Investigating Stress-Dependent Permeability in Longmaxi Shale Through Integrated Acoustic-Permeability Experiments and Microstructural Analysis

Shale permeability declines with increasing effective stress during production, leading to impairment of well productivity and gas recovery. However, conventional effective stress theory fails to fully capture the stress-dependent permeability in heterogeneous shales, leading to prediction uncertainties. To address this limitation, integrated acoustic-permeability experiments were conducted on Longmaxi shale cores under two pressure paths: constant confining pressure with variable pore pressure and constant pore pressure with variable confining pressure. A modified Response Surface Technique was employed to quantify the effective stress coefficient, while acoustic velocities were used to derive dynamic Young’s modulus and Poisson’s ratio. Microstructural interpretation based on a micropore model yielded aspect ratios that linked microstructure to stress sensitivity. Industry-standard evaluation and a sensitivity coefficient were used to assess stress sensitivity. The key findings are as follows: (1) Both permeability and acoustic parameters stabilized after the initial loading cycle. (2) Macroscopically, fracture-developed samples exhibited more pronounced nonlinear variations in the effective stress coefficient and stronger stress sensitivity. Microstructural analysis of aspect ratio evolution further confirmed that fracture closure was the dominant mechanism for permeability reduction. These results collectively demonstrate the fracture-dominated nature of stress sensitivity in the Longmaxi Formation shale. (3) By comparing different evaluation methods, the Stress Sensitivity Coefficient Method was shown to offer broader applicability and greater practicality for engineering applications. These insights provide a mechanistic basis for optimizing production strategies in Longmaxi shale gas reservoirs.