A method for quantifying vertical stratification in crude oil emulsions by rotational viscometry

Vertical stratification during quiescent storage can strongly affect the stability and handling of crude oil-produced water-surfactant emulsions, yet routine methods rarely provide quantitative, height-resolved information on the evolving mechanical state. This paper describes a practical workflow to characterize stratification and aging using a Brookfield-type rotational viscometer operated under fixed spindle-speed-temperature conditions. Apparent viscosity is measured at predefined relative height positions along a liquid column and repeated at scheduled aging times, yielding a spatiotemporal apparent viscosity map. Vertical heterogeneity is quantified by a dimensionless Stratification Index, SI(t), defined as the mean absolute deviation of the height-dependent viscosity profile from the column-averaged viscosity, normalized by the column average at each time point. The resulting SI(t) time series enables direct, comparable assessment of stratification magnitude and evolution across formulations with different absolute viscosity levels. Optional kinetic descriptors, including stratification onset time determined via an objective threshold and characteristic stratification rates from simple saturation models, can be extracted to support comparative screening. Method validation is addressed through technical repeatability, batch-to-batch reproducibility, and null testing with homogeneous reference fluids, with optional comparison to independent stratification indicators.