Enhanced oil recovery in carbonate reservoirs using Cyphos 104: Interfacial, Wettability, and core-flooding studies with comparative assessment of Cyphos 101

Under harsh reservoir conditions, conventional surfactants used in chemical enhanced oil recovery frequently lose their efficiency due to precipitation in high salinity brines and thermal degradation. This study presents the first integrated assessment of trihexyl(tetradecyl)phosphonium bis(2,4,4-trimethylpentyl)phosphinate (Cyphos IL 104) for enhanced oil recovery (EOR) using crude oil and carbonate cores under realistic reservoir conditions in order to address these challenges. Cyphos IL 101 ([P 6,6,6,14 ][Cl]) was also tested to study the influence of the anion structure. Both commercial phosphonium ionic liquids (PILs) were evaluated through interfacial tension, wettability alteration, microemulsion stability, spontaneous imbibition, and core flooding tests. In comparison to [P 6,6,6,14 ][Cl], the results revealed that [P 6,6,6,14 ][(iC 8 H 17 ) 2 PO 2 ], which contains a larger phosphinate anion, had better surface activity. Having a lower critical micelle concentration (CMC), greater interfacial tension (IFT) reduction (up to 96 %), and stronger wettability alteration toward water-wet conditions. Additionally, the development of stable Winsor III emulsions with fine droplets even at high salinity and temperature was validated by microemulsion and microscopy analyses. Furthermore, core flooding achieved an additional oil recovery of 23.57 % OOIP in SFB, outperforming other PILs and conventional surfactant systems. These findings demonstrate that Cyphos 104 can be an attractive alternative for EOR in carbonate reservoirs exhibiting strong salinity tolerance and thermal resistance.