Breaking of Water-In-Crude Oil Emulsions. Part 9. New Interfacial Rheology Characteristics Measured Using a Spinning Drop Rheometer at Optimum Formulation

Water–crude oil interfaces often exhibit a viscoelastic layer with a high mechanical resistance, consisting of natural surfactants in crude oil, mainly asphaltenes, which stabilize water-in-oil emulsions. Shear and dilational interfacial rheological properties of these systems have been studied for more than 40 years. However, a clear understanding of the role of interfacial rheological behavior in water-in-oil emulsion destabilization at optimal formulation (when the hydrophilic lipophilic deviation, HLD = 0) has been established only a few months ago thanks to the use of an oscillatory spinning drop rheometer. In the first studies using this equipment, the dilational interfacial rheological properties of water–oil interfaces have been measured for very simple systems, with pure cyclohexane and pure surfactant, showing a very consistent new behavior. In the present work, different oils are used and the system complexity is increased up to be close to actual petroleum cases by including asphaltenes. As in systems without asphaltenes, a deep minimum in dilational modulus and phase angle is found at optimum formulation, thus showing that the reported interfacial rheology phenomenon is very general. Then, a practical approach for crude oil dehydration is designed, and the formulation scan is carried out by adding an increasing concentration of a surfactant acting as a demulsifier, as in the bottle test method usually used. For the first time with oil-containing asphaltenes, the occurrence of a deep minimum of interfacial dilational rheological properties is shown to happen at the point of minimum interfacial tension and maximum instability of emulsions, that is, at optimum formulation. This is a significant advance in the evaluation of the selected demulsifier performance for crude oil dehydration.