Optimized Guerbet Alkoxy Carboxylate Surfactant Formulations for EOR in Carbonates Under Harsh Conditions

Surfactant flooding is a proven chemical flooding technique with limited applications in carbonates, especially in the Middle East, due to prevailing conditions of high temperature, high salinity (HTHS), and mixed-to-oil wettability. Under such conditions, surfactants break down or become inefficient, limiting their effectiveness for enhanced oil recovery (EOR) applications. This study evaluates the efficiency and stability of superior chemistry carboxylate-based anionic surfactant blends for EOR applications in Middle East HTHS carbonate reservoirs. The study employed a range of tests to identify optimal surfactant blends for HTHS carbonate conditions, including thermal and aqueous stability tests and phase behavior analyses for salinity optimization. Both carboxylate and internal olefin sulfonate surfactants were utilized at elevated temperatures and salinities, up to 120°C and 170 kppm, respectively. Stable formulations were selected based on their performance under these representative field conditions. Interfacial tension was measured using a spinning drop tensiometer and verified using Huh correlation during phase behavior studies. The screening was further supported by surfactant retention to ensure efficiency and economic viability through both static and dynamic studies. Moreover, a coreflooding experiment was conducted to evaluate incremental oil recovery through this optimized surfactant formulation. The study demonstrated that specific blends of alkyl ethoxy carboxylate surfactants with internal olefin sulfonate co-surfactants are highly effective in terms of thermal and aqueous stability. These blends consistently maintained transparent, homogeneous solutions without any noticeable turbidity, even when exposed to rigorous reservoir conditions characterized by elevated temperature and salinity. However, for reservoirs with higher salinity, seawater pre-flush is recommended to condition the formation before surfactant injection. The surfactant blends also demonstrated ultra-low interfacial tensions in the order of 10⁻³ at their optimal salinity levels, close to seawater salinity (43 kppm). This was confirmed by spinning drop measurements and Huh correlation. In parallel, contact-angle measurements showed a shift from strongly oil-wet (~161 °) to water-wet (~69°) conditions, confirming that the formulations induce favorable wettability alteration alongside IFT reduction. Additionally, the surfactant blends exhibited low adsorption onto carbonate rock surfaces, with retention values within the generally accepted economic viability range (~0.1 to 0.2 mg/g-rock), underscoring their suitability and promise for practical EOR applications in harsh, high-temperature, and high-salinity carbonate reservoir environments. Coreflooding experiments further validated the effectiveness of the optimized surfactant blend, achieving a significant incremental oil recovery of 22.9% OOIP (≈ 76 % of S_orw) -16.5 % OOIP from the surfactant slug (≈ 55 % of S_orw) and a further 6.4 % OOIP from the salinity-gradient post-flush (≈ 21 % of S_orw). This study expands the envelope of surfactant flooding EOR applications to Middle East HTHS carbonate reservoirs. By introducing a holistic evaluation framework integrating thermal and aqueous stability, interfacial tension, phase behavior, and salinity optimization, this research highlights the potential of surfactant-based EOR for harsh reservoir conditions. Although further research is necessary to address all challenges associated with surfactant-based EOR in carbonates, the insights gained establish a foundation and pave the way for possible future field-scale applications.