Pore-scale study of fluids flow and fluid-fluid interactions during near-miscible CO₂ EOR and storage in oil reservoirs

CO₂ injection in oil reservoirs has been widely accepted as an effective EOR and CO₂ storage technique. While oil recovery and CO₂ storage potential of this technique at the core scale has been widely studied, complex fluids flow and fluid-fluid interactions at the pore-scale during near-miscible CO₂ injection require further study. For this aim, a unique high-pressure and high-temperature microfluidic system was used to conduct experiments at 2,500 psi and 40°C using live reservoir crude oil. According to results, during tertiary CO₂ injection, due to the positive value of spreading coefficient, CO₂ flowed only inside the oil and oil spread over CO₂ and prevented CO₂ contact the water. Due to unfavourable mobility ratios and permeability heterogeneities, displacement during tertiary CO₂ flooding was unstable and viscous fingering occurred which led to an early breakthrough of CO₂ and bypassing of a large amount of oil. However, after CO₂ breakthrough, CO₂ gradually started to flow inside the bypassed oil zones in the transverse/backward directions which is a characteristic of capillary fingering. Due to the gradual diffusion of CO₂ into the bypassed oil, IFT between oil and CO₂ decreased which led to a reduction of threshold capillary pressure, thus CO₂ (non-wetting phase) entered the bypassed oil-filled pores. As a result of this unique mechanism, oil recovery after CO₂ breakthrough significantly increased and almost all the bypassed oil was produced. The extent of this oil recovery mechanism depends on the extent of CO₂-oil IFT reduction which depends on injection pressure. During CO₂ flow in pores, CO₂ displaced the water through multiple displacement mechanism. CO₂ displaced the oil in the open-end pores thorough bulk flow, and the spreading oil layers were gradually produced by film flow. Uniquely, CO₂ produced the oil in dead-end pores through a mix of bulk flow and film flow. The outcomes of this study provide an in-depth understanding of fluids flow and fluid-fluid interactions during near-miscible CO₂ EOR-storage.