## 3 Results and Discussion
### Chemicals Screening
#### 3.1.2 Phase Behavior and IFT Results  
This section presents the results of the phase behavior study for the stable surfactants 1S, 2S, and 2S/2A. The screening criteria for this phase was to select surfactant solutions based on the development of the oil/brine microemulsion middle phase after 7 days of aging at reservoir temperature. As shown in Figure 6a, none of the 1S concentrations showed the formation of a microemulsion phase, and hence, surfactant 1S was fully discarded from the study. Likewise, the standalone 2S at a stable concentration of 0.1 wt% also did not show any microemulsion formation (Figure 6b) and thus was dropped from further analysis. Microemulsion was developed at different 2S/2A ratios; however, it was difficult to compare the microemulsion size due to the smaller volume of samples. To measure the volume of the middle phase more accurately, 20 mL vials were used. Then, 10 mL of brine/surfactant solution and 10 mL of oil were mixed in each case. Table 8 shows the phase distribution for all five 2S/2A ratios at four overall concentrations after 7 days under reservoir temperature. The IFT of all cases was also measured. The size of the middle phase and the IFT value were used as screening criteria to select the most effective surfactant formulation for the next phase of the study. Figure 5: The 7 th day aqueous stability results for six 3S samples after (**a**) 10 times and (**b**) 20 times dilution with CSW. Figure 6: Phase behavior results of (**a**) 1S and (**b**) 2S at reservoir temperature on the 7 th day. The phase behavior results showed that the samples with 1.5 wt% and 2 wt% overall concentration had higher microemulsion volumes. Most of these samples developed Winsor Type II microemulsions while only the 1.5 wt% samples with 2S/2A ratios of 30/70 and 40/60 exhibited Winsor Type III microemulsions. Figure 7 shows a comparison of IFT results as a function of the 2S/2A ratio for the four overall concentrations studied. The initial oil-water IFT between Field A crude oil and CSW was 14 mN/m. Although \begin{table}
\begin{tabular}{c c c} \hline \hline
**Overall Concentration** & \multicolumn{2}{c}{**2S/2A Ratio**} \\ \cline{2-3}
**centration** & 10/90 & 20/80 & 30/70 & 40/60 & 50/50 \\ \hline \hline \end{tabular}
\end{table} 
Table 8: Phase behavior of all 2S/2A formulations after 7 days at 63 \({}^{\circ}\)C.  
all 2S/2A formulations showed a reduction in IFT compared to original oil-water IFT, the reduction was not appreciable for 0.5 and 1.0 wt% concentrations. The results of this screening phase showed that lower 2S/2A concentrations were not efficient in forming microemulsion since their IFT values ranged between 4-8 mN/m and the phase behavior showed Winsor type II microemulsion. As a result, all the 2S/2A ratios at 0.5 wt% and 1.0 wt% over all concentrations were discarded from the study. It can be seen from Figure 7 that higher overall 2S/2A concentrations of 1.5 and 2.0 wt% showed a considerable reduction in IFT; however, only 1.5 wt% concentration with a 2S/2A ratio of 30/70 caused a reduction two orders of magnitude in IFT with an ultralow IFT of 0.2 mN/m. The rest of the samples showed a tendency to form Winsor type II microemulsion with higher IFT values. Overall, the addition of co-surfactant 2A to surfactant 2S improved the stability of this anionic surfactant and resulted in desired microemulsion formation. Based on the IFT and phase behavior results, 1.5 wt% 2S/2A at a surfactant/co-surfactant ratio of 30/70 was selected as an optimum surfactant formulation for ASP/NSP flooding design for Field A. After screening the most suitable surfactant, the next step was to select the optimum alkali and NPs to control surfactant adsorption. To select the alkali and nanoparticles concentrations for the static adsorption study, the aqueous stability of various alkali-surfactant and NP-surfactant solutions was analyzed using the optimum surfactant 2S/2A at 1.5 wt% concentration and a surfactant/co-surfactant ratio of 30/70.