Mechanisms of wettability for crude oil/brine/mica system
Yang, Shih-Yi Steve
Hirasaki, George J.
Doctor of Philosophy
The system under investigation is the crude oil/brine/mineral (COBR) system. Deposition of polar asphaltenes from oil causes mineral wetting to alter from strongly water-wet to less water-wet. The degree of alteration depends on the hydrophobicity of asphaltene and the amount of deposition, which depends on many factors such as asphaltene content and solvency of the oil, acid and base content of the oil, brine pH and salinity, aging condition, and mineral type. Bulk and interfacial physico-chemical properties that affect mineral surface wetting are examined. Acid and base content are determined by potentiometric titration. Asphaltene solvency is quantified with the refractive indices of the oil and its solution with paraffinic precipitants. Interfacial tension and zeta potential of the oil/brine interface are measured to investigate the effect of surface-active compounds on interfacial properties. Two experimental methods to characterize surface wetting (contact angle and adhesion) are conducted. Wetting transition pH is calculated based on the DLVO theory and good agreement is observed in comparison with experimental results for two crude oils. Statistical analysis is preformed on the parameters affecting wettability. When the candidate parameter set is properly chosen, the two important factors (asphaltene content and base number/acid number) that most affect wetting emerge from the analysis. Atomic force microscopy (AFM is used to characterize mica surfaces that have first been equilibrated in 0.01 M NaCl pH 6 brine and then aged in crude oil at elevated temperature. The AFM images show the mixed-wet surface to be patches of bare mica and patches of asphaltene with a characteristic areal dimension of about 200 nm. A contact-angle model is developed based on the hypothesis that mineral surface is covered with circular, hydrophobic patches on a hydrophilic mineral surface in hexagonal pattern. A numerical simulation combined thin-film hydrodynamics (lubrication theory) with creeping flow model (local-wedge) to simulate advancing/receding motion of a gas/liquid interface over a solid. The equilibrium and apparent (advancing, receding) angles are examined. Equilibrium angle can be calculated exactly utilizing disjoining pressure. The apparent dynamic angles are observed to be a function of disjoining pressure (equilibrium angle) and line-speed of the interface (capillary number).