## 3 Results and Discussion
### Effect of Oil Viscosity on LSW/EW Performance  
Data measured by our experiments (4 points for 967 cp viscosity), shown in Table 6, are added to the data list in Table 1. Totally 29 measured/collected data points were used for the study. The primary objective is to investigate the effect of oil viscosity on recovery factors by LSWI at secondary and tertiary injection stages. The dependence of the recovery factor on the oil's viscosity for all measured/collected data was plotted, as shown in Figure 7. The figure contains the viscosity/recovery tertiary recovery data from papers listed in Table 4 [28; 30; 34] and the results of our core flooding tests. Small triangle points show actual recovery data for the tertiary development stage. These data can be categorized into three groups, one group for the recovery factor of oil samples with the viscosity in the range of 200 cp, the second the group with viscosity in the range of 600 cp, and the last one in the range of 1000 cp. The average arithmetic value of the recovery factor for each category is shown in Figure 7 by the grey rectangle point. These points clearly show the recovery trend with increasing viscosity. On average, tertiary recovery in porous media containing heavy oil with a viscosity higher than 150 cp cannot produce more than 6%, which is much lower than the typical recovery by LSW/EW. This number becomes much lower at viscosity values of more than 800 cp. In Figure 8, the same data from Table 4 are reported but for the secondary recovery development (9 points) [28; 31; 32; 34; 37]. Similar to the previous graph, actual and average data are shown for different oil type categories. Oil samples are in the range of less than 1000 cp, 1000-10,000 cp, and more than 10,000 cp. For the highest viscosity range, the recovery factor is around 20%, which is low for the secondary recovery stage. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Data \#** & **Papers** & **Rock type** & **Viscosity, cp** & **Acid** & **Injection** & **Recovery Factor by LSWI\%** \\ \hline
1 & & & & & & 0 \\ \hline
2 & & & & & & 0 \\ \hline
3 & & & & & & 1.98\% \\ \hline
4 & & & & & & 3.36\% \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Core flooding results. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Test** & **Core \(N\)** & **RF by Seawater (\%** & **RF by LSW/EW (\%** \\  & & **OOIP)** & **OOIP)** \\ \hline LSW (5\(\times\) Diluted CSW) & 1 & 67.51 & 0 \\ EW1(5\(\times\) Diluted CSW with 2 Ca\(*\) 2 SO\({}_{4}\) ions) & 3 & 72.7 & 0 \\ EW2 (5\(\times\) Diluted CSW with 2 Mg* 2 SO\({}_{4}\) ions) & 2 & 74.95 & 1.98 \\ EW3 (5\(\times\) Diluted CSW with 2 Ca\(*\) 2 Mg* 2 SO\({}_{4}\) ions) & 4 & 66.79 & 3.36 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Core flooding results summary. Figure 9 shows the normal distribution graph for the oil recovery during the EOR approach. This one-sided normal distribution curve shows that the observed data of the recovery factor has a limit with a certain value of viscosity. It can be observed that for 90% of collected data, the recovery is less than 10%, and for half of the data, the recovery is less than 4%, which is a poor indicator of a successful EOR method. Figure 8: Incremental recovery factor and viscosity dependence for secondary recovery. Figure 7: Incremental recovery factor and viscosity dependence for tertiary recovery. Our results show the poor performance of LSW/EW flooding when the oil viscosity is high. At high oil viscosity, the mobility ratio of the injected fluid and the oil is high, resulting in the propagation of channels, fingering, unfavorable flow, and low macroscopic sweep efficiency. The analysis showed that the wettability alteration mechanism is active during the LSW/EW flooding, and the presence of PDIs can even enhance it. However, due to the oil's high viscosity, the viscous force is still weak to overcome the capillary force and decrease the residual oil. Hence, the microscopic sweep efficiency is also low, leading to low oil recovery.