Numerical analysis of pore-scale CO₂-EOR at near-miscible flow condition to perceive the displacement mechanism

Gas flooding through the injection of CO₂ is generally performed to achieve optimum oil recovery from underground hydrocarbon reservoirs. However, miscible flooding, which is the most efficient way to achieve maximum oil recovery, is not suitable for all reservoirs due to challenge in maintaining pressure conditions. In this circumstances, a near-miscible process may be more practical. This study focuses on pore-scale near-miscible CO₂–Oil displacement, using available literature criteria to determine the effective near-miscible region. For the first time, two separate numerical approaches are coupled to examine the behavior of CO₂–oil at the lower-pressure boundary of the specified region. The first one, the Phase-field module, was implemented to trace the movement of fluids in the displacement CO₂–Oil process by applying the Navier–Stokes equation. Next is the TDS module which incorporates the effect of CO₂ mass transfer into the oil phase by coupling classical Fick’s law to the fluids interface to track the variation of CO₂ difusion coefficient. To better recognize the oil recovery mechanism in pore-scale, qualitative analysis indicates that interface is moved into the by-passed oil due to low interfacial tension in the near-miscible region. Moreover, behind the front ahead of the main flow stream, the CO₂ phase can signifcantly displace almost all the bypassed oil in normal pores and efectively decrease the large amounts in small pores. The results show that by incorporating mass transfer and capillary cross-flow mechanisms in the simulations, the displacement of by-passed oil in pores can be signifcantly improved, leading to an increase in oil recovery from 92 to over 98%, which is comparable to the result of miscible gas injection. The outcome of this research emphasizes the significance of applying the CO₂-EOR process under near-miscible operating conditions.