Comparative Evaluation of the Effects of Long-Term and Short-Term Water–Rock Interaction on the Microstructure and Mechanical Properties of Shale

Accurately determining the stability of shale gas reservoirs requires an understanding of the mechanical behavior and microstructural changes exposed to both short-term and long-term water–rock interactions during hydraulic fracturing. After soaking Longmaxi shales in slick water for 0, 5, 15, 30, 60, 120, and 180 days, we use laboratory uniaxial and triaxial compression experiments across T–Pc (high temperature and high pressure) conditions to examine these changes. The results indicate significant differences in the effects of long-term versus short-term water–rock interaction on the mechanical properties of shale. Furthermore, as the duration of water–rock interaction increases, the correlation among mechanical properties, brittle mineral content, and various fractal dimensions gradually diminishes, especially after immersion exceeding 15 days. In contrast, the correlation between surface roughness, pore structure, and shale mechanical properties remains consistently stable, with surface roughness being particularly notable. Based on surface roughness, we propose a multiscale quantitative characterization method for rock damage using the analytical hierarchy process (AHP) calculation method, grounded in fractal damage mechanics theory (covering macroscopic and mesoscopic scales). Consequently, a shale damage evolution model under water–rock interaction is formulated, which can predict and assess the degree of damage in shale gas reservoirs following different durations of water–rock interaction. In addition, we propose the mechanisms to demonstrate how the microstructure and mechanical behavior of shale vary depending on the duration of the water–rock interaction: (1) surface hydration and ionic hydration (mineral dissolution and detachment, ion exchange), predominantly occurring under conditions of short-term water–rock interaction, and (2) osmotic hydration (pore pressure fluctuations, alterations in stress distribution, crack propagation, and clay swelling), which typically takes place over a longer duration and is primarily observed in long-term water–rock interaction scenarios.