Mechanical and Acoustic Dynamic Evolution of Shale under Water-Shale Interaction

Water-based drilling fluid invasion-induced shale hydration is one of the key mechanisms leading to shale formation instability. Its essence lies in the microstructural damage caused by hydration, which further leads to the deterioration of mechanical and acoustic properties. To thoroughly investigate the intrinsic relationship of the evolutionary characteristics of “hydration–structural damage–mechanical weakening,” shale samples from the Da’anzhai Formation in the Sichuan Basin were selected. A series of comprehensive experiments were designed, covering different fluid systems, immersion pressures , and immersion times. By conducting simultaneous acoustic (velocity and attenuation), rock mechanical (triaxial), and CT scanning tests, the evolution trends of macro- and microstructures, mechanical properties, and acoustic characteristics of shale under water–rock interaction were quantitatively characterized. The response relationship between mechanical and acoustic parameters was clarified, and the influence of different drilling fluid systems on formation collapse pressure was compared, providing a quantitative basis for drilling fluid optimization. The main conclusions are as follows:
(1) Macro- and microstructural damage exhibits time- and pressure-dependent behavior. After immersion in drilling fluid, macroscopic fractures appeared on the shale surface. CT scanning indicated that hydration-induced fractures primarily formed during the initial immersion stage. With prolonged immersion time or increased pressure, existing fractures continued to expand, while the number of new fractures gradually decreased, reflecting irreversible cumulative hydration damage within the shale.
(2) Mechanical and acoustic parameters exhibit a synergistic deterioration trend. As immersion time and pressure increase, shale acoustic wave velocity, peak amplitude, compressive strength, and elastic modulus all decrease, while the acoustic attenuation coefficient increases. Microscopically, this is attributed to hydration-induced microcrack propagation and mineral interface weakening, which complicate wave propagation paths and intensify energy dissipation, leading to a simultaneous reduction in mechanical performance.
(3) Drilling fluid modification effectively inhibits hydration damage. Compared with the original water-based drilling fluid, adding 3% plugging agent increased shale longitudinal wave velocity and compressive strength by 8.2% and 10.2%, respectively. Using 50% organic salt as an inhibitor increased these values by 12.4% and 22.3%, respectively. When both were used in combination, the improvements further increased to 14.7% and 27.2%. This indicates that physical plugging and chemical inhibition can significantly mitigate structural damage and mechanical weakening.
(4) The acoustic attenuation coefficient is a sensitive indicator for evaluating structural damage. After water–rock interaction, the correlation between shale compressive strength, elastic modulus, and acoustic wave velocity is weaker, while the correlation with the acoustic attenuation coefficient is stronger. This suggests that the acoustic attenuation coefficient responds more sensitively to microstructural damage and can serve as a non-destructive evaluation method for optimizing drilling fluid systems.
(5) Drilling fluid optimization significantly reduces collapse pressure risk. As immersion time or pressure increases, the incremental collapse pressure of the shale formation gradually rises, reaching 0.248 g/cm³ and 0.201 g/cm³ under conditions of 15 days and 7 MPa, respectively. After adding 3% plugging agent, 50% organic salt, and their combination, the incremental collapse pressure decreased to 0.167, 0.151, and 0.113 g/cm³, respectively. This confirms that optimizing drilling fluids through physical–chemical synergy can effectively enhance wellbore stability.