Critical stress related to coalbed methane migration pattern: Model development and experimental validation

Increased stress induces the evolution of coal fracture structure, which may result in a transition of gas flow from seepage-dominated to diffusion-dominated in deep coal seams. For clarifying the mechanisms of this transformation, based on the assumption of penny-shaped crack for primary fracture in coal, a new model describing the critical stress that cause the gas flow pattern transformation in coal is proposed. Then, we conduct seepage experiment under confining pressures ranging from 5 to 50 MPa to verify its validation. The permeability decay characteristics with stress demonstrate the existence of critical stress and the validation of the novel model (23.62 MPa, 24.02 MPa, and 31.45 MPa for the selected coal samples). By calculating the critical stress for 42 samples of anthracite coal from references, the distribution of critical stress is obtained. The result shows that 73.82% of the samples have a critical stress between 20 and 30 MPa, indicating that the in-situ stress in deep coal seams may have already reached the critical stress, leading to diffusion dominating gas flow. The new model also reveals that an increase in coal matrix hardness and fracture aspect ratio induces an increase in critical stress, with coal matrix hardness having a greater influence.