# Hybrid Engineered Water-Polymer Flooding in Carbonates: A Review of Mechanisms and Case Studies
## 3 Engineered Water Flooding (EWF)  
At present, research work on enhanced oil production involving the modification of the chemistry of the injected brine is being extended to carbonates, which is apparent from an increasing number of related research publications [55; 56; 57; 58; 59]. A number of coreflood studies have shown incremental oil recovery ranging values from 5 to 30% from different carbonate rocks, including limestone and dolomite, using low-salinity or chemically tuned water (Table 1). Despite the variation in additional oil recovery rates, the EOR effect of EW is supported by all of the studies. Table S1 in the Supplementary Materials provides incremental recoveries by EWF obtained in different studies. The different recovery percentages can be attributed to variations in the oil, rock, compositions of the formation and injection water, and temperature. The EWF method gives better performance in strongly oil-wet to intermediate-wet systems and at temperatures greater than 70 \({}^{\circ}\)C [60]. Spiking of the low-salinity water with SO\({}_{4}\)\({}^{2-}\) ions has led to higher incremental recovery in most of the studies [61; 62; 63]. Despite the increased interest in the use of EWF EOR in carbonates, there is still a debate on the principal driving mechanisms. In some cases, mineral dissolution acts as a primary mechanism for  
Figure 3: Effects of salinity on the adsorption of different polymers [43]. TDS: Total dissolved solids.  
Figure 2: Schematic illustration of polymer degradation and adsorption in high-salinity water.  
wettability change [64; 65] and incremental recovery, while in other cases multi-ion exchange (MIE) reactions and surface charge modifications are dominant [66; 67; 68]. The characteristics of a reservoir fluid system have a strong influence on the success or failure of the EWF process.  
The research conducted to date indicates that more than one mechanism is responsible and that certain conditions need to be met to achieve the desired effects of EWF EOR. Additionally, the mechanisms in carbonates are different from those reported in sandstones, possibly due to the absence of clay in the carbonates [69; 70]. In sandstones, the presence of clay is considered to be critical to achieving the low-salinity EOR effect, as the clay particles are released from the rock's surface due to the salinity difference between the injected and in situ brines, detaching the organic oil components and altering the sandstone's wettability [60; 6; 71]. However, low salinity alone may not be enough to recover trapped or adsorbed oil in carbonates due to their positive surface charge.  
\begin{table}
\begin{tabular}{l l l l l l l l l l} \hline  
\begin{table}
\begin{tabular}{l l l l l l l l l l} \hline
**Study** & **Back Type** & **Bione TPS (g/l)** & **Potential Documenting** & **Add Number (AN)** & **Temperature (\(\times\)C)** & **lipidemia Mode** & **Isometrical** & **Remarks** \\  & & & **loss (PDG)** & **(ng/KMIQ)** & **(ng/KMIQ)** & & **Resency (COOPT)** & **Romans** & \\ \hline \hline \multirow{3}{*}{[PERSON], [PERSON] [60]} & \multirow{3}{*}{Limeestone} & PW: 106 & - & - & 87 & Tertiary & 22.5 &
\begin{tabular}{l} IP \\ \end{tabular  
The presence of active ions, e.g., Ca\({}^{2+}\) and SO\({}_{4}\)\({}^{2-}\), in injected water is necessary to aid oil detachment from the carbonate surface [85]. Different factors govern incremental oil recovery using EW, and those that are discussed in the literature are summarized in the next section.