(175i) Experimental Studies on Adsorption of Surfactants on Carbonates
Soumik Das1, Quoc Nguyen2 and Roger T. Bonnecaze*
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
Adsorption of surfactants on mineral surfaces is a critical factor determining their efficiency for wettability alteration. There should be a substantial amount of adsorption for the onset of wettability alteration. On the other hand, very high adsorption means a potential loss of surfactants at unwanted locations and consequent depletion downstream in the reservoir. Here we present an experimental study measuring the extent of adsorption of different nonionic surfactants onto a calcium carbonate surface. The surfactants selected were previously found to be effective in altering the wettability of oil-wet calcite surfaces. Adsorption isotherms are determined for two families of surfactants with distinct hydrophobic components and varying lengths of hydrophilic ethoxylates. The extent of adsorption is strongly determined by the hydrophilicity of the surfactant molecules and the temperature of the system. Low hydrophilic surfactants have higher adsorption and exhibit adsorption isotherms similar to bilayer adsorption, particularly at higher temperatures. Estimates of the surfactant coverages are also presented. Experimental observations are summarized by a few thermodynamic parameters that give more insight into the energetics involved in the process. The maximum adsorption is found to correlate directly with the difference between the cloud point of the surfactant and the temperature. A simple scaling law exhibits that the adsorption increases significantly as the temperature nears the cloud point of the system. In order to increase the effectiveness of the surfactant by preventing excess adsorption at such high temperatures, a gemini anionic surfactant is added to promote surfactant stability in the solution. Here we also report the adsorption behavior of such mixed-surfactant systems for different combinations of individual surfactant components.