(75f) Effect of Asphaltenes on the Structure and Surface Properties of Wax Crystals in Waxy Oils

As the temperature of waxy crude oils decreases, the dissolved wax molecules precipitate because of supersaturation. When the precipitated wax crystals reach a certain amount, cross-links occur among wax crystals, gradually forming a three-dimensional network, which leads to the gelation of crude oils. Asphaltenes, as the most polar component of crude oils, are able to interact with wax molecules during the wax crystallization, thus changing wax precipitation properties and wax crystal structure, and affecting the gelation behavior of crude oils. In the present work, the effect of asphaltenes on waxy model oil systems was carried out from four aspects: macroscopic wax precipitation and gelation properties, mesoscopic particle size and distribution of wax crystals, microscopic lattice structure, and zeta potential on wax crystal surfaces.

The model oils were prepared using mineral oils and mixed paraffines with variable asphaltene concentrations (0~0.20wt%). The results of the differential scanning calorimetry (DSC) and rheological measurements showed that asphaltenes could inhibit wax precipitation, delay gelation, and weaken gel structure. This impact would be magnified with the increase of asphaltenes. Through the X-ray diffraction (XRD) analysis, it was concluded that the crystallinity index (CI) decreases with the asphaltene increase as well, indicating asphaltenes hinder the wax precipitation, which confirms the DSC results. Analysis on the gel structure of waxy oils was performed via the stress sweep test and scaling theory. It was found that as more wax crystals precipitate out, the gel structure changes into the weak-link regime from the strong-link regime, and the critical amount of precipitated wax corresponding to the transition point will increase when there are more asphaltenes.

Quantitative analysis on the morphology of wax crystals was conducted by means of polarized optical microscopy (POM), Image J software, and Nano Measure 1.2 software. As the asphaltene concentration rises, the average size of wax crystals gradually reduces, and the size distribution tends to smaller particle size; furthermore, it is obvious that the wax crystals transform from distinct acicular shape to the elliptical or punctiform. The aspect ratio and average perimeter of wax crystals decreases while the boundary box fractal dimension increases, demonstrating that the morphology of wax crystals becomes more complex. Based on the XRD pattern, the grain size of wax crystals was calculated by Scherrer equation. The result showed that the grain size presents a drop trend with the increase of asphaltenes. Since the grain is considered as the basic unit of wax crystals, it is inferred that the reduction of grain size is one of the reasons leading to wax crystals getting smaller.

The lattice structure of wax crystals was studied by the XRD and MDI Jade 6.0 software. As the asphaltene is more concentrated, the lattice parameters a and b rarely varies whereas the lattice parameter c significantly increases. According to the end-gauche defect, the increase of lattice parameter c indicates the extent of conformational disorder of wax crystals is strengthened, resulting in the weakening of gel strength, which shows up as the decline of yield stress and the delay of gelation.

Finally, the zeta potential of wax crystals was detected. Based on the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, it was observed that with higher asphaltene concentrations, the zeta potential of wax crystals enhances. More negative charges are absorbed on the surface of wax crystals, and larger electrostatic repulsion generates between wax crystals due to the electrostatic double layers. Therefore, the growth and aggregation of wax crystals are interfered, which counts against gelation.

This paper is supported by the National Natural Science Foundation of China (No.51534007 and No.51134006)