We report for the first time the results from a systematic investigation of how asphaltenes of different polarity affect crystallization and gelation of waxy oils. The more polar asphaltenes were found to be more aromatic in nature and more highly self-aggregated in the solvent. The presence of less polar asphaltenes in the waxy oil reduced the wax appearance temperature and wax precipitation to a greater degree compared to more polar asphaltenes, which was mainly attributed to the difference in the aggregation state of asphaltenes of different polarity. Reducing the polarity of asphaltenes present in the oil also resulted in a lower gelation temperature, lower storage modulus, and lower yield stress, which was probably because the less polar asphaltenes were more similar to wax on the molecular level and, thus, more readily interacting with wax. Notably, a 99% reduction in the yield stress was observed upon the addition of the least polar asphaltenes examined in the present work, in contrast to the 62% yield stress reduction upon the addition of the most polar asphaltenes. This observation may be of industrial significance because it suggests that the crude oil containing less polar asphaltenes may form a softer gel or deposit that is more easily broken or removed. Microscopic analysis showed that the wax crystals precipitated in the presence of less polar asphaltenes have a smaller aspect ratio.

Asphaltene can exist in both the dispersed state and the aggregated state in crude oil. Because of the changes in crude oil composition, pressure, or temperature, the asphaltene transition from dispersed asphaltene to aggregated asphaltene will occur and then influence the wax crystallization, gelation, and flow behavior of crude oil. In this paper, the asphaltene transition was realized by mixing two different crude oils for different times. The aggregated asphaltene was characterized by the optical microscopy and centrifugation-based separation method. The effects of asphaltene transition on wax crystallization, gelation, and flow behavior of crude oil were investigated by differential scanning calorimetry and rheological measurements. The results show that the aggregated asphaltene can serve as a crystal nucleus for wax molecules, promoting the wax precipitation, weakening the strength of the network of wax crystals, and delaying the gelation process of crude oil. On the other hand, the dispersed asphaltene can serve as the connecting point between wax crystals, accelerating the gelation of crude oil, and increasing the gel strength. The viscosity measurements below the wax appearance temperature show that the viscosity of crude oil increases because of the interaction between aggregated asphaltene and wax.

No abstract

Wax precipitation and deposition are major flow assurance problem in crude oil production and transportation. Knowledge of the amount of wax that precipitates from the crude oil at different temperatures, delineated by the solubility curve, is necessary for accurate predictions of wax deposition in subsea pipelines. The solubility curve is obtained from the wax precipitation curve, which is determined using centrifugation and high temperature gas chromatography (HTGC). However, previous studies have overestimated the amount of precipitated wax by assuming that all heavy alkanes or heavy n-alkanes exist solely in the solid phase. This work addresses this issue by conducting mass balances on the noncrystallized carbon numbers in the centrifuged cake and in the crude oil. A new equation was developed to obtain the solid fraction of the centrifuged cake and crude oil. The wax precipitation curve developed by this new method was compared with the curves determined using the previous methods and differential scanning calorimetry (DSC). The curve determined by this new method was consistent with the results obtained by DSC, while previous methods overpredicted the amount of precipitated wax.

Asphaltenes, as the most polar component of crude oils, may interact with wax molecules during the wax crystallization, thus changing wax precipitation characteristics and wax crystal structure and affecting the gelation behavior of crude oils. In the present work, waxy model oils of variable asphaltene contents (0–0.20 wt %) were prepared in order to systematically examine the effect of asphaltenes on the wax precipitation characteristics and the gelation behavior of waxy oils and to look into the structure and the surface electrical properties of wax crystals formed in oils of different asphaltene contents. By means of differential scanning calorimetry and multiple rheometries, it was found that asphaltenes play a significant role in inhibiting the precipitation of wax crystals and delaying the gelation, as well as in weakening the strength of the gel structure. It was observed by polarized optical microscopy that as the asphaltene content increases, the size of wax crystals gradually decreases, and the shape of wax crystals transforms from rodlike into elliptical. Furthermore, both the average aspect ratio and the average perimeter of wax crystals decrease, while the boundary box fractal dimension increases, demonstrating that the morphology of wax crystals becomes more complex. Based on the X-ray diffraction (XRD) pattern, the grain size obtained with the Scherrer equation presents a descending trend with the increase of asphaltenes. It could be inferred that the reduction of grain size is one of the reasons leading to the smaller sizes of wax crystals. The XRD result also showed that with higher asphaltene content, the lattice parameter c of wax crystals obviously increases, indicating that the conformation of wax crystals gets more disordered, which results in the weakening of gel strength, corresponding to the decline of the measured yield stress of the waxy oil gel. With higher asphaltene content, the zeta potential of wax crystals goes up, which indicates that with more negative charges adsorbed on the wax crystal surfaces, stronger electrostatic double layers have formed. The greater electrostatic repulsion among wax crystals would interfere with the growth and aggregation of wax crystals, thus holding up the gelation of waxy oils.