Gas flow characteristics of coal samples with different levels of fracture network complexity under triaxial loading and unloading conditions

Research on the characteristics of gas flow in coal rocks is fundamentally important for coalbed methane exploitation. Laboratory tests of prefractured coal samples may be a practical means of simulating the fractured media encountered in actual engineering applications. In this study, coal samples were mechanically prefractured, and the complexity of the prefractured structures was described quantitatively. Several gas flow experiments were conducted on these samples under loading and loading-unloading conditions. The experimental results were analyzed in terms of the Klinkenberg effect, the evolution mechanisms of coal rock damage, the coal permeability and the permeability enhancement rate (PER). The main results suggest that the Klinkenberg effect gradually weakens as the fracture complexity indices increase. The prefractured coal samples clearly exhibited a deformation capacity under both loading and loading-unloading conditions, with significant transient instability leading to failure and volume expansion. Under conventional triaxial stress conditions, the permeability of the prefractured coal samples typically evolved via an initial rapid decrease followed by a rebound and a slow increase. The permeability was clearly enhanced during this evolution process. The final permeabilities of fractured coal samples with complexity indices of 0.600, 0.565, 0.549 and 0.522 increased by factors of 2.98, 6.79, 7.54 and 53.33, respectively, over that of a nonprefractured coal sample. Therefore, the more complex a fractured coal sample is, the less distinct the permeability enhancement is. The evolution mechanism of coal permeability under loading-unloading conditions was analyzed based on the porosity-damage relationship and a PER calculation model. The onset of damage corresponded to the onset of the volume change and permeability enhancement.