Experimental study on the CO2-decane displacement front behavior in high permeability sand evaluated by magnetic resonance imaging

Highlights: • MRI data can directly reflect the fluid interfacial behavior in porous media. • A stable displacement interface benefits oil recovery and CO₂ utilization factor. • A secondary flooding phenomenon was observed for the first time. • Correlation of CO₂-oil mixing layer and oil recovery was proposed. CO₂ flooding has been proven to be an effective method to improve oil recovery, and the interface characteristics of the displacement front directly affect the oil recovery and CO₂ utilization efficiency in reservoirs. In this study, we aimed to investigate the in situ interfacial behavior during CO₂ flooding using high-resolution magnetic resonance imaging (MRI). A series of CO₂ flooding tests were carried out using upward and downward injection methods consisting of different rates (28, 56 and 112 pore volume (PV)/day) in porous media. The unstable CO₂ front is indicated to significantly impact the gas sweep efficiency, which is not favorable for oil recovery and leads to low energy utilization efficiency. For downward injection, subordinate displacement of CO₂ was observed after breakthrough, which can improve oil recovery. The optimum oil recovery and CO₂ utilization factor were obtained under upward injection at a low rate in the immiscible state, which was controlled by the stable interface behavior. From the perspective of CO₂ storage, miscible flooding improved the storage volume by more than 25% compared to immiscible flooding. For high permeability oil reservoirs, the oil recovery and gas sequestration can be synthetically optimized by adjusting the injection rates and phase state.