The axial and radial permeability testing of coal under cyclic loading and unloading
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ORIGINAL PAPER
The axial and radial permeability testing of coal under cyclic loading and unloading Cun Zhang 1,2,3 & Lei Zhang 4 & Wei Wang 2 Received: 31 January 2018 / Revised: 24 January 2019 / Accepted: 16 May 2019 / Published online: 1 June 2019 # Saudi Society for Geosciences 2019
Abstract 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 Bmatchstick 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). Keywords Permeability . Coal seams . Stress sensitivity . Anisotropy . Cyclic loading and unloading
Introduction The stress sensitivity of coal seam permeability refers to the degree to which coal seam permeability is affected by an applied stress (Pan et al. 2010; Chen et al. 2016). With the application of pressure relief mining in high-gas and lowpermeability coal seams, the stress sensitivity of high-gas and low-permeability coal seams affects directly the
effectiveness of pressure relief mining (Yang et al. 2011; Zhang et al. 2015a). The study of stress sensitivity of coal seams generally is based on oil and gas extraction. Stress models such as P&M, S&D, C&B, and others were applied to predict the change in stress permeability during the process of
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