Dominant governing mechanisms of deformation-seepage and dynamic evolution model of permeability in gas-containing coal under coupled stress-pore pressure

Abstract

• Triaxial seepage experiments under stress-pore pressure coupling (SPPC) loading and unloading are conducted. • The dominant governing mechanisms of coal deformation-seepage during SPPC loading–unloading processes are investigated. • A SPPC damage variable is proposed, and damage constitutive model and permeability model for coal under SPPC are developed. During coal mining, the pore gas pressure and geostress in coal seams exhibit synergistic evolution characteristics of coupled loading or unloading. These two factors jointly drive coal deformation instability and gas migration. The deformation and seepage evolution characteristics of coal under single load conditions (stress or pore pressure) is difficult to accurately reveal the underlying mechanism of coal instability. Based on measured coal seam stress data, this study designs triaxial seepage experiments involving only stress (OS) and stress-pore pressure coupling (SPPC) loading–unloading. It systematically investigates the deformation-seepage evolution characteristics of coal and their dominant governing mechanisms under SPPC. Results indicate that SPPC-affected coal enters plastic damage stage earlier than OS-affected coal during loading, thereby leading to more residual strains (axial 0.173 %, radial 0.158 %, volumetric 0.143 %) during unloading. During loading, OS-affected coal undergoes continuous volumetric compression, resulting in a reduced flow rate and a permeability reduction rate of 67.24 %. By contrast, SPPC-affected coal experiences volumetric dilatation, with increased flow rate; its permeability first decreases and then increases due to the Klinkenberg effect, with a growth rate of 37.17 %. During unloading, OS-affected coal shifts from volumetric compression to expansion, leading to an increased flow rate and a permeability growth rate of up to 3742.21 %. SPPC-affected coal exhibits continuous volumetric expansion but a reduced flow rate, as the primary cause is the weakened gas driving force due to synchronous pore pressure unloading. Its permeability also shows a trend of decreasing first and then increasing, with a growth rate of 41.12 %. During SPPC loading, stress dominates axial deformation throughout the process, while pore pressure dominates radial deformation in the middle stage. These two factors exhibit opposing competitive mechanisms for volumetric deformation: stress dominates volumetric compression in early-stage, whereas pore pressure dominates volumetric expansion in late-stage. During SPPC unloading, stress serves as the dominant factor governing axial, radial, and volumetric deformations. Notably, pore pressure consistently governs seepage across loading–unloading phases. We propose a SPPC damage variable to construct a permeability evolution model for damaged coal under SPPC. This model accurately describes the dynamic evolution of coal permeability under SPPC. The research findings can offer theoretical underpinnings for developing gas disaster early-warning systems, designing roadway supports, and optimizing extraction processes.