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

Abstract

The morphology and structure of wax crystals are among the factors dominatingrheological characteristics of a waxy crude oil at temperatures below the wax appearancetemperature (WAT). In several reported researches fractal dimensions were employed indescribing the waxy crude oil microstructures; however, they were all determined via theindirect approach, i.e. deduced from the rheological data. This paper presents a directfractal characterization approach based on micrographs of wax crystals. The box-countingmethod is applied to the wax crystal images of three waxy crude oils beneficiated with andwithout pour-point-depressants (PPDs), and for the fractal measurements thet-distribution tests of hypothesis on linear regression are performed at the significance levelof 0.01. It is demonstrated that the boundary fractal dimensions from micrographsof different visual fields of a specimen are almost identical, with the maximumand minimum relative ranges being 9.97% and 1.88% respectively, and with thestandard deviation ranging from 0.0549 to 0.0107. Then the wax crystal structuresare determined as fractal at the confidence level of 99%. All the listed absolutet-statistics with the minimum of 29.568 are much higher than the correspondingt-quantiles with the maximum of 3.4995. The results also show that the larger value of theboundary box dimension represents the higher complexity and irregularity of the waxcrystal morphology. The box dimension increases with decreasing oil temperature for eachwaxy crude oil. After the oil is beneficiated with a PPD, the box dimension increases ateach given temperature. Thus, it is feasible to use fractal dimensions to characterize thewaxy crude oil microstructures. This helps to probe the rheology–microstructure relation.