# Hybrid Engineered Water-Polymer Flooding in Carbonates: A Review of Mechanisms and Case Studies
## 4 Hybrid Engineered Water-Polymer Flooding (EWFF)
### Numerical Modeling Studies  
A systematic analytical and numerical modeling study was conducted to investigate the potential synergy between LSW and PF [132]. The comparison of conventional WF, HSW-PF, and LSW-PF showed that the combined LSPF method gave the best results, having the highest incremental oil recovery rate and lowest water cut rate. This can be attributed to the improved fractional flow, stable frontal advance, and reduced viscous fingering for the LSPF method. Overall, the combined EOR process resulted in incremental oil recovery values of 15-42% and 11-48% reductions in the water cut after one pore volume (PV) of injection, depending on the extent of the reservoir layering and heterogeneity. Figure 19a presents the correlation between the shock front mobility ratio and the cumulative oil production. As the mobility ratio decreased from 1.1 (for high-salinity water flooding) to 0.9 (for LSPF), the total oil recovery increased from 48% to 70%, proving the better polymer performance in low-salinity environments. The effect of salinity on oil recovery using PF was also studied and the 3D numerical modeling results for different injection water salinity rates clearly showed the higher ultimate recovery rates for low-salinity-polymer cases, as highlighted in Figure 19b. A reduction in salinity from 30,000 to 700 ppm resulted in 23% extra oil production compared to the high-salinity base case, confirming the synergy between the two processes. Figure 17: Oil recovery as a function of the zeta potential, showing higher recovery using the hybrid method compared to standalone processes. Coreflood-1 shows a comparison between PF and LSPF, while coreflood-2 displays a comparison between LSW and LSPF [139]. Figure 18: Comparison of oil recovery, S\({}_{\text{or}}\), and pressure drop values using LSWF followed by LSPF in a Kuwaiti carbonate oil reservoir composite core [15]. [PERSON] and [PERSON] [131] presented an analytical solution for modeling LSPF by incorporating the non-Newtonian properties, adsorption, and ionic strength of water of the polymer. [PERSON]'s sorption equation and modified [PERSON]'s law were used to formulate the model. To study the effect of the LSP slug size, a splitting technique presented by [PERSON] and [PERSON] [186] was used to solve problems that were non-self-similar. The results showed \(\sim\)14% higher ultimate oil recovery when using LSPF, delayed water breakthrough, and the lowest water cut (Figure 20). Polymer adsorption was also higher with the high-salinity polymer (HSP) method, as polymer breakthrough occurred later (after 8.1 PV) compared to LSPF (after 7.8 PV). The results showed that the hybrid LSPF technique performed better than conventional PF, both in terms of the reduced polymer consumption and higher oil cut rates. A comprehensive numerical simulation study using empirical modeling of LSWF was carried out by [PERSON] and [PERSON] [129] to simulate the hybrid LSPF process in Landmark Solutions and Computer Modeling Group Ltd. (CMG) reservoir simulators. Polymer rheology and adsorption measurements were included to model the effects of salinity and hardness on the viscosity degradation, reduction in permeability, and IPV caused by polymer retention in porous media. The study investigated the influences of the injection sequence, polymer and LSW slug sizes, oil viscosity, and heterogeneity of the reservoir on oil recovery (Figure 21). The hybrid method was more beneficial for high-viscosity oil. Based on the simulation results, the optimum LSP slug size was 0.5-0.7 PV. An additional 20% of oil recovery was obtained using LSPF as compared to the conventional WF, mainly due to the wettability modification using LSW, the better polymer stability, and the efficient front displacement caused by polymer, improving the overall fractional flow. The 1D simulation results showed the improved fractional flow and better performance of the hybrid process compared to LSWF and PF alone.