(694a) 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 T6G2G6,
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_gvalues 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.