Direct image-based fractal characterization of morphologies and structures of wax crystals in waxy crude oils

Abstract

The morphology and structure of wax crystals are among the factors dominating rheological characteristics of a waxy crude oil at temperatures below the wax appearance temperature (WAT). In several reported researches fractal dimensions were employed in describing the waxy crude oil microstructures; however, they were all determined via the indirect approach, i.e. deduced from the rheological data. This paper presents a direct fractal characterization approach based on micrographs of wax crystals. The box-counting method is applied to the wax crystal images of three waxy crude oils beneficiated with and without pour-point-depressants (PPDs), and for the fractal measurements the t-distribution tests of hypothesis on linear regression are performed at the significance level of 0.01. It is demonstrated that the boundary fractal dimensions from micrographs of different visual fields of a specimen are almost identical, with the maximum and minimum relative ranges being 9.97% and 1.88% respectively, and with the standard deviation ranging from 0.0549 to 0.0107. Then the wax crystal structures are determined as fractal at the confidence level of 99%. All the listed absolute t-statistics with the minimum of 29.568 are much higher than the corresponding t-quantiles with the maximum of 3.4995. The results also show that the larger value of the boundary box dimension represents the higher complexity and irregularity of the wax crystal morphology. The box dimension increases with decreasing oil temperature for each waxy crude oil. After the oil is beneficiated with a PPD, the box dimension increases at each given temperature. Thus, it is feasible to use fractal dimensions to characterize the waxy crude oil microstructures. This helps to probe the rheology–microstructure relation.

Rheo-PIV Analysis of the Yielding and Flow of Model Waxy Crude Oils

“…[6][7][8] Some studies have also suggested that the gel structure formed by the precipitates is a fractal network. 9 There is considerable interest in developing an improved understanding of the rheology of waxy crude oils. 10,11 These particular types of crude oil are often encountered in deep water or ultra-deep water production scenarios where the temperature of the ambient sea water around tiebacks is particularly low.…”

A Nonlinear Viscoelastic Model for the Yielding of Gelled Waxy Crude Oil

“…The deviation of the value of fractal dimension from the dimension of Euclidean space D = 3 may be used as a measure for the complexity of the gel structure, which depends on the mechanism of aggregation. The actual value of fractal dimension found for waxy crude gel is often between 1.5 and 2.5 [40,44]. In the prediction results shown in Section 4.4, as well as in the anylasis in Sections 5.1-5.3, we have taken it as D = 1.5, which is noted in the literature as possible value of D [44]; here we compare it with the results obtained for D = 2.…”

“…Pioneer studies in References [8,39,40] show that rheological properties, for instance, the viscosity and elastic modulus of waxy crude oil, may be well described by fractal models. Also, waxy crude gels and fractal systems have been found to have many morphological features as well as gelation mechanisms in common [9].…”

Effect of the amount and the composition of precipitated n-alkanes on the yield stress of wax-decane gels

“…On the other hand, the yield stress of wax-oil gel is related with the breakdown and recovery of the wax crystalline microstructure formed during different shearing and thermal conditions (Bai and Zhang, 2013;Gao and Zhang, 2006;Livescu, 2012;Kane et al, 2003;Oh et al, 2009;Visintin et al, 2005). The morphology of wax crystals microstructure has been paid great attention.…”

“…The boundary fractal dimension has been further proposed to quantitatively characterize the morphology of wax crystal microstructure (Bai and Zhang, 2013;Gao and Zhang, 2006). Bai and Zhang (2013) found that the aggregated wax crystals of lower boundary fractal dimension favors for a lower yield stress.…”