# Data-Driven Analyses of Low Salinity Waterflooding in Carbonates
## 3 Results and Discussion
### Linking Mechanisms to Parameters
There were 17 measurements of IFT collected from experiments after the secondary and tertiary stages of flooding. In the coreflooding tests, measurements mostly indicated a decrease in IFT. We also analyzed RF for different ranges of IFT decrease and noticed that higher incremental oil is recovered by greater changes in IFT, as shown in Figure 9. Hence, this mechanism can be considered effective only if the change in IFT is large enough. Changes in zeta potential (due to the reduced concentration of cations, such as Mg\({}^{2+}\) and Ca\({}^{2+}\), in LSW) results in the predominance of repulsive forces [9]. Thus, the EDL expands, and water-wet films become thicker and more stable. As a result, oil components are desorbed, and oil recovery is improved [9]. There are 14 experimental measurements of zeta potential of carbonate rock before and after the contact with LSW; 12 of them reported that zeta potential changed for more than 6 mV and became more negative, as shown in Figure 10. Even a small change in zeta potential can yield a noticeable improvement in oil recovery. Hence, zeta potential cannot solely show the LSW active mechanism. During LSW injection, pressure drop is expected to decrease due to a change in relative permeability as a result of switching from high to low salinity brines [56]. We collected the recordings of pressure change and found 56 data points reporting decreases in pressure drop and no change was observed in only two experiments. Approximately the same average RF was obtained for both cases, which shows that pressure drop should be analyzed along with other parameters to evaluate the performance of LSW. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Paper** & **Mg\({}^{2+}\)** & **Ca\({}^{2+}\)** & **SO\({}_{4}\)\({}^{2-}\)** & **Proposed Mechanism** \\ \hline [PERSON] et al., 2012 & & Increase & Increase & Rock dissolution \\ [PERSON] et al., 2016 & No change & Increase & Increase & Rock dissolution \\ [PERSON] et al., 2014 & & Decrease & Decrease & MIE \\ [PERSON] et al., 2016 & & Increase & Decrease & Rock dissolution \\ [PERSON] et al., 2011 & No change & Increase & & Rock dissolution \\ [PERSON] et al., 2018 & Increase & Decrease & & MIE \\ [PERSON] et al., 2012 & No change & Increase & & Rock dissolution \\ [PERSON] et al., 2018 & Decrease & Decrease & & MIE \\ \hline \hline \end{tabular}
\end{table} 
Table 9: Proposed mechanisms based on PDI concentration change. Figure 9: Average incremental RF vs. Change in IFT. Alteration of the wettability to a more water-wet state was also observed in almost all experimental studies, by comparing the contact angle and wettability index values before and after LSW tests. From a total of 61 data points, 57 core flooding tests reported a change of wettability toward a more water-wet state. Alteration toward more oil-wet conditions was only found in four experiments. More water-wet conditions were achieved due to alteration in the rock surface by MIE, rock dissolution, and expansion of EDL mechanisms that detach the oil from the rock. Different mechanisms have been suggested by researchers. There are 54 recordings of mechanisms proposed for LSW flooding tests in the literature. Figure 11 compares the number of tests that mentioned different mechanisms. The most popular mechanisms were rock dissolution and EDL expansion. MIE was suggested as an active mechanism for LSW injection on eleven occasions, based on PDI concentration measurements. IFT reduction is the least popular mechanism in experimental studies. Reduction in IFT is not large enough to significantly change the capillary number. LSW performance cannot be explained by one mechanism, as different parameters (such as PDI concentration change, IFT reduction, and zeta potential) are found to have a correlation with oil recovery. However, all of these mechanisms contribute to the wetta  
Figure 11: Number of data points for proposed mechanisms. Figure 10: Average incremental RF vs. zeta potential change.