Experimental study of hybrid surfactant-gas enhanced oil recovery for liquid–rich tight reservoirs

Enhanced oil recovery (EOR) in unconventional reservoirs commonly uses gas injection, while surfactant-based methods have recently gained attention. This study investigates a hybrid approach that combines surfactant-assisted spontaneous imbibition (SASI) with CO2 injection in silica-rich, oil-wet tight rocks. Core flooding experiments were conducted on Scioto sandstone and Meramec shale, with surfactants selected through contact angle, interfacial tension, and imbibition tests. Results show that surfactant injection followed by CO2 consistently improves oil recovery compared to either method alone. Gas-first sequences recovered oil but caused delayed and reduced surfactant response. Gas chromatography confirmed that surfactants primarily mobilize heavier hydrocarbons, while CO2 recovers lighter fractions. Gas utilization analysis further indicated higher efficiency when surfactant preceded gas injection. These findings demonstrate that the surfactant–gas sequence is more effective for hybrid EOR in liquid-rich, tight reservoirs.