Geological CO₂ sequestration in abandoned coal mine goafs is regarded as one of the key approaches to facilitating China’s carbon peaking and carbon neutrality goals. Investigating the leakage mechanisms of caprocks is essential for developing effective reinforcement strategies and guaranteeing the long−term stability and efficiency of CO₂ sequestration. To clarify the fluid flow evolution in fractured caprocks with spatially variable thickness, a gas−liquid two−phase pressure equilibrium mechanism is derived based on seepage mechanics theory. A geological model containing through fractures and aquicludes with gradual thickness variation is established accordingly. COMSOL Multiphysics is adopted to simulate the spatiotemporal evolution of the seepage field within the initial 100 days of CO₂ injection.

The numerical results show that: (1) Caprock thickness, fracture development characteristics and permeability dominate the CO₂ leakage behavior; (2) When the internal pressure of the goaf exceeds the pore water pressure of the overlying caprock, an upward pressure gradient forms from the goaf to the overlying strata; (3) In the thickness transition zone of the aquiclude, the pressure distribution shifts from linear to nonlinear, and pressure concentration occurs near fracture tips, forming dominant leakage pathways for CO₂; (4) Based on the simulation findings, targeted caprock reinforcement measures are proposed, including targeted fracture grouting and sealing, differentiated zonal remediation, graded permeability regulation, and closed−loop management integrating dynamic monitoring and adaptive intervention.