Effect of formation water and surfactant characteristics on reservoir rock surface charges and their zeta potentials

Abstract

The loss of injected surfactants in the oil reservoir during a surfactant flooding due to adsorption onto the rock surface weighs heavily on economics. Surfactant adsorption weakens the efficiency for the oil-water interfacial tension reduction. Several factors that influence the surfactant adsorption have been investigated, including the effect of temperature, as well as the concentration and charge of the surfactants. However, the impact of rock surface charge and zeta potential on surfactant adsorption has remained uncertain. This is because the surface charge development of the rock is complex depending on the effect of rock mineralogy, pH, brine salinity, and ionic strength of the ions present. This project aimed to develop an improved understanding of these effects on the rock surface charge and its contribution to the surfactant adsorption. These objectives were achieved through detailed laboratory measurement of surface charge density and zeta potential of silica and kaolinite by electrophoretic light scattering at different pH, brine salinities, and ionic strength. There were three different surfactant types used: anionic, zwitterionic, and non-ionic, which are represented by sodium dodecyl sulphate, coco betaine, and triton X-100, respectively, to investigate their adsorption on the surface of silica and kaolinite. The mineral characterization study shows that SiO2 and Al2O3 were dominant chemical compound in both silica and kaolinite, which reflect their surface charge. The experimental work demonstrates that silica has a negative surface charge for pH exceeding 4.4 at all NaCl concentrations from 0.01M to 1M. However, in CaCl2 solutions, the negative surface charge of silica turns to a positive charge at 0.1M for pH exceeding 8.7 and becomes more positively charged at a concentration of 1M. Different surface charge behavior was observed for kaolinite, which it is negatively charged at all NaCl concentrations from 0.01M to 1M. Moreover, both negative and positive charges were observed in CaCl2 solutions. At pH of less than 10.8, the kaolinite surface indicates a negative charge at both concentrations of 0.01M and 0.1M. The positive charge of kaolinite was observed at a concentration of 1M with pH exceeding 4.4. These results indicate that pH, salt concentration (salinities), and salt types (ionic strength) have a major influence on the surface charge behavior of silica and kaolinite.