Measurement of Wax Appearance Temperature Using Near-Infrared (NIR) Scattering

A near-infrared (NIR) scattering technique is used to measure the wax appearance temperature of several petroleum fluids under nonquiescent conditions. Within the Rayleigh scattering limit, NIR attenuation measurements at a wavelength of I 100 nm can theoretically detect wax crystallites < 55 nm in size. In comparison, commonly used cross-polarized microscopy (CPM) observations are limited by a resolution of similar to 0.5 mu m. The NIR scattering technique readily allows for application of nonquiescent and thermal equilibrium conditions, effectively accelerating the crystal growth process and overcoming subcooling effects. Wax appearance temperature measurements are demonstrated using a waxy crude oil, a waxy gas condensate fluid, and model fluids consisting of macrocrystalline or microcrystalline paraffin wax dissolved in dodecane. Light scattering by wax crystals is evidenced by a baseline elevation in the measured NIR attenuation spectra, with higher shifts observed at lower wavelengths. For opaque crude oils, WAT determination requires delineation of the radiation attenuation originating from the precipitated wax crystals and the mother crude oil. The NIR scattering technique yields WAT values similar to the classical CPM technique. In addition, NIR scattering is shown to be an appropriate technique for measuring the time necessary to melt paraffin wax solids from waxy petroleum fluids at warm temperatures and under nonquiescent processing conditions.