Enhanced Oil Recovery (EOR) poses a significant challenge for carbonate rock reservoirs in the oil and gas industry. To address this, researchers have introduced methods such as smart water and surfactant-assisted injections. The complex and heterogeneous nature of these rock formations requires a comprehensive understanding of the processes during injection. In recent years, researchers have used phosphate and sulfate anions for smart water injections. While sulfate anion injections have received considerable attention, a significant gap remains in understanding the impact of phosphate anions. This gap calls for further exploration and study to uncover the potential effects of phosphate anion injections. A comprehensive understanding of their EOR mechanisms has yet to be efficiently achieved. This study aimed to examine the impact of sulfate ( \({\text{SO}}_{4}^{2-}\) ), mono- ( \({\text{HPO}}_{4}^{2-}\) ) and dihydrogen ( \({\text{H}}_{2}{\text{PO}}_{4}^{-}\) ) phosphate anions on altering wettability in oil-wet carbonate rock and reducing water/oil interfacial tension. The simultaneous use of these anions with a cationic surfactant, Cetyl Trimethyl Ammonium Bromide (CTAB) was also investigated. These mechanisms were explored through experimental tests, including contact angle measurements, fourier-transform infrared spectroscopy-attenuated total reflectance (FTIR-ATR) imaging, zeta potential measurements, and spontaneous imbibition. Contact angle measurements showed that \({\text{HPO}}_{4}^{2-}\) and \({\text{H}}_{2}{\text{PO}}_{4}^{-}\) anions perform better than \({\text{SO}}_{4}^{2-}\) in restoring water-wetness to carbonate rocks. FTIR-ATR analysis confirmed that carbonate rock exhibits higher water-wettability in solutions enriched with \({\text{H}}_{2}{\text{PO}}_{4}^{-}\) . Zeta potential assessments showed a shift in the charge of oil-wet carbonate rock: from -54.9 mV to -32.4 mV with \({\text{SO}}_{4}^{2-}\) , -16.4 mV with \({\text{H}}_{2}{\text{PO}}_{4}^{-}\) , and -24.8 mV with \({\text{HPO}}_{4}^{2-}\) . Moreover, the spontaneous imbibition test enabled us to calculate oil recovery. The computed oil recovery values for low-salinity water enriched with \({\text{H}}_{2}{\text{PO}}_{4}^{-}\) , \({\text{HPO}}_{4}^{2-}\) , and \({\text{SO}}_{4}^{2-}\) were 48%, 44%, and 36%, respectively. Finally, these recovery values significantly increased to 78%, 74%, and 66% for \({\text{H}}_{2}{\text{PO}}_{4}^{-}\) , \({\text{HPO}}_{4}^{2-}\) , and \({\text{SO}}_{4}^{2-}\) solutions after exposure of the core plug sample to CTAB. Therefore, the presence of \({\text{H}}_{2}{\text{PO}}_{4}^{-}\) ion, in conjunction with CTAB, yielded the most favorable results across all conducted tests.
