Asphaltene Deposition

Although asphaltenes are always present in crude oils, they create problems only when they become insoluble in the remainder of the oil. A change in asphaltene solubility can occur because of changes in pressure, composition, or temperature. Whether asphaltene insolubility constitutes a problem, however, depends primarily on when and where in the oil production, transportation, and processing stream it occurs. Instability leading to deposition in a deepwater well is high on the list of potentially problematic situations. While insolubility is a necessary condition, it is not sufficient to predict the buildup of an asphaltene deposit in a production well. Flocculated asphaltenes may deposit or be carried out of the well in the flowing oil phase. To improve understanding and, hence, prediction of asphaltene deposition, several tests have been reported. A capillary deposition test in long stainless steel capillaries has been used to investigate the tendency of a wide variety of oils to create deposits. Observations, in addition to pressure increases during flow through a capillary, include changes in morphology of asphaltene flocs as they slowly separate from unstable oils, the amount of asphaltene produced as a function of time, and chemical analysis of deposits recovered from capillary tests. Other tests have used glass capillaries and a Taylor-Couette cell. As insight grows, the data produced are being used to develop simulators to predict the rate of growth of arterial asphaltene deposits under wellbore conditions.