## 5 Conclusions  
The mechanical properties of water-sensitive shale under high confining pressures and water-rock coupling were here experimentally studied, and the strength variation law of water-sensitive shale during drilling was discussed, providing a scientific basis for addressing the mechanism of wellbore instability in shale and analyzing changes in casing external pressure. The main conclusions are as follows:  
(1) Under the same confining pressure, the triaxial compressive strength of shale is closely related to its water content. As the water content increases, the triaxial compressive strength of shale decreases by up to 77.6%, so we can infer that hydration leads to significant deteriorations in the compressive strength of shale. Under the same water content, with the increase in confining pressure, the triaxial compressive strength of rocks increased by up to 22.1%, suggesting the strength of shale significantly increases under high confining pressures. The effect of confining pressure on the strength of saturated samples is more significant than under dry conditions;  
(2) The water content has a significant impact on the failure mode of shale. With the increase in water content, the failure mode of shale under triaxial compression load gradually transitions from brittle failure to plastic failure. Under the constraint of high confining pressure, the rock sample generates shear bands, leading to viscosity and shear failure;  
(3) The influence of the water-rock interaction on the strength of shale was analyzed from macro and micro perspectives. The main effects of the water-rock interaction on the  
Figure 7: Microstructure SEM images of different cement-containing shales. Note: red circles represent pores, and yellow lines represent the bedding structure. macroscopic strength of shale are 1 water absorption and expansion, 2 cementation or disintegration, 3 dissolution/latent corrosion, 4 pore pressure, 5 seepage drag force, and 6 strength degradation. As a result of the water absorption and expansion of clay mineral particles, the microstructure is destroyed, the clay cementitious material becomes muddy, and the bonding force is reduced. At the same time, the pore water pressure also reduces the effective stress placed on the rock skeleton, and the water-rock interaction changes the internal structure of shale, ultimately leading to the deterioration of its mechanical properties. (4) Through SEM experiments, it was found that the action of water causes changes in the microstructure of shale, which determines its macroscopic mechanical response and leads to the deterioration of its mechanical properties under external loads. Conceptualization, M.L. ; methodology, M.L. ; formal analysis, J.L. ; resources, Y.D. ; supervision, M.L. ; project administration, M.L. ; funding acquisition, M.L. All authors have read and agreed to the published version of the manuscript. This research was funded by The key research and development project in Shaanxi Province, \"Research on the Optimization Design Method of Ultrahigh Low-damage Perforating Projectile Based on Improved SPH Particle Tracking\", grant number 2024 GX-YBXM-500; [PERSON]'s \"Chief Scientist + Engineer\" project, \"Research and Application of Efficient Perforation Technology for Unconventional Oil and Gas Reservoirs under the \"Dual Carbon\" Background\", grant number 2024 QCY-KYJ144; Research and Practice Project on Comprehensive Reform of Graduate Education at Xi'an Shiyou University, \"Exploration of Graduate Innovative Practice Ability Cultivation Model Based on Research Team\", grant number 2023-X-YJG-010. Not applicable. The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author. Thanks are given to the following authors for their indelible contributions to this paper: [PERSON] and [PERSON] and [PERSON] (PetroChina Tariim Oilfield Company, Kuerle, 841000, China).