LAUSR

Abstract Seepage is the main gas migration form in coal seam. The experiments in this study show that there exist threshold pressure gradient and low-velocity nonlinear seepage phenomenon when non-adsorbable helium flows in coal briquette. To further explain that from microscopic view, we establish a capillary seepage model and use a new experimental method to characterize the equivalent seepage channel size distribution (ESCSD) of coal under different effective stress conditions (3–48 MPa). The experimental results prove that the ESCSD of coal obeys fractal distribution. The fractal dimensions in this study are in the range of 2.30–3.36, and decrease significantly with the increase of effective stress. Under the condition of effective stress 1 μm in the experimental coal sample always control most seepage discharge (>90%). Based on the theoretically simplified seepage model and the measured threshold pressure gradient for seepage, the relationship between the critical seepage channel size (dcr) of helium in coal and the pressure gradient ( ∂ P / ∂ L ) is obtained, that is, d c r = α ∂ P / ∂ L and α is in the range of 0.0024–0.0033 Pa. Note that the critical condition here represents the transition not from flow to no flow but from seepage to diffusion.