# Hybrid Engineered Water-Polymer Flooding in Carbonates: A Review of Mechanisms and Case Studies
## 4 Hybrid Engineered Water-Polymer Flooding (EWFF)  
Literature and field case studies have proven that the synergistic combination of two or more EOR techniques provides better results in terms of oil recovery and economics. For instance, the combined use of an alkali, surfactant, and polymer in the alkaline-surfactant-polymer (ASP) flooding technique results in enhanced macroscopic and microscopic sweep efficiencies, owing to the favorable mobility ratio of the polymer, the microscopic sweep efficiency due to pH change caused by alkali, and the reduction in IFT caused by surfactants [115; 116]. Any EOR method involving two or more EOR techniques is known as a hybrid EOR method. Much research is being carried out in the field of hybrid EOR, particularly aiming to increase the applicability of chemical methods. As discussed in previous sections, both PF and EWF have associated challenges that limit their field-scale applications. In order to overcome these limitations and take advantage of the synergetic effects of the two methods, the hybrid EWPF technology comes into play. The hybrid process can be simply represented as:  
\[ \begin{array}{l}\textit{Engineered Water}\ (EW)+\textit{Polymer Flooding}\ (PF)=\textit{Incremental Oil Recovery}\\ (\textit{Wettabilityitability Modification})\ (\textit{Reduction in Mobility Ratio})\end{array} \]  
This hybrid EWPF method provides the following obvious benefits:  
* The addition of a polymer to low-salinity water flooding enhances the volumetric sweep and can also mobilize the oil released due to wettability modification after LSWF, which otherwise would be trapped [117];
* Various studies have shown the added benefit of the reduced polymer concentration needed to attain the desired in situ viscosity by using low-salinity brine compared to high-salinity water [118]. This can result in a significant cost reduction;
* Other advantages include the improved polymer stability (particularly at high temperatures), reduced sensitivity to shear-rate-induced degradation, less polymer adsorption, improved viscoelasticity of the polymer, and decreased scaling and souring tendency [119; 120];
* Most of the studies have been conducted on sandstones [121; 122; 123; 124], however the experiments performed on carbonates have also proved the synergetic effects of EWPF. ;
* Recently, osmosis was proposed as the mechanism for the LSW EOR effect instead of wettability alteration in a sandstone sample [125; 126; 127; 128]. Various experiment-based and modeling studies have been carried out in recent years to prove the effectiveness of hybrid EWPF as an EOR method [129; 130; 131; 132; 133; 134; 135]. These studies have shown an average of 11% incremental oil recovery using the hybrid method in different carbonate formations (Figure 4). The supporting data and references for the figure can be found in Table S2 of the Supplementary Materials. The limited literature on the application of EWPF EOR in carbonates shows that it can increase oil recovery by equal to or more than the sum of the recovery from each process. [PERSON] [10] and [PERSON] et al. [136] studied the combined effects of EWPF in terms of the better polymer stability and yield, improved microscopic and macroscopic sweep efficiency, and reduction in chemical costs. A seawater desalination process was developed for combined EWPF applications in an offshore field [137]. EW hybrid methods can achieve up to 30% of OOIP incremental recovery [138]. Figure 4: Summary of experimental and simulation studies involving incremental oil recovery using engineered water–polymer flooding (EWPF) in carbonates.4.1. Enhanced Oil Recovery using Hybrid EWPF in Carbonates. [PERSON] et al. [135] performed experiments on carbonate samples using LSW with ion adjustments followed by PF. The S\({}_{\text{or}}\) value was considerably decreased by using low-salinity polymer flooding (LSPF) compared to conventional high-salinity water flooding and EWF.