## 3 Results and Discussion
### Eor
#### 3.4.5 Oil Recovery  
A core flooding run was performed in a fresh sandstone core, and the main results are summarized in Table 6. The recovery test was started with a secondary water flooding step, injecting 5 PV of brine (0.5 wt% NaCl) until no more oil was recovered. Subsequently, 4.41 PV of the optimized formulation were injected during tertiary recovery, until reaching the residual oil saturation (\(S_{or}\)). An additional oil recovery of 34.18% of the OOIP was achieved, thus reducing the residual oil saturation down to 8.51%. The results obtained in the core flooding experiment show that the use of the eutectic mixture leads to a significant improvement in oil recovery.  
\begin{table}
\begin{tabular}{l c} \hline \hline
**Parameter** & **Value** \\ \hline Pore volume, PV (cm\({}^{3}\)) & 16.20 \\ Porosity of the core (\%) & 18.32 \\ Permeability (mD) & 80.88 \\ OOIP (cm\({}^{3}\)) & 12.73 \\ Initial oil saturation, \(S_{oil}\) (\%) & 78.72 \\ Initial water saturation, \(S_{wi}\) (\%) & 21.28 \\ Brine injected during secondary recovery (PV) & 4.99 \\ ORWF (\%OOIP) & 55.01 \\ Residual oil saturation after water flooding, & 35.42 \\ \(S_{out}\) (\%) & Optimized formulation injected during tertiary & 4.41 \\ recovery (PV) & 34.18 \\ AOR (\%OOIP) & 8.51 \\ Residual oil saturation after surfactant flooding & 89.19 \\ \(S_{out}\) (\%) & 35.42 \\ Cumulative oil recovery (\%OOIP) & 4.41 \\ \hline \hline \end{tabular}
\end{table}
Table 6: Summary of core-flooding test.  
Figure 7: Freundlich isotherm fit for the adsorption of AOT and the AOT + PEG-600 eutectic. Dots represent experimental data for: AOT and A eutectic. Black, solid line and red, dashed line represent the Freundlich isotherm fit for AOT and the eutectic mixture, respectively.  
The main oil recovery mechanism was the IFT reduction that led to an increase in the capillary number. The increase of the viscosity did not play a significant role, since the viscosity of the injected fluid was similar to the viscosity of water (Table 3). However, the use of the polymer reduced surfactant adsorption, thus improving the economy of the process.  
A comparison of the main results obtained in this investigation and previous experimental studies with eutectics is shown in Table 7. It should be highlighted that previously proposed eutectics did not combine surfactant and polymer, as in this case. Rather, they combined ChCl with different hydrogen bond donors. None of the previous works [15; 16; 18; 19] achieved ultra-low IFT or a value as low as 0.051 mN/m reported herein. Additional oil recoveries in these literature precedents ranged from 15.8 to 48.4% OOIP, with this upper limit being represented by the only value greater than our result herein. However, the authors of that work [19] had to resort to the addition of a traditional surfactant (5 ppm of CTAB) to the eutectic in order to improve the formulation. Static adsorption tests were performed for the eutectic and CTAB separately, but no adsorption study of the optimized formulation was carried out. On the other hand, concentrations previously proposed of 50% of eutectic (see Table 7) are far too high for real applications.