Surfactant flooding for enhanced oil recovery – Insights on oil solubilization through optical spectroscopic approach

Surfactant flooding is a proven method employed to improve the recovery of crude oil from subsurface reservoirs but the knowledge about microemulsion formation, factors related to their stability, and the trapping mechanism of crude oil components in a microemulsion is still not clear for designing of injection fluid for specific crude type. The interaction mechanisms of four crude oils with surfactant-brine solutions were studied in this work for better insight into the molecular level mechanisms during surfactant flooding. UV–visible absorption and fluorescence spectra of the separated maltene and asphaltene fractions indicate the structural difference of the components in the molecular level, particularly, the size of the fused aromatic rings (FAR) present within individual bulk crude sample. Moreover, the FTIR spectra of the separated asphaltenes from the crude samples clearly showed the subtle difference in the functional groups present in respective FAR structures. Well-characterized compositionally different crude oils were mixed with four different surfactant-brine solutions (anionic – SDS, AOS, cationic - CTAB, and neutral - TX-100) and examined using UV–visible absorption and fluorescence spectroscopic approach. Detailed analysis of the acquired absorption and fluorescence spectra of the crude oil-surfactant-brine solutions distinctly demonstrate the entrapment of different crude oil components within the micelles formed by surfactants of different charges while the synchronous fluorescence spectra show the entrapment of specific size FARs within the specific micellar microenvironment. The results exhibit that the anionic surfactant entraps the smaller size (2–4) FARs whereas cationic and neutral surfactants entrap small as well as medium size (2–6) FARs within the formed micelles in the microemulsions. The present spectroscopic study signifies the importance of molecular-level interaction mechanisms for customized designing of surfactant solutions for maximizing crude oil recovery from petroleum reservoirs.