## 3 Results and Discussion
### Hybrid Huff-n-Puff in Micromodels
In conventional CO\({}_{2}\)-based oil swelling methods, such as huff-n-puff, oil recovery is primarily driven by the volume increase and reduced viscosity due to the swelling of the oil. However, the proposed hybrid approach can potentially enhance oil recovery further by incorporating this second flooding stage after the intermediate oil swelling stage. Chronologically, the first flooding stage recovers some oil but leaves a significant portion trapped in tiny pores. Subsequently, CO\({}_{2}\) is injected into the reservoir, causing the oil to swell and become more mobile, which adds to total oil recovery. After this intermediate swelling stage, the second flooding stage targets the remaining oil that was previously inaccessible. The removal of the swollen oil creates new pathways for trapped oil into tiny pores and reduces oil saturation, allowing the second flooding to sweep out additional oil that was previously trapped. This sequential  
Figure 8: Comparison of oil displacement from the pore network between DI water and SDS–DI water flooding. The red circle shows the same area of the pore network in both cases where DI water flooding shows no oil displacement, whereas in DI water-SDS flooding, the oil shows being imbibed by flooding fluid. The image contrast has been changed for the visualization purpose. process maximizes oil recovery by combining the benefits of multiple operations. We would like to clarify that due to the low pressure of CO2 and low SDS concentration, CO2 foam was not observed. We conducted a total of six different experiments, namely, DI water floodings with intermediate oil swelling and SDS-DI water floodings with intermediate oil swelling. Each case was repeated with three different CO2 pressures of 5, 10, and 15 psig. The recorded experiments lasted 25,200 s, consisting of two 1800 s (30 min) of floodings and 21,600 s (6 h) of oil swelling. Figure 9a shows DI water flooding and oil swelling at 5 psig CO2 pressure and second DI water flooding with their cumulative oil recovery in three different colors. The blue line shows the first flooding, which was 30 min long, and the line shows a steeper curve due to initial oil displacement. Following that, the orange-colored line shows CO2-based oil swelling for 6 h. During this time, oil is not being recovered due to its swelling, the intensity value changes, and that change can be attributed to the slope of this line. Lastly, the yellow-colored line shows the second flooding, where the swelled oil is displaced. Due to the different time scales between flooding and oil swelling, it could be challenging to analyze the plot. Therefore, modifications were made to represent the complete plot of an experiment. In Figure 9b, the time axis has been trimmed from 5000 to 20,000 s to make the cumulative oil recovery visible in the plot for six different tests (Figure 10); for complete data, see Supplementary Information (SI). Figure 10a,b show the cumulative oil recovery by DI water and SDS-DI water flooding at 5 psig CO2 pressure oil swelling. DI water flooding-based CO2 oil swelling produced around 16% of OOIP, whereas SDS-DI water flooding-based CO2 oil swelling showed around 25% of oil recovery. This suggests that adding nonionic surfactant has been proven to improve oil recovery by 9%. Similarly, Figure 10c,d show the DI and SDS-DI water flooding at 10 psig pressured CO2 oil swelling. The cumulative oil recovery in the DI water flooding case increased from 16% to 20%, whereas in the SDS-DI water flooding case, the cumulative oil recovery increased from 25% to 36% of OOIP. The improved oil recovery at 10 psig suggests that oil swelling is improved along with the higher pressure. However, the oil swelling at higher pressure yielded an increase of only 4% in the DI water case, whereas it was 10% for the SDS-DI water case, which shows a positive effect of SDS in oil swelling. Lastly, cumulative oil recovery at 15 psig CO2 oil swelling is shown in Figure 10e,f. The DI water flooding case showed around 28% oil recovery.