Article

# Controls of gas slippage and effective stress on the gas permeability of coal

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## Abstract

Dynamic coal reservoir permeability impacts the production of coalbed methane (CBM) and has been extensively studied. Gas (Helium) permeability change was measured under a 4.3MPa confining stress condition for 6 anthracite coal cores from the southern Qinshui Basin. Furthermore, controlling factors of the permeability change were investigated by comprehensively analyzing the effects of gas slippage and effective stress on the permeability. Results show that during gas pressure decline: (1) the permeability initially decreases but subsequently increases, during which the rebound begins at an inlet gas pressure of about 1.9MPa (corresponding to a mean gas pressure of 1.0MPa); (2) gas slippage phenomenon appears as the mean gas pressure falls bellow 1.0MPa; (3) the permeability is approximately negatively related to effective stress; (4) the permeability decreases due to the negative effect from effective stress as the inlet gas pressure is greater than 1.9MPa; while it increases when the pressure falls below 1.9MPa because the positive effect from gas slippage is stronger than the negative effect from effective stress.

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... Yin et al 15 found that the higher the gas pressure is, the shorter the time from the beginning of unloading confining pressure to stop failure, that is, the greater the strength of the coal sample. In terms of effective stress, Li et al 16 analyzed the effects of effective stress and slippage on the gas permeability of coal and rock. Meng 17 and Chen 18 carried out the study on stress sensitivity of permeability. ...
... where, σ 1 is the maximum principal stress, σ 3 is the minimum principal stress, and σ 3 are the maximum and minimum principal stress of coal failure, respectively; M is the strength parameter corresponding to the complete shear failure of uniaxial compression, N is the influence coefficient of confining pressure on axial-bearing capacity, which shows the influence of stress state on the bearing capacity of coal sample, equation (16) shows that the maximum axial stress σ 1 that can be carried by a given coal sample is linear with the confining pressure σ 3 , denoted as: ...
... According to equation (16), the data in Table 3 are analyzed by regression, the curve is shown in Figure 4D. From this, the internal friction angle φ = 43.05°, the cohesion c = 3.4 MPa. ...
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