Comparative Study on the Seepage Characteristics of Gas-Containing Briquette and Raw Coal in Complete Stress–Strain Process
Abstract
:1. Introduction
2. Test Process
2.1. Test Materials
2.2. Test Device and Principle
2.3. Test Procedure
- (1)
- The sample, whose cling film was unwrapped before the test, was fixed with the upper and lower ventilative plates by a thin heat shrinkable film. Then, the fixed sample was wrapped with insulating tape against the hydraulic oil in the pressure chamber during the test. Finally, the sample was wrapped with a thick heat shrinkable film to ensure its air tightness (Figure 4);
- (2)
- Test instruments including the gas pipe, the flow meter, etc., were connected. Axial pressure, confining pressure, and gas pressure were applied to the coal sample in turn, where the amounts of three pressures followed the order: axial pressure of 4 MPa = confining pressure of 4 MPa > gas pressure of 1.2 MPa. Afterwards, the air tightness of the equipment was checked again. After each equipment in the system operated normally, the sample was allowed to fully adsorb gas for 24 h;
- (3)
- The outlet valve was opened to release gas for 30 min until the gas flow stabilized, and then the test started. The loading, controlled by the displacement, proceeded at a rate of 0.002 mm/s until the coal sample finally failed.
3. Test Results and Analysis
3.1. Comparative Analysis on the Complete Stress–Strain Curves
- (1)
- Initial compaction stage (OA section): With the rise of axial pressure, the stiffnesses of the two coal samples are enhanced gradually, and the primary fractures are gradually compacted and closed, leading to the shrink of seepage channels. As a result, the permeabilities of both briquette and raw coal decline to some extent;
- (2)
- Elastic deformation stage (AB section): The stress–strain curves of the two coal samples show approximately linear variations. Raw coal is barely damaged internally, so all its primary damages only deform elastically. As its primary micropores and microfractures further close, the coal permeability to gas continues to decline, but such a decline is insignificant owing to its low initial permeability. In contrast, under the action of the external load, the cohesive force of briquette is reduced by the extrusion and dislocation of its particles. Resultantly, the primary fractures between the particles are filled, leading to a rapid decline in its permeability. Besides, its permeability is the most sensitive to stress in this stage;
- (3)
- Plastic deformation stage (BC section): The permeabilities of the two coal samples begin to grow. With the rise of axial pressure, the continuous distributed damages that occurred inside the raw coal create a condition for the stable extension of more and more microfractures, causing plastic deformation. At this time, the permeability of raw coal grows rapidly due to the further development of primary fractures and the formation of new fractures, and the permeability is the most sensitive to stress in the plastic deformation stage. For briquette, the shear movement of its particles facilitates the stable extension of fractures. However, the newly generated fractures are blocked by the detached particles (as they squeeze and displace each other), and thus the permeability grows slowly;
- (4)
- Instability damage stage (CD section): As raw coal experiences a stress drop, where its damage develops from continuous damage to local damage, its fractures with elastic deformation undergo elastic unloading deformation. Consequently, the inelastic strain borne by primary fractures gradually focuses on few fractures generated by the local damage. These large instability-induced fractures enable the gas to pass through smoothly and promote the permeability of raw coal rapidly. However, as briquette only develops based on shear damage, its bearing capacity begins to decline, and its internal structure disenables a sudden stress drop, so its permeability grows only gently.
3.2. Relation between Permeability and Axial Stress
4. Discussion
4.1. Sensitivity Analysis on Gas Permeabilities of Briquette and Raw Coal
4.2. Analysis of the Suddenness of Coal and Gas Outbursts
5. Conclusions
- (1)
- The complete stress–strain curves of briquette and raw coal have similar trends and can be divided into four developing stages: initial compaction stage, elastic deformation stage, plastic deformation stage, and instability damage stage. However, due to the difference between the two samples in structural property, their deformation and damage mechanisms are different. Briquette has a lower compressive strength and experiences a much more severe deformation than raw coal. Therefore, the applicability of briquette in place of raw coal for the simulation study on mechanical behavior needs to be investigated further;
- (2)
- Permeability changes with the deformation and damage of coal, and the variation trend corresponds to the deformation and damage developing stage under load. As the axial pressure rises in the loading process, the permeability of raw coal declines or remains almost unchanged in the initial compaction stage and the elastic deformation stage, and then surges in the plastic deformation stage and the instability damage stage. In contrast, that of briquette plunges in the initial compaction stage and the elastic deformation stage, remains almost unchanged in the plastic deformation stage, and grows steadily in the instability damage stage, but its final permeability is lower than the initial value;
- (3)
- Under triaxial compression, the permeability of briquette is the most sensitive to axial pressure in the elastic deformation stage, while that of raw coal is the most sensitive in the plastic deformation stage. In the test process, compared with the stable damage of briquette, the sudden damage of raw coal is closer to the suddenness of on-site coal and gas outbursts;
- (4)
- The variation law of coal permeability is related to the law of coal deformation, and the four stages of the complete stress–strain-seepage test of raw coal can well explain the four stages of the coal and gas outburst process, i.e., preparation, initiation, development, and termination. Therefore, coal and gas outbursts can be predicted by utilizing the variation laws of coal deformation and damage and permeability in the field.
Author Contributions
Funding
Informed Consent Statement
Data Availability Statement
Acknowledgments
Conflicts of Interest
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Ding, K.; Wang, L.; Li, Z.; Guo, J.; Ren, B.; Jiang, C.; Wang, S. Comparative Study on the Seepage Characteristics of Gas-Containing Briquette and Raw Coal in Complete Stress–Strain Process. Materials 2022, 15, 6205. https://doi.org/10.3390/ma15186205
Ding K, Wang L, Li Z, Guo J, Ren B, Jiang C, Wang S. Comparative Study on the Seepage Characteristics of Gas-Containing Briquette and Raw Coal in Complete Stress–Strain Process. Materials. 2022; 15(18):6205. https://doi.org/10.3390/ma15186205
Chicago/Turabian StyleDing, Ke, Lianguo Wang, Zhaolin Li, Jiaxing Guo, Bo Ren, Chongyang Jiang, and Shuai Wang. 2022. "Comparative Study on the Seepage Characteristics of Gas-Containing Briquette and Raw Coal in Complete Stress–Strain Process" Materials 15, no. 18: 6205. https://doi.org/10.3390/ma15186205