(166b) The Surprising Mechanism for Wettability Alteration By Non-Ionic Surfactants

Soumik Das¹, Fardin Khabaz¹, Quoc Nguyen² and Roger T. Bonnecaze^[1]*

¹ McKetta Department of Chemical Engineering, The University of Texas at Austin, Austin, TX, 78712

²Hildebrand Department of Petroleum and Geosystems Engineering, The University of Texas at Austin, Austin, TX, 78712

Nonionic surfactants that do not form oil-water microemulsions have been found to be effective in altering the wettability of oil-wet carbonate surfaces. It is typically thought that these surfactants displace the acidic components of the oil, making the carbonate surface oil wet. Here we show the mechanism for the wettability alteration is very different. We use atomistic molecular dynamics simulations of surfactant, model oils, and water with calcite as a representative carbonate surface. Adsorption of surfactants on bare calcite is first examined and binding energies presented for different hydrophobes and hydrophilic chain lengths as well as surfactant coverages. The presence of a thin intercalated water layer indicates a weak physical interaction between the surfactants and calcite. Oil-wet surfaces are prepared by adding model oil molecules onto bare calcite. Oils of different acidities are investigated by varying the contribution of acidic components in them. Strongly acidic molecules form well-defined arrangements near the surface owing to favorable electrostatic interactions with the calcite. It is found that the amphiphilic nature of the surfactant is critical to displace the oil and promote the adsorption of water onto the surface. The extent of oil removal is found to be strongly dependent on both the type of oil and surfactant. Through the simulations, we postulate a molecular-scale mechanism to explain wettability alteration of oil-wet calcite by nonionic surfactants.

* rtb@che.utexas.edu