CO₂ injection into deep saline aquifer reservoirs is promising for long-term storage of greenhouse gases. To reveal the pore-scale mechanisms underlying CO₂-brine displacement in subsurface formations, computational fluid dynamics simulations were performed in digitally reconstructed homogeneous and fractured porous media. Results showed that the displacement processes in the two types of porous media were governed by fundamentally different mechanisms. In homogeneous media, capillary forces associated with complex pore-throat geometries dominated the displacement behavior. Under low driving forces, the migration of CO₂ was strongly restricted by capillary trapping, resulting in limited removal of brine. As the driving force increased, the injection of CO₂ could overcome local pore-throat resistance and achieve effective displacement of brine. In fractured porous media, fracture structures provided preferential flow paths with lower hydraulic resistance, allowing the breakthrough of CO₂ to occur under relatively low driving forces. However, after the breakthrough, fractures contributed only marginally to additional displacement of brine from the rock matrix, as CO₂ preferentially flowed through the fracture channels. The present work provides quantitative and mechanistic insights into CO₂-brine displacement processes in porous media, offering valuable guidance for the assessment and optimization of geological carbon storage strategies.