# Hybrid Engineered Water-Polymer Flooding in Carbonates: A Review of Mechanisms and Case Studies
## 1 Introduction  
The World Energy Outlook has reported that about 60% of oil reserves globally are concentrated in carbonate reservoirs [1]. However, due to the presence of organic and polar acidic compounds in crude oil, the majority of carbonate reservoirs tend to be oil-wet [2; 3; 4]. This results in higher residual oil saturations in carbonates, even after water flooding. During the primary recovery phase, only 10-15% of oil originally in place (OOIP) is recovered. Secondary recovery using water or gas injection can further extract 10-30% of the remaining oil. However, almost 40-60% of oil remains trapped and inaccessible, even after secondary recovery. Various enhanced oil recovery (EOR) practices are in use to recover the trapped oil volume, such as polymer flooding (PF), surfactant EOR, miscible gas injection, and steam injection. Each EOR method modifies the rock-fluid interaction (oil-water-rock, OWR) properties in a certain manner, resulting in improved recovery. For example, polymer floodingincreases the displacing fluid viscosity, resulting in a favorable mobility ratio, whereas surfactant EOR causes reductions in the oil-water interfacial tension (IFT) and residual oil saturation (S\({}_{\text{or}}\)) [5]. Engineered water flooding (EWF) is a relatively new EOR technique, which involves the injection of properly designed low-salinity water (LSW), disturbing the original equilibrium state of the OWR system, resulting in incremental oil recovery, which is mainly caused by the wettability modification. Additional oil recovery resulting from EWF in sandstone was first documented in 1967 [6]. In 2004, the first ever field application of EWF in sandstone as an EOR technique was published. The idea of EWF as a potential EOR process in carbonates was triggered first in the 1980s, when exceptionally high oil recovery rates were observed from the injection of seawater into fractured chalk formations under the North Sea [7; 8; 9]. Engineered water for use in carbonates is designed by tuning the injected water salinity, ionic strength, and the concentrations of potential determining ions (PDIs) Ca\({}^{2+}\), Mg\({}^{2+}\), and SO\({}_{4}\)\({}^{2-}\). EWF also has some inherent limitations, such as viscous fingering due to unfavorable mobility ratios, oil trapping, and fines migration [10]. Hence, this method may not qualify as an optimum EOR option under certain conditions, even though it is a low-cost technique. The development and implementation of novel, cost-effective EOR methods is critical for the sustainable growth of the oil industry. Research is underway on various levels to develop economically viable hybrid EOR methods and overcome the limitations of individual methods, such as hybrid engineered water-polymer flooding (EWPF) or surfactant flooding. The idea behind hybrid EWPF is to enhance oil recovery via the combined effects of optimization of the injection water composition and PF, mainly through two mechanisms. (1) Engineered water modifies the wetting characteristics of the rock surface due to the salinity and composition differences, which affects the bonding of the crude oil polar components (carboxylic material, -COOH group) with the carbonate surface and helps in oil detachment [11; 12; 13]. (2) Polymer flooding improves the macroscopic sweep efficiency by decreasing the mobility ratio [14]. In addition, EW promotes polymer stability and a reduction in the polymer concentration requirement, making PF applicable to high-salinity, high-temperature carbonate formations [15]. Hence, the hybrid method results in greater incremental oil recovery than that obtained by standalone methods. This review paper is focused on the experimental and modeling work performed in the field of hybrid EWPF for carbonates. This paper is divided into four sections. The first section consists of a brief overview of polymer flooding, its recovery mechanism, and its associated limitations.