Changes in microstructure and mechanical properties of low-permeability coal induced by pulsating nitrogen fatigue fracturing tests

Yi Xue, Jia Liu, P. G. Ranjith, Feng Gao, Heping Xie, Jun Wang

Research output: Contribution to journalArticleResearchpeer-review

54 Citations (Scopus)

Abstract

Effective methane extraction in underground coal seams can improve the efficient utilization of fossil energy and reduce the risk of safety accidents in coal mines. Pulsating nitrogen fatigue fracturing technology is proposed as a novel and effective method to enhance gas production in low-permeability coal seams and improve gas extraction efficiency. In this study, a pulsating gas test system was established to apply fatigue fracturing of pulsating nitrogen to low-permeability coal. Mercury intrusion tests, wave velocity tests, and triaxial compression tests were used to reveal the changes in microstructure and mechanical properties of low-permeability coal under fatigue fracturing tests. Results show that the residual deformation of the coal changes considerably under fatigue fracturing. The strain of the coal is characterized by a periodic “expansion–contraction” variation with the intrusion and discharge of the pulsating nitrogen, and the residual strain increases gradually in this process. After fatigue fracturing, facilitates the seepage of gas and the development of micropores and transition pores toward mesopores and macropores enhances the permeability of the coal. The specific surface area of the pores is considerably improved in the transition pores and mesopores. The pore fractal dimension of the coal tends to decrease under fatigue fracturing, resulting in a more uniform distribution of pores and enhanced interpore connectivity within the coal. The fatigue period is negatively correlated with the strength and wave velocity evolution of the coal, and the peak intensity first decreases rapidly and then gradually stabilizes. The fatigue fracturing results in a substantial increase in the spatial complexity and connectivity of the fracture distribution throughout the specimen. On the basis of the evolution characteristics of residual volumetric strain, a fatigue damage model was constructed to analyze the characteristics of fatigue deformation and failure of coal.

Original languageEnglish
Pages (from-to)7469-7488
Number of pages20
JournalRock Mechanics and Rock Engineering
Volume55
Issue number12
DOIs
Publication statusPublished - Dec 2022

Keywords

  • Damage
  • Low-permeability coal
  • Mercury intrusion tests
  • Microstructure
  • Residual volumetric strain

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