A Geochemical Framework for Understanding Residue Properties

Abstract

Crude oils, and the residual fractions thereof, vary widely in many relevant properties. Thus, a detailed characterization of a single residue stream or even a number of residue streams from one region may not be very relevant in a different part of the world. We have therefore attempted to develop a framework for understanding residue properties in a generic fashion. A detailed analysis of 11 vacuum residue fractions from a wide variety of crude oils resulted in a general model for understanding the composition and processability of residue streams. This model is based on the geochemical origin (kerogen type) and the maturity of the crude oil. The different origin of kerogen I (paraffinic) residues, compared with the more conventional kerogen II residues, is reflected in a large number of properties and also in a larger variability in these properties. However, upon maturation, the average properties of kerogen I residues and kerogen II residues converge so that mature residues of both kerogen types have much in common (low S content, low metals content, high H/C ratio etc). Maturation is found to have a negative effect on the stability and coking tendency of the residue fraction. NMR data reveal that the asphalthenes become more aromatic upon ageing, whereas the maltenes (non-asphalthenes) become less aromatic, thus causing an increasing gap in aromaticity between these fractions. Significant differences in asphaltene molecular structure between kerogen I and kerogen II residues were observed. The analytical data suggest that the concept of kerogen type and maturity may be a useful tool in describing and understanding residue characteristics and assist in optimizing feedstock selection for residue conversion processes and other residue applications (fuel oils, bitumen), as well as in understanding fouling and coking phenomena. Further research is required to establish the trends shown in this paper.