Size and Structure of Asphaltene Aggregates in the Bubble Pressure Region

Size and Structure of Asphaltene Aggregates in Saturated Diluents at High Temperature and Pressure

Jes¨²s Leonardo Amundara¨ªn Hurtado*, Martin Chodakowski, Bingwen Long**, J. M. Shaw***

Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta T6G 2G6, Canada

* Now at working at Schlumberger, Russia

** Visiting professor, Beijing University of Chemical Technology, presenter

*** Corresponding Author:  E-mail: jmshaw@ualberta.ca.

Abstract:

Asphaltene precipitation is typically most pronounced adjacent to the liquid-vapour to liquid phase boundary of reservoir fluids. In this investigation, the bubble pressure region is probed using n-pentane asphaltenes from Athabasca vacuum residue (AVR) prepared based on the ASTM D3279 protocol. The size and structure of asphaltene aggregates in 1-methyl naphthalene and n-dodecane were studied using small-angle X-ray scattering (SAXS) and the results are compared with values obtained at near ambient conditions for the same samples and from the literature. The experiments were carried out from 50 to 310 °æ at 20°æ intervals with the asphaltene volume % in solution ranging from 0.6 to 9.7% where 13 sealed sample vials, equipped with magnetic mixers were inserted into a custom heated multi-sample holder device placed in the BSSERC-CAT 12 beam line at the American Photon Source (Argonne National Laboratories) simultaneously. All SAXS measurements were carried out along the saturation curves of the solutions at fixed global compositions. The scattering spectra obtained, were subject to calibration, normalization, averaging and solvent background subtraction, prior to analysis. The radius of gyration (R_g) and power-law slope (P) were extracted simultaneously from the experimental scattering data by fitting Beaucage's unified model. The R_g values fall in the range 1.6 to 3.5 nm, while P values range from 1.8 to 2.4, though in some cases values as low as 1 are obtained.

The thermal and concentration variations of asphaltene properties in the two diluents are shown to exhibit complex and contrasting relationships. For example, in 1-methyl naphthalene, the power law slope is insensitive to variations in concentration and temperature, while the radius of gyration is sensitive to both variables. In n-dodecane, the radius of gyration is not readily measured but the power law slope is sensitive to variations in temperature and concentration. Asphaltene structural differences in aromatic-rich and n-alkane-rich liquids persist at high pressure and temperature, and in both cases the properties are shown to differ from those observed at ambient conditions. The results are discussed in the context of the phase behavior of neat asphaltenes, and asphaltene + diluent mixtures.