Coal Permeability Evolution Under True Triaxial Principal Stress and Pore Pressure Competition
摘要
Gas migration in deep, low-permeability coal seams is governed by the simultaneous evolution of the three principal stresses (σ1, σ2, σ3) and pore pressure, yet most earlier studies simplify the loading path or treat the stresses as interchangeable. In this work, a fully coupled true triaxial apparatus—capable of imposing multistage, non-proportional σ1–σ2−σ3 histories while recording the complete permeability in real time—was employed to clarify how competitive principal stresses control permeability. Experiments revealed a previously undocumented lag-then-surge transition: Permeability remains nearly constant until the shear stress approaches 3 MPa, whereupon σ3-induced dilation suddenly outweighs σ1 + σ2 compression and triggers a rapid increase in flow capacity. In addition, σ2 exerts a nonlinear, pore pressure-dependent influence and σ1 mainly controls the compaction effect. Guided by these mechanisms, an anisotropic permeability model grounded in principal stress competition was formulated. Model parameters were calibrated independently for the σ1, σ2 and σ3 directions and validated against multistage, non-proportional loading–unloading data, accurately reproducing the full permeability and outperforming classical single-effective-stress approaches. This paper proposes a permeability model based on the principal stress-competition mechanism, which can be used to predict permeability evolution under complex stress fields and guide deep mine gas control and gas extraction optimization.