Microfluidic Experimental Study on Oil Phase Mobility during CO₂-Hydrocarbon Gas Injection in Tight Oil Reservoirs

Tight oil reservoirs, characterized by low porosity and permeability, frequently exhibit a significant mismatch between substantial resource potential and constrained production due to the water injection. Gas injection, particularly with CO₂ and CO₂-hydrocarbon mixture gases, presents a promising enhanced oil recovery strategy. In this study, we employed a high-pressure microfluidic platform to visualize and compare the displacement behavior of pure CO₂ and CO₂-CH₄ (7:3 molar ratio) mixtures under reservoir-representative conditions (75 °C, 18.1 MPa). Results show that pure CO₂ achieved superior oil recovery (∼80%), with less residual oil and weaker gas fingering, while the CO₂-CH₄ mixture displayed pronounced gas channeling, stronger phase separation, and lower oil recovery (∼50%), especially in smaller pores (<500 μm). Residual oil morphology evolved from continuous clusters to disconnected droplets, films, and corner-trapped states, highlighting the interplay between pore geometry, capillarity, and fluid properties. This work provides direct experimental insights into gas injection efficiency in tight formations and evaluating gas-based EOR mechanisms at the microscale.