# Repeated Low-Intensity Shock Wave Simulation Experiment for Fracture Development and Propagation of Coal

Authors: Taofeng Xie; Xiaoting Zhou; Qingya Wang; Shuai Cao; Huiming Cao; Wengang Huang
Journal: Shock and Vibration
 (2019)

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## Abstract

For low-permeability coal seam, the gas extraction rate is relatively low. The repeated low-intensity shock wave can improve the permeability of coal and raise the rate of coal seam gas drainage. A simulation test platform was set up to carry out repeated low-intensity shock wave simulation experiment. Under the effect of repeated low-intensity shock wave, the development process of the macrofracture, pore evolution, and the law of microcrack propagation was analyzed to study the law of coal fracture propagation. Research shows that the expansion of the pore of the coal is made by the development of large, medium, and micropores by the repeated low-strength shock wave. The main contribution of the total pore volume increase comes from the micropore growth. The microcrack of the coal mainly begins to sprout and develops from the telocollinite where the fracture is more developed. With the increase of impact times, the microcracks extend to other components. Under the impact of different times, the fractal dimension of the coal sample increases with the increase in the number and length of cracks.


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## Body

## 1. Introduction

Improving the permeability of coal seam and increasing gas drainage have always been the main research direction of gas disaster control and gas resource extraction technology [1, 2]. Hydraulic sand fracturing is the main measure to improve permeability of coal seam at present [3–6]. However, due to its own characteristics, the fracturing effect is severely constrained by the conditions of the coal seam. Many injuries caused by fracturing to coal seams will affect subsequent mining, reduce production efficiency, and increase production costs [7–10]. It is an effective method to improve the efficiency of gas extraction by adopting explosions increasing permeability of coal seam [11–13]. De-yong et al. applied deep hole energy blasting technology to increase the permeability of the coal seam [14]. Ren et al. developed high-voltage electrohydraulic discharge (EHD) [15]. Strong pressure shock waves were generated by underwater electrical discharges or high energy explosion, which then served to damage the associated solids, creating fractures and cracks. But the secondary disasters such as roof fall were easily caused by strong pressure shock waves [16–18].Xu et al. used the explosive fracturing experiment device developed by them to carry out the explosive fracturing simulation experiment for low-permeability reservoir and analyze the growth of the cracks produced by the explosive fracturing in the samples [19]. Lin et al. studied the phenomenon of the rock damage and fracture by the shock wave of exploding in fractures via the experiment of exploding on the surface of the cement sample in deep water [20].The permeability of the coal seam can be increased in a softer way by using repeated low-intensity shock wave. By establishing a simulation test platform, we simulated the law of coal rock fracture expansion under repeated low-strength shocks and further studied the mechanism of coal cracking, which provides a basis for optimizing the parameters and analyzing the effects of increasing permeability of coal under repeated low-strength impact.


## 2. Materials and Methods

To study the influence of the low-intensity shock wave on fracture development of coal fissure and pore, repeated low-intensity impact tests were carried out on the coal samples under three kinds of low-intensity shock wave loading conditions using a self-built experimental platform under the condition of no confining pressure.


### 2.1. Experimental Device

The low-intensity explosion impact simulation experiment device mainly consists of three parts, such as a water tank (1.2 m × 1.2 m × 1.2 m in size, water depth 0.8 m), a simulated shock wave generator, and a shock wave pressure sensor (Figure1). The simulated drilling casing size is 139.7 mm, the hole density is 60 holes per metre, and the hole diameter is 10 mm. The shock wave generating device triggers the energetic bomb with electric energy to form a low-intensity shock wave. The shock wave pressure sensor is fixed on the water tank. The probe position is parallel to the center point of the shock wave generating window, and the distance is 300 mm.Figure 1
Schematic diagram of an experimental device.
(a)
(b)
(c)We use the small pieces dropped during the impact process under different impact times to make a polished block (0.3 m × 0.3 m × 0.3 m in size) and use NIS-Elements Documentation optical microscope for quantitative observation (Figure2). The NIS-Elements Documentation optical microscope has the function of phototaking and splicing.Figure 2
Microphotometer and polished lump coal section: (a) NIS-Elements Documentation optical microscope; (b) polished block.
(a)
(b)


### 2.2. Properties of Coal Samples

The sample of the experimental coal was taken from No. 7 coal seam of Wulan Coal Mine in Hulusitai Mining Area of Ningxia, which belongs to fat coal. Proximate analysis, determination of reflectance of mirror mass, and quantitative experiments on the microscopic composition are carried out on coal samples. The basic properties of coal samples are shown in Table1.Table 1
Coal sample and basic properties.
Coal rank
Sampling sites
Sample
R
0,max (%)
Proximate analysis
Permeability (md)
Porosity (%)
M
ad (%)
A
d (%)
V
daf (%)
Fat coal
Wulan coal mine
F1
1.18
0.86
6.42
35.21
0.025
3.43
F2
1.19
1.03
4.71
29.70
0.105
3.99
F3
1.17
1.30
5.81
29.68
0.065
3.85The large coal samples were cut into samples of 30 cm × 30 cm × 30 cm size along parallel and vertical lamination directions and polished, and then the cut scratches were removed. The shock wave loading positions of all samples are facing the center area of plane B (Figure3).Figure 3
Photos of coal samples: (a) F1; (b) F2; (c) F3.
(a)
(b)
(c)


### 2.3. Mechanical Properties of Shock Wave

The shock waveforms were measured during the experiment in the case of 5 g, 10 g, and 15 g energetic projectiles (Figure4). The trend of attenuation of all waveforms is basically similar. And the impact is instantaneous. The impact pressure becomes more than 4 MPa within 0.1 ms and instantly becomes 0 MPa. The maximum peak pressure at 5 g energetic bombs is 4.0 MPa, at 10 g energetic bomb is 5.0 MPa, and at 15 g energetic bomb is 5.4 MPa.Figure 4
Waveforms of electric pulse wave: (a) 5 g energetic bomb; (b) 10 g energetic bomb; (c) 15 g energetic bomb.
(a)
(b)
(c)


### 2.4. Approaches and Processing Procedures

The processed coal sample needs to be immersed in the tap water for 72 hours, simulating the impact work environment of the saturated water coal to reduce the influence of water environment error on the process and degree of coal sample rupture. In order to prevent the coal from falling into the water due to cracking during the impact, a metal mesh with a mesh size of 10 mm × 10 mm was used to wrap the coal block. Then, the coal block was immersed in the water tank along the parallel bedding direction, and the occurrence window of the shock wave was consistent with the central position of the coal sample. The distance between coal sample and shock wave source is 100 mm. Different intensity shock waves were simulated with 5 g, 10 g, and 15 g bombs, respectively. The power supply of the power control cabinet to generate an explosion shock wave was turned on. Then, the power supply was turned off, and the shock wave pressure detected by the oscilloscope was saved. The coal block breakage was checked, and a coal sample impact experiment was completed.The experiment step was repeated, and the coal block was placed out of the water tank at certain impact times. The surface of the coal body was photographed and sampled. The entire process was repeated until the coal sample was broken, and the experiment was ended.


## 2.1. Experimental Device

The low-intensity explosion impact simulation experiment device mainly consists of three parts, such as a water tank (1.2 m × 1.2 m × 1.2 m in size, water depth 0.8 m), a simulated shock wave generator, and a shock wave pressure sensor (Figure1). The simulated drilling casing size is 139.7 mm, the hole density is 60 holes per metre, and the hole diameter is 10 mm. The shock wave generating device triggers the energetic bomb with electric energy to form a low-intensity shock wave. The shock wave pressure sensor is fixed on the water tank. The probe position is parallel to the center point of the shock wave generating window, and the distance is 300 mm.Figure 1
Schematic diagram of an experimental device.
(a)
(b)
(c)We use the small pieces dropped during the impact process under different impact times to make a polished block (0.3 m × 0.3 m × 0.3 m in size) and use NIS-Elements Documentation optical microscope for quantitative observation (Figure2). The NIS-Elements Documentation optical microscope has the function of phototaking and splicing.Figure 2
Microphotometer and polished lump coal section: (a) NIS-Elements Documentation optical microscope; (b) polished block.
(a)
(b)


## 2.2. Properties of Coal Samples

The sample of the experimental coal was taken from No. 7 coal seam of Wulan Coal Mine in Hulusitai Mining Area of Ningxia, which belongs to fat coal. Proximate analysis, determination of reflectance of mirror mass, and quantitative experiments on the microscopic composition are carried out on coal samples. The basic properties of coal samples are shown in Table1.Table 1
Coal sample and basic properties.
Coal rank
Sampling sites
Sample
R
0,max (%)
Proximate analysis
Permeability (md)
Porosity (%)
M
ad (%)
A
d (%)
V
daf (%)
Fat coal
Wulan coal mine
F1
1.18
0.86
6.42
35.21
0.025
3.43
F2
1.19
1.03
4.71
29.70
0.105
3.99
F3
1.17
1.30
5.81
29.68
0.065
3.85The large coal samples were cut into samples of 30 cm × 30 cm × 30 cm size along parallel and vertical lamination directions and polished, and then the cut scratches were removed. The shock wave loading positions of all samples are facing the center area of plane B (Figure3).Figure 3
Photos of coal samples: (a) F1; (b) F2; (c) F3.
(a)
(b)
(c)


## 2.3. Mechanical Properties of Shock Wave

The shock waveforms were measured during the experiment in the case of 5 g, 10 g, and 15 g energetic projectiles (Figure4). The trend of attenuation of all waveforms is basically similar. And the impact is instantaneous. The impact pressure becomes more than 4 MPa within 0.1 ms and instantly becomes 0 MPa. The maximum peak pressure at 5 g energetic bombs is 4.0 MPa, at 10 g energetic bomb is 5.0 MPa, and at 15 g energetic bomb is 5.4 MPa.Figure 4
Waveforms of electric pulse wave: (a) 5 g energetic bomb; (b) 10 g energetic bomb; (c) 15 g energetic bomb.
(a)
(b)
(c)


## 2.4. Approaches and Processing Procedures

The processed coal sample needs to be immersed in the tap water for 72 hours, simulating the impact work environment of the saturated water coal to reduce the influence of water environment error on the process and degree of coal sample rupture. In order to prevent the coal from falling into the water due to cracking during the impact, a metal mesh with a mesh size of 10 mm × 10 mm was used to wrap the coal block. Then, the coal block was immersed in the water tank along the parallel bedding direction, and the occurrence window of the shock wave was consistent with the central position of the coal sample. The distance between coal sample and shock wave source is 100 mm. Different intensity shock waves were simulated with 5 g, 10 g, and 15 g bombs, respectively. The power supply of the power control cabinet to generate an explosion shock wave was turned on. Then, the power supply was turned off, and the shock wave pressure detected by the oscilloscope was saved. The coal block breakage was checked, and a coal sample impact experiment was completed.The experiment step was repeated, and the coal block was placed out of the water tank at certain impact times. The surface of the coal body was photographed and sampled. The entire process was repeated until the coal sample was broken, and the experiment was ended.


## 3. Results and Discussion

Coal is a three-element porous medium composed of macroscopic fissure, microfissure, and pore. Microfissure is the bridge between pore and fissure. Pore and fissure are discontinuous spaces in coal rock, which determine the permeability of coal rock fissure [21].


### 3.1. Analysis of Growth of Macrocracks

Under the action of different energy shock waves, the law of fracture generation and propagation of the coal body is approximately the same, all of which are around the primary crack to further expand and develop. Finally, the coal structure ruptures and disintegrates (Figure5).Figure 5
Development of macroscopic cracks: (a) F1; (b) F2; (c) F3. (A) Impact 0 times; (B) impact 2 times; (C) impact 6 times; (D) impact 8 times.
(a)
(b)
(c)At first, the number of cracks slowly increased, and the cracks mainly extended from the end of the cracking. Then, the fissure increases rapidly, mainly by open fork hyperplasia, and it showed that the crack width increased and the coal body broke. The complexity of the crack network tended to maximize, and the connectivity between the cracks increased. It can be seen from the law of macroscopic crack expansion that, with the increase of the number of shocks, the degree of coal fracture gradually deepened and finally disintegrated. All experimental coal samples were fractured and disintegrated after 8 impacts. There is a threshold in the number of shocks. If the number of shocks exceeds the threshold, the coal body will be destroyed rapidly, and the macrocracks will increase obviously. Before that, the coal has been in the period of fracture incubation under the impact.


### 3.2. Pore Evolution Law

The pore structure changes of coal samples F1 and F2 under the impact of 5 g and 10 g energetic shock waves before and after loading were studied by means of mercury injection experiments. The mercury injection experiment results of the pore structure of the sample are shown in Table2. The aperture structure is divided by B. B. Hodort’s decimal scheme (micropores <10 nm, small pores 10–100 nm, medium pores 100–1000 nm, and large pores >1000 nm.).Table 2
Pore structure parameters of fat coals by using mercury intrusion porosimetry.
Loading conditions
Sample
Impact number
Total pore volume (10−4 cm3/g)
Porosity (%)
Pore volume (10−4 cm3/g)
Large pores
Medium pores
Small pores
Micropores
5 g energetic projectiles
F1-0
0
334
3.99
29
14
98
193
F1-2
2
339
4.01
44
17
94
184
F1-4
4
339
4.30
58
16
89
176
F1-6
6
351
4.07
38
13
102
198
F1-8
8
332
4.03
48
15
90
179
F1-10
10
379
4.37
47
9
108
215
10 g energetic projectiles
F2-0
0
313
3.85
30
17
95
171
F2-2
2
355
3.92
47
14
105
189
F2-4
4
333
3.99
27
14
101
191
F2-6
6
379
4.39
54
17
108
200
F2-8
8
356
4.18
54
18
100
184
F2-10
10
354
3.98
31
15
107
201Under the action of repeated shock waves, the expansion of coal pores was alternated between the development of large and medium pores and micropores and small pores. The increase of pore volume at each stage is due to the transformation of smaller pore size into larger pore size. The fluctuation degree of the pore volume of the large and medium pores and the micropores and small pores with the number of impacts reflects the degree of adjustment of the pore structure during the shock wave action. The greater the degree of fluctuation, the stronger the impact of the explosive shock wave on the pore structure of coal body, and the more favorable the pore improvement (Figure6).Figure 6
Plots of pore volume to shock times for fat coal samples: (a) 5 g energetic projectiles; (b) 10 g energetic projectiles.
(a)
(b)


### 3.3. Evolution Law of Fracture Propagation

The development of microcracks in coal samples F1 and F3 with different energies and impact times under the action of low-intensity explosion shock waves was counted [14], and the results are shown in Table 3. The development of microcracks was described and analyzed by using the results of the double objective lens of the polished block under different loading conditions.Table 3
Statistics of microcracks of coal samples.
Coal rank
Sample
Loading conditions
Impact number
Linear density (mm)
Areal density (mm2)
Length between 0.625 and 20 mm
Crack number,N
Fractal dimension,D
Fat coal
F1
5 g energetic projectiles
0
0.694
0.4025
161
1.810
4
0.784
0.8625
345
1.938
6
0.808
0.8375
335
1.716
10
0.867
0.97
388
1.679
F3
15 g energetic projectiles
0
0.8
0.5375
215
2.007
2
0.922
0.5525
221
1.829
6
1.244
0.745
298
1.688
8
1.389
1.365
546
2.065As shown in Figure7, under the action of the explosive shock wave of 5 g or 15 g energetic bomb, the microcracks of coal samples mainly began to germinate and develop from the telocollinite with more developed cracks. And with the increase of impact times, it expanded to other components. The boundaries of the components of the fat coal are clear, and there are a lot of telocollinite bands in fat coal. In the early stages of the impact, it was mainly short and small fissure development. With the increase of impact times, cracks expanded through components and connected to each other. The length of cracks continued to increase.Figure 7
Evolution of microfractures in No. F1 and F3 fat coal samples (×20): (a) F1-0; (b) F1-4; (c) F1-6; (d) F1-10; (e) F3-0; (f) F3-2; (g) F3-6; (h) F3-8.
(a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)As shown in Figures8 and 9, under the impact of coal samples, linear density and areal density of microfissure increased significantly and showed a good trend of fracture development.Figure 8
Plots of linear density to shock times for fat coal samples.Figure 9
Plots of areal density to shock times for fat coal samples.


### 3.4. Permeability

The air permeability of 3 coal samples was tested to study the effect of the shock wave on the coal seam. After the experiment, the porosity and permeability of coal were higher than the original coal samples. The three coal samples were impacted 10 times with 5 g, 10 g, and 15 g energy bombs, respectively, and the permeability of the coal body increased significantly. Under the impact of 15 g energy bombs, the permeability of the coal body increased by more than 12 times (Figure10).Figure 10
Testing results of porosity and permeability of coal samples.


## 3.1. Analysis of Growth of Macrocracks

Under the action of different energy shock waves, the law of fracture generation and propagation of the coal body is approximately the same, all of which are around the primary crack to further expand and develop. Finally, the coal structure ruptures and disintegrates (Figure5).Figure 5
Development of macroscopic cracks: (a) F1; (b) F2; (c) F3. (A) Impact 0 times; (B) impact 2 times; (C) impact 6 times; (D) impact 8 times.
(a)
(b)
(c)At first, the number of cracks slowly increased, and the cracks mainly extended from the end of the cracking. Then, the fissure increases rapidly, mainly by open fork hyperplasia, and it showed that the crack width increased and the coal body broke. The complexity of the crack network tended to maximize, and the connectivity between the cracks increased. It can be seen from the law of macroscopic crack expansion that, with the increase of the number of shocks, the degree of coal fracture gradually deepened and finally disintegrated. All experimental coal samples were fractured and disintegrated after 8 impacts. There is a threshold in the number of shocks. If the number of shocks exceeds the threshold, the coal body will be destroyed rapidly, and the macrocracks will increase obviously. Before that, the coal has been in the period of fracture incubation under the impact.


## 3.2. Pore Evolution Law

The pore structure changes of coal samples F1 and F2 under the impact of 5 g and 10 g energetic shock waves before and after loading were studied by means of mercury injection experiments. The mercury injection experiment results of the pore structure of the sample are shown in Table2. The aperture structure is divided by B. B. Hodort’s decimal scheme (micropores <10 nm, small pores 10–100 nm, medium pores 100–1000 nm, and large pores >1000 nm.).Table 2
Pore structure parameters of fat coals by using mercury intrusion porosimetry.
Loading conditions
Sample
Impact number
Total pore volume (10−4 cm3/g)
Porosity (%)
Pore volume (10−4 cm3/g)
Large pores
Medium pores
Small pores
Micropores
5 g energetic projectiles
F1-0
0
334
3.99
29
14
98
193
F1-2
2
339
4.01
44
17
94
184
F1-4
4
339
4.30
58
16
89
176
F1-6
6
351
4.07
38
13
102
198
F1-8
8
332
4.03
48
15
90
179
F1-10
10
379
4.37
47
9
108
215
10 g energetic projectiles
F2-0
0
313
3.85
30
17
95
171
F2-2
2
355
3.92
47
14
105
189
F2-4
4
333
3.99
27
14
101
191
F2-6
6
379
4.39
54
17
108
200
F2-8
8
356
4.18
54
18
100
184
F2-10
10
354
3.98
31
15
107
201Under the action of repeated shock waves, the expansion of coal pores was alternated between the development of large and medium pores and micropores and small pores. The increase of pore volume at each stage is due to the transformation of smaller pore size into larger pore size. The fluctuation degree of the pore volume of the large and medium pores and the micropores and small pores with the number of impacts reflects the degree of adjustment of the pore structure during the shock wave action. The greater the degree of fluctuation, the stronger the impact of the explosive shock wave on the pore structure of coal body, and the more favorable the pore improvement (Figure6).Figure 6
Plots of pore volume to shock times for fat coal samples: (a) 5 g energetic projectiles; (b) 10 g energetic projectiles.
(a)
(b)


## 3.3. Evolution Law of Fracture Propagation

The development of microcracks in coal samples F1 and F3 with different energies and impact times under the action of low-intensity explosion shock waves was counted [14], and the results are shown in Table 3. The development of microcracks was described and analyzed by using the results of the double objective lens of the polished block under different loading conditions.Table 3
Statistics of microcracks of coal samples.
Coal rank
Sample
Loading conditions
Impact number
Linear density (mm)
Areal density (mm2)
Length between 0.625 and 20 mm
Crack number,N
Fractal dimension,D
Fat coal
F1
5 g energetic projectiles
0
0.694
0.4025
161
1.810
4
0.784
0.8625
345
1.938
6
0.808
0.8375
335
1.716
10
0.867
0.97
388
1.679
F3
15 g energetic projectiles
0
0.8
0.5375
215
2.007
2
0.922
0.5525
221
1.829
6
1.244
0.745
298
1.688
8
1.389
1.365
546
2.065As shown in Figure7, under the action of the explosive shock wave of 5 g or 15 g energetic bomb, the microcracks of coal samples mainly began to germinate and develop from the telocollinite with more developed cracks. And with the increase of impact times, it expanded to other components. The boundaries of the components of the fat coal are clear, and there are a lot of telocollinite bands in fat coal. In the early stages of the impact, it was mainly short and small fissure development. With the increase of impact times, cracks expanded through components and connected to each other. The length of cracks continued to increase.Figure 7
Evolution of microfractures in No. F1 and F3 fat coal samples (×20): (a) F1-0; (b) F1-4; (c) F1-6; (d) F1-10; (e) F3-0; (f) F3-2; (g) F3-6; (h) F3-8.
(a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)As shown in Figures8 and 9, under the impact of coal samples, linear density and areal density of microfissure increased significantly and showed a good trend of fracture development.Figure 8
Plots of linear density to shock times for fat coal samples.Figure 9
Plots of areal density to shock times for fat coal samples.


## 3.4. Permeability

The air permeability of 3 coal samples was tested to study the effect of the shock wave on the coal seam. After the experiment, the porosity and permeability of coal were higher than the original coal samples. The three coal samples were impacted 10 times with 5 g, 10 g, and 15 g energy bombs, respectively, and the permeability of the coal body increased significantly. Under the impact of 15 g energy bombs, the permeability of the coal body increased by more than 12 times (Figure10).Figure 10
Testing results of porosity and permeability of coal samples.


## 4. Conclusions

Under repeated shock waves, the pore expansion of the coal body is carried out alternately with the development of large, medium, and small holes. The fluctuation trend of total pore volume is basically the same as that of micropores. The change of total pore volume mainly comes from micropores, and the impact can affect the expansion and development of micropores.The microcracks of coal samples mainly began to germinate and develop from the telocollinite with more developed cracks. With the increase of impact times, cracks expanded through components and connected with each other and the length increases.With different impact times, the number of microcracks increases and the length of cracks increases. The alternation of crack initiation and expansion makes the cracks develop continuously and improves the coal permeability.



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*Source: 3023702-2019-05-26.xml*