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Fracturing of Coal and Rock Mass
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Fracturing of Coal and Rock Mass

A special issue of Minerals (ISSN 2075-163X).

Deadline for manuscript submissions: closed (23 September 2022) | Viewed by 28164

Special Issue Editors

Prof. Dr. Bingxiang Huang

E-Mail Website
Guest Editor
State Key Laboratory of Coal Resources and Safe Mining(CUMT), China University of Mining & Technology, Xuzhou, China
Interests: hydraulic fracturing; fracture propagation; displacement methane; hard roof control; top coal weakening
Dr. Yuekun Xing

E-Mail Website
Guest Editor
State Key Laboratory of Coal Resources and Safe Mining(CUMT), China University of Mining & Technology, Xuzhou, China
Interests: rock fracture mechanics; hydraulic fracture propagation; rock fracturing characterization; rock thermoplasticity; acoustic emission and microseismics
Dr. Xinglong Zhao

E-Mail Website
Guest Editor
State Key Laboratory for Fine Exploration and Intelligent Development of Coal Resources, China University of Mining & Technology, Xuzhou, China
Interests: rock mechanics; hydraulic fracturing; stress disturbance; fracture propagation
Special Issues, Collections and Topics in MDPI journals

Special Issue Information

Dear Colleagues,

On behalf of Minerals, I would like to invite you to contribute to a Special Issue related to the fracturing of coal and rock mass. Fracturing is considered as one of the most favorable stimulation approaches towards coal, dramatically mitigating its inherent low permeability. Over the last several decades, huge successes have been achieved by the use of hydraulic fracturing technology in coal mining and in the oil industry, with capacity to considerably improve methane extraction efficiency and production performance for CBM reservoirs. At the same time, although a great deal of effort has been devoted and many excellent advances have been made, a good understanding of a couple of key issues continues to elude us, including underlying mechanisms or control methods for directional hydraulic fracturing, the methane-driven characteristics caused by fracturing, newly emerged gas fracturing, and so on.

This Special Issue aims to collect recent advances in the hydro-mechanical behavior of coal-rock fractures or fracture networks. We expect to bring together researchers in the aforementioned fields to highlight the current development of new techniques, to exchange the latest understanding of the underlying mechanisms, to present advanced algorithms for modeling, and to facilitate collaboration between researchers in different fields. We invite you to submit comprehensive review papers and original articles.

Potential topics include but are not limited to the following:

  • Theoretical modeling of fracturing in coal and rock mass;
  • Advanced techniques for monitoring or characterizing hydraulic fracture propagation;
  • Permeability evolution and multiphase fluid flow in fracturing coal-rock;
  • Novel laboratory testing approaches in the hydraulic fracturing of coal;
  • Field test of multistage hydraulic fracturing in the directional borehole;
  • Evaluation of fracturing effect in laboratory testing or field experiments.

Prof. Dr. Bingxiang Huang
Dr. Yuekun Xing
Dr. Xinglong Zhao  
Guest Editors

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All submissions that pass pre-check are peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Minerals is an international peer-reviewed open access monthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 2400 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

  • hydraulic fracture propagation mechanism
  • multiphase fluid flow in fracturing coal-rock
  • directional hydraulic fracturing
  • novel laboratory testing approaches or field experiments
  • predictive models for permeability of rock fractures

Published Papers (15 papers)

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Research

16 pages, 3906 KiB  
Article
Investigation on the Fragmentation and Outburst Mechanism of Coal Sample with Pore Gas Using CDEM
by Qunlei Zhang, Zhiming Wang, Chun Feng, Xinguang Zhu and Jun Zhou
Minerals 2023, 13(3), 351; https://doi.org/10.3390/min13030351 - 01 Mar 2023
Viewed by 1024
Abstract
In this paper, using the continuum-discontinuum element method (CDEM), the fragmentation and outburst process of coal specimen are simulated, and the main factors affecting coal breaking and outburst are explored. The results show that after the coal seam is uncovered, coal generates obvious [...] Read more.
In this paper, using the continuum-discontinuum element method (CDEM), the fragmentation and outburst process of coal specimen are simulated, and the main factors affecting coal breaking and outburst are explored. The results show that after the coal seam is uncovered, coal generates obvious failure and outburst trend. Near coal-free surface, the fracture coal blocks generate significant displacement, resulting in larger opening widths of coal cracks. Coal deep generates the cracks without an obvious opening width. The crack density of coal with pore gas is larger than those of coal without gas, and it is larger than those of coal without pores. However, in the early stage of coal failure, the obvious separation and outburst ranges of coal with gas are smaller than those of coal without gas, and are smaller than those of coal without pores. The numbers of fracture coal blocks show an increase with the growth of in situ stress, pore ratio and gas pressure. The effect of in situ stress on fracture coal block number (517–10,203) is larger than the effect (7589–15,170) of pore ratio and is larger than the effect (5803–6836) of gas pressure. The effect of in situ stress on a maximum size (0.0387–0.138 m) of fracture blocks is larger than the effect (0.0342–0.0733 m) of pore ratio and is larger than the effect (0.0454–0.0578 m) of gas pressure. The coal outburst velocity and range show an increase with the growth of gas pressure and in situ stress (3.77–5.65 m/s); however, the coal outburst shows a slow decrease with a growth of pore ratio. The effect of gas pressure on the coal outburst velocity (11.51–21.9 m/s) is larger than the effect (3.77–5.65 m/s) of in situ stress and is larger than the effect (4.52–5.23 m/s) of pore ratio. This investigation is beneficial to understand the mechanisms of coal–gas outburst in coal mining and roadway excavation. Full article
(This article belongs to the Special Issue Fracturing of Coal and Rock Mass)
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25 pages, 7881 KiB  
Article
Dynamic Deformation and Failure Characteristics of Deep Underground Coal Measures Sandstone: The Influence of Accumulated Damage
by Ziheng Sha, Hai Pu and Junce Xu
Minerals 2022, 12(12), 1589; https://doi.org/10.3390/min12121589 - 11 Dec 2022
Cited by 1 | Viewed by 1173
Abstract
Understanding accumulated damage effects is essential when undertaking deep underground rock engineering, as complex in situ environments and intense engineering disturbances realistically affect the physical and mechanical properties of rocks. Accumulated damage mainly causes the extension of micro-cracks and the sprouting of specific [...] Read more.
Understanding accumulated damage effects is essential when undertaking deep underground rock engineering, as complex in situ environments and intense engineering disturbances realistically affect the physical and mechanical properties of rocks. Accumulated damage mainly causes the extension of micro-cracks and the sprouting of specific defects in the rocks, altering the microstructural parameters. In this investigation, loading and unloading tests were used to simulate the damage states of the deep underground coal measures sandstone. The accumulated damage factor was formed by combining the P-wave and energy damage variables. The effect of accumulated damage on the bearing capacity and deformation behavior of sandstone was particularly pronounced after experiencing impact loading. The experimental results demonstrate that the accumulated damage factor can depict the initial damage state of sandstone as well as the subsequent dynamic and progressive damage. There is a mutually governing effect between accumulated damage and strain rate. In contrast, accumulated damage significantly extends the range of strain rates, which is fed back into the dynamic uniaxial compressive strength of the sandstone. There is a negative correlation between dynamic fracture energy and accumulated damage, which strongly agrees with the sandstone’s deformation mechanism. The combination of accumulated damage and impact loads can be used to assess the long-term safety of deep underground rock engineering. Full article
(This article belongs to the Special Issue Fracturing of Coal and Rock Mass)
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24 pages, 12670 KiB  
Article
The Numerical Simulation and Characterization of Complex Fracture Network Propagation in Multistage Fracturing with Fractal Theory
by Peng Zhang, Chunsheng Pu, Xian Shi, Zhiqian Xu and Zhengqin Ye
Minerals 2022, 12(8), 955; https://doi.org/10.3390/min12080955 - 28 Jul 2022
Cited by 2 | Viewed by 1222
Abstract
To investigate complex fracturing and the influencing factors of simultaneous fracture propagation in horizontal wells, a three-cluster fracture propagation model that is controlled by fracture surface displacement parameters is established. When performing multistage fracturing on reservoirs with a relatively high development degree of [...] Read more.
To investigate complex fracturing and the influencing factors of simultaneous fracture propagation in horizontal wells, a three-cluster fracture propagation model that is controlled by fracture surface displacement parameters is established. When performing multistage fracturing on reservoirs with a relatively high development degree of natural fractures, staged multicluster fracturing in horizontal wells is one of the commonly used technical methods for volume fracturing. Two frequently encountered problems are multifracture extension and interfracture stress interference between fractures. The characteristics of the coal mechanics parameters of coalbed methane (CBM) blocks in northwestern China are analyzed by probability statistics to obtain the elastic modulus and Poisson’s ratio. With the interactive development environment of the MATLAB-PYTHON-FEM platform, a numerical model of fracture network expansion under the staged fracturing of horizontal wells is constructed. The stress interference level between fractures and the fractal expansion mechanism of fracture networks are analyzed under different influencing factors, including the fractal dimensions of natural joints, fracturing fluid pumping rate, and inhomogeneity coefficient of the in situ stress. Full article
(This article belongs to the Special Issue Fracturing of Coal and Rock Mass)
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19 pages, 57903 KiB  
Article
Impact of Stimulated Fractures on Tree-Type Borehole Methane Drainage from Low-Permeability Coal Reservoirs
by Liang Zhang, Qingjie Qi, Xuexi Chen, Shaojie Zuo, Kai Deng, Ruiqing Bi and Jiamei Chai
Minerals 2022, 12(8), 940; https://doi.org/10.3390/min12080940 - 26 Jul 2022
Cited by 1 | Viewed by 1205
Abstract
Tree-type hydraulic fracturing (TTHF) is a promising method applicable to the effective development of methane in low-permeability coal seams. However, a large-scale application of this technique is limited due to the unclear impact of stimulated fractures by TTHF on the effect of post-fracturing [...] Read more.
Tree-type hydraulic fracturing (TTHF) is a promising method applicable to the effective development of methane in low-permeability coal seams. However, a large-scale application of this technique is limited due to the unclear impact of stimulated fractures by TTHF on the effect of post-fracturing methane drainage. To address this issue, a multi-scale methane flow model of coupled thermo-hydro-mechanical (THM) processes in stimulated coal seams by TTHF was developed and verified against laboratory-based measurements. Using this proposed model, a systematic evaluation of the influence extent of hydraulic fractures connecting sub-boreholes in a tree-type borehole on the drainage effect under different fracture apertures, initial permeabilities of the cleat system, and remnant methane pressures was performed. Detailed simulated results showed that the presence of highly permeable fractures induced by TTHF greatly enhanced, as expected, the drainage efficiency of coal seam methane between the ends of adjacent sub-boreholes, and led to a significant increase in the homogeneity coefficient β. Furthermore, increasing the stimulated fracture aperture and initial cleat permeability or reducing the remnant methane pressure also resulted in a larger value of β, but in turn shortened the lead time of the tree-type borehole. The β’s growth rate for different investigated cases compared to identical simulations without stimulated fractures presented an overall trend of increasing at first and then slowly decreasing with sustained drainage time. Meanwhile, large-aperture hydraulic fractures and lower remnant methane pressure are more beneficial to the drainage effect of tree-type boreholes in the initial stages of drainage. These results portrayed herein can be employed to better understand how fractures generated by TTHF play a role in post-fracturing drainage programs and provide theoretical assistance in engineering applications. Full article
(This article belongs to the Special Issue Fracturing of Coal and Rock Mass)
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13 pages, 3658 KiB  
Article
Study on Damage Characteristics of Deep Coal Based on Loading Rate Effect
by Junhua Xue, Shulou Wang, Quanlin Yang, Yutong Du and Zongxuan Hou
Minerals 2022, 12(4), 402; https://doi.org/10.3390/min12040402 - 25 Mar 2022
Cited by 3 | Viewed by 1363
Abstract
In this paper, the uniaxial compression tests of coal under different loading rates are carried out by using the MTS rock mechanics loading system and the DS5 acoustic emission instrument, and the mechanical parameters and damage characteristics of coal samples under five loading [...] Read more.
In this paper, the uniaxial compression tests of coal under different loading rates are carried out by using the MTS rock mechanics loading system and the DS5 acoustic emission instrument, and the mechanical parameters and damage characteristics of coal samples under five loading rates are studied. The conclusions are as follows: The uniaxial compressive strength and the elastic modulus of the sample increase with the increase in the loading rate, whereas the strain at the peak point of stress decreases with the increase in the loading rate; at a low loading rate, the AE ring count of the coal samples is widely distributed, but the maximum AE ring count value is small. At a high loading rate, the AE ring count of the coal samples decreases, whereas the maximum AE ring count value increases significantly. With the increase in the loading rate, the maximum AE ring count and cumulative AE ring count of the sample increase. According to the damage variable curves of the coal samples under different loading rates, the damage evolution process of the samples before the stress peak is roughly divided into three stages: initial damage stage, stable damage development stage and rapid damage development stage. It is found that when the loading rate is low, the deformation time of the coal sample is relatively sufficient, whereas when the loading rate is high, the micro cracks have no time to develop and expand, the sample is destroyed instantly and the destruction of the coal sample is sudden. Therefore, with the increase in the loading rate, the damage variable value at the peak stress of the coal sample generally shows an increasing trend. Full article
(This article belongs to the Special Issue Fracturing of Coal and Rock Mass)
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19 pages, 5768 KiB  
Article
Experimental Study on the Influence of Confining Pressure and Bedding Angles on Mechanical Properties in Coal
by Yongfei Li, Baoyun Zhao, Jiaosheng Yang, Junchang Sun, Wei Huang, Ziyun Li and Bingyuan Wang
Minerals 2022, 12(3), 345; https://doi.org/10.3390/min12030345 - 11 Mar 2022
Cited by 7 | Viewed by 1540
Abstract
Extensive bedding planes have a great influence on the mechanical properties of coal. In order to study the mechanism of the effects of bedding angles on the mechanical properties and failure characteristics of coal in the Shanxi Baode coal mine, uniaxial and triaxial [...] Read more.
Extensive bedding planes have a great influence on the mechanical properties of coal. In order to study the mechanism of the effects of bedding angles on the mechanical properties and failure characteristics of coal in the Shanxi Baode coal mine, uniaxial and triaxial compression tests and numerical simulations were conducted. The strength deterioration and microstructural changes in the samples were then analyzed with discrete element method (DEM) numerical simulation. The experimental results reveal that the power function strength criterion has good applicability to the strength characteristics of this coal. It was also found that the bedding angles have a great influence on the mechanical properties of coal. The properties of peak strength at different bedding angles roughly showed a U-shaped changing trend. The maximum strength occurred at a bedding angle of 0°, whereas the minimum strength occurred at a bedding angle of 60°. The numerical simulation and test results prove that the forms of failure of different bedding coal samples are complicated and are mainly represented by tensile and shear failures. Full article
(This article belongs to the Special Issue Fracturing of Coal and Rock Mass)
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17 pages, 4696 KiB  
Article
Study on the Stress Field and Crack Propagation of Coal Mass Induced by High-Pressure Air Blasting
by Xiaolin Yang, Chang Wang, Huaibao Chu, Shaoyang Yan, Haixia Wei and Mengfei Yu
Minerals 2022, 12(3), 300; https://doi.org/10.3390/min12030300 - 27 Feb 2022
Cited by 6 | Viewed by 2120
Abstract
High-pressure air blasting (HPAB) is one type of physical blasting technique that enhances the extraction rate of coalbed methane by impacting the coal mass with high-pressure gas to create cracks within it. First, based on the physical and mechanical parameters of the simulated [...] Read more.
High-pressure air blasting (HPAB) is one type of physical blasting technique that enhances the extraction rate of coalbed methane by impacting the coal mass with high-pressure gas to create cracks within it. First, based on the physical and mechanical parameters of the simulated coal rock mass, the RHT constitutive model of the coal rock mass was established, and its parameters were determined. Then, the laws of crack propagation and stress wave decay in coal induced by high-pressure air blasting were revealed by comparing the effect with that of equivalent explosive blasting. Next, the HPAB experiment was simulated to explore the coal crack propagation law under in-situ stress conditions. Finally, the HPAB experiment was carried out and the results of this experiment were compared with the numerical simulation results. The results indicate that the crack propagation induced by high-pressure air blasting is considered as two major stages, i.e., the crack initiation and crack propagation stage induced by the stress wave and the crack stable propagation stage induced by the duration high-pressure gas. In the case of equal energy, the peak stress wave of high-pressure gas is smaller, decays more slowly and has a longer action time, compared to explosive blasting. Therefore, the number of initial random cracks in coal mass induced by high-pressure air blasting is less, and the range of crack propagation induced by high-pressure air blasting is larger. When λ = 0 (λ is the ratio of the horizontal in-situ stress to the vertical in-situ stress), the in-situ stress in the coal seam can promote the propagation of vertical cracks but inhibit the propagation of horizontal cracks. When λ = 0.5 and 1, the in-situ stress inhibits the propagation of both horizontal and vertical cracks. Full article
(This article belongs to the Special Issue Fracturing of Coal and Rock Mass)
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18 pages, 7067 KiB  
Article
Dynamic Mechanics and Crystal Structure Fracture Characteristics of Rock-like Materials in Coal Mines
by Pengfei Gao, Mengxiang Wang, Xiaolei Lei and Qi Zong
Minerals 2022, 12(3), 290; https://doi.org/10.3390/min12030290 - 25 Feb 2022
Cited by 3 | Viewed by 1452
Abstract
Deep rock bears dynamic loads such as machinery, blasting and disturbance in the mining process. The dynamic fracture mechanism of deep rock is a necessary prerequisite for engineering design and analysis. To study the dynamic fracture mechanism of rock under high in situ [...] Read more.
Deep rock bears dynamic loads such as machinery, blasting and disturbance in the mining process. The dynamic fracture mechanism of deep rock is a necessary prerequisite for engineering design and analysis. To study the dynamic fracture mechanism of rock under high in situ stress, deep mudstone and sandstone were selected as research objects. The dynamic mechanical properties and energy dissipation of mudstone sandstone were assessed by using a 50 mm diameter separated Hopkinson test device. According to the similarity criterion, the similarity of strength was assumed as primary factor to prepare similar model materials. Then, dynamic mechanical tests of these similar materials were carried out under dynamic compression splitting and active confining pressure. The results show that materials similar to mudstone and sandstone mainly show axial fracture tensile failure and crushing failure. Both the average strain rate dynamic strength and peak strain of these similar materials increase with increasing impact pressure, and the dynamic strength of similar materials increases exponentially with increasing strain rate. This result is consistent with the regularity of original rock. The dynamic splitting of mudstone-like materials is dominated by the failure of intermediate cracks, and sandstone-like materials also show secondary cracks in addition to intermediate splitting cracks. The dynamic peak strength of mudstone-like materials increases with increasing active confining pressure, and the dynamic peak strength of sandstone-like materials increases nearly twofold under the action of active confining pressure. Full article
(This article belongs to the Special Issue Fracturing of Coal and Rock Mass)
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17 pages, 5449 KiB  
Article
Evaluation of Vibration Effect Caused by Carbon Dioxide Phase-Transition Fracturing Based on the Hilbert–Huang Transform
by Baolin Li, Enyuan Wang, Shaobin Hu and Ali Muhammad
Minerals 2022, 12(2), 242; https://doi.org/10.3390/min12020242 - 13 Feb 2022
Viewed by 1764
Abstract
To evaluate the vibration effect caused by carbon dioxide phase-transition fracturing, the Hilbert–Huang transform was used to study the change of vibration energy with distance in different frequency bands. The results are drawn as follows: (1) The peak particle velocity (PPV) decreases as [...] Read more.
To evaluate the vibration effect caused by carbon dioxide phase-transition fracturing, the Hilbert–Huang transform was used to study the change of vibration energy with distance in different frequency bands. The results are drawn as follows: (1) The peak particle velocity (PPV) decreases as a power function with an increase in distance and has fallen below 25 mm/s at 2.8 m. (2) The energy of vibration signals induced by carbon dioxide phase-transition fracturing is mainly distributed at the frequency band of 10–50 Hz. With the increase in distance, the energy distribution of vibration signals falls in four phases: Propagation to high frequency (0–13.9 m); a rapid high-frequency energy decrease (20–30 m); energy fluctuation (30–47.2 m) and a stable energy distribution (larger than 50 m). (3) The proportion of the low-frequency vibration energy (0–10 Hz) increases as a result of the increase in distance (less than 8.7 m), which should be paid more attention. Full article
(This article belongs to the Special Issue Fracturing of Coal and Rock Mass)
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14 pages, 2831 KiB  
Article
An Experimental Study on the Effects of True Triaxial Loading and Unloading Stress Paths on the Mechanical Properties of Red Sandstone
by Shuai Wang, Lianguo Wang, Jiansheng Tian, Hao Fan, Chongyang Jiang and Ke Ding
Minerals 2022, 12(2), 204; https://doi.org/10.3390/min12020204 - 05 Feb 2022
Cited by 7 | Viewed by 1663
Abstract
Loading and unloading stress paths play critical roles in investigating the deformation and failure of roadway excavation. In this study, tests under four different loading and unloading stress paths were conducted on red sandstone samples, with the aid of a self-developed true triaxial [...] Read more.
Loading and unloading stress paths play critical roles in investigating the deformation and failure of roadway excavation. In this study, tests under four different loading and unloading stress paths were conducted on red sandstone samples, with the aid of a self-developed true triaxial test system. Meanwhile, the deformation and failure characteristics of the samples were monitored during the tests. The following research conclusions were obtained: The octahedral shear stress is linearly correlated with the average effective stress, and the correlation coefficient R2 is 0.9825. The Mogi–Coulomb strength criterion is superior to the Drucker–Prager strength criterion in reflecting strength failure characteristics of red sandstone during loading and unloading. Shear failure tends to occur under uniaxial compression, whereas shear–tensile composite failure occurs under loading and unloading conditions. Compared with the true triaxial loading test, loading and unloading tests produce a larger strain in the unloading direction. Under loading and unloading stress paths, with the increase in intermediate principal stress (IPS), the strain in the direction of IPS gradually changes from expansion to compression, and the peak strength gradually increases. The state of IPS affects the failure strength of the sample and reflects the strengthening effect of IPS. This paper boasts a certain value and significance for research on the deformation and failure characteristics of sandstone in the actual in situ stress environment with triaxial dynamic changes. Full article
(This article belongs to the Special Issue Fracturing of Coal and Rock Mass)
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13 pages, 8037 KiB  
Article
Damage and Failure Characteristics of Surrounding Rock in Deep Circular Cavern under Cyclic Dynamic Load: A True Triaxial Experiment Investigation
by Binglei Li, Pengfei Gao, Yangbing Cao, Weiguo Gong, Sui Zhang and Jianzhi Zhang
Minerals 2022, 12(2), 134; https://doi.org/10.3390/min12020134 - 24 Jan 2022
Cited by 3 | Viewed by 2202
Abstract
For ensuring safety and efficiency during the construction of deep engineering, it is essential to explore the failure mode of the surrounding rock mass under dynamic disturbance and high geo-stress. We conducted true triaxial load tests for rock-like material with a preexisting circular [...] Read more.
For ensuring safety and efficiency during the construction of deep engineering, it is essential to explore the failure mode of the surrounding rock mass under dynamic disturbance and high geo-stress. We conducted true triaxial load tests for rock-like material with a preexisting circular hole, and monitored the acoustic emission (AE) signal during the whole test. The result demonstrates the evolution characteristics of damage and failure mode with different cyclic dynamic load amplitudes and intermediate principal stress. With the increase in cyclic dynamic load amplitude or the decrease in intermediate principal stress, the failure source mainly occurs at the two horizontal side walls of the surrounding rock where the failure patterns change from the slabbing to wall caving and, finally, to rockburst. The former failure mode can actually serve as an important precursor for the latter. Based on such mechanisms, the precursor can be indirectly detected in forms of AE signal released by microcracking. The research can provide a reliable guidance for the rock stability control and faithfully forecasting the larger-scale failure during the excavation of deep circular cavern. Full article
(This article belongs to the Special Issue Fracturing of Coal and Rock Mass)
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22 pages, 6880 KiB  
Article
Characteristics of Acoustic Emission Waveforms Induced by Hydraulic Fracturing of Coal under True Triaxial Stress in a Laboratory-Scale Experiment
by Nan Li, Liulin Fang, Bingxiang Huang, Peng Chen, Chao Cai, Yunpeng Zhang, Xuan Liu, Zhihuai Li, Yaolin Wen and Yanli Qin
Minerals 2022, 12(1), 104; https://doi.org/10.3390/min12010104 - 16 Jan 2022
Cited by 3 | Viewed by 1799
Abstract
Hydraulic fracturing (HF) is an effective technology to prevent and control coal dynamic disaster. The process of coal hydraulic fracturing (HF) induces a large number of microseismic/acoustic emission (MS/AE) waveforms. Understanding the characteristic of AE waveforms’ parameters is essential for evaluating the fracturing [...] Read more.
Hydraulic fracturing (HF) is an effective technology to prevent and control coal dynamic disaster. The process of coal hydraulic fracturing (HF) induces a large number of microseismic/acoustic emission (MS/AE) waveforms. Understanding the characteristic of AE waveforms’ parameters is essential for evaluating the fracturing effect and optimizing the HF strategy in coal formation. In this study, laboratory hydraulic fracturing under true triaxial stress was performed on a cubic coal sample combined with AE monitoring. The injection pressure curve and temporal variation of AE waveforms’ parameters in different stages were analyzed in detail. The experimental results show that the characteristics of the AE waveforms’ parameters well reflect the HF growth behavior in coal. The majority of AE waveforms’ dominant frequency is distributed between 145 and 160 kHz during HF. The sharp decrease of the injection pressure curve and the sharp increase of the AE waveforms’ amplitude show that the fracture already runs through the coal sample during the initial fracture stage. The “trapezoidal” rise pattern of cumulative energy and most AE waveforms with low amplitude may indicate the stage of liquid storage space expansion. The largest proportion of AE waveforms’ energy and higher overall level of AE waveforms’ amplitude occur during the secondary fracture stage, which indicates the most severe degree of coal fracture and complex activity of internal fracture. The phenomenon shows the difference in fracture mechanism between the initial and secondary fracture stage. We propose a window-number index of AE waveforms for better response to hydraulic fracture, which can improve the accuracy of the HF process division. Full article
(This article belongs to the Special Issue Fracturing of Coal and Rock Mass)
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19 pages, 7971 KiB  
Article
Experimental Investigation on the Energy Properties and Failure Process of Thermal Shock Treated Sandstone Subjected to Coupled Dynamic and Static Loads
by Xiang Li, Si Huang, Tubing Yin, Xibing Li, Kang Peng and Xiaodong Fan
Minerals 2022, 12(1), 25; https://doi.org/10.3390/min12010025 - 23 Dec 2021
Cited by 2 | Viewed by 2308
Abstract
Thermal shock (TS) is known as the process where fractures are generated when rocks go through sudden temperature changes. In the field of deep rock engineering, the rock mass can be subjected to the TS process in various circumstances. To study the influence [...] Read more.
Thermal shock (TS) is known as the process where fractures are generated when rocks go through sudden temperature changes. In the field of deep rock engineering, the rock mass can be subjected to the TS process in various circumstances. To study the influence of TS on the mechanical behaviors of rock, sandstone specimens are heated at different high temperatures and three cooling methods (stove cooling, air cooling, and freezer cooling) are adopted to provide different cooling rates. The coupled dynamic and static loading tests are performed on the heated sandstone through a modified split Hopkinson pressure bar (SHPB) system. The influence of heating level and cooling rate on the dynamic compressive strength, energy dissipations, and fracturing characteristics is investigated based on the experimental data. The development of the microcracks of the sandstone specimens after the experiment is analyzed utilizing a scanning electron microscope (SEM). The extent of the development of the microcracks serves to explain the variation pattern of the mechanical responses and energy dissipations of the specimens obtained from the loading test. The findings of this study are valuable for practices in rock engineering involving high temperature and fast cooling. Full article
(This article belongs to the Special Issue Fracturing of Coal and Rock Mass)
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18 pages, 6457 KiB  
Article
Mining-Induced Stress Control by Advanced Hydraulic Fracking under a Thick Hard Roof for Top Coal Caving Method: A Case Study in the Shendong Mining Area, China
by Kaige Zheng, Yu Liu, Tong Zhang and Jingzhong Zhu
Minerals 2021, 11(12), 1405; https://doi.org/10.3390/min11121405 - 11 Dec 2021
Cited by 11 | Viewed by 2899
Abstract
Fully mechanized top-coal caving mining with high mining height, hard roofs and strong mining pressure are popular in the Shendong mining area, China. The occurrence of dynamic disasters, such as rock burst, coal and gas outburst, mine earthquakes and goaf hurricanes during the [...] Read more.
Fully mechanized top-coal caving mining with high mining height, hard roofs and strong mining pressure are popular in the Shendong mining area, China. The occurrence of dynamic disasters, such as rock burst, coal and gas outburst, mine earthquakes and goaf hurricanes during the coal exploitation process under hard roof conditions, pose a threat to the safe production of mines. In this study, the characteristics of overburden fracture in fully mechanized top-coal caving with a hard roof and high mining height are studied, and the technology of advanced weakening by hard roof staged fracturing was proposed. The results show that the hard roof strata collapse in the form of large “cantilever beams”, and it is easy to release huge impact kinetic energy, forming impact disasters. After the implementation of advanced hydraulic fracturing, the periodic weighting length decreases by 32.16%, and the length of overhang is reasonably and effectively controlled. Ellipsoidal fracture networks in the mining direction of the vertical working face, horizontal fracture networks perpendicular to the direction of the working face, and near-linear fracture planes dominated by vertical fractures were observed, with the accumulated energy greatly reduced. The effectiveness of innovation technology is validated, and stress transfer, dissipation and dynamic roof disasters were effectively controlled. Full article
(This article belongs to the Special Issue Fracturing of Coal and Rock Mass)
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19 pages, 5738 KiB  
Article
Energy Dissipation and Electromagnetic Radiation Response of Sandstone Samples with a Pre-Existing Crack of Various Inclinations under an Impact Load
by Zesheng Zang, Zhonghui Li, Yue Niu, He Tian, Xin Zhang, Xiaoliang Li and Muhammad Ali
Minerals 2021, 11(12), 1363; https://doi.org/10.3390/min11121363 - 02 Dec 2021
Cited by 11 | Viewed by 2659
Abstract
Various primary fissures and defects are widely present in a rock mass and have a significant impact on the stability of the rock mass. We studied the influence of the crack inclination angle on the energy dissipation and electromagnetic radiation (EMR) response of [...] Read more.
Various primary fissures and defects are widely present in a rock mass and have a significant impact on the stability of the rock mass. We studied the influence of the crack inclination angle on the energy dissipation and electromagnetic radiation (EMR) response of sandstone under an impact load. Impact tests were conducted on red sandstone samples with different inclination angles, in addition to test energy dissipation and EMR signals. The results showed that as the energy of the stress wave increased, the energy consumption density and damage variables of the sample gradually increased, and the electromagnetic radiation energy also increased. As the crack inclination increased, the energy consumption density first decreased and then increased, while the damage variable and electromagnetic radiation energy first increased and then decreased. In the process of impact damage, the main frequency of EMR was 0~5 kHz. As the energy of the stress wave increased, the dominant frequency band of the main frequency expanded from low frequency to high frequency, and the amplitude signal gradually increased; the α = 45° specimen frequency domain was the widest, and the amplitude was the largest. The crack inclination significantly changed the failure state of the sample, resulting in changes in the energy dissipation and the electromagnetic radiation response of the sample. Full article
(This article belongs to the Special Issue Fracturing of Coal and Rock Mass)
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