The axial and radial permeability testing of coal under cyclic loading and unloading

  • Cun ZhangEmail author
  • Lei Zhang
  • Wei Wang
Original Paper


Porous reservoir rocks are often anisotropic in permeability. It is necessary, therefore, to study the effect of anisotropy on the permeability of porous coal seams that are subjected to stress during repeated mining of grouped coal seams. In the present investigation, specialized permeability evaluation equipment, known as gas injection to flush methane testing apparatus (GIFMTA), was used to study the directional permeability characteristics of the coal under various stress states according to the repeated mining of group coal seams. The stress states include three cyclic loading and unloading (0.5 MPa–16 MPa–0.5 MPa in a single cycle), and different axial stresses (0.5 MPa to 10 MPa) with the same confining stress, and different confining stresses (2 MPa to 10 MPa) with the same axial stress. The test results showed that the axial and radial permeability of the coal samples decreased with the increase in effective stress. The radial permeability of coal samples (parallel to the bedding structure) and their stress sensitivity were significantly larger than the axial permeability of the samples at the same effective stress level. It was observed that axial permeability was more sensitive to the confining stress than radial permeability, while the sensitivity of radial permeability to axial stress was much larger than that of axial permeability. The axial and radial permeability and their decrease during first loading were much greater than the case during later loading processes. With an increase in cycle time, the losses in axial and radial permeability, and their stress sensitivities, were observed to decrease gradually. Based on the laboratory test data and on a “matchstick model,” the axial and radial permeability models of the coal samples during a three-cycle loading/unloading procedure were obtained in this paper. Additionally, an anisotropic model was established to investigate the mechanisms that led to the test results using a digital evaluation model (DEM).


Permeability Coal seams Stress sensitivity Anisotropy Cyclic loading and unloading 


Funding information

Financial support for this work was supported by the Beijing Natural Science Foundation (8184082), the National Natural Science Foundation of China (Nos. 51874281, 51874312, and 51861145403), the Young Elite Scientists Sponsorship Program by CAST (2017QNRC001), the Open Fund of State Key Laboratory of Coal Resources and Safe Mining (No. SKLCRSM19KFA17), the Independent Research Project of State Key Laboratory of Water Resources Protection and Utilization in Coal Mining (NO. GJNY-18-77) and the Yue Qi Distinguished Scholar Project, China University of Mining and Technology, Beijing.


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Copyright information

© Saudi Society for Geosciences 2019

Authors and Affiliations

  1. 1.State Key Laboratory of Coal Resources and Safe MiningChina University of Mining and TechnologyBeijingChina
  2. 2.School of Resource and Safety EngineeringChina University of Mining and TechnologyBeijingChina
  3. 3.State Key Laboratory of Groundwater Protection and Utilization by Coal MiningNational Energy Group Co., Ltd.BeijingChina
  4. 4.Key Laboratory of Deep Coal Resource Ministry of Education of China, School of MinesChina University of Mining and TechnologyXuzhouChina

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