Mobilization and micellar solubilization of NAPL contaminants in aquifer rocks

• Asphaltenes contained in NAPLs can alter the wettability of aquifer rocks from strongly water-wet to weakly water-wet.
• Surfactant micelles can desorb DNAPL (i.e., 4–6 wt.% asphaltenes) from rocks by forming Winsor type I microemulsions.
• Surfactant can also mobilize bulk NAPL (containing 9 wt.% asphaltenes) by forming Winsor type III microemulsions.
• Spontaneous imbibition tests indicate that the ratio of mobilized to solubilized NAPL is about 6:1.
• With colloidically stable asphaltenes, aging cores in NAPL beyond 3 days do not affect the amount of NAPL recovered by solubilization.

Surfactant-enhanced aquifer remediation is often performed to overcome the capillary forces that keep residual NAPL phases trapped within contaminated aquifers. The surfactant selection and displacement mechanism usually depend on the nature of NAPL constituents. For example, micellar solubilization is often used to cleanup DNAPLs from aquifers whereas mobilization is desirable in aquifers contaminated by LNAPLs. Although the majority of crude oils are LNAPLs, they often contain heavy organic macromolecules such as asphaltenes that are classified as DNAPLs. Asphaltenes contain surface-active components that tend to adsorb on rocks, altering their wettability. Previous studies revealed that surfactants that formed Winsor type III microemulsions could promote both mobilization and solubilization. However the extent by which these two mechanisms occur is still unclear, particularly in oil-contaminated aquifers.In this study we investigated the remediation of oil-contaminated aquifers using an environmentally friendly surfactant such as n-Dodecyl β-D-maltoside. Focus was given on asphaltenes to better understand the mechanisms of surfactant cleanup. Through phase behavior, spontaneous imbibition, dynamic interfacial tension and contact angle measurements, we showed that microemulsions formed by this surfactant are able to mobilize bulk NAPL (containing 9 wt.% asphaltenes) in the porous rock and solubilize DNAPL (i.e., 4–6 wt.% adsorbed asphaltenes) from the rock surface. Spontaneous imbibition tests, in particular, indicated that the ratio of mobilized to solubilized NAPL is about 6:1. Furthermore, aging the cores in NAPL beyond 3 days allowed for more NAPL to be trapped in the large pores of the rock but did not alter the amount of asphaltenes adsorbed on the mineral surface.

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