Dynamic Permeability Response and Pore-Fracture Structure Evolution of Deep Coal Reservoirs Under Stress Release

The utilization of horizontal wells to generate cavities and induce stress release is a potent technique for increasing deep coalbed methane (CBM) production. The evolution of pore-fracture structure (PFS) during stress release is crucial for the efficient development of deep CBM. Therefore, in this study, the unloading–seeping test system, nuclear magnetic resonance and X-ray computed tomography scanning technology were combined, and a conceptual model depicting the tensile rupture conditions and permeability evolution mechanism induced by the coupling effect of unloading–seeping was formulated. The results show that the evolution of PFS in deep coal reservoirs primarily depends on the fracture mechanism of compression–tension stress conversion, which manifests as rapid fractures propagation and contraction of micropores and mesopores. As for shallow coal reservoirs, the evolution of PFS is mainly decided by the non-uniform rebound of coal matrix, with its impact on the PFS limited to expansion and rebound of the pore system. Therefore, the increase in deep coal permeability under the stress release cannot be solely attributed to “stress release–coal expansion–permeability increase.” Rather, the coupling effect of unloading–seeping induces the transformation of tensile–compressive stress, resulting in the formation of macro- and microfractures which is the key factor controlling its evolution. However, the formation of fractures can also result in instantaneous collapse and closure of mesopores, making it difficult for CBM adsorbed in micropores to be produced through mesopores. Therefore, to prevent the sudden closure of a mesoporous system, the rapid generation of large caves on the coal seam roof should be avoided.