Experimental Study on the Mechanical Properties of Tectonic Coal Using Reconstituted Coal Specimens

Abstract

Tectonic coal, an aggregate of coal particles formed by compacting pulverized coal, has been developed extensively in China. Currently, reconstituted coal specimens are widely adopted to investigate the mechanical properties of tectonic coal, but they have a low compaction degree compared to the tectonic coal in the field. Therefore, the current understanding of the mechanical properties of tectonic coal is not accurate. Herein, a new high–pressure–resistant mold was developed, and a heavy press was developed to prepare highly compacted reconstituted coal specimens. Based on the reconstituted coal specimens and the intact coal specimens obtained through coring, the mechanical properties of tectonic coal and intact coal were measured and compared systematically. The results show that the compaction degree of reconstituted coal specimen can be improved significantly by increasing the external force. For Sijiazhuang coal, the compaction degree of the reconstituted coal specimen almost reaches that of the tectonic coal in the field when the external force is increased to 550 KN. Moreover, the tectonic coal exhibits a low elastic modulus and low strength but high stress sensitivity. The elastic modulus and cohesion of tectonic coal are 22.08% and 43.47% of the corresponding values for intact coal. However, with the increase in the confining pressure from 5 to 20 MPa, the elastic modulus of tectonic coal increases by 1.14 times, while that of the intact coal increases just by 8.70%. In addition, tectonic coal and intact coal present different post-peak failure modes under the triaxial compression stress path. Typical shear failure occurs in the intact coal, while multiple shear failure occurs in the tectonic coal.

1. Introduction

Coal is the main source of energy in China, which plays an important role in ensuring energy security and rapid economic development. However, the coal–bearing strata in China experienced multiple tectonic actions and stress fields, such as Indosinian, Yanshanian, and Himalayan periods, after their formation. During these tectonic movements, the coal underwent various degrees of brittle fracture, broken or ductile deformation, or superimposed destruction under the effect of tectonic stress fields, leading to severe crushing and even pulverization of the coal [1], as shown in Figure 1. The severely crushed or pulverized coal is usually called tectonic coal in China, i.e., tectonic coal is an aggregate of pulverized coal particles. After the formation of pulverized tectonic coal, it can be recompacted under tectonic stress. However, recompaction of the tectonic coal lowers its permeability and makes its gas extraction becomes rather more difficult. Therefore, a large amount of coalbed gas is expected to be released during the recovery of the tectonic coal reservoir. It is well known that coalbed gas is a major greenhouse gas. Thus, the gas emission in the tectonic coal reservoir can induce a serious greenhouse effect.

In order to achieve efficient gas extraction in the tectonic coal reservoir and thus reduce the greenhouse gas emissions, mechanical–based permeability–improving technologies, such as protective layer mining [2], hydraulic flushing [3], and hydraulic slotting [4], have been widely adopted in China. The basic principle of these technologies involves the achievement of stress unloading and plastic failure in the tectonic coal. After stress unloading and plastic failure in the coal, primary fractures get opened and many secondary fractures can be generated, which results in a significant improvement of the coal permeability. As a result, it is extremely necessary to fully understand the mechanical properties of tectonic coal in order to realize the efficient application of mechanical–based permeability–improving technologies in tectonic coal reservoirs. Recently, reconstituted coal specimens have been widely adopted to investigate the mechanical properties of tectonic coal, and some meaningful conclusions have been obtained [1]. For instance, Skoczylas et al. [5], Li et al. [6], Dong et al. [7], and Liu et al. [8] systematically measured the deformation behavior of tectonic coal. Their research results indicated that the elastic modulus of tectonic coal increased with confining pressure and loading speed, and its uniaxial compressive strength and elastic modulus were about one order of magnitude lower than those of the intact coal. Zhao et al. [9], Liu et al. [10], and Lu et al. [11] also evaluated the effect of gas on the mechanical properties of tectonic coal, and they reported that free gas has a greater weakening effect on the mechanical strength of coal than adsorbed gas. Lin et al. [12] and Tu et al. [13] tested the post–peak dilatancy of tectonic coal and found that the specimen expanded steadily due to the breaking of the bonds of coal particles in the post–peak stage. Gao et al. [14] investigated the energy evolution law of tectonic coal subjected to cyclic loads. They found that the elastic energy density of tectonic coal increased linearly with deviatoric stress and was proportional to the confining pressure and loading rate. However, the dissipated energy density increased exponentially with deviatoric stress and was inversely proportional to the confining pressure and loading rate.

The above–mentioned research results have greatly deepened the understanding of the mechanical properties of tectonic coal. However, the reconstituted coal specimens adopted in the above studies have a low compaction degree. Their densities and permeabilities are 1.1–1.2 m³/t and 1 mD, respectively. However, to the best of our knowledge, the actual densities and permeabilities of tectonic coals in the field are about 1.4 m³/t and 0.01–0.001 mD [15,16,17,18], respectively. Thus, the densities of reconstituted coal specimens are significantly lower than the actual densities of tectonic coals in the field, while their permeabilities are significantly higher than the actual permeabilities. Therefore, the current measurement results based on the low compacted reconstituted coal specimens cannot represent the mechanical properties of the tectonic coal in the field.

Based on the above discussion and analysis, a new high–pressure–resistant mold was developed herein, and a heavy press was adopted to prepare highly compacted reconstituted coal specimens. By using these coal specimens, the mechanical properties of tectonic coal and intact coal were measured and compared systematically. The research in this field can deepen the understanding of the mechanical properties of tectonic coal and would be of great significance to achieving efficient gas extraction and safe production in the tectonic coal reservoir.

2. Reconstitution of Tectonic Coal Specimens

2.1. Basic Physical Parameters of Coal Sample

In this study, the samples were obtained from the Sijiazhuang coal mine in the Yangquan coal field. In this coal mine, the 15# coal seam is the only one with mining value, and its thickness is ~6.0 m. In the 15# coal seam, tectonic coal has been widely developed. Before the reconstitution of tectonic coal specimens, the basic physical parameters of the 15# coal seam, including the proximate analysis, firmness coefficient (f value), initial gas diffusion velocity (ΔP), and the gas adsorption constant (V_L and P_L), were measured systematically. The measurement results are listed in

Table 1. Basic physical parameters of the 15# coal seam.

Coal	Proximate Analysis				f Value	ΔP (mmHg)	Gas Adsorption Constant
	Mad	Aad	Vdaf	Fcad			VL (m3/t)	PL (MPa)
Intact	2.96	5.86	10.72	84.31	1.32	28.6	35.75	1.41
Tectonic	2.64	8.32	8.64	83.68	0.18	41.3	41.58	1.27

The table presents the moisture content M_ad; the ash content A_ad; the volatile content V_daf; the fixed carbon content F_cad; maximum gas adsorption amount V_L; and gas adsorption pressure P_L.

.

Table 1 reveals that the volatile contents of intact coal and tectonic coal are both close to 10%, indicating that the 15# coal seam has a high metamorphic degree. Moreover, the volatile content of the tectonic coal is lower than that of the intact coal, indicating that dynamic metamorphism may occur during the formation process of tectonic coal. The firmness coefficient of tectonic coal is just 0.18, showing that it has low strength. Furthermore, the initial gas diffusion velocity and maximum gas adsorption amount of tectonic coal are both higher than those of intact coal, which indicates that the tectonic coal has better gas diffusion and adsorption capacities. In sum, tectonic coal exhibits low strength, high diffusion, and high gas adsorption capacities. Consequently, coal and gas outbursts often occur.

2.2. Reconstitution Equipment and Process

In order to measure the mechanical properties of tectonic coal, the first task is to prepare the standard tectonic coal specimen (Φ50 × 100 mm). Owing to the complete structure and high strength of intact coal, coring and cutting can be achieved [19,20,21]. However, tectonic coal is formed by the compaction of pulverized coal, so it is easily broken and impossible to obtain a core specimen. Therefore, reconstituted coal specimens have been widely adopted to measure the mechanical properties of tectonic coal. The reconstruction principle is shown in Figure 2.

As mentioned above, the currently available reconstituted coal specimens possess a low compaction degree. To overcome this problem, a new high–pressure–resistant mold was developed and a heavy press was adopted in this study to reconstitute the coal specimens, as shown in Figure 3. The mold was made of high–strength steel and fixed with two rows of fastening screws for safety. The inner diameter of the mold was 48 mm. During the reconstitution process, the mold could expand to a certain degree under the effect of external force. Therefore, the radius of the reconstituted coal specimen was expected to be very close to 50 mm. The detailed reconstitution process for a single coal specimen is as follows:

(1)

Weighing: the first step involves the weighing of a certain mass of pulverized tectonic coal particles. As for its quality, it should be adjusted repeatedly to ensure that the size of each reconstituted coal specimen is the standard size (Φ50 × 100 mm).

(2)

Wetting: a certain amount of distilled water is injected into the coal sample and evenly mixed by hand. During the reconstitution process, the distilled water can act as a binder.

(3)

Reconstituting: the wet coal particles are placed into the high–pressure–resistant mold, and a heavy press is used to provide external force to reconstitute the coal specimen. After the targeted force is achieved, the force is maintained for 12 h.

(4)

Demolding: after reconstitution, the screw of the mold is loosened, and the coal specimen is removed from the mold. To ensure a successful demolding, some lubricating oil can be applied on the inner surface of the mold before reconstitution.

(5)

Vacuum drying: the coal specimen is placed in a vacuum oven, and it is dried at 60 °C for 48 h.

2.3. Investigation of the Compaction Degree

Based on the above–mentioned method and equipment, a series of tectonic coal specimens were reconstituted in this study by adopting different external forces (150, 250, 350, 450, and 550 KN). Next, the internal structural characteristics, density, and permeability of these coal specimens were measured systematically to investigate the compaction degree.

2.3.1. Internal Structural Characteristics

To obtain the internal structural characteristics of the reconstituted coal specimens, computed tomography (CT) scanning experiments with low resolution (50 μm) were conducted in the Taiyuan University of Technology. For brevity, only the CT scan images of three coal samples (i.e., corresponding external force is 150, 350, and 550 KN, respectively) are provided in Figure 4. It is seen from Figure 4 that some clear large–scale fractures exist in the middle or at the bottom of the coal specimens under an external force of 150 and 350 KN, indicating that these specimens have a relatively low compaction degree. Moreover, the large–scale fractures almost vanish in the coal specimen reconstituted at an external force of 550 KN. Therefore, it can be inferred that the compaction degree of the reconstituted coal specimen improves with the increase in the applied external force.

2.3.2. Density and Permeability Data

In addition to the CT scanning experiments, the density and permeability of reconstituted coal specimens were also measured. The measurement principle of coal specimen density is shown in Figure 5. First, a high–precision electronic balance was used to accurately measure the quantity of the specimen. Then, the coal specimen was wrapped in plastic film, and its volume was measured using a measuring cylinder. Furthermore, the permeability of the specimen was measured by adopting the steady–state method. During the specimen permeability measurement process, methane with a pressure of 1.5 MPa was utilized, and the confining pressure was increased from 4 to 24 MPa.

The density and permeability measurement results are shown in Figure 6. Figure 6a exhibits the density measurement results, indicating that the specimen density increases from 1.18 to 1.38 m³/t when the external force increases from 150 to 550 KN. In the Sijiazhuang coalmine, the density of the 15# coal seam is ~1.41 m³/t. Therefore, when the external force increases to 550 KN, the density of the reconstituted specimen is very close to that of the tectonic coal in the field. As seen from the permeability measurement results in Figure 6b, the specimen permeability is negatively correlated with confining pressure, which is consistent with the current perception. Moreover, the greater the external force, the lower the specimen permeability. Considering the confining pressure of 24 MPa as an example, the permeability of the specimen reconstituted under 150 KN external force is 0.04479 mD, while that of the specimen reconstituted under 550 KN decreases to 0.00651 mD. Under the current mining depth in the Sijiazhuang coalmine, the coal permeability ranges from 0.003 to 0.01 mD, and the average in situ stress is approximately 20 MPa. Figure 6b illustrates that the permeability reconstituted at 550 KN is ~0.00824 mD under a confining pressure of 20 MPa, which is very close to field permeability data. According to the density and permeability measurement results, it can also be concluded that the compaction degree of the reconstituted coal specimen improves with the external force. The greater the external force, the higher the compaction degree. Moreover, when the external force increases to 550 KN, the compaction degree of the reconstituted specimen is very close to that of the tectonic coal in the field.

3. Mechanical Properties of Tectonic Coal

The above results indicate that the compaction degree of the reconstituted coal specimen is almost the same as that of the tectonic coal in the field under an external force of 550 KN. Therefore, in this study, the external force of 550 KN was adopted to reconstitute the tectonic coal specimen. Based on the reconstituted coal specimens and the intact coal specimens obtained through coring, the mechanical properties of tectonic coal and intact coal were measured and compared. In this study, the uniaxial and triaxial compressive stress paths were adopted. The mechanical and seepage experiment system introduced by Liu et al. [8] was used herein as the experimental equipment. Under the triaxial compressive stress path, the following confining stress (σ₃) values were adopted: 5, 10, 15, and 20 MPa. During the experimental process, the confining stress was first loaded to the target value at a rate of 0.05 MPa/s. For the loading of the axial stress, the stress loading mode was adopted up to the yield point. Subsequently, the displacement loading mode was adopted until the end of the experiment. Under the stress loading mode, the loading rate of axial stress was 0.05 MPa/s. Under the displacement loading mode, the displacement loading rate was 5 mm/min. Moreover, a set of large range extensometers was used in this study considering the large deformation property of tectonic coal, as shown in Figure 7. The range of the axial extensometer is 0–25 mm and that of the radial extensometer is 0–12.5 mm.

3.1. Total Stress–Strain Curve

Figure 8 exhibits the total stress–strain curves of intact coal and tectonic coal. Both intact coal and tectonic coal exhibit an elastic–brittle property under the uniaxial compression stress. After reaching the peak point, the axial stress drops rapidly. However, the total stress–strain curves of intact coal and tectonic coal are obviously different under the triaxial compression stress path. Notably, in the pre–peak stage, the stress–strain curve of tectonic coal is much flatter than that of the intact coal, indicating that tectonic coal shows weak resistance to deformation. Moreover, the tectonic coal shows significant stress–hardening characteristics in the pre–peak stage. According to the study on rock mechanics, irreversible plastic deformation has already occurred in the strain–hardening stage of coal mass. Therefore, new fractures are generated in the pre–peak stage of tectonic coal, which is very important for the permeability increasing of tectonic coal. In the post–peak stage, intact coal shows a typical stress–softening characteristic. However, the tectonic coal just shows a certain strain–softening characteristic under a low confining pressure. After the confining pressure reaches 15 MPa, the stress drop is rather weak in the post–peak stage, and the coal tends to be perfectly plastic.

3.2. Deformation and Strength Properties

3.2.1. Deformation Property

According to the total stress–strain curve measurement results shown in Figure 8, the deformation parameters, including the elastic modulus (E) and Poisson’s ratio (μ), were calculated carefully. The calculation results are listed in

Table 2. Deformation parameters of intact coal and tectonic coal.

Coal	σ3 (MPa)	E (MPa)	Eav (MPa)	μ (Dimensionless)	μav (Dimensionless)
Intact	0	1364	2939	0.35	0.33
	5	3196		0.31
	10	3268		0.38
	15	3392		0.30
	20	3474		0.33
Tectonic	0	363	649	0.33	0.35
	5	478		0.35
	10	632		0.36
	15	753		0.33
	20	1021		0.38

. The results indicate that the elastic modulus under the triaxial condition is greater than that under the uniaxial condition for both the intact coal and the tectonic coal. Furthermore, the elastic modulus increases with confining stress under the triaxial condition. The elastic modulus of the intact coal increases by 8.70% from 3196 to 3474 MPa with an increase in the confining stress from 5 to 20 MPa. In contrast, the elastic modulus of the tectonic coal increases from 478 to 1021 MPa under the same condition, i.e., the elastic modulus increases by 1.14 times. Therefore, the elastic modulus of tectonic coal shows stronger stress sensitivity. Moreover, the elastic modulus of intact coal is significantly greater than that of tectonic coal. The average elastic modulus (E_av) of intact coal is 2939 MPa, which is 4.53 times that of the tectonic coal. In other words, the average elastic modulus of tectonic coal is just ~22.08% of that of intact coal. The low value and high confining stress sensitivity of the elastic modulus of tectonic coal are closely related to its structure. As mentioned above, tectonic coal is an aggregate of coal particles formed by compacting pulverized coal. As a result, micro–fractures are more developed in tectonic coal and its stiffness is relatively low, which results in the low value and high confining stress sensitivity of its elastic modulus.

The average Poisson’s ratio of tectonic coal is also greater than that of the intact coal, indicating that tectonic coal also shows a stronger lateral deformation capacity; however, it is not significant.

3.2.2. Strength Property

Currently, the Mohr–Coulomb criterion has been widely adopted to judge the failure of coal or rock mass. According to the Mohr–Coulomb criterion, the coal strength can be described by the internal friction angle and cohesion, as presented by Equation (1).

$${\sigma }_1=\frac{2c\cos \phi }{1-\sin \phi }+{\tan }^2(45+\frac{\phi }{2}){\sigma }_3$$

(1)

where $c$ is the cohesion in MPa and $\phi$ denotes the internal friction angle.

According to the peak strengths of coal under different confining stresses shown in Figure 8, Equation (1) can be adopted to calculate internal friction and cohesion. Figure 9 shows the fitting results of axial stress and confining stress at the peak point, which indicates that the existence of a significant linear relationship between the axial stress and the confining stress. Equation (1) reveals that the slope and intercept of the fitting curve are functions of internal friction angle and cohesion. Therefore, the internal friction angle and cohesion of coal can be further calculated by solving the slope and the intercept data. It should be noted that the uniaxial data were not adopted during this process because shear failure did not occur in the coal under the uniaxial condition.

The internal friction angle and cohesion obtained by the above–mentioned method are presented in

Table 3. Internal friction angle and cohesion calculation results.

Coal	Internal Friction Angle (°)	Cohesion (MPa)
Intact	38.46	4.21
Tectonic	34.28	1.83

. The results indicate that the internal friction angle and cohesion of tectonic coal are lower than those of the intact coal. In particular, the cohesion (1.83 MPa) of tectonic coal is just 43.47% of that of intact coal (4.21 MPa). Under the current mining depth (500–600 m) in China, the coal strength is mainly controlled by cohesion [8]. Therefore, tectonic coal has low strength compared to intact coal. The low strength and weak cohesive property of tectonic coal are also closely related to its structure. In the tectonic coal, cohesion mainly occurs by the mechanical biting force between coal particles due to its particle aggregation property. In contrast, the intact coal basically retains its primary structure, so the cohesion is provided by the chemical bonding force between coal molecules. Compared to the chemical bonding force, the mechanical biting force is rather weak. Therefore, tectonic coal has a low strength and weak cohesive property.

3.3. Post–Peak Failure Mode and Mechanism

3.3.1. Post–Peak Failure Mode

Plastic failure occurs in the coal mass in the post–peak stage. Figure 10 shows the post–peak failure characteristics of intact coal and tectonic coal. Under the uniaxial compression stress path, axial fracture develops both in the intact coal and the tectonic coal. The difference is that one large fracture develops in the intact coal, while several small fractures are developed in the tectonic coal. After opening the heat shrink tube, the intact coal breaks into two halves and the tectonic coal is highly crushed. Under the triaxial compressive stress path, one macroscopic shear fracture is developed in the intact coal, and some coal particles are formed around the shear fracture. Obviously, the failure of the intact coal corresponds to brittle failure characteristics. For tectonic coal, no obvious shear fracture is found under the triaxial compression stress path. However, after opening the heat shrink tube, the tectonic coal is found to be highly pulverized, indicating the formation of an implicit fracture group during the coal failure process. The failure of tectonic coal obviously has ductile failure characteristics.

Figure 11 exhibits the typical failure modes of coal and rock masses, indicating that axial splitting failure occurs both in the intact coal and tectonic coal under the uniaxial compression stress path. However, under the triaxial compression stress path, shear failure occurs in the intact coal, while multiple shear failure occurs in the tectonic coal.

3.3.2. Post–Peak Failure Mechanism

In general, the axial splitting failure of the coal mass under the uniaxial compression stress path is believed to be caused by the radial tension deformation. However, under the triaxial compression stress path, the difference in the failure modes of intact coal and tectonic coal is also related to their structures. For the intact coal, the coal mass basically retains its primary structure and is closely connected with chemical bonds. Thus, its failure mechanism can be explained by adopting the typical Mohr–Coulomb criterion. Figure 12a shows that the shear failure always occurs at the most unfavorable plane under the coupling effect of normal stress and shear stress. The tectonic coal is an aggregate of coal particles, so its failure can be explained by adopting the particle flow theory, as shown in Figure 12b. The figure illustrates that the failure of granular materials mainly includes three modes, i.e., tensile failure, shear failure, and compression failure. Obviously, the failure mode of the tectonic coal particle is shear failure. With the continuous loading of the axial stress, the shear stress suffered by the tectonic coal particles increases gradually. When the shear stress exceeds its own low shear strength that is mainly provided by the mechanical biting force between adjacent particles, shear failure occurs along the coal particle. Then, the residual shear strength of the coal particle is just provided by the friction force, which is poor. Therefore, rotation and sliding can further occur in the failed coal particles, inducing a wide range of failure and coal pulverization. This is denoted as the multiple shear failure mechanism of tectonic coal.

3.4. Failure Angle Evolution of Intact Coal

In Section 3.3.1, the post–peak failure modes of intact coal and tectonic coal under the triaxial compression stress were illustrated, and their analysis is presented by considering the confining pressure of 15 MPa as an example. Under different confining pressure values, the failure angle of tectonic coal barely changes. However, the failure angle of intact coal changes with the confining pressure. Figure 13 shows that the failure angle of intact coal decreases with the increase in confining pressure. When the confining pressure increases from 5 to 20 MPa, the failure angle decreases from 69° to 46°. The experimental results are in good agreement with the results on rock materials reported by Ma et al. [23] and Zhou et al. [24].

According to the Mohr–Coulomb criterion, the failure angle of coal and rock masses can be calculated as follows:

$$\beta =\frac{\pi }{4}+\frac{\phi }{2}$$

(2)

where $\beta$ is the failure angle in °.

According to the calculation results of internal friction angle presented in Section 3.2.2, the failure angle of intact coal should be 64.2°. The difference between the theoretical and measurement results is caused by the oversimplification of the Mohr–Coulomb criterion. In the Mohr–Coulomb criterion, the envelope is a straight line, so the internal friction angle is a constant. However, the real envelope is not a straight line, and the internal friction angle decreases with the increase in confining pressure. Therefore, the failure angle of intact coal decreases with the increase in confining pressure.

4. Conclusions

In this study, a new high–pressure–resistant mold was developed, and a heavy press was adopted to prepare highly compacted reconstituted coal specimens based on the density and permeability indexes. Then, the basic mechanical properties of tectonic coal and intact coal were measured and comprehensively compared. The main conclusions of this study are as follows:

(1)

Increasing the external stress is an effective method to improve the compaction degree of the reconstructed coal sample. With the increase in the external force, the inner cracks close, the specimen density increases, and the specimen permeability decreases. When the external force is increased to 550 KN, the specimen density increases to 1.38 m³/t, and its permeability decreases to 0.00824 mD under the field stress level. Density and permeability measurement results are fairly close to field permeability data. Therefore, for the Sijiazhuang coal, a 550 KN external force can be used to reconstitute the tectonic coal specimen.

(2)

Compared with intact coal, tectonic coal exhibits a low elastic modulus but a high stress sensitivity due to its particle aggregation property. Its average elastic modulus is only 22.08% of that of tectonic coal. Moreover, with the increase in the confining pressure from 5 to 20 MPa, the elastic modulus of intact coal increases by just 8.70%, while that of tectonic coal increases by 1.14 times. In addition, tectonic coal also exhibits a low strength and weak cohesive properties due to the fact that its cohesion is only provided by the mechanical biting force between the coal particles. Its cohesion is only 43.47% of that of intact coal.

(3)

Tectonic coal and intact coal present different post–peak failure modes under the triaxial compression stress path. Typical shear failure occurs in intact coal at the most unfavorable plane under the coupling effect of normal stress and shear stress, while multiple shear failure occurs in tectonic coal due to its particle aggregation property. In addition, the failure angle of intact coal decreases with the increase in confining pressure. As the confining pressure increases from 5 to 20 MPa, the failure angle decreases from 69° to 46°.