# Data-Driven Analyses of Low Salinity Waterflooding in Carbonates
## 3 Results and Discussion
### Linking Mechanisms to Parameters  
In previous studies, different mechanisms were proposed for governing LSW performance in carbonates [6; 9; 12; 14]. The change in PDI concentration (Ca\({}^{2+}\), Mg\({}^{2+}\) and SO\({}_{4}\)\({}^{2-}\) ions) in the injected and effluent brine can be used to study the MIE and rock dissolution mechanisms. When MIE is dominant, Ca\({}^{2+}\), Mg\({}^{2+}\) and SO\({}_{4}\)\({}^{2-}\) decrease due to the adsorption of ions onto the rock surface. In contrast, the rock dissolution mechanism involves a rise in the effluent Ca\({}^{2+}\) and SO\({}_{4}\)\({}^{2-}\) concentrations. Alterations in IFT values explain the IFT reduction mechanism. Change in zeta potential can be evidence to show the EDL expansion mechanism. A total of 24 data recordings of Ca\({}^{2+}\) concentration changes in the effluent were found in the literature. RF values observed for cases with either an increase, a decrease or no change in effluent Ca\({}^{2+}\) concentration are compared in Figure 6. A relatively equal number of rises and reductions of Ca\({}^{2+}\) concentration were found in the experimental studies (13 and 10 data points). A similar RF is achieved when either an increase or a decrease in Ca\({}^{2+}\) concentration was measured, which means that MIE and rock dissolution mechanisms have almost the same strength in detaching and recovering oil. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Parameters**} & \multirow{2}{*}{**Model**} & \multicolumn{2}{c}{**Strength of Relationship \({}^{*}\)**} \\ \cline{3-4}  & & **Training Data** & **Testing Data** \\ \hline  & ANN & Weak & Negligible \\ DC + DS + DTDS & SVM & Weak & Negligible \\  & DT & Strong & Moderate \\  & ANN & Very strong & Strong \\ DC + DS + DTDS + AB & SVM & Very strong & Strong \\  & DT & Strong & Strong \\ \hline \hline \end{tabular} \({}^{*}\)[0.01,0.2)—Negligible, [0.2,0.3)—Weak, [0.3,0.4)—Moderate, [0.4,0.7)—Strong, [0.7,1.0]—Very strong.  
\end{table} 
Table 8: Strength of nonlinear relationship for brine and oil/brine parameters. Figure 6: Average incremental RF vs. effluent Ca\({}^{2+}\) concentration change. There are nine data points containing the effluent Mg\({}^{2+}\) concentration collected from experimental studies. Approximately the same number of data points reported an increase and a decrease in Mg\({}^{2+}\) concentrations. Average RF and temperature were higher when Mg\({}^{2+}\) ion concentrations decreased in the effluent (Figure 7). This can be explained by the effect of temperature on increased Mg\({}^{2+}\) activity towards the carbonate surface, which results in the adsorption of cations [23]. SO\({}_{4}\)\({}^{2-}\) concentration change in the effluent brine was reported in 14 coreflooding tests, as shown in Figure 8. Reduction of SO\({}_{4}\)\({}^{2-}\) concentrations in effluent was recorded in seven tests and an increase in anion concentration was measured in six experiments. Higher average RF corresponded to decreases in SO\({}_{4}\)\({}^{2-}\) concentration which, in combination with Ca\({}^{2+}\) reduction, supports the MIE mechanism. Changes in the concentrations of all PDIs should be analyzed together, to evaluate the possible active mechanism. Table 9 shows the recordings of ion changes in the effluent brine and proposed mechanisms for these cases. Alteration in PDIs is an indicator of the MIE/rock dissolution mechanism. It is generally accepted that a simultaneous decrease in cations and anion concentrations in the effluent shows the ion exchange on the carbonate rock surface. On the other hand, an increase in these ions is due to the dissolution of the carbonate surface. Figure 8: Average incremental RF vs. SO\({}_{4}\)\({}^{2-}\) concentration change. Figure 7: Average incremental RF vs. Mg\({}^{2+}\) concentration change. Brine/oil interfacial tension reduction was suggested as a mechanism that is active during LSW flooding, which affects the capillary force and reduces the residual oil.