Uncovering Mysteries of Waxphaltenes: Meticulous Experimental Studies of Field and Lab Deposits Unveil Nature of Wax-Asphaltene Intermolecular Interactions

Abstract

In recent years, the utilization of modern sampling tools provided access to the field deposits from several offshore and onshore wells producing asphaltenic crudes. Compositional analysis of field deposits revealed the presence of asphaltenes and wax as major fractions, while system conditions traditionally implied precipitation and deposition of asphaltenes only. Most of the previous studies on organic deposition have been conducted with the key assumption that aggregation and precipitation of wax and asphaltene occur independently. A few researchers investigated the solubility parameter's alteration, but they did not incorporate waxes found in the oilfield deposits. This study aims to investigate the nature of "waxphaltenes"; from intermolecular interactions between asphaltenes and wax in samples collected from fields and made in the laboratory. Asphaltenes samples were extracted and fully characterized by proton nuclear magnetic resonance (NMR) and Fourier-transform infrared spectroscopy (FTIR). Paraffin waxes were identified using gas chromatography (GC), differential scanning calorimetry (DSC), NMR, and FTIR. Precipitation tests of asphaltenes with n-heptane at high temperature were performed both in the presence and absence of wax; GC, NMR and FTIR techniques evaluated the precipitates and the material dispersed in solution. It was found that asphaltenes co-precipitated with waxes even at higher temperatures than the normal wax appearance temperature (WAT) of the crude oil or the model solutions and that long and medium size paraffin waxes had higher tendencies to coprecipitate with asphaltenes than either short chain or very long chain paraffin hydrocarbons. The results also indicated that the amount of wax that co-precipitates with asphaltenes was more related to asphaltene structure but is independent of the asphaltenes or wax content. Heteroatoms played an important role in the interactions between wax and asphaltenes during precipitation and separation.