Coupled Evolution of Coal Damage–Permeability–Mechanical Response Induced by Alternating N2/CO2 Injection

Abstract

To reveal the coupled evolution of coal damage–permeability–mechanical response during alternating N2/CO2 injection (ANI), a thermo-hydro-mechanical-damage (THMD) fully coupled model incorporating three distinct damage mechanisms (stress-induced, temperature-induced, and CO2 adsorption-induced damage) is established in this study, and the dynamic damage feedback effects on permeability and mechanical parameters are quantified. Validation against triaxial compression-seepage experiments confirms the model’s reliability in characterizing the permeability and mechanical response evolution. Results show that, compared with single-gas injection, ANI reveals a synergistic mechanism, termed “N2 mechanical pioneering-CO2 adsorption expansion”, which effectively enhances the seepage capacity while minimizing the final damage area. The whole ANI evolution process consists of four typical phases, which sequentially evolve in an orderly manner. Such evolutionary characteristics expose the inherent coupling mechanism among coal rock damage development, permeability variation, and mechanical behavior response. Parameter analysis indicates that the in situ stress ratio governs the spatial direction of coal damage expansion. An increase in gas injection pressure markedly accelerates damage accumulation and induces a transition in failure mode from brittle to ductile. In contrast, the cycle period exerts a nonmonotonic regulatory effect on both damage and permeability evolution.