A coupled stress-permeability model for hydraulic fracturing in layered shale: Insights into damage and permeability evolution

Abstract

Understanding the evolution of damage, fracturing, and permeability in shale under stress-permeability coupling is crucial for safe and efficient shale gas extraction. In this study, a coupled stress-permeability numerical model was developed to describe the fracturing behavior and permeability evolution of shale under varying bedding angles and stress conditions. The model incorporates mesoscale heterogeneity, anisotropy, and a local material degradation law to capture progressive failure. Validation against experimental data confirmed its reliability. The interlaminar strength ratio (b) was introduced to investigate the damage and failure processes of layered shale, as well as permeability evolution, under uniaxial and triaxial compression. Results indicate significant variations in failure modes and mechanical properties across bedding configurations, with permeability increasing as microcracks propagate until failure. The model was also extended to simulate hydraulic fracturing, showing that the coefficient of lateral stress (λ = σ₃/σ₁) strongly affects fracture propagation. When λ > 0.83, fractures propagate in a markedly more complex and random manner; when λ < 0.67, they predominantly align with the maximum principal stress. Five types of hydraulic fractures were identified, highlighting that trans-layer and bifurcated fractures are essential for complex fracture networks. These findings aid hydraulic fracturing design and shale gas recovery.

Keywords

damage evolution / bedding plane / hydraulic fracturing / crack propagation / mechanical characteristics

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