Surfactant–Polymer EOR Formulations for Harsh Carbonate Reservoirs

Chemical flooding in high-temperature, high-salinity (HTHS) carbonate reservoirs remains challenging because many surfactants and polymers either precipitate or adsorb strongly on rock. This work evaluates in-house zwitterionic surfactant, oleyl polyoxyethylene amidopropyl carboxybetaine (OPAC), and its surfactant–polymer (SP) blends with ATBS-based polyacrylamide (2,000 mg·L−1) in seawater (SW) and its 2-, 5-, and 10-fold dilutions at 70–90°C. OPAC solutions were thermally stable up to 80°C in all tested brines and remained single-phase at 90°C in SW and its dilutions; all SP formulations stayed clear after 30 days at 70–90°C. Static adsorption on crushed carbonate was modest. OPAC reduced crude-oil/brine interfacial tension (IFT) from ≈14–17 mN·m−1 to 0.47–1.27 mN·m−1 at 70°C, while polymer addition changed IFT by only a few tenths of mN·m−1. Aging at 70°C for 30 days lowered viscosity of SP fluid from about 7.5–8.5 to 3–4 cP at 23°C, but polymer still provided clear mobility control. On oil-aged calcite substrates, 0.25 wt% OPAC in 10DSW achieved the largest wettability shift in low-coverage regime (contact angle drop ≈59°), whereas at 0.5–1.0 wt% the strongest alteration occurred in SW (contact angle drop was up to ≈117°). Adding polymer (0.5 wt% OPAC + 2,000 mg·L−1 polymer) further changed zeta potential at calcite-brine interface to ≈−13 mV in 10DSW and ≈−22 mV in SW and yielded small but systematic gains in wettability alteration.The combined interfacial data support mechanistic picture in which OPAC activates all three wettability-alteration pathways identified in this work: (1) ion-pair-driven stripping of acidic oil films, (2) hydrophobic tail insertion that weakens and detaches the residual oil layer, and (3) surface-charge conditioning as adsorbed carboxybetaine layer turns the calcite–brine zeta potential from slightly positive to clearly negative. In SP formulations, the ATBS-based polymer does not replace OPAC in peeling off oil film; instead, it co-adsorbs on the mineral surface, deepens the negative zeta potential, adds a steric barrier that helps to stabilize the water film, and provides viscosity needed for mobility control. Together, these roles explain why OPAC alone can strongly alter wettability and why adding polymer gives a small but systematic extra decrease in contact angle while delivering sweep improvement.