Wax precipitation from North Sea crude oils. 1. Crystallization and dissolution temperatures, and Newtonian and non-Newtonian flow properties

This paper is the first in a series which summarizes experimental work on wax precipitation in North Sea crude oils, leading to an improved thermodynamic model for prediction of wax formation. Seventeen crude oils and condensates were characterized analytically and rheologically with emphasis on properties related to the content of wax. In particular, three different methods for determination of wax precipitation temperature (WPT), namely polarization microscopy, differential scanning calorimetry (DSC), and viscometry, are discussed and results obtained by the three methods are compared. Microscopy invariably gave the highest WPTs and probably the most relevant values for predicting the onset of wax deposition on cold surfaces. The WPTs from microscopy were found to depend on factors such as thickness of the sample film and cooling rate. DSC and viscometry are likely to underestimate the onset temperature of initial wax deposition. Activation energies of viscous flow in the Newtonian temperature range (usually above 30-35 °C), calculated by fitting viscosity data to a simple Arrhenius type exponential equation, are presented and found to be closely correlated with type of petroleum fluid. Light condensates had activation energies approaching those of pure n-alkanes (≤10 kJ mol-1) while a highly biodegraded heavy oil had an activation energy of nearly 40 kJ mol-1. Finally, compositional analyses of wax precipitated from one of the oils indicated that wax crystallizing just below the WPT was richer in condensed naphthenes and poorer in isoalkanes than wax formed at lower temperatures. Isoalkanes appeared to be the most abundant class at all temperatures.