Experimental investigation on CO2 flooding for enhanced light oil recovery in low-permeability sandstones under multiple controlling factors by online nuclear magnetic resonance testing

As an efficacious medium for enhancing oil recovery, CO2 has been extensively employed in the domain of petroleum extraction and can effectively improve the oil recovery rate to a certain extent. However, the mechanisms of oil-gas distribution and migration in low-permeability reservoirs remain unclear and require further study. The development of nuclear magnetic resonance (NMR) testing provides a new method for visualizing displacement experiments. This research investigates CO2 displacement and NMR testing on low-permeability cores, simultaneously focusing on multiple controlling factors such as injection pressure, temperature, core permeability, and CO2 phase state. Cores were selected based on NMR assessments of pore-permeability characteristics, followed by four sets of displacement experiments. Results show that injection pressure and core permeability are the primary factors influencing oil recovery. High injection pressure can enhance oil recovery and effective recovery occurs when pressure exceeds the threshold (about 6 MPa for core #3 in this study), with recovery rates above 30%. Low-permeability cores do not form effective displacement channels, while high-permeability cores, especially those with fractures, are prone to CO2 breakthrough, reducing recovery efficiency, such as core #6. During low-pressure recovery, there is a discernible migration of oil from smaller to larger pores before extraction, totally differing from the pattern observed in high-pressure recovery. Additionally, temperature increases were found to improve recovery slightly, but the impact of CO2 phase on recovery remains inconclusive due to the limitation of the equipment and need further discussion.