Steady-state and time-resolved fluorescence spectroscopic study of petroleum crudes in aqueous-surfactant solutions: Its implications for enhanced oil recovery (EOR) during surfactant flooding

Surfactant flooding, a widely used chemical enhanced oil recovery (EOR) method, involves reduction of interfacial tension of the crude oil-aqueous surfactant system. During the surfactant flooding micelle structures have been formed in aqueous media where crude oil components are entrapped inside it. The nature of the surfactants in the aqueous injection fluids and the crude oil compositions are the key controlling factors for understanding the efficacy of the surfactant flooding during EOR. Therefore, understanding of the molecular level interaction between crude oils and aqueous surfactant solutions is very crucial for designing injection fluids during surfactant flooding for subsurface reservoirs having different types of crude oils. Two crude oils (Cambay basin, India) and three surfactants: cationic (cetyltrimethylammonium bromide i.e. CTAB), anionic (sodium dodecyl sulfate i.e. SDS) and neutral (Triton X-100 i.e. TX100) were used in this study for understanding their interaction behavior. The chemical characterization by various analytical methods supports the presence of different sizes of the fused aromatic ring (FAR) associated with the polycyclic aromatic hydrocarbons (PAH) of the used crude oils. In this study, we have investigated the type of oil components present in the micelles formed by the surfactant solutions of various charges using steady-state and time-resolved fluorescence spectroscopic measurements. The extracted oil components in the micelles of different charges show different emission peaks with varying excitation wavelength and different fluorescence lifetime, indicating incorporation of PAH with variable sizes of FAR. Ionic surfactants (CTAB and SDS) are more compatible to exploit small (2-3 FAR) as well as medium (4-7 FAR) PAH structures whereas non-ionic surfactant (TX100) dominantly entraps medium PAH structures within their micelles. This study established that the intrinsic fluorescence of the crude oil can monitor its interaction with surfactant micelle of different charges and helps to design effective injection fluids for optimized crude oil recovery during surfactant flooding.