Geochemical Investigation of Surfactant-Polymer Flooding in CarbonateUnder Harsh Conditions

Surfactant-polymer (SP) flooding has emerged as a promising enhanced oil recovery (EOR) techniquefor carbonate reservoirs characterized by high-temperature and high-salinity (HTHS) conditions. Thesereservoirs, often defined by their low permeability, heterogeneity, and predominantly oil-wet nature, posesignificant challenges to conventional recovery methods. This study explores the potential of combining azwitterionic carboxybetaine surfactant with an ATBS-based polymer to improve mobility control, enhancesweep efficiency, and optimize flow dynamics within carbonate reservoirs. Comprehensive corefloodingexperiments were conducted on Indiana limestone cores under controlled laboratory conditions. Theinfluence of surfactant concentration, polymer addition, and brine salinity on pressure drop, rheology,and geochemical interactions was investigated. Resistance factor (RF) enhancements observed during SPflooding were substantial, particularly at reduced salinity, with RF values increasing from 3.46 at seawatersalinity to 10.06 at 10-times diluted seawater (10DSW). Effluent analyses highlighted the critical role ofgeochemical interactions, such as ion exchange and mineral dissolution. Calcium and magnesium ions,released during rock-fluid interactions, actively influenced the equilibrium, promoting favorable changesin flow dynamics. Rheological evaluations revealed the ATBS polymer's thermal resilience, with the solutions retainingeffective viscosities at reservoir-relevant temperatures (70°C) and showing only moderate reductions at90°C. The presence of surfactants in SP formulations did not compromise polymer stability, ensuringrobust viscosifying performance. Furthermore, low-salinity brines not only enhanced polymer viscosity butalso reduced polymer retention, as evidenced by residual resistance factors (RRF) consistently below 1.35across all injection scenarios. This highlights the SP system's ability to maintain injectivity while deliveringsignificant mobility control benefits. This study demonstrates the potential of surfactant-polymer floodingas a transformative EOR approach tailored to carbonate reservoirs under HTHS conditions. The findingsunderscore the importance of salinity optimization, chemical formulation tuning, and the integration of rheological and geochemical insights to maximize recovery efficiency. Future work will focus on extendingthese laboratory findings to field-scale applications, ensuring cost-effectiveness and operational feasibilityin diverse reservoir environments.