Interfacial and electrokinetic properties of asphaltenes and alkali/surfactant/polymer in produced water system

• Alkali/surfactant/polymer coexist with asphaltene in bulk phase.
• Asphaltene dissolved in toluene–heptane mixtures was employed as model oil system.
• Oil-in-water emulsion was used to mimic produced water from ASP flooding.

Oil-in-water (O/W) emulsions were prepared to mimic water produced by alkali/surfactant/polymer (ASP) flooding. We used a model oil system consisting of asphaltenes precipitated from bitumen and that were then dissolved in toluene–heptane mixtures (6:4, v/v). Different amounts of ASP agents were initially added to brine and these solutions were used as the aqueous phase. Static and dynamic interfacial tension, interfacial rheology, and zeta potential measurements were used to investigate emulsion stability. At an asphaltene concentration of 8 g/L, when ASP coexists with asphaltenes in the bulk phase, a much higher elastic modulus was obtained. After aging for 10 h, the nearly constant values of the elastic modulus indicated that the interfacial film was quite stable. The static and dynamic interfacial tension rapidly decreased with an increase in ASP concentrations, which can be attributed to the combination of diffusion, adsorption, and cross-linking interactions between asphaltenes and ASP to form a three-dimensional network structure. Asphaltene molecules that adsorbed at oil/brine interfaces rearranged with increased aging time. These factors should enhance the elastic modulus. The elastic modulus is an important property of emulsion stability, and a rigid and viscoelastic interfacial film inhibits emulsion droplet coalescence. Addition of ASP had the most significant effect on the zeta potential of the emulsion droplets. The zeta potential of the emulsion droplets became more negative, resulting in an increase in electrostatic repulsion. These observations further confirmed the role of ASP in controlling interfacial electrokinetic properties through specific interactions with hydroxyl groups, hydrophilic headgroups and hydrophobic chains of the surfactant and polymer at oil/brine interfaces. The synergetic effect of electrostatic repulsion, adsorption, cross-linking, and molecular rearrangement were the main mechanisms that stabilized the O/W emulsions.