<p>The integrity evolution of cement–caprock systems in CO₂-rich environments is critical for assessing wellbore leakage risks in CO₂ geological utilization and storage projects. While existing studies have provided valuable insights into this issue, limited attention has been given to shale—a common caprock—and especially to the role of pre-existing fractures in governing the reaction processes within cement-shale-brine-CO₂ systems. Therefore, this study conducted a 35-day corrosion experiment on fracture-bearing cement-shale composites under typical CO₂ geological storage conditions (50&#xa0;°C, 10&#xa0;MPa, 1 wt% NaCl solution). The evolution of composition, porosity, and permeability of the samples was analyzed, and the underlying reaction mechanism was further investigated through geochemical modeling. The results indicate that cement components undergo intense carbonation, whereas shale minerals exhibit limited alteration. Widespread CaCO₃ precipitation occurs within the samples, particularly on shale surfaces and inside fractures, which inhibits further shale-fluid reactions. Notably, CaCO₃ precipitation within the primary connected fracture results in a significant reduction in sample permeability, ranging from 42 to 77%. The key mechanism controlling the behavior of the fracture-bearing cement-shale-brine-CO₂ system is the spatially heterogeneous reactivity, which drives the diffusion of ions—especially Ca²⁺ and H⁺—along concentration gradients. This process promotes CaCO₃ precipitation in the shale region, especially in highly connected fractures, while slightly delaying precipitation within the cement domain. The findings provide a scientific basis for evaluating the sealing performance of the wellbore-caprock system in CO₂ geological utilization and storage applications.</p>