Study of Surface Complexation Modeling on Low Salinity Polymer Flooding in High-Temperature High-Salinity Carbonate Reservoirs

The low salinity polymer (LSP) injection is a hybrid enhanced oil recovery (EOR) technique, which synergistically enhances the displacement and sweep efficiencies through compounding the advantages of low-salinity water (LSW) and polymer floodings (PF). While an appropriate LSP-flooding field-scale design typically requires a predictive mechanistic model for capturing the polymer-brine-rock (PBR) interactions, few studies have focused on this issue till date. Therefore, the present study investigates the impact of water chemistry on polymer behavior in porous media using a surface complexation model (SCM), with the purpose of refining our understanding of the PBR-system. In particular, this work examines the effect of salinity and hardness on polymer viscosity and adsorption in dolomite formations during LSP-injection with the use of our in-house developed coupled MRST-IPHREEQC simulator. Hence, to comprehensively capture the geochemistry of the LSP process, the coupled MRST-IPHREEQC simulator included the chemical reactions, such as aqueous, mineral dissolution and/or precipitation, along with the surface complexation reactions. The findings of this study showed polymer viscosity losses of 82% and 63% for the 10-times spiked salinity (6230 ppm) and 10-times spiked hardness (110 ppm) cases, respectively. Thus, the base case low-salinity (LS) brine of 623 ppm was more effective in reducing the risk of polymer viscosity loss for the dolomite model (i.e., viscosity loss of 55%). The polymer viscosity losses calculated for the various potential determining ions (PDIs) concentrations of 10-times spiked Mg²⁺ (40 ppm) and 2-times spiked SO₄^2-(156 ppm) were 61%, and 46%, respectively. Moreover, investigating the impact of salinity on polymer adsorption revealed that dynamic polymer adsorption increased from 53 μg/g-rock to 68 mg/g-rock and 64 mg/g-rock, when the salinity and hardness were increased from the base case (623 ppm) to 10-times spiked salinity and 10-times spiked hardness cases, respectively. Furthermore, the analysis showed that the 10-times spiked magnesium case exhibited higher polymer adsorption (87 μg/g-rock) compared to the 2-times spiked sulfate case (64 mg/g-rock), which is related to the formation of Mg-polymer surface complexes as a result of surface complexation processes between polymer molecules and magnesium surface species at the surface of dolomite rock. Overall, the surface complexation model has demonstrated that during LSP-injection, the stability of the water-film is enhanced, suggesting a significant alteration in wettability towards a more water-wetting state. This wettability alteration plays a crucial role in increasing oil production. Consequently, our findings underscore the effectiveness of LSP-flooding in enhancing oil recovery processes by modifying the wettability of the reservoir rock surfaces, leading to a more efficient displacement of oil.