With the aim at finding a method for the selection of a suitable demulsifier for crude oil dehydration, five crude/associated water/effective demulsifier real systems were evaluated in this study. Likewise, based on the existing knowledge about the fundamentals of emulsion stabilization process, the typical characteristics of each element of the systems, which represent the variables or factors governing the process, were chosen. These variables were: in the crude oil, the acidity number, as a measure of its polarity; in the associated water, the salinity; and in the demulsifier, the relative solubility number (RSN).
A hypothesis based on the surfactant affinity difference (SAD) model is proposed. This model was adapted to the specific case of crude oil chemical demulsification. A simplified mathematical expression was obtained, which establishes a relation between the crude oil nature, the associated water salinity, and the demulsifier hydrophilicity. The data obtained from the evaluation of the selected characteristics were introduced into the model. As a result, the following empirical relationship was obtained:
where S is the associated water salinity and A is the crude oil acidity number, variables which are easily measured.
RSN values were calculated and compared with those obtained from experiments. Except in one case, the calculated RSN value approximates very closely the experimental one. It is therefore concluded that this correlation serves to select the appropriate demulsifier to dehydrate a given crude oil, based on the simple procedure outlined above.
Introduction
Design and selection of demulsifiers for dehydrating crude oils have been performed using the classical bottle test. This consists of a large number of laboratory essays. The results of these tests should be confirmed, validated and improved by field tests.
The empirical approach that forces the execution of a large number of rigorous essays, is a consequence of: the multiple factors involved in the emulsion stabilization/destabilization process, the difference between crude oil emulsions, and the changes that take place in these emulsions during the life of the production well. Besides, the techniques used to investigate the main factors related to demulsifier performance are tedious and complicated. Instead, the use of an empirical procedure, like the bottle test, gives the most practical solution.
Because of the increasing knowledge of the factors related to formation/breaking of the crude oil emulsion, demulsifier selection could be carried out in the future on a more scientific base.
Background
Emulsion stability is governed by different factors that characterize a component or have effects on the system:–Water phase salinity and pH,–Oil organic nature (paraffinic, aromatic or asphaltenic), - Relative water/oil surfactant affinity,–Oil/water ratio,–Oil and water viscosity and density difference,–Temperature.
At the same time, the affinity or relative solubility of the surfactant in the oil or the water phase depends on the salinity of the water phase, the oil nature, and the temperature. Therefore, in a water/oil/surfactant system, if the surfactant affinity to the oil phase is greater than to the water phase, the system forms an water-in-oil emulsion.
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