Effects of asphaltenes and molecular composition on the adsorption mechanisms of surfactant mixtures onto basal sandstone of the carbonera formation

• Adsorption is governed by surfactant hydrophilicity, interactions with asphaltenes, and the balance between monomeric and micellar regions. • Asphaltenes enhanced interfacial excess for specific formulations. • Asphaltenes influenced the selective adsorption of anionic surfactants and stabilized the anionic/nonionic surfactant molar ratio. • Asphaltenes could help maintain interfacial tension. The adsorption of surfactants from aqueous solutions in porous media is a determining parameter in enhanced oil recovery processes. Surfactant loss due to adsorption on the reservoir rocks is associated to an efficiency decrease during traditional chemical flooding, as it diminishes the reduction of oil–water interfacial tension. However, in the case of oilfields stimulations by cyclic injections of surfactant formulations, adsorption can lead to surface wettability changes. Natural crude-oil polar molecules, such as asphaltenes adsorbed on the mineral are associated to oil-wet surfaces. In such cases, surfactants molecules can compete with asphaltenes for adsorption onto the porous medium or being adsorbed onto these molecules by hydrophobic interactions. Traditional studies focus on the static adsorption isotherms of solid/surfactants systems. This study investigates the adsorption behavior of anionic/nonionic surfactant mixtures including asphaltenes molecules, focusing on the influence of molecular composition and critical micelle concentration. Three surfactant formulations were evaluated: two highly hydrophilic formulations, each composed of two molecules with significantly different water affinities, and one formulation with reduced hydrophilicity. Adsorption mechanisms were analyzed across concentration ranges, with particular attention to monomeric and micellar regions. Results revealed that adsorption behavior is governed by the surfactants’ hydrophilicity and interactions with the substrate or asphaltenes. In the hydrophilic Formulation 1, asphaltenes had minimal impact on total adsorption, but selective adsorption of anionic surfactants on asphaltenes was observed. In contrast, the less hydrophilic Formulation 2 exhibited reduced adsorption in the presence of asphaltenes, with enhanced interfacial excess below the critical micelle concentration attributed to electrostatic and hydrophobic interactions. Finally, the hydrophilic Formulation 3 showed significantly increased adsorption in the presence of asphaltenes due to the reduced water affinity of its components. These findings suggest that formulations with balanced hydrophilicity and molecular interactions can enhance adsorption stability and maintain interfacial tension, providing valuable insights for optimizing surfactant performance in enhanced oil recovery applications.