A Study on the Effect of Different Charge Structures on the Permeability Enhancement of Coal Seam Blasting

Abstract

With the increasing depth of coal mining, the ground stress of coal seams rises, the gas content rises, and the permeability decreases, which is unfavorable to gas extraction and increases the possibility of gas accidents in coal mines. Blasting technology is often used to improve the permeability of coal seams and increase the effect of gas extraction, but it is difficult to control the blasting effect of ordinary flux coils, and the direction of the fissure and the degree of development of randomness, so it is important to study the effect of different flux coil structures on the blasting of coal seams to increase the permeability of the coal seam, in this paper, first of all, through the numerical simulation to analyze the effect of the blasting of the ordinary structure flux coils and the fissure change in the polygonal structure flux coils, and then make the experimental module. Then, we make the test module, build the test platform, and analyze the effect of blasting penetration of different drug coil structures by comparing the strain after blasting and the change in resistivity before and after blasting of two kinds of drug coil structures, and the results show that the polymerized drug coil can play the role of directional fracturing, and compared with the ordinary structure of the drug coil, the polymerized drug shows a better effect of fracturing and penetration increase.

1. Introduction

Blasting technology is an important engineering technique that utilizes the tremendous energy generated by explosions to break rocks or structures, achieving predetermined engineering purposes. The application of blasting technology is extensive, including mining [1,2], shaft construction, [3], highway tunnel construction [4,5,6], underwater demolition of structures [7,8], and urban subway construction [9].

Due to China’s energy resource situation characterized by “abundant coal, scarce oil, and limited gas”, coal will continue to play a dominant role in the country’s energy structure for a long time [10]. As the depth of coal mining continues to increase [11], the geological conditions of coal seams have significantly changed compared to shallow coal seams. These changes include higher ground stress, reduced permeability, and increased gas content [12]. These alterations lead to higher mining costs and reduced safety. Therefore, in deep coal seam mining, enhancing coal seam permeability and reducing gas content are crucial for the safe production of coal mines.

Currently, techniques such as hydraulic fracturing [13], hydraulic punching [14,15], and deep-hole blasting [16] are commonly used to improve the permeability of high-gas, low-permeability coal seams.

Li et al. monitored and characterized the spatial shapes of fractures induced by hydraulic fracturing in coal seams underground [17]. Peng et al. analyzed the changes in the physical fields during the hydraulic fracturing process and proposed a multiscale, multimodal detection method based on variations in wave velocity and resistivity caused by fracture propagation to determine the effective range of hydraulic fracturing in coal seams [13]. Huang et al. studied the influence of stress fields, coal seam inclination, and fracturing fluid displacement on the expansion patterns of hydraulic fractures in interlayer coal, analyzing the formation process and expansion mechanism of hydraulic fractures in interlayer coal [18]. Zhang et al. developed a permeability model for coal-containing hydraulic support fractures and concluded that the higher the mass concentration of coal powder in the suspension and the larger the volume fraction of deposited coal powder, the lower the porosity and permeability of the support fractures [19].

Liu et al. studied the parameters of hydraulic punching. The study results indicate that the variation in the flaw parameters changes the propagation pattern of cracks, resulting in different compressive strengths [20]. Zhao et al. developed a multifield model that integrates stress, damage, gas diffusion, and gas flow in heterogeneous coal. They simulated coal damage induced by stress release around slot-shaped boreholes and conducted engineering field validation [21]. Shi et al. proposed an enhanced coalbed methane (ECBM) synergistic extraction technology based on the combination of high-frequency and gas injection displacement (GD), which can eliminate the stress concentration zones (SC) generated by hydraulic fracturing (HF) and improve gas extraction efficiency [22]. Liu et al. established a gas flow-geomechanics coupling model based on an equivalent fractured coal model. The stress release zone, plastic zone, and permeability enhancement zone around boreholes in soft coal are significantly larger than those in hard coal. The degree of stress concentration and permeability reduction in soft coal are also greater than in hard coal [23].

Chen et al. applied deep-hole pre-splitting blasting technology at the Xinji No. 2 Mine, which doubled the gas drainage volume, confirming the effectiveness of this technology in enhancing coal seam permeability [24]. Liu et al., through numerical simulations and theoretical analysis, concluded that blasting in hard rock induces numerous fractures and significant displacements due to the explosive stress. Additionally, the desorption gas flow from the top coal cavities and along the fractures improves the gas extraction capability [25].

Directional blasting technology uses concentrated explosive energy and high-penetration shaped jets formed during the explosion to induce rock fractures [26,27,28]. Shaped charge blasting technology employs grooving on both sides of the charge to achieve directional fracture control [26,27]. Directional energy-concentrated blasting technology is mostly used in rock fracturing research and related engineering applications [28,29,30,31]. Zhu et al. investigated the synergistic effect of shaped charge blasting and control hole [32], as well as the mechanical impact of shaped charge jet in the process of coal fracturing [26].

In summary, there are many technical methods to increase the permeability of high gas coal seams, and blasting still occupies an important position, but blasting technology also has its shortcomings, such as too large a range of crushing zone, random distribution of fissure direction. Energy-concentrated blasting has the effect of directional fracturing, this technology is mainly used in other fields, it is rare to apply it to increase the permeability of coal seams, so it is of great significance to carry out the research on blasting penetration enhancement by changing the structure of the pill rolls for high-gas and low-permeability coal seams.

2. Numerical Simulation Study of Permeability Enhancement by Blasting

Using the ANSYS/LS-DYNA (v. 18.2) three-dimensional numerical simulation analysis software, a coal-rock model with dimensions of 50 cm × 50 cm × 50 cm was established. The model includes a roof thickness of 15 cm, a coal seam thickness of 20 cm, and a floor thickness of 15 cm. The blasting hole is located at the center of the coal seam with a diameter of 20 mm. Non-reflective boundary conditions were applied to all faces of the model. The explosive material was modeled using the high-energy material model (MAT_HIGH_EXPLOSIVE_BURN) available in ANSYS/LS-DYNA., and the JWL equation was chosen as the equation of state for the explosive [33,34]. The JWL equation is as follows:

$$\mathrm{P}=\mathrm{A}(1-{\displaystyle \frac{\mathsf{\omega }}{{\mathrm{R}}_1\mathrm{V}}}){\mathrm{e}}^{-{\mathrm{R}}_1\mathrm{V}}+\mathrm{B}(1-{\displaystyle \frac{\mathsf{\omega }}{{\mathrm{R}}_2\mathrm{V}}}){\mathrm{e}}^{-{\mathrm{R}}_2\mathrm{V}}+{\displaystyle \frac{\mathsf{\omega }{\mathrm{E}}_0}{\mathrm{V}}}$$

(1)

In the equation: A and B are explosive characteristic parameters in GPa. R₁, R₂, and ω are dimensionless explosive characteristic parameters. P is the pressure generated by the explosive detonation in MPa. E₀ is the initial internal energy ratio of the detonation products in GPa. V is the relative volume in m³.

In numerical calculations, the parameters of the coal-rock layer are as shown in

Table 1. Physical and mechanical parameters of coal and rock.

Medium	Elastic Modulus E/(GPa)	Poisson’s Ratio	Compressive Strengths/(MPa)	Tensile Strength/(MPa)
coal seam	4.5	0.38	1.06	0.2
rock stratum	9.6	0.24	7.34	0.5

, and the specific parameters of the explosive during the simulation process are shown in

Table 2. Explosive material parameter.

ρ/(g/cm3)	D/(m·s−1)	A/(GPa)	B/(GPa)	R1	R2	ω	V
1.3	2800	0.36	28	4.15	0.95	0.3	1

.

The material of the shaped charge is copper, and the MAT_JOHNSON_COOK model is utilized. The EOS_GRUNEISEN model is used to define its equation of state. The material parameters of the shaped charge are shown in

Table 3. Material parameter table of shaped medicine rolls.

ρ/(g/cm3)	C	S1	S2	S3	γ0	a	E0	V0
8.96	0.46	1.489	0	0	2.02	0.47	0	1

, and the schematic diagram of different shaped charge structures is depicted in Figure 1.

2.1. Comparative Analysis of Cloud Diagrams of Different Shaped Charge Structures after Blasting

Using the LS-Pre Post post-processing software to read the d3plot header file of the model and analyze the simulation results. Utilizing the post-processing software Fcomp to demonstrate the evolution process of effective stress near the blasting hole during the initial stage of blasting. The cloud diagrams of effective stress at different time points for two sets of models are shown in Figure 2 and Figure 3.

From Figure 2, it can be observed that after the conventional shaped charge detonates, the blasting shock wave first acts on the walls of the blasting hole, creating a large crushing zone around the blasting hole. As the stress wave continues to propagate, noticeable fracture development occurs around the blasting hole.

From Figure 3, it can be observed that after the detonation of the shaped charge with energy concentration, the energy focusing hood first forms a metal jet acting on the blasting hole, causing cracks to form in the direction of energy concentration, creating guiding fissures. The overall shape around the blasting hole appears to be elliptical.

By comparing Figure 2 and Figure 3, it can be seen that by changing the shaped charge structure, the evolution process of stress changes around the blasting hole can be altered. The fragmentation zone of the shaped charge with energy concentration is smaller than that of the conventional shaped charge. Therefore, the shaped charge with energy concentration can apply more energy to the generation and development of fractures.

2.2. Comparative Analysis of Fractures after Blasting with Different Shaped Charge Structures

Previously, we discussed the different stress cloud diagrams near the blasting hole due to different shaped charge structures during the blasting process. Through post-processing software, the development and expansion process of fractures within the model can be further displayed, as shown in Figure 4 and Figure 5.

From Figure 4, it can be observed that after the detonation of the conventional shaped charge, fractures around the blasting hole develop sufficiently, but the direction of fractures is relatively random, exhibiting a radial pattern.

From Figure 5, it can be observed that after the detonation of the shaped charge with energy concentration, fractures initially occur in the direction of energy concentration around the blasting hole and continue to develop. The degree of fracture development in the direction of energy concentration is greater than in the non-energy concentration direction.

Figure 4d and Figure 5d were processed using Matlab to statistically calculate the fractal dimension of different roll structures, and the statistical results are shown in

Table 4. Fractal dimensions after blasting for different drug coil structures.

Structure of the Pill Rolls	Position	Fractal Dimension
Conventional Shaped Charge	Down (base plate)	1.1298
	Mid (a coal seam)	1.5397
	Up (roof plate)	1.1425
Shaped Charge with Energy Concentration	Down (base plate)	0.91685
	Mid (a coal seam)	1.5593
	Up (roof plate)	0.59612

. According to the statistical results, it can be concluded that the fractal dimension of the coal seam is small and the fractal dimension of the top and bottom plate is large after blasting with ordinary drug rolls, and the fractal dimension of the coal seam is large and the fractal dimension of the top and bottom plate is small after blasting with polymerized drug rolls, which means that polymerized drug rolls enhance the destruction of the coal seam and reduce the damage of the top and bottom plate, and they can play a role of directional fracture effect.

Overall, from Figure 4 and Figure 5, it can be concluded that changing the shaped charge structure can alter the degree of fracture development around the blasting hole. Shaped charges with energy concentration can play a role in directional fracturing.

3. Similar Simulation Study on Permeability Enhancement by Blasting

In order to investigate the permeability enhancement effect after blasting with different shaped charge structures, a similar simulation experiment on blasting-induced permeability enhancement was conducted. The experiment simulated a coal-rock formation at a certain location in the Pansan Mine, Anhui Province. After determining the similar materials and proportions, an experimental platform was constructed, with the overall design scheme shown in Figure 6. Firstly, the positions of strain collection points, parallel electrical method measurement points, and shaped charge structures were designed. Secondly, test blocks were fabricated according to the design scheme. Finally, blasting experiments were conducted, and data were collected. Before both blasting models completed the blasting tests, the NPEI-DHZI network parallel electrical resistivity instrumentation was used to conduct data measurements on the models after blasting.

According to the experimental plan, precast strain bricks measuring 2 cm × 2 cm × 1 cm were prepared. During the production process of the blasting test blocks, the strain bricks were arranged in the coal seam. Specifically, strain brick 1 and strain brick 2 were horizontally placed on the left and right sides of the blasting hole, respectively, with the blasting hole as the axis. The distance between strain brick 1 and the blasting hole was 80 mm, while the distance between strain brick 2 and the blasting hole was 160 mm. Strain brick 3 was placed directly above the blasting hole, in the same plane as strain brick 1, and at a distance of 80 mm from the blasting hole.

Test module in the production of layered filling method, in the filling to the design of the height of the blast hole, pre-buried 25 mm diameter, length of 30 cm PVC pipe.

According to the experimental plan, the parallel electrical method measurement points were arranged at the interface between the coal seam and the roof. When filling the similar materials to the corresponding height, 32 measurement points were arranged in sequence, and each measurement point was numbered consecutively to prepare for the measurement of the electrical method before and after blasting.

The explosive charges used in this experiment are conventional shaped charges and directional energy-concentrated shaped charges. The main difference between them lies in the internal charge structure. For the directional energy-concentrated shaped charges, PVC pipes with a diameter of 20 mm, a wall thickness of 1 mm, and an inner diameter of 18 mm are used as the charge casing. The energy-concentrating hood inside the charge casing is made of 0.1 mm thick copper sheet. After the completion of the charge casing, it is filled with black powder and detonated using a detonator before the blasting experiment. Conventional shaped charges are also made using PVC pipes of the same specifications, with no special treatment to the internal structure.

After 28 days of curing in the similar model used for the experiment, the pre-embedded PVC pipes are removed to form blast holes. The depth of the holes is 30 cm, with a diameter of 25 mm. The experimentally self-made PVC pipe charges with a diameter of 20 mm are inserted into the blast holes, and a mixture of yellow clay, fine sand, and lime in certain proportions is used to seal the blast holes.

3.1. Comparative Analysis of Strain after Blasting with Different Shaped Charge Structures

The experiment collected strain data from different points using the SDY2107A super dynamic strain gauge. After organizing the collected strain data, it was plotted into Figure 7, Figure 8 and Figure 9.

At Point 1 of the two similar model test blocks, a comparison between Figure 7a,b reveals that before 400 μs is the dynamic loading phase of the blasting. During this phase, the coal body is mainly affected by the explosion stress wave, resulting in drastic changes in stress values. Both simulated test blocks showed two stress peaks during this phase. In the simulated test block filled with conventional shaped charges, the strain peak at Point 1 was −29,693 με, while in the simulated test block filled with directional energy-concentrated shaped charges, the strain peak at Point 1 was −36,230 με. It can be calculated that the strain peak in the horizontal energy-concentrating direction using directional energy-concentrated shaped charges is approximately 1.22 times that of conventional shaped charges. During the static loading phase, the coal body is mainly affected by the detonation gases, resulting in a smaller range of strain changes that gradually attenuate over time. In Figure 7a, the static loading phase of the model test block filled with conventional shaped charges is from 400 to 1000 μs, with a duration of 600 μs. In Figure 7b, the static loading phase of the model test block filled with directional energy-concentrated shaped charges is from 400 to 1200 μs, with a duration of 800 μs. This indicates that in the horizontal direction, the directional energy-concentrated shaped charges have a longer duration of detonation gas action compared to conventional shaped charges, extending the time by approximately 200 μs, reflecting the ability of directional energy-concentrated shaped charges to guide more detonation gases towards the energy-concentrating direction.

At Point 2 of the two similar model test blocks, a comparison between Figure 8a,b reveals that before 600 μs is the dynamic loading phase of the blasting. During this phase, the coal body is mainly affected by the explosion stress wave, resulting in drastic changes in stress values. Both simulated test blocks showed two stress peaks during this phase. In the simulated test block filled with conventional shaped charges, the strain peak at Point 2 was −26,511 με, while in the simulated test block filled with directional energy-concentrated shaped charges, the strain peak at Point 2 was −32,304 με. It can be calculated that the strain peak in the horizontal energy-concentrating direction using directional energy-concentrated shaped charges is approximately 1.22 times that of conventional shaped charges. During the static loading phase, in Figure 8a, the static loading phase of the model test block filled with conventional shaped charges is from 600 μs to 1200 μs, with a duration of 600 μs. In Figure 8b, the static loading phase of the model test block filled with directional energy-concentrated shaped charges is from 600 μs to 1400 μs, with a duration of 800 μs. This reflects that in the horizontal direction, directional energy-concentrated shaped charges can prolong the duration of detonation gas action, guiding more detonation gases towards the energy-concentrating direction.

At Point 3 of the two similar model test blocks, a comparison between Figure 9a,b reveals that before 400 μs is the dynamic loading phase of the blasting. During this phase, the coal body is mainly affected by the explosion stress wave, resulting in drastic changes in stress values. Both simulated test blocks showed two stress peaks during this phase. In the simulated test block filled with conventional shaped charges, the strain peak at Point 3 was −28,974 με, while in the simulated test block filled with directional energy-concentrated shaped charges, the strain peak at Point 3 was −22,378 με. It can be observed that in the vertical direction, the strain peak of the conventional shaped charges is greater than that of the directional energy-concentrated shaped charges, approximately 1.29 times higher. During the static loading phase, in Figure 9a, the static loading phase of the model test block filled with conventional shaped charges is from 400 μs to 1200 μs, with a duration of 800 μs. In Figure 9b, the static loading phase of the model test block filled with directional energy-concentrated shaped charges is from 400 μs to 900 μs, with a duration of 500 μs. This indicates that in the vertical direction, the duration of detonation gas action for directional energy-concentrated shaped charges is shorter than that of conventional shaped charges, approximately 300 μs less, indicating a weaker ability of energy-concentrated shaped charges to promote coal body fracture development in the non-energy-concentrating direction.

3.2. Analysis of Parallel Electrical Resistivity Instrumentation after Blasting with Different Cartridge Structures

After both blasting models completed the blasting tests, the NPEI-DHZI network parallel electrical resistivity instrumentation was used to conduct data measurements on the models after blasting, as shown in Figure 10. The collected data were exported using software and decrypted using the corresponding decryption software.

The decrypted data were imported into the WBD2.0 (Network Parallel Electrical Resistivity Processing System) software for slicing and extraction of data slices at a depth of 15 cm below the electrical resistivity instrument measurement points, corresponding to the XY plane of the blasting hole. The extracted data files were then processed in Surfer software to generate contour maps of electrical resistivity.

As shown above, where Figure 11a depicts the contour map of resistivity before blasting for the block with ordinary explosive charge, and Figure 11b shows the contour map of resistivity after blasting for the block with ordinary explosive charge. From Figure 11a, it can be observed that the resistivity in most areas is approximately 360 Ω·m, distributed relatively uniformly. From Figure 11b, it can be seen that after blasting, the maximum resistivity at the center position is 500 Ω·m, while the resistivity values around it are approximately 450 Ω·m. Comparing Figure 11a,b, it is evident that after blasting with ordinary explosive charge, a crushed zone appears near the blasting hole. Therefore, the resistivity value at the center position is higher than that of its surroundings. The resistivity at the center position has increased by approximately 140 Ω·m after blasting, representing an increase of approximately 38.9%. The resistivity around the center has increased by approximately 90 Ω·m, representing an increase of approximately 38.9%. There is a slight increase in resistivity in the four corner areas, but the variation is relatively small compared to other positions.

As shown above, Figure 12a displays the contour map of resistivity before blasting for the test block filled with directional energy-aggregated explosive charges, while Figure 12b shows the contour map of resistivity after blasting for the same block. From Figure 12a, it can be observed that the resistivity at the center of this plane is 300 Ω·m, with values ranging between 250 Ω·m and 300 Ω·m in its vicinity. In Figure 12b, after blasting, the resistivity at the center of this plane is 500 Ω·m, with surrounding resistivity ranging between 480 Ω·m and 490 Ω·m. By comparing Figure 12a,b, it is evident that the resistivity at the center of this plane has increased by approximately 200 Ω·m after blasting, representing an increase of approximately 66.7%. The resistivity in the surrounding area has increased by approximately 280 Ω·m, with an increase ranging from 93.3% to 112%, indicating sufficient development of fractures around the blasting hole. Although there is an increase in resistivity in the four corner regions, it is relatively smaller compared to the center. By comparing Figure 11b and Figure 12b, it can be seen that there is a significant difference in the impact of the two types of charge structures on the surrounding fracture zones after blasting, with the directional energy-aggregated explosive charge demonstrating superior effectiveness in promoting fracture development in the direction of energy aggregation compared to the ordinary explosive charge.

4. Conclusions

In this study, the following conclusions were obtained from the previous numerical simulations and blasting tests:

(1)

Through numerical simulations and comparing the stress and fracture conditions after blasting with two types of explosive charges, it can be concluded that directional energy-aggregated explosive charges can achieve directional fracturing, reduce energy loss in the crushing zone, and promote higher degree of fracture development in the direction of energy aggregation.

(2)

Analysis of data at position 1# of the two test blocks leads to the following conclusions: During the dynamic loading phase, the coal body is mainly affected by the explosion stress wave, resulting in drastic changes in stress values. The peak strain value in the horizontal energy aggregation direction using directional energy-aggregated explosive charges is approximately 1.22 times that of ordinary explosive charges. Analysis of data at position 2# indicates that directional energy-aggregated explosive charges can prolong the action time of the explosion gas in the horizontal direction. Analysis of data at position 3# reveals that in the non-energy aggregation direction, the peak strain value of ordinary explosive charges is greater than that of directional energy-aggregated explosive charges, indicating a weakening effect of directional energy-aggregated explosive charges. The action time of directional energy-aggregated explosive charges in the non-energy aggregation direction is shorter than that of ordinary explosive charges.

(3)

Analysis of resistivity data inverted by the electrical resistivity instrument reveals that for the test block filled with ordinary explosive charges, the resistance jump amplitude on the selected plane ranges from 90 Ω·m to 140 Ω·m. For the test block filled with directional energy-aggregated explosive charges, the resistance jump amplitude on the selected plane ranges from 180 Ω·m to 200 Ω·m. Comparing the resistivity contour maps before and after the action of two different explosive charges, it can be concluded that directional energy-aggregated explosive charges are more effective in promoting fracture development in the direction of energy aggregation than ordinary explosive charges.

(4)

Compared to ordinary explosive charges, directional energy-aggregated explosive charges demonstrate better directional fracturing effects, promote higher degree of coal seam fracture development, and are conducive to increasing coal seam permeability, thereby improving gas extraction efficiency.

(5)

The gas content of the coal seam and gas pressure is also a factor that affects the effect of blasting penetration, limited by the test conditions, the research carried out in this paper will be simplified, to be continued in the future research.