A Simulation Test Method for Fracture Propagation in Large-Diameter Boreholes for Decompression

Abstract

This invention discloses a simulation test method for fracture propagation in coal seams under large-diameter borehole pressure relief. Specifically, the method involves selecting and processing coal samples from rockburst-prone coal seams, determining the original fracture distribution of the coal samples using CT scanning, applying stress to a set value using a testing machine, drilling the coal samples to relieve pressure, monitoring the acoustic emission localization of fractures, and finally performing another CT scan on the coal and rock after pressure relief. The differences in fracture distribution before and after pressure relief are compared to optimize the acoustic emission localization velocity model. Based on the evolution of acoustic emission localization, the fracture propagation mode of the borehole-relieved coal seam is analyzed and determined. This method can realistically reflect the fracture propagation of coal seams under borehole action, thereby guiding the design and optimization of large-diameter borehole pressure relief parameters in the field, ultimately achieving the goal of guiding safe and efficient production in the field.

Description

Technical Field

[0001] This invention relates to the field of rockburst prevention and control technology, specifically to a simulation test method for fracture propagation in coal seams with large-diameter borehole pressure relief. Background Technology

[0002] With the deepening of coal mining and the increasing complexity of geology, dynamic phenomena such as rockbursts have become one of the major hazards threatening safe production in mines. Based on existing theories of rockburst prevention, borehole stress relief can reduce or eliminate the risk of rockbursts by adjusting the stress distribution of the coal body, increasing the energy release of the coal body, and changing the rockburst hazard. However, unreasonable design of stress relief borehole parameters can easily lead to excessive weakening of the coal body strength, which is detrimental to the control of the surrounding rock in the roadway.

[0003] The rational design of borehole decompression parameters and the effective control of coal decompression intensity are crucial for coordinating the prevention of impact hazards and the control of roadway surrounding rock deformation. During borehole decompression, excavation causes a redistribution of stress in the nearby coal, leading to coal deformation and failure accompanied by the release of elastic energy. Essentially, this is the development and expansion of fractures caused by differential deformation of the coal during stress adjustment, resulting in a weakening of coal strength. Current research on the plastic failure, stress transfer, and elastic energy release of coal under borehole decompression from a macroscopic perspective struggles to clearly pinpoint the characteristics of fracture expansion in decompressed coal and to quantitatively analyze the strength of decompressed coal. Therefore, it is necessary to study the fracture expansion of coal under borehole decompression to provide theoretical guidance for the design of borehole decompression parameters in the field.

[0004] The distribution and development characteristics of coal fractures are decisive factors affecting coal strength and are also important indicators for analyzing the propagation and attenuation characteristics of dynamic load vibration waves. Influenced by factors such as the original stress distribution, the heterogeneity and inelasticity of the coal body, coal strength, and borehole parameters, the characteristics of coal fracture propagation and the strength weakening effect under borehole stress relief vary significantly. Therefore, analyzing the fracture propagation law of coal under borehole stress relief, studying the relationship between fracture propagation and the stress state of the unstressed coal body, and the controlling effect of fracture propagation on coal strength, revealing the fracture propagation of coal under borehole stress relief, is a theoretical prerequisite for the rational design of borehole stress relief parameters. It has significant theoretical and scientific value for the control of surrounding rock deformation in borehole-stressed coal bodies and the study of the propagation and attenuation effect of fracture propagation in borehole-stressed coal bodies on dynamic load vibration waves. Summary of the Invention

[0005] To address the aforementioned technical shortcomings, the purpose of this invention is to provide a simulation test method for the propagation of coal fractures in large-diameter boreholes under pressure relief. This method can realistically reflect the propagation of coal fractures under borehole action, thereby guiding the design and optimization of pressure relief parameters for large-diameter boreholes in the field, and ultimately achieving the goal of guiding safe and efficient production in the field.

[0006] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0007] This invention provides a method for simulating fracture propagation in large-diameter borehole-decompressed coal seams, comprising the following steps:

[0008] S1: Coal samples were selected from the rockburst coal seam on site and processed into cubic specimens, and the surface of the specimens was polished smooth.

[0009] S2: Perform CT scanning and 3D image reconstruction on the sample to determine the original fracture distribution of the coal sample; use an acoustic emission system to roughly model the sample and obtain the initial acoustic emission positioning velocity model;

[0010] S3: Apply stress to the specimen in one of the circumferential horizontal and vertical directions using a testing machine to the set value. After the loading pressure is completed, let it stand for a period of time, and then drill a hole in one of the unpressurized end faces of the specimen to relieve the pressure.

[0011] S4: After the drilling and decompression are completed, monitor the acoustic emission location of the cracks in the sample, and perform another CT scan on the decompressed sample to compare the differences in crack distribution before and after decompression.

[0012] S5: Optimize the acoustic emission positioning velocity model, and analyze and determine the fracture propagation mode of the borehole pressure-relieved coal body based on the acoustic emission positioning evolution.

[0013] Preferably, in step S2, a high-resolution three-dimensional X-ray microscopy system at the nanoscale is used to scan the distribution of internal fractures in the coal sample before decompression.

[0014] Preferably, in step S3, the borehole diameter is set to be variable, and the drill bit is kept perpendicular to the sample surface and drilled at a uniform speed until it penetrates the sample during the pressure relief process; in order to analyze the interaction between multiple borehole pressure relief and crack propagation, the number of boreholes is set to be variable and evenly arranged along the horizontal line, and after each borehole pressure relief is completed, it is necessary to let it stand for a period of time before the next borehole pressure relief is carried out.

[0015] Preferably, in step S4, an acoustic emission system is used to collect acoustic emission signals throughout the entire process;

[0016] The location, energy release, and vibration waveform parameters of acoustic emission events were analyzed to identify and analyze the crack initiation type and propagation scale characteristics, accurately locate the crack propagation path, and then a high-resolution three-dimensional X-ray microscopy system was used to scan the internal crack distribution of the coal sample after decompression, and the differences in crack distribution before and after decompression were compared.

[0017] Preferably, in step S5, based on the fracture propagation characteristics and CT scan results of the unstressed coal sample, and the real-time monitoring of the micro-fracture development inside the unstressed coal sample by the acoustic emission system, the propagation speed of ultrasonic waves at different stress levels is monitored, the influencing factors are analyzed, and the acoustic emission positioning velocity model is optimized. The stress-strain curve characteristics under fracture propagation are analyzed, the influence mechanism of different fracture propagation characteristics on coal strength is studied, the acoustic emission positioning velocity model is verified and optimized, and the fracture propagation mode of the borehole unstressed coal is analyzed and determined.

[0018] The beneficial effects of this invention are as follows: By utilizing CT scanning of fracture distribution and real-time acoustic emission monitoring, accurate analysis can be achieved of the development and propagation patterns of coal fractures and the evolution characteristics of coal stress distribution under borehole pressure relief. Simultaneously, combined with theoretical modeling, the relationship between coal fracture propagation characteristics and coal stress state under borehole pressure relief can be analyzed and solved, and verified using numerical simulations, revealing the coal fracture propagation mechanism under borehole pressure relief. This guides the design and optimization of pressure relief parameters for large-diameter boreholes in coal seams, ultimately achieving the goal of guiding safe and efficient on-site production. Attached Figure Description

[0019] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0020] Figure 1 This is a schematic diagram of a large-diameter borehole pressure relief coal seam fracture propagation simulation test method provided in an embodiment of the present invention.

[0021] Explanation of reference numerals in the attached figures:

[0022] 1. Experimental machine; 2. PCI-2 acoustic emission system; 3. Drilling rig; 4. Vertical clamping plate; 5. Nano30 high-precision acoustic emission sensor; 6. Drilling hole; 7. Horizontal clamping plate. Detailed Implementation

[0023] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0024] like Figure 1 As shown, a simulation test method for fracture propagation in large-diameter borehole-relieved coal seams includes the following steps:

[0025] S1: Collect coal samples from the coal seam where rock bursts occurred on site, process them into cubic specimens with dimensions of 100mm×100mm×100mm, and grind the surface of the specimens smooth according to standards.

[0026] S2: Using a nanoscale high-resolution three-dimensional X-ray microscopy system, CT scans and three-dimensional image reconstructions were performed on the crack distribution of the sample before pressure relief to determine the internal crack distribution of the sample before pressure relief.

[0027] S3: Using the Landmark 370.50 test system, the tester applies stress to the sample in one of the circumferential horizontal and vertical directions to the set value through the plumb line pressure plate 4 and the horizontal line pressure plate 7 corresponding to the tester 1. After the loading pressure is completed, the sample is left to stand for a period of time, and the pressure is released by drilling a hole in one of the ends of the sample where no pressure is applied.

[0028] Vertical and horizontal stresses are applied to the set values, with the initial stress designed based on in-situ stress tests and the average value of uniaxial compressive strength. After the initial stress loading is completed, the area is allowed to stand for a period of time before drilling to relieve the pressure.

[0029] The borehole diameter was set to be variable (8mm, 10mm, and 12mm). During the decompression process, the drill bit was kept perpendicular to the sample surface and drilled at a constant speed until it penetrated the sample. In order to analyze the interaction between multiple borehole decompression and fracture propagation induced by multiple boreholes, the number of boreholes 6 was set to be variable (1, 2, and 3), and they were arranged along a horizontal line. The distance between two boreholes 6 was 25mm. After each borehole was decompressed, it was necessary to let it stand for a period of time before the next borehole was decompressed.

[0030] S4: Acoustic emission signals were acquired throughout the entire process using a PCI-2 acoustic emission system 2. A total of eight Nano30 high-precision acoustic emission sensors 5 (monitoring frequency 125-750KHz) were arranged before and after the sample, with a sampling frequency of 1MHz. The sensors monitored signals such as event location, number of impacts, amplitude, energy, and impact event waveforms during the test. Then, a high-resolution three-dimensional X-ray microscopic imaging system was used to scan the distribution of internal fractures in the coal body after decompression, and the differences in fracture distribution before and after decompression were compared.

[0031] S5: Using a high-resolution three-dimensional X-ray microscopy system, CT scans and three-dimensional image reconstructions were performed on the fracture distribution of the sample after decompression to determine the internal fracture distribution of the sample. Based on the fracture propagation characteristics and CT scan results of the decompressed coal body, as well as the microscopic fracture development inside the borehole decompressed coal body monitored in real time by acoustic emission, the propagation velocity of ultrasonic waves at different stress levels was monitored, its influencing factors were analyzed, and the acoustic emission positioning velocity model was optimized. Combined with the acoustic emission positioning evolution, the fracture propagation mode of the borehole decompressed coal body was analyzed and determined.

[0032] Obviously, those skilled in the art can make various modifications and variations to this invention without departing from its spirit and scope. Therefore, if these modifications and variations fall within the scope of the claims of this invention and their equivalents, this invention also intends to include these modifications and variations.

Claims

1. A method for simulating fracture propagation in large-diameter borehole-drilled, pressure-relieved coal seams, characterized in that... Includes the following steps: S1: Coal samples were selected from the rockburst coal seam on site and processed into cubic specimens, and the surface of the specimens was polished smooth. S2: Perform CT scanning and 3D image reconstruction on the sample to determine the original fracture distribution of the coal sample; use an acoustic emission system to roughly model the sample and obtain the initial acoustic emission positioning velocity model; S3: Apply stress to the specimen in one of the circumferential horizontal and vertical directions using a testing machine to the set value. After the loading pressure is completed, let it stand for a period of time, and then drill a hole in one of the unpressurized end faces of the specimen to relieve the pressure. S4: After the drilling and decompression are completed, monitor the acoustic emission location of the cracks in the sample, and perform another CT scan on the decompressed sample to compare the differences in crack distribution before and after decompression. S5: Optimize the acoustic emission positioning velocity model, and analyze and determine the fracture propagation mode of the borehole pressure-relieved coal body based on the acoustic emission positioning evolution.

2. The method for simulating fracture propagation in large-diameter borehole-relieved coal seams as described in claim 1, characterized in that, In step S2, a high-resolution three-dimensional X-ray microscopy system at the nanoscale is used to scan the distribution of internal fractures in the coal sample before decompression.

3. The method for simulating fracture propagation in large-diameter borehole-relieved coal seams as described in claim 1, characterized in that, In step S3, the borehole diameter is set to be variable, and the drill bit is kept perpendicular to the sample surface and drilled at a constant speed until it penetrates the sample during the pressure relief process. In order to analyze the interaction between multiple borehole pressure relief and crack propagation, the number of boreholes is set to be variable and evenly arranged along the horizontal line. After each borehole pressure relief is completed, it is necessary to let it stand for a period of time before the next borehole pressure relief is carried out.

4. The method for simulating fracture propagation in large-diameter borehole-relieved coal seams as described in claim 1, characterized in that, In step S4, an acoustic emission system is used to collect acoustic emission signals throughout the entire process; The location, energy release, and vibration waveform parameters of acoustic emission events were analyzed to identify and analyze the crack initiation type and propagation scale characteristics, accurately locate the crack propagation path, and then a high-resolution three-dimensional X-ray microscopy system was used to scan the internal crack distribution of the coal sample after decompression, and the differences in crack distribution before and after decompression were compared.

5. The method for simulating fracture propagation in large-diameter borehole-relieved coal seams as described in claim 1, characterized in that, In step S5, based on the fracture propagation characteristics and CT scan results of the unstressed coal sample, and the real-time monitoring of the micro-fracture development inside the unstressed coal sample by the acoustic emission system, the propagation speed of ultrasonic waves at different stress levels is monitored, the influencing factors are analyzed, and the acoustic emission positioning velocity model is optimized. The stress-strain curve characteristics under fracture propagation are analyzed, the influence mechanism of different fracture propagation characteristics on coal strength is studied, the acoustic emission positioning velocity model is verified and optimized, and the fracture propagation mode of the borehole unstressed coal body is analyzed and determined.

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