## 3 Results and Discussion
### Oil Displacement Results  
To evaluate the effectiveness of the selected chemicals, the first coreflood test was conducted with standalone surfactant. After reaching S\({}_{\text{or}}\) by waterflooding, surfactant flooding started and continued for around 23 pore volumes. This phase recovered an additional 17% OOIP, increasing the total recovery factor to around 60% OOIP. Figure 15 shows a comparison of the oil recovery and pressure drop profiles of the three coreflood tests. An important observation from Figure 15a is that the pressure drop during surfactant flooding exhibited a slightly increasing trend at each injection rate and was closer to the pressure drop during waterflooding. This is an indication of surfactant adsorption and surfactant loss from the injected surfactant solution, resulting in a pressure drop similar to that of waterflooding. Figure 14: Microemulsion formation tendency of (**a**) surfactant, (**b**) alkali–surfactant, and (**c**) NP–surfactant. The second coreflood test was a combination of the selected alkaline Na\({}_{2}\)CO\({}_{3}\) and surfactant 2S/2A. After injecting a total of 21 pore volumes of seawater, oil recovery was around 41% OOIP. The injection fluid was then switched to the alkali-surfactant solution (0.02 wt% Na\({}_{2}\)CO\({}_{3}\) and 1.5 wt% 2S/2A at 30/70 ratio), and AS flooding was continued for additional 26 pore volumes. The alkali-surfactant flooding resulted in 13% OOIP incremental recovery, increasing the total recovery factor to 54% at the end of AS flooding. The incremental recovery during alkali-surfactant flooding can be credited to the mobilization of residual oil in the form of microemulsion by surfactant action and a reduction in surfactant adsorption by the action of alkali. As shown in Figure 15b, the incremental oil recovery during the first couple of pore volumes was relatively slow as the surfactant solution needed some time to interact with the oil and generate microemulsion. Moreover, the pressure drop during AS flooding at 0.5 cc/min was significantly lower and was probably not sufficient to produce the mobilized oil and microemulsion, which was later produced at higher rates. Overall, the pressure drop during AS flooding was 3-5 times lower than the pressure drop during seawater flooding, indicating the efficient performance of alkali in controlling surfactant adsorption. As a result, the surfactant solution maintained its lower viscosity and helped in recovering residual oil by generating microemulsion. The purpose of the third flooding experiment was to assess the NPs' effect of lowering surfactant adsorption on recovering more oil by comparing the results of this test with an SF and AS flooding test. As depicted in Figure 15c, waterflooding recovered 40% OOIP from this core. The NS flooding was then started by injection of NPs-surfactant solution (0.05 wt% SiO\({}_{2}\) and 1.5 wt% 2S/2A at 30/70 ratio) for a total of 24 pore volumes. The  
Figure 15: Comparison of oil recovery performance of (**a**) SF, (**b**) ASF, and (**c**) NSF. total recovery factor was increased to 63%, translating to 22% incremental oil recovery by NP-surfactant flooding. This incremental oil recovery can be attributed to IFT reduction by surfactant and adsorption control by NPs. NPs could have also helped by blocking some pores and diverting the surfactant to previously un-contacted areas, as evident by repeated fluctuations in the pressure drop during NP-surfactant flooding. Table 10 presents a comparison of the incremental oil recovery by SF, ASF, and NSF. It can be observed from Table 10, the incremental recovery in terms of the remaining oil in core (ROIC) by NP-surfactant flooding was almost 7% higher compared to standalone surfactant flooding; meanwhile, it was around 16% higher compared to alkali-surfactant flooding. The better performance of the NSF scenario can be explained by the fact that the selected NPs were successful in lowering surfactant adsorption as well as lowering the oil-water IFT.