Studies on crude oil and petroleum product emulsions: Water resolution and rheology

Water-in-oil mixtures from more than 300 crude oils and petroleum products were made in the laboratory and studied over time. More than 140 of these yielded products resembling emulsion. Water-in-oil types were characterized by resolution of water at 1 and 7 days, and some after 1 year. Rheology measurements were carried out at the same intervals. The objective of the study was to characterize the water-in-oil types and relate these to starting oil properties.It was found that water-in-oil types can be grouped into four categories: stable, unstable, meso-stable and entrained. Each of these has distinct physical properties. A stable emulsion remains so for at least 30 days and takes up typically 80% water. The viscosity of a stable emulsion is at least two orders-of-magnitude greater than that of the starting oil. An unstable emulsion does not retain water volume greater than 10%. For an unstable emulsion, the viscosity is typically less than two times greater than the viscosity of the starting oil. A meso-stable emulsion has properties between unstable and stable emulsions and typically takes up 60% water, breaking down within 7 days, losing most of this water. A meso-stable emulsion usually has a viscosity no more than one order-of-magnitude greater than that of the starting oil. Another retention mechanism for water in oil is that of viscous entrainment of water droplets. This type of water uptake, not a true emulsion, has been called entrained. Entrained water to the extent of typically 45% is taken up initially and this water content declines slowly over a period of days.A stability index was developed to characterize these water-in-oil types. This consists of the product of the ratio of viscosity increase and a ratio of the elasticity increase. This index was used to describe the water-in-oil types and to correlate stability with starting oil composition and properties.A comparison of the properties of the starting oils before mixing with water shows that the most important factors for water uptake and emulsion formation are asphaltene and resin content, starting oil viscosity and density. Other factors that are shown to be important are the saturate content and asphaltene-to-resin ratio. The relationship between these factors and stability gives insights into the nature of water-in-oil types and emulsions.