This study examined how methanol (MeOH) can improve the sweep efficiency of miscible CO₂ flooding in enhanced oil recovery (EOR) applications—a process characterized by efficient miscible oil displacement but poor areal sweep efficiency, in which the viscosity contrast between the injected and resident fluids causes CO2 to channel through preferential paths. Vertical core flooding simulations were conducted using CMG-GEM software to evaluate the effect of MeOH slug size on displacement behavior and oil recovery at one pore volume (1 PV) injected. The analysis showed that small MeOH fractions primarily increased the supercritical CO₂ (scCO₂)–oil miscibility, density, and viscosity, producing only minor improvements in macroscopic sweep efficiency. By contrast, larger fractions sufficiently increased the scCO₂ viscosity, shifting the mobility contrast (M) to stabilize the advancing front, delay breakthrough (BT), and consolidate the scCO₂ channels. At sufficiently high MeOH volumes, the displacement transitioned into a near-piston-like advance with complete suppression of fingering and near-complete sweep of the core. These shifts in flow behavior increased recovery factors (RF) from approximately 73% for pure scCO₂ injection to nearly 99% when MeOH occupied half or more of the injected PV. Overall, the findings demonstrate that MeOH acts as both a microscopic efficiency modifier and a hydrodynamic stabilizer, with its effectiveness strongly dependent on the injected proportion. This work suggests that MeOHassisted scCO₂ flooding may offer a practical pathway to improving sweep efficiency in miscible EOR processes and motivates future laboratory validation and evaluation of hybrid strategies such as water-alternating-gas (WAG) with MeOH buffering.