Research on the Gangxi III area in the Dagang Oilfield shows that there was still a significant amount of oil remaining in oil reservoirs after many years of polymer flooding. This is a potential target for enhanced oil recovery (EOR). Surfactant–polymer (SP) flooding is an effective chemical EOR method for mobilizing residual oil and improving displacement efficiency macroscopically, but the microscopic oil displacement efficiency in pores of different sizes is unclear. Nuclear magnetic resonance (NMR) is an efficient method for quantifying oil saturation in the rock matrix and analyzing pore structures. In this paper, the threshold values of different pore sizes were established from the relationship between mercury injection curves and NMR T ₂ spectrums. The distribution and migration of residual oil in different flooding processes was evaluated by quantitatively analyzing the change of the relaxation time. The oil displaced from pores of different sizes after the water flood, polymer flood, and the SP flood was calculated, respectively. Experimental results indicate that (1) the residual oil in medium pores contributed the most to the incremental oil recovery for the SP flood, ranging from 40 % to 49 %, and small pores usually contributed <30 %; (2) the residual oil after the SP flood was mainly distributed in small and medium pores; the residual oil in medium pores accounted for 47.3 %–54.7 %, while that trapped in small pores was 25.7 %–42.5 %. The residual oil in small and medium pores was the main target for EOR after the SP flood in oilfields.

CO₂ flooding is a favorable technical means for the efficient development of low-permeability reservoirs, and it can also contribute to the realization of net-zero CO₂ emissions. However, due to the unfavorable viscosity ratio and gravity overriding effect, CO₂ channeling will inevitably occur, seriously affecting its storage and displacement effects. This paper conducts a systematic study on the application of polymer-assisted CO₂ flooding in low-permeability reservoirs. Firstly, the polymer agent suitable for low-permeability reservoirs is optimized through the viscosity-increasing, rheological, and temperature- and salt-resistant properties of the solution. Then, the injectivity performance, resistance-increasing ability, and profile-improving effect of the polymer solution were evaluated through core experiments, and the optimum concentration was optimized. Finally, the enhanced oil recovery (EOR) effects of polymer-assisted and water-assisted CO₂ flooding were compared. The results show that the temperature-resistant polymer surfactant (TRPS) has a certain viscosity-increasing performance, good temperature resistance performance, and can react with CO₂ to increase the solution viscosity significantly. Meanwhile, TRPS has good injection performance and resistance-increasing effect. The resistance increasing factor (η and η′) of TRPS-assisted CO₂ flooding increases with increased permeability, the concentration of TRPS solution, and injection rounds. Considering η′ and the profile improvement effect comprehensively, the application concentration of TRPS should be 1000 mg/L. The EOR effect of TRPS-assisted CO₂ flooding is 8.21% higher than that of water-assisted CO₂ flooding. The main effective period is in the first and second rounds, and the best injection round is three. The research content of this paper can provide data support for the field application of polymer-assisted CO₂ flooding in low-permeability reservoirs.