Multiscale Structural Evolution of Stressed Coal and Its Regulation Mechanisms on CH4 Migration

Summary The dynamic behavior of coal structure under stress is of significant importance for enhancing coalbed methane (CBM) production in low-permeability and highly heterogeneous reservoirs. In this study, we systematically investigated the multiscale pore evolution of coal by analyzing four key parameters—pore volume, pore volume increment, equivalent length increment, and dominant transport channels under confining stress–gas pressure coupled conditions. More critically, the fundamental regulating mechanisms of methane (CH4) migration within coal seams were elucidated. The key findings reveal that stress-dependent pore compression exhibits scale-selective characteristics. Pores with diameters greater than 5.3 μm are proved to be of heightened sensitivity to confining stress variations. The pore volume decreases linearly under low confining stress and exponentially under high confining stress. Based on the magnitude of pore volume increment, the pores can be classified into the three categories of limited compression pores, transitional pores, and strong compression pores. The cumulative pore volume increments of these pores all demonstrate an exponential decrease with increasing confining stress. The strong compression pores contribute the most to pore volume change, with a cumulative pore volume increment of approximately −87.41%. In contrast, the equivalent length compression is more pronounced in limited compression pores, with a maximum equivalent length compression of about 31.20 mm occurring at a pore diameter of 0.03 μm. In addition, two fundamental regulatory mechanisms of gas seepage pressure on CBM migration are quantitatively characterized. First, the distribution of pores responsible for transport is dynamically reconstructed, and second, the threshold aperture for CH4 migration is regulated. The results demonstrate a good exponential relationship between the porosity of pores responsible for transport (ηx) and gas seepage pressure. As gas seepage pressure increases, the pore diameter threshold decreases, leading to changes in the dominant mechanism of CH4 migration.