Formulation of an alkali-surfactant slug for enhanced oil recovery in depleted reservoirs of the Assam shelf basin

Chemical Enhanced Oil Recovery (CEOR) has shown strong potential for improving oil production in the Assam Shelf Basin. This study aims to develop an optimized Alkali–Surfactant (AS) formulation by combining the anionic surfactant Sodium Dodecyl Benzene Sulfonate (SDBS) with cost-effective synthetic alkalis Sodium Carbonate (Na₂CO₃), Sodium Hydroxide (NaOH), and Sodium Orthosilicate (Na₄SiO₄). Formulation screening included measurements of interfacial tension (IFT), wettability alteration (WA), zeta potential (ZP), coreflooding performance, conductivity behavior, turbidity and monovalent/divalent ion exchange before and after flooding. The critical micelle concentration (CMC) of SDBS in 7000 ppm formation brine (FB) was identified as 0.5 w/v %, achieving a minimum IFT of 7.4 × 10⁻² mN/m. Introducing alkalis into the CMC level SDBS slug further reduced IFT through in situ surfactant generation via saponification with acidic crude oil components. Among the alkalis, NaOH yielded the lowest IFT, followed by Na₂CO₃ and Na₄SiO₄. Reservoir rock (RR) samples from the basin characterized by XRD and SEM showed smectite-rich mineralogy dominated by montmorillonite. Three AS slugs containing 0.5 w/v% SDBS with 0.5 w/v% NaOH, Na₂CO₃, or Na₄SiO₄ (AS-1, AS-2, and AS-3) were further evaluated for WA, ZP response, core flooding efficiency, and formation compatibility. Conductivity and ionic composition changes provided insights into clay swelling and injectivity. Integrating interfacial properties, rock–fluid interactions, and dynamic coreflooding results, the Na₂CO₃ based formulation (AS-2) demonstrated the most favorable overall performance and is recommended as the optimum AS slug for enhancing oil recovery in depleted reservoirs of the Assam Shelf Basin.