cut-off, installing a kilometer-level drilling rig, and then drilling and conducting kilometer-level horizontal borehole and hydraulic fracturing on site; S5, after hydraulic fracturing is completed, starting to install the support and simultaneously removing the anchor at the working face, then starting longwall mining, and specifically analyzing the shortening distance of the initial pressure step of the fully mechanized longwall face after hydraulic fracturing, as well as the amount of coal mined in multiple longwalls.

Description

Technical Field

[0001] This invention relates to the field of coal mining, and more specifically, to a method for reducing the initial pressure step distance of the roof in a fully mechanized longwall face and improving the top coal recovery rate. Background Technology

[0002] After the initial excavation of the coal face, the initial mining stage begins. During this stage, if the roof is hard and unlikely to collapse, a large initial pressure step distance can lead to a large area of ​​suspended roof in the goaf. When this suspended roof reaches a certain length, it can suddenly fracture, causing a large-scale or concentrated roof collapse, generating a hurricane and posing a significant safety hazard to workers. Simultaneously, it can cause a sudden increase in gas emissions, leading to localized gas exceedances and greatly increasing the risk of rockbursts and gas explosions. Furthermore, a large initial pressure step distance in a fully mechanized longwall face results in significant top coal waste, affecting recovery rates. Therefore, shortening the initial pressure step distance in a fully mechanized longwall face is of great importance.

[0003] Currently, there are three main methods to shorten the initial pressing step distance in a fully mechanized longwall mining face:

[0004] (1) Deep-hole pre-splitting blasting method, which involves drilling holes in the roof rock strata and placing explosives in the holes to use the explosive energy to induce rock strata fracturing in order to reduce roof pressure. This method itself has certain safety risks, which may cause the top coal to fall directly, causing accidents. It is also prone to producing a large amount of toxic and harmful gases, causing personnel poisoning and causing great environmental pollution, which does not meet the requirements of green mine construction.

[0005] (2) Vertical drilling and hydraulic fracturing method: This method involves drilling vertical holes in the pre-cut section and applying high-pressure water to create fissures in hard rock strata, thereby reducing pressure on the roof. Traditional construction of this method requires vertical drilling through soft rock strata to reach key hard rock strata, resulting in a large workload and low efficiency.

[0006] (3) Liquid carbon dioxide pre-splitting method: This method is safe, environmentally friendly, does not pollute the surrounding environment, has a low vibration velocity, and the transportation, storage and use of liquid carbon dioxide do not require approval. However, the cracking effect produced by this method is generally average and the cost is high.

[0007] To address the above issues, this invention proposes combining a kilometer-level drilling rig with hydraulic fracturing. The kilometer-level drilling rig drills horizontal holes in hard rock strata and then performs hydraulic fracturing on these holes, thereby shortening the initial fracturing step distance and improving top coal recovery. Because the kilometer-level drilling rig eliminates the need for pre-drilling and offers high efficiency, this method effectively reduces the workload, improves fracturing efficiency, and aligns with the principles of green and environmentally friendly mine construction. Summary of the Invention

[0008] This invention provides a method for reducing the initial pressure step distance of the roof in a fully mechanized longwall mining face and improving the top coal recovery rate, aiming to solve the problems of existing technologies. Specifically, in the initial mining stage, a series of horizontal boreholes are drilled into the hard rock strata along the working face direction using a kilometer-level drilling rig in the drilling site. Then, hydraulic fracturing is carried out in the horizontal boreholes to cut off the hard roof, thereby effectively shortening the initial pressure step distance and improving the top coal recovery rate.

[0009] A method for reducing the initial pressure step distance of the roof in a fully mechanized longwall mining face and improving the top coal recovery rate includes the following steps:

[0010] S1, collect engineering geological data of the working face and the cut-in hole;

[0011] S2, determine the layer and thickness of the top plate that needs to be fracturing;

[0012] S3, Analyze and design parameters for kilometer-level horizontal borehole and hydraulic fracturing;

[0013] S4, set up a drilling site near the cut-out, install a kilometer drilling rig in the drilling site, then drill holes and carry out kilometer horizontal drilling and water pressure fracturing on site;

[0014] S5. After hydraulic fracturing is completed, the support is installed and the anchor is removed at the working face. Then mining begins. The reduction distance of the initial pressure step of the fully mechanized longwall face after hydraulic fracturing and the amount of top coal recovered are specifically analyzed.

[0015] Furthermore, the basic data for step S1 includes: the opening and tunneling operation procedure and support cross-section diagram, the longwall mining operation procedure, the mining engineering plan, the surrounding borehole columnar section, the geological report, and the mine preliminary design specification.

[0016] Furthermore, step S2 includes: observing the top plate at certain intervals within the cut hole, analyzing the lithology and thickness of the top plate, and determining the hard, thick rock layers that need to be fractured.

[0017] Step S2 specifically includes the following sub-steps:

[0018] S2.1, Conduct geological exploration, including geological profiling, core drilling, etc. Through geological feature analysis, potential target strata that need to be fracturing can be identified;

[0019] S2.2 Core Analysis: Analyze the physical and mechanical properties and geological and mineralogical characteristics of the collected core samples;

[0020] S2.3 Based on the geological exploration results and the key layer theory, determine the main key layer and sub-key layer in the overlying strata, and then, based on the mining pressure manifestation pattern of previous working faces, determine the specific strata that need to be fractured.

[0021] Furthermore, step S3 includes: the drilling site location is the location of the two mining roadways on both sides corresponding to the horizontal borehole for the planned roof cutting; the interval between the horizontal boreholes is set in the range of 0 to 5m.

[0022] Furthermore, step S3 includes the following sub-steps:

[0023] S3.1, Determine the number and location of holes according to the thickness of the hard rock layer;

[0024] When the thickness of the hard top plate being cut is less than 5m, one borehole is drilled in the top plate at 1 / 2 of the rock layer; when the thickness of the hard top plate being cut is 5-10m, two boreholes are drilled in the top plate, each at 1 / 3 of the distance from the upper and lower boundaries of the rock layer; when the thickness of the hard top plate being cut is 10-15m, three boreholes are drilled in the top plate, each at 1 / 4, 1 / 2, and 3 / 4 of the distance from the boundary of the rock layer, respectively.

[0025] S3.2, through laboratory or field tests, test the compressive and tensile strength of the required thick rock layer to obtain the required water injection pressure;

[0026] S3.3, when the vertical distance between two boreholes is between 1.6 and 5 m, according to the formula:

[0027] x=cotαh

[0028] It can be seen that when the drilling layer is located in conglomerate, limestone, etc., the horizontal offset x is taken as 2.8 to 8.5 m; when the drilling layer is located in fine-grained sandstone, medium-grained sandstone, etc., the horizontal offset x is taken as 1.6 to 5 m; when the drilling layer is located in coarse-grained sandstone, siltstone, etc., the horizontal offset x is taken as 0.9 to 2.9 m.

[0029] Where x is the horizontal offset between the two boreholes, h is the vertical distance between the two boreholes, and α is the collapse angle. When the uniaxial compressive strength of the rock is greater than 60 MPa, i.e., when the required drilling layer is located in conglomerate, limestone, etc., the collapse angle α = 30°; when the uniaxial compressive strength of the rock is between 50 and 60 MPa, i.e., when the required drilling layer is located in fine-grained sandstone, medium-grained sandstone, etc., the collapse angle α = 45°; when the uniaxial compressive strength of the rock is between 40 and 50 MPa, i.e., when the required drilling layer is located in coarse-grained sandstone, siltstone, etc., the collapse angle α = 60°.

[0030] Furthermore, step S4 includes the following sub-steps:

[0031] S4.1, Set up a drilling site near the cut-out hole and install a kilometer-long drilling rig in the drilling site;

[0032] S4.2, Install a high-pressure water injection pump in the drilling site, drill a hole into the hard rock layer to be fractured, and then start water pressure fracturing. Each hole adopts the retreating multiple fracturing method, and fracturing is carried out every 10 to 30 meters. Each fracturing time is not less than 30 minutes.

[0033] S4.3 During fracturing, the direction of the hydraulic head must be perpendicular to the roadway roof; as the fracturing time increases, the water pressure gradually rises and reaches its maximum value.

[0034] S4.4, due to the high pressure, new cracks appeared in the fractured rock layer and began to expand, and the water pressure decreased slightly; the cracks continued to spread in all directions, and then the water pressure decreased significantly, at which point the fracturing was completed.

[0035] Furthermore, step S5 includes:

[0036] After hydraulic fracturing is completed, the support is installed and the anchor is simultaneously removed from the working face. After mining begins, the support resistance is monitored during mining, the initial pressure step distance of the working face after fracturing is analyzed, and compared with the initial pressure step distance of adjacent working faces to obtain the shortened distance of the initial pressure step distance and the amount of coal mined in additional mining.

[0037] The beneficial effects of this application are as follows:

[0038] First, the basic concept of this application is: This application proposes a method to reduce the initial pressure step distance of the roof in a fully mechanized longwall mining face and improve the top coal recovery rate. Specifically, in the initial mining stage, a number of sets of horizontal boreholes are drilled into the hard rock strata along the working face direction using a kilometer-long drilling rig in the drilling site. Then, water pressure fracturing is carried out in the horizontal boreholes. This method effectively shortens the initial pressure step distance and improves the top coal recovery rate.

[0039] Secondly, this invention utilizes a kilometer-long borehole near the coal face during the initial coal mining stage to simultaneously induce hydraulic fracturing in two or more working faces. This effectively reduces the initial pressure step distance and improves coal recovery rate. Simultaneously, it reduces the likelihood of large-area roof overhangs in the goaf, decreasing roof pressure and enhancing safety.

[0040] Third, this application breaks with the conventional method of vertical hydraulic fracturing and roof cutting in the early stages of coal mining. Traditional vertical roof cutting requires drilling through soft rock strata to reach the critical hard rock strata, which is particularly labor-intensive and economically inefficient when the cutting height exceeds 30m. The proposed method uses a kilometer-long drilling rig near the cut-out point to directly reach the critical hard rock strata by drilling kilometer-long directional horizontal boreholes, thereby reducing the amount of drilling work.

[0041] Fourth, in the initial stage of coal mining, holes are drilled near the coal face at the cut-out point. When the drilled holes penetrate the roof, the roof collapses at different angles due to its hardness. By utilizing the different collapse angles in different rock strata, the horizontal offset between the two drill holes is calculated using the formula x = cotαh (where x is the horizontal offset between the two drill holes, h is the vertical distance between the two drill holes, and α is the collapse angle). This effectively reduces the initial pressure step distance and improves the top coal recovery rate. Attached Figure Description

[0042] Figure 1 This is a design schematic diagram of Example 1.

[0043] Figure 2 yes Figure 1 Section I-I in the diagram.

[0044] Figure 3 yes Figure 1 Section II-II in the diagram.

[0045] Figure 4 This is a design schematic diagram of Example 2.

[0046] Figure 5 yes Figure 4 Section I-I in the diagram.

[0047] Figure 6 yes Figure 4 Section II-II in the diagram. Detailed Implementation

[0048] To better understand the above-mentioned objectives, features, and advantages of the present invention, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be noted that, unless otherwise specified, the embodiments and features described in these embodiments can be combined with each other.

[0049] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein, and therefore the scope of protection of the invention is not limited to the specific embodiments disclosed below.

[0050] The specific embodiments of the present invention will be further described below with reference to the accompanying drawings and specific examples.

[0051] Example 1: The 10102 longwall mining face of a certain mine

[0052] As shown in the figure, the method for reducing the initial pressure step distance of the roof in a fully mechanized longwall mining face and improving the top coal recovery rate provided by the present invention is carried out according to the following steps:

[0053] Step I: Collect on-site information including the opening cut tunneling operation procedures and support cross-section diagrams, longwall mining operation procedures, mining engineering plan, surrounding borehole columnar section diagrams, geological report, and preliminary mine design specifications. Based on the collected mine data, it is known that the No. 10 coal seam has an average thickness of 6.5m, employs fully mechanized top-coal caving mining technology, with a mining height of 3.2m, a coal release height of 3.3m, and an average mining-to-release ratio of 1:1. Additionally, the 10102 opening cut is driven along the bottom, 150m long, 7.5m wide, and 3.2m high. Its roof consists of, in sequence: top coal (3.3m thick), siltstone (2.7m thick), medium sandstone (4.9m thick), siltstone (5.8m thick), mudstone (0.8m thick), limestone (5.0m thick), and mudstone (2.1m thick).

[0054] Step II: Use a viewing instrument to inspect the roof at different locations in the roadways on both sides of the working face, and record the lithology of the roof at different locations. Specific analysis is needed to determine the layers and thicknesses that require hydraulic fracturing. Based on the borehole columnar section and previous mining conditions, the No. 10 coal seam has a mining height of 3.2m and a roof caving height of 3.3m. It is determined that the key hard rock layers are siltstone 10.9m from the roadway roof with a thickness of 5.8m and limestone 17.5m from the roadway roof with a thickness of 5.0m.

[0055] Step III: The drilling site is located in the chamber near the pre-cut hole. The interval between horizontal boreholes can be set within the range of 0-5m. Boreholes 1 and 2 are to be drilled in siltstone at distances of 12.8m and 14.7m from the tunnel roof, respectively, with a collapse angle α = 60°. Borehole 3 is to be drilled in limestone at distances of 20.0m from the tunnel roof, with a collapse angle α = 45°. The vertical distance between boreholes 1 and 2 is calculated to be 1.9m, and the vertical distance between boreholes 2 and 3 is 5.3m. The horizontal offset between boreholes 1 and 2 is calculated to be 1.1m, and the horizontal offset between boreholes 2 and 3 is 3.6m. The required design hydraulic fracturing parameters need to be determined by comprehensively considering the thickness of the hard rock layer and the uniaxial compressive strength results.

[0056] Step IV: Set up a drilling site near the pre-cut hole, install a kilometer-long drilling rig within the drilling site, and then install a high-pressure water injection pump within the drilling site. Afterward, begin hydraulic fracturing. The initial part of the borehole trajectory is a curve, while the main part is an approximately horizontal straight line.

[0057] Step V: After hydraulic fracturing is completed, the support system is installed while the anchor removal operation is carried out on the working face. After all preparations are completed, mining begins. During mining, the support resistance is monitored, and the initial pressure step distance of the working face after fracturing is analyzed. The actual fracturing effect is further analyzed. Through monitoring and calculation, it is found that the initial pressure step distance was reduced from 72.5m before hydraulic fracturing to 54.0m, a reduction of 18.5m. It is calculated that hydraulic fracturing resulted in the recovery of an additional 9712.5 tons of coal.

[0058] Example 2: The 100301 fully mechanized longwall face of a certain No. 2 mine

[0059] As shown in the figure, the method for reducing the initial pressure step distance of the roof in a fully mechanized longwall mining face and improving the top coal recovery rate provided by the present invention is carried out according to the following steps:

[0060] Step I: Collect on-site information including the opening cut tunneling operation procedures and support cross-section diagrams, longwall mining operation procedures, mining engineering plan, surrounding borehole columnar section diagrams, geological report, and preliminary mine design specifications. Based on the collected mine data, the 100301 working face is 175m long. The No. 10 coal seam has an average thickness of 5.9m, employing a fully mechanized top-coal caving mining technique with a mining height of 3.1m, a coal release height of 2.8m, and an average mining-to-release ratio of 1.1:1. Furthermore, the 100301 opening cut is driven along the bottom, 175m long, 5.0m wide, and 3.1m high. Its roof consists of, in sequence: top coal (2.8m thick), siltstone (5.9m thick), mudstone (2.7m thick), limestone (2.1m thick), sandy mudstone (4.4m thick), limestone (7.0m thick), and sandy mudstone (2.5m thick).

[0061] Step II: Use a viewing instrument to inspect the roof at different locations in the roadways on both sides of the working face, and record the lithology of the roof at different locations. For specific analysis, the layers and thickness that need to be hydraulically fracturing are determined. Based on the borehole columnar section provided by the mine and the previous mining face conditions, considering that the No. 10 coal seam has a mining height of 3.1m and a roof caving height of 2.8m, the key hard rock layer is determined to be limestone 17.9m away from the roadway roof, with a thickness of 7.0m.

[0062] Step III: Based on the roof inspection and lithological analysis from Step II, design the parameters for the kilometer-long horizontal boreholes and hydraulic fracturing. The drilling site is located at the side chamber corresponding to the planned horizontal borehole for roof cutting. The interval between horizontal boreholes can be set within the range of 0–5 m. Since the required drilling location is limestone, the caving angle α = 45°. Borehole No. 1 is drilled in limestone at a depth of 20.2 m from the roadway roof, and borehole No. 2 is drilled in limestone at a depth of 22.5 m from the roadway roof. The vertical distance between the two boreholes is found to be 2.3 m, and the horizontal offset between the two boreholes is calculated to be 2.3 m. The required hydraulic fracturing parameters need to be determined by comprehensively considering the thickness of the hard rock layer and the uniaxial compressive strength results.

[0063] Step IV: Set up a drilling site near the pre-cut hole and install a kilometer-long drilling rig within the site. Then, install a high-pressure water injection pump within the site, and begin hydraulic fracturing. The borehole trajectory starts as a curve, while the main body is an approximately horizontal straight line.

[0064] Step V: After hydraulic fracturing is completed, the support system is installed while the anchor removal operation is carried out on the working face. After all preparations are completed, mining begins. During mining, the support resistance is monitored, and the initial pressure step distance of the working face after fracturing is analyzed. The actual fracturing effect is further analyzed. Through monitoring and calculation, it is found that the initial pressure step distance was reduced from 85.0m before hydraulic fracturing to 57.3m, a reduction of 27.7m. It is calculated that hydraulic fracturing resulted in the recovery of an additional 13,330.6 tons of coal.

[0065] The above-described embodiments are preferred embodiments of the present invention and are only used to facilitate the illustration of the present invention. They are not intended to limit the present invention in any way. Any person skilled in the art who makes local modifications or alterations to the technical content disclosed in the present invention without departing from the scope of the technical features of the present invention shall still fall within the scope of the technical features of the present invention.

Claims

1. A method for reducing the initial pressure step distance of the roof in a fully mechanized longwall mining face and improving the top coal recovery rate, characterized in that, It includes the following steps: S1, collect engineering geological data of the working face and the cut-in hole; S2, determine the layer and thickness of the top plate that needs to be fracturing; S3, Analyze and design parameters for kilometer-level horizontal borehole and hydraulic fracturing; That Includes the following sub-steps: S3.1 The number and location of boreholes are determined according to the thickness of the hard rock layer; when the thickness of the hard top plate being cut is less than 5m, one borehole is drilled on the top plate at 1 / 2 of the rock layer; when the thickness of the hard top plate being cut is 5~10m, two boreholes are drilled on the top plate, each at 1 / 3 of the distance from the upper and lower boundaries of the rock layer; when the thickness of the hard top plate being cut is 10~15m, three boreholes are drilled on the top plate, each at 1 / 4, 1 / 2, and 3 / 4 of the distance from the boundary of the rock layer, respectively. S3.2, through laboratory or field tests, test the compressive and tensile strength of the required thick rock layer to obtain the required water injection pressure; S3.3 When the vertical distance between two boreholes is 1.6~5m, according to the formula: x=h·cotα; where x is the horizontal offset between the two boreholes; h is the vertical distance between the two boreholes; α is the collapse angle. When the uniaxial compressive strength of the rock is greater than 60 MPa, the collapse angle α = 30°; when the uniaxial compressive strength of the rock is between 50 and 60 MPa, the collapse angle α = 45°; when the uniaxial compressive strength of the rock is between 40 and 50 MPa, the collapse angle α = 60°. S4. Set up a drilling site near the cut-out, install a kilometer-long drilling rig within the drilling site, and then drill and construct a kilometer-long horizontal borehole on-site, followed by hydraulic fracturing. This includes the following sub-steps: S4.

1. Set up a drilling site near the cut-out and install a kilometer-long drilling rig within the drilling site; S4.

2. Install a high-pressure water pump within the drilling site, drill into the hard rock strata to be fracturing, and then begin hydraulic fracturing. Each borehole uses a retreating, multiple-fracturing method, fracturing every 10-30 meters, with each fracturing session lasting at least 30 minutes; S4.

3. During fracturing, the water head direction must be perpendicular to the roadway roof; as the fracturing time increases, the water pressure gradually rises, reaching its maximum value; S4.

4. Due to the high pressure, new cracks appear in the fracturing rock strata and begin to expand, causing a slight decrease in water pressure; the cracks continue to spread outwards, and then the water pressure drops significantly, at which point fracturing is complete. S5. After the hydraulic fracturing is completed, the support is installed and the anchor is removed at the working face. Then the mining begins. The reduction distance of the initial pressure step of the fully mechanized longwall face after hydraulic fracturing and the amount of top coal recovered are specifically analyzed. After the hydraulic fracturing is completed, the support frame is installed and the anchor is simultaneously removed from the working face; After the start of mining, the support resistance is monitored during mining, the initial pressure step distance of the working face after fracturing is analyzed, and it is compared with the initial pressure step distance of the adjacent working face to obtain the shortening distance of the initial pressure step distance and the amount of coal mined in multiple mining operations.

2. The method for reducing the initial pressure step distance of the roof in a fully mechanized longwall mining face and improving the top coal recovery rate according to claim 1, characterized in that, The basic data for step S1 includes: the opening and tunneling operation procedure and support cross-section diagram, the longwall mining operation procedure, the mining engineering plan, the surrounding borehole columnar section, the geological report, and the mine preliminary design specification.

3. The method for reducing the initial pressure step distance of the roof in a fully mechanized longwall mining face and improving the top coal recovery rate according to claim 1, characterized in that, Step S2 includes: observing the top plate at certain intervals within the cut hole, analyzing the lithology and thickness of the top plate, and determining the hard, thick rock layers that need to be fractured; Step S2 specifically includes the following sub-steps: S2.1, Conduct geological exploration, including geological profiling, core drilling, etc. Through geological feature analysis, potential target strata that need to be fracturing can be identified; S2.2 Core Analysis: Analyze the physical and mechanical properties and geological and mineralogical characteristics of the collected core samples; S2.3 Based on the geological exploration results and the key layer theory, determine the main key layer and sub-key layer in the overlying strata. Then, based on the mining pressure manifestation patterns of previous working faces, determine the specific strata that need to be fractured.

4. The method for reducing the initial pressure step distance of the roof in a fully mechanized longwall mining face and improving the top coal recovery rate according to claim 1, characterized in that, Step S3 includes: the drilling site is located near the opening; the interval between horizontal boreholes is generally set in the range of 0~5m.

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