Physicochemical Approach to Pour Point Depressant Treatment of Waxy Crudes

A physicochemical approach was taken to show wax-based and crude oil-based differences between three waxy crudes (Amenam, Norne, and Varandey), that had similar amounts of wax according to DSC crystallizable areas, that were responsible for very different responses to pour point depressant additive treatments. Varandey crude oil was the most difficult to treat, and the effectiveness of the additives decreased with time when the oil had been aged for 2 years prior to treating with the additives. Detailed compositional analysis of these three crude oils, and their waxes, showed that Varandey contains the highest concentration of longer n-paraffins, significantly more total n-paraffins, a higher wax appearance temperature, the highest amount of wax that can be centrifuged at ambient temperature, a bimodal distribution of crystallizable waxes by DSC, more saturates in the heavy ends by the Saturates, Aromatics, Resins-Asphaltene Determinator, a very high amount of Waxphaltene Determinator Waxy B (–24 °C methyl ethyl ketone precipitated waxes that do not dissolve in heptane at that temperature but melt at 60 °C in heptane) and very little asphaltenes that do not flocculate. These additional characterization methods, including the wax appearance temperature, show that the amount of wax by DSC is not discriminating for some crude oils. Microscopy of Varandey showed large fractal crystalline domains that were different from Amenam, which showed classic large macrocrystalline needle-like crystals, or Norne that had a gelled network with much smaller needle-like crystals. Norne is less responsive to additive treatment than Amenam despite having significantly less n-paraffins, Waxphaltene Determinator Waxy B waxes, and asphaltenes. Simulated distillation shows that Norne is the heaviest oil and has the highest amount of isomeric material in the C24–C34 range and a higher temperature wax appearance temperature. Furthermore, its asphaltenes flocculated with the addition of isooctane at 60 °C, whereas the asphaltenes did not flocculate in Amenam. The higher amount of isomeric material by GCMS and the much smaller needle-like crystallites suggest that Norne wax is more microcrystalline in nature. Centrifuging Varandey waxes and doping them into Amenam and Norne resulted in the doped crude oils having a bimodal DSC profile with a higher wax appearance temperature. The presence of Varandey wax caused the oils to be significantly less responsive to pour point additive treatments. Overall, for these relatively similar crude oils, it is shown that the nature of the highest carbon number waxes, the wax appearance temperature, and the amount of the least soluble waxes are significantly more important than the crude oil composition when treating with additives since they will dictate the temperature at which n-paraffins first self-associate leading to crystal nucleation.