Oil recovery by low-salinity polymer flooding in carbonate oil reservoirs

Low-salinity polymer flooding (LSPF) is a promising enhanced oil recovery (EOR) method with a synergetic effect of combining the polymer injection and low-salinity water injection methods. To maximize EOR efficiency, it is essential to design the compositions of low-salinity water appropriately while considering polymer properties. The goal of this study was to investigate the effects of pH and potential determining ions (PDI; SO42- and Ca2+) in low-salinity polymer solution used as injection water on oil production when applying LSPF to a carbonate oil reservoir. From the experimental results, the SO42- ions interfered with polymer adsorption in the neutral SO42--containing polymer solution, while the adsorption was greatly promoted in the acidic conditions, reducing the permeability. In the Ca2+-containing polymer solution, the adsorption trend was different from the SO42- case. In examining the wettability alteration, the change in residual oil saturation between after-LSWF and after-LSPF (Delta S-or) was greater with higher SO42- concentration under the neutral state. This means that the end-point of the relative permeability-saturation curves shifted to the right, indicating that wettability was altered to a more water-wet state. Under acidic conditions, Delta S-or was smaller compared to the neutral state due to greater polymer adsorption. A small Delta S-or was observed when Ca2+ ions were contained in the neutral polymer solution, causing less wettability alteration. This is because Ca2+ ions are involved in precipitation as well as adsorption, thus reducing the permeability more than SO42--containing case. We confirmed these changes also in the contact angle analysis. In conclusion, since SO42- ions interfered with adsorption of polymer molecules and adsorption occurred less in the neutral state, a neutral polymer solution with high SO42- ion concentration as an injection water provided the highest enhancement in the EOR efficiency.