Background Strong heterogeneity in porous media creates pronounced differences in flow capacity between high- and low-permeability regions. During immiscible CO₂ displacement, this contrast promotes preferential migration through high-permeability pathways, leading to insufficient utilization of low-permeability regions and a pronounced interlayer sweep imbalance. Clarifying how flow is dynamically partitioned between layers under different capillary and wettability conditions is therefore important for understanding displacement behavior in heterogeneous porous media.

Methods To elucidate the dynamic flow-partitioning behavior between regions with contrasting permeabilities, a parallel dual-permeability porous-medium model was established. The Navier–Stokes/Cahn–Hilliard phase-field model was employed to investigate the effects of the capillary number (Ca) and wettability on layered oil recovery, sweep efficiency, dynamic flow partitioning, and CO₂ breakthrough.

Results At low capillary numbers, CO₂ preferentially migrated through the high-permeability region, and the sweep efficiency of the low-permeability layer was only 5.6%–5.8%. When Ca increased to 10⁻⁴, the sweep efficiency of the low-permeability layer increased to 39.8%–45.0%, and the relative sweep index increased from 0.071–0.085 to 0.574–0.585, indicating a marked reduction in the interlayer sweep disparity. At Ca = 10⁻⁶–10⁻⁵, interface propagation, stagnation, and preferential path switching continuously altered the instantaneous flow resistance of the two layers, causing pronounced fluctuations in the net flow partitioning ratio. When Ca increased to 10⁻⁴, these fluctuations weakened substantially, and the net flow fraction of the high-permeability layer stabilized at 0.85–0.90. Wettability mainly influenced the displacement process by modifying local interface migration and remaining-oil distribution, and its effect varied with the capillary–viscous competition regime. At Ca = 10⁻⁶, increasing the contact angle from 105° to 150° reduced the breakthrough hydrocarbon pore volume (HCPV) from 0.346 to 0.311, whereas at Ca = 10⁻⁴, it increased from 0.403 to 0.461. Under low-Ca conditions, stronger oil-wetness enhanced selective CO₂ invasion along preferential pathways, favoring earlier breakthrough. Under high-Ca conditions, stronger viscous forces promoted the remobilization of trapped oil in larger pores, while interfacial constraints and remaining-oil trapping persisted in smaller pore throats, inducing additional pathway adjustments before breakthrough. These results show that the influence of wettability on breakthrough is not monotonic but depends on the prevailing balance between capillary and viscous forces.

Conclusions The capillary number primarily controls the overall evolution of interlayer flow partitioning and sweep balance, whereas wettability further regulates local interfacial behavior and CO₂ breakthrough characteristics. Increasing the capillary number promotes the participation of low-permeability regions, reduces interlayer sweep imbalance, and drives the flow-partitioning state from strongly unsteady competition toward a more stable regime. The results further demonstrate that the roles of the capillary number and wettability are distinct but coupled during immiscible displacement in heterogeneous porous media.