Mechanistic Insights into CO ₂ Miscible Front Migration and Efficiency Optimization in Offshore Complex Reservoirs

CO2 miscible flooding is currently a key technology for enhancing oil and gas recovery, while also facilitating carbon storage. However, in offshore complex reservoirs, variations in reservoir thickness, strong formation heterogeneity, and differences in injection-production processes result in substantial discrepancies in the formation characteristics of miscible zones and displacement efficiency across various development stages. Therefore, based on geological and experimental data from the offshore X oil reservoir, this study integrates slim-tube experiments with compositional numerical simulations to quantitatively delineate the phase front, compositional front, and miscibility-pressure front. From a mechanistic perspective, it elucidates the migration behavior and controlling factors of the CO2 miscible front. The results indicate that gravity segregation and reservoir heterogeneity are the fundamental causes of the nonuniform expansion of the miscible zone. The miscible volume efficiency decreases by approximately 0.6% for every 10 m increase in reservoir thickness, and by about 1.6% for each unit increase in permeability ratio. Dynamic injection-production parameters regulate miscibility efficiency by affecting oil-gas contact time and pressure maintenance, among which the gas injection rate is the key factor controlling miscible-zone stability. Orthogonal multifactor analysis shows that production rate and reservoir heterogeneity dominate miscible-zone stability after breakthrough, whereas injection rate and well spacing determine the front advancement behavior before breakthrough. Considering the coupling between injection-production balance and reservoir conditions, an optimized CO2 miscible flooding strategy for offshore reservoirs is proposed. Specifically, maintaining a gas injection rate of 10–14 × 104 m3/d, a production rate of 50–90 m3/d, and a well spacing of 300–350 m can effectively suppress gravity segregation and gas channelling, preserve the continuity of the miscible zone, and achieve the dual objectives of enhancing oil recovery and improving carbon storage efficiency.