# Hybrid Engineered Water-Polymer Flooding in Carbonates: A Review of Mechanisms and Case Studies
## 4 Hybrid Engineered Water-Polymer Flooding (EWFF)
### Effect of Hybrid EWPF on Residual Oil Saturation: Capillary Desturation  
Capillary desaturation is the process of recovering capillary-trapped residual oil. Generally, conventional PF is considered to affect only the macroscopic efficiency. A typical procedure for examining the microscopic sweep efficiency of any improved oil recovery (IOR) process is to construct a capillary desaturation curve (CDC) by performing corefloods at different rates. The residual oil saturation at the end of each stabilized pressure interval is plotted against the capillary number (N\({}_{c}\)) [140; 141]. The capillary number is the ratio of viscous to capillary forces [142; 143; 144; 145; 146; 147], as given in Equation (2) [148]:  
\[N_{c}=\frac{K\Delta p}{\sigma L} \tag{2}\]  
where \(K\) is the absolute permeability, \(\Delta p\)/\(L\) is the pressure gradient across the core, and \(\sigma\) is the oil-water IFT. Equation (1) shows that in order to have an acceptable range of \(N_{c}\), either the pressure gradient should be high enough or the IFT should be extremely low, both of which are generally not achievable by conventional water flooding (WF) and PF. From the CDC, the critical N\({}_{c}\) can be identified for the process under study. The critical N\({}_{c}\) is usually in the range of \(10^{-4}\) to \(10^{-3}\)[143; 149], whereas the typical N\({}_{c}\) obtainable using WF and PF in the field ranges from \(10^{-7}\) to \(10^{-5}\)[150]. Analysis of CDCs from  
Figure 5: **(a)** Reduction in S\({}_{\text{or}}\) after LSPF [135] **(b)** additional recovery using EWPF in carbonate reservoir cores [139].  
different coreflood studies has shown that the typical field operating constraints for PF are generally not sufficient to cause any reduction in capillary-trapped residual oil [33; 151; 152; 153; 154].  
A comparison of the capillary desaturation tendency of PF [155; 156; 157; 158; 159] and LSPF [15; 117; 119; 122; 124; 134; 135; 160] has been made by plotting the coreflood end-points from various LSPF studies. It was observed that LSPF can result in a significant \(S_{\rm ov}\) reduction, even at smaller values of \(N_{\rm c}\) (Figure 6). Almost all the points in Figure 6 fall below PF CDCs, indicating that the \(S_{\rm ov}\) values are lower than standalone PF and that the hybrid method is more effective in recovering trapped oil. This can be attributed to the synergetic effects of wettability modification and efficient oil bank displacement using EW and polymer. Data for Figure 6 can be found in Table S3 of the Supplementary Materials. Recently, it has been suggested by some researchers that the viscoelastic properties of polymers can cause a reduction in \(S_{\rm ov}\) by stripping the trapped oil [155; 156; 159; 161; 162; 163; 164; 165; 166; 167]. However, this is debatable and beyond the scope of this review.  
Hence, different researchers have confirmed the synergy and additional oil recovery that can be achieved by combining low-salinity or engineered water with polymer in carbonates, but more work is required to fully understand the recovery mechanisms driving incremental oil recovery using this hybrid process. The dominant recovery mechanisms for EWPF are discussed in the following section.