Influences of path control effects on characteristics of gas migration in a coal reservoir

In this paper, the influences of path control effects on characteristics of gas migration in coal samples with different sizes were investigated by using self-developed test device. Moreover, the mathematical model regarding path control effects on the gas migration was established. The results show that the curves of gas diffusion velocity in the coal samples with different sizes gradually decrease and then flatten with diffusion time. At the initial stage of diffusion, the gas diffusion velocity in coal samples with a small geometric size (short migration path) is larger than that in the coal samples with a large geometric size (long migration path). However, with the test proceeding, the phenomenon of velocity transcendence appears. The cumulative gas diffusion from the coal samples increases sharply with the rise of gas diffusion velocity, and its growth rate decreases gradually with the decrease of gas diffusion velocity. Finally, the gas diffusion curve tends to be flat. Although there is a phenomenon of transcendence in the gas diffusion velocity, transcendence in the cumulative gas diffusion has not been found. By equating the free space around the coal samples into a large fracture connecting to the coal samples, a fracture system was formed together with pore structures in the coal samples to affect gas migration process.