Dilute Surfactant Injection for Chemical EOR in High-Salinity, High-Temperature Carbonate Reservoirs of Sultanate of Oman

Summary Carbonate reservoirs represent more than 50% of the world's oil reserves and are predominantly oil-wet, which limits recovery efficiency under conventional primary and secondary recovery methods. In the Sultanate of Oman, oil recovery from carbonate reservoirs is further challenged by high-salinity formation waters (up to 180,000 ppm), elevated reservoir temperatures (60-85 °C), and significant geological heterogeneity. This study evaluates the applicability of dilute surfactant injections as a chemical enhanced oil recovery (CEOR) method to alter wettability and improve oil recovery in two representative high-salinity, high-temperature (HSHT) carbonate reservoirs. The proposed methodology involves analytical pre-screening based on a comprehensive literature review, guided by surfactant selection criteria commonly recommended in SPE literature. Published data on CEOR performance were integrated into the screening process, followed by comprehensive laboratory evaluation program conducted under the reservoir-representative conditions. An initial set of 33 surfactants was screened for compatibility, thermal stability testing, emulsion and turbidity assessments. Based on chemical stability and phase behavior results, 12 surfactants were shortlisted for spontaneous imbibition experiments. The results indicate that nonionic surfactants generally outperformed other surfactant classes under HSHT conditions. Among the tested formulations, surfactant N25 exhibited the highest incremental oil recovery, achieving 32.6% in Reservoir A and 34.8% in Reservoir B. Other surfactants showed moderate recovery, while cationic surfactants exhibited limited effectiveness. The findings confirm that diluted surfactant injections can significantly enhance oil recovery in HSHT carbonate reservoirs when appropriate surfactants are selected. While the study established technical feasibility, field-scale performance and long-term chemical retention were beyond its scope and warrant further investigation. The proposed methodology serves as a practical benchmark for analogous CEOR initiatives across the Middle East and other regions dominated by carbonate reservoirs worldwide.