THE ASPHALTENE AND WAX DEPOSITION ENVELOPES

Asphaltene and wax phase behavior is quite different than the conventi onal ''PVT'' phase behavior. Asphaltenes exhibit a behavior at some th ermodynamic states called flocculation. That is, asphaltene particles or micelles aggregate or flocculate into larger aggregates or flocs. T he locus of all thermodynamic points in a P-T-x phase diagram at which flocculation occurs is called the Asphaltene Deposition Envelope (ADE ). Paraffin waxes, on the other hand, at some thermodynamic states, ex hibit the phenomenon of crystallization. The locus of all thermodynami c points in a P-T-x phase diagram at which wax crystallization occurs is called the Wax Deposition Envelope (WDE). Asphaltene flocculation c an be both reversible and irreversible (as expected from the asphalten e colloidal nature). Wax crystallization is generally a reversible pro cess. However, paraffin waxes more than often precipitate together wit h resins and asphaltenes (which are said to be responsible for the obs erved irreversible thermodynamic phenomena). Hence, some wax precipita tion is occasionally reported as irreversible. Measurement of the ADE and WDE boundaries, at in-situ conditions, is a challenging task. Meas urement of asphaltene and wax solubility at in-situ conditions inside the ADE and WDE is even more challenging. The ADE and WDE data have pr actical significance and are very useful for modeling of reservoir flu id behavior. Asphaltene and wax data for a number of oils are presente d and analyzed. This paper introduces and compares two powerful thermo dynamic diagrams that summarize the phase behavior of asphaltenes and waxes, the ADE and WDE.