Seepage laws of two kinds of disastrous gas in complete stress–strain process of coal

The similarities and differences in seepage flow evolution laws of CH₄ and CO₂ during complete stress–strain process of samples were comparatively analyzed. The results show that the seepage flow evolution laws of CH₄ and CO₂ are extremely similar during the stress–strain process, showing that the characteristic first decreased and then increased. A mathematical model was also established according to the relationship of seepage velocity and axial strain. However, due to the strong adsorption ability of CO₂, the coal samples generated a more serious “Klinkenberg effect” under the condition of CO₂. Owing to this, the CO₂ seepage flow resulted into occurrence of “stagnation” phenomenon during the late linear elastic stage II. In the strain consolidation stage III, the increment rate of CH₄ seepage velocity was significantly greater than that of CO₂. In the stress descent stage IV, when the axial load reached the peak pressure of coal, the increment rates of CH₄ seepage velocity presented a turning point. But the changing rate of CO₂ seepage velocity still remained slow and a turning point was presented at one time after the peak of the strain pressure, which showed an obvious feature of hysteresis.