# Flow Dynamics of Sulfate-Modified Water/Polymer Flooding in Micromodels with Modified Wettability
## 1 Introduction  
The concept of smart/modified water flooding was first proposed by [PERSON] et al. [1]; leading to further investigations with regards to the impact of injected water composition on oil recovery [2; 3; 4; 5; 6; 7; 8; 9]. In simple words, smart/modified water flooding deals with either the addition of active ions/salts or removal of in-active ions/salts from the injection brine. This addition or removal of some specific salts changes not only the salinity but also the hardness of the brine [10; 11]. Active and non-active ions are known as potential determining ions (PDI) and non-PDI, respectively [12], and their impact on oil recovery have already been investigated by researchers [12; 13]. The manipulation of injected brine composition is believed to be able to disturb the established rock-oil-brine (ROB) equilibrium, and detach the oleic polar compounds from the rock surface. [PERSON] et al., [PERSON] and [PERSON] and [PERSON] et al. [14; 15; 16] showed that sulfate, as the most effective PDI, helps to detach the long chain carboxylic group from the oil-wet surface, and control the further adsorption of these compounds on the rock surface. Clay and quartz content in the rock matrix are negatively charged surfaces, whereas oil can have positively or negatively charged compounds. The negative polar compounds of oil are attached to rock-surface with the divalent ions bridging (Ca\({}^{2+}\) and Mg\({}^{2+}\) present in formation brine) [17], as shown in Figure 1A. The positive polar compounds of the oil are attached to the negatively charged rocksurfaces, and the created chemical bondage results in the oil wetting condition of the sandstone reservoir. The negative polar compounds of the oil are replaced by SO\({}_{4}\)\({}^{2-}\) through Ca\({}^{2+}\) and Mg\({}^{2+}\), causing bridging of the rock surface, and subsequently altering wettability towards a water-wet state, as presented in Figure 1B. However, the wettability alteration process is catalyzed if low salinity sulfate-modified water flooding is performed. Low salt will further dilute the divalent cations in the formation brine, and hence weaken the bondage force. Interfacial interaction between fluids is another recovery mechanism responsible for the additional oil recovery during modified water flooding [18; 19; 20]. This interfacial interaction (layer) is developed at the oil-brine interface (fluid-fluid interaction) that resulted from the ionic activity between the modified brine and oil polar compounds [21; 22]. However, this layer is sensitive to the salinity of injected brine. [PERSON] and [PERSON] [23], [PERSON] et al. [24] and [PERSON] et al. [25] demonstrated that low salinity flooding produces a more stable and viscoelastic surface at the oil-brine interface. For instance, [PERSON] et al. [24] found that this stable layer is resistant to rupture, and assists the continuous oil phase transportation in the porous media, hence it contributes to the higher oil recovery. Based on the common recovery mechanisms of wettability alteration, multicomponent ion exchange (MIE) and change in pH and clay swelling [26; 27; 28; 29], some researchers refer to low salinity flood (LSF) and smart/modified water flood (SWF) as the same technology [30]. Others believe both technologies are completely different, based on the microscale ionic activity between fluid-fluid interactions [31; 32]. Further developments of smart/modified water technology led to the concept of interfacial viscoelasticity (IFV) at the fluids interface as the main recovery mechanism [16; 20; 22; 33; 34; 35; 36]. Table 1 reviews a list of studies that deal with interfacial interaction between oil and brine, and investigate its impact on oil recovery from sandstone and carbonates. We have addressed this topic in a previous work, where the results of IFV are presented for the same oil sample and different brine compositions used in this work [32].