Synergistic effects of surfactant mixtures on the displacement of nonaqueous phase liquids in porous media

Binary surfactant mixtures have often been used to enhance the recovery or cleanup of nonaqueous phase liquids (NAPLs) such as crude oil from subsurface formations. Through synergistic effects, these mixtures further promoted NAPL mobilization and solubilization by lowing NAPL/brine interfacial tension (IFT) and desorbing NAPL from mineral surfaces, thereby altering their wettability. While the impact of mixing synergy on IFT has been well documented, its effect on wettability alteration in rocks under different test conditions (i.e., mineralogy and pore topology) is still unclear. The objective of this study was to establish structure-function relationships in these systems and identify the test conditions under which binary mixtures outperform single surfactants. Four different nonionic surfactants (alkyl glucosides, linear/branched alcohol ethoxylates) were selected with a hydrophile-lipophile balance number of (similar to)13. We used IFT, contact angle (CA), and spontaneous imbibition tests with NAPL-aged cores to understand the mechanisms of interaction and examine the efficiency of surfactants and their mixtures in promoting NAPL displacement in three sedimentary rocks with different mineralogy and pore topology. Synergistic effects were investigated by calculating cross-sectional areas and interaction parameters of surfactant mixtures and relating them to IFT and CA. Alkylphenol ethoxylates with relatively short aliphatic tails could fill the gaps between the large sugar-based heads of alkyl glucosides and enhance their geometrical packing at NAPL/brine interfaces. The denser interfacial layer formed by these mixtures led to the largest decrease in IFT and promoted the highest NAPL mobilization in low permeability sandstone and carbonate samples. The ability of nonionic surfactants to restore the wettability of oil-wet mineral surfaces was a function of their molecular structure and rock topology. In homogeneous sandstones, alkyl glucosides with their strong hydrogen bonding ability with silanol groups of quartz, could restore the wettability to a greater extent and consequently exhibited the highest NAPL removal. However, they were ineffective in altering the wettability of carbonates due to their strong self-association tendency into large micelles which prevented them from penetrating rough and nanoporous regions. The synergistic mixing of alkyl glucosides with alkylphenol ethoxylates could overcome these limitations as they exhibited a complementary mechanism of action with respect to IFT reduction and wettability alteration, recovering the largest amounts of NAPL in carbonate-bearing rocks.