Составители:
et al., 2004). Moreover, our studies of viscosity and NMR relaxa-
tion in dilute solutions (Evdokimov et al., 2003c) indicated that the
phenomenon of asphaltene aggregation at ≈100 mg/l may be de-
scribed as a liquid-liquid phase separation (demixing transition), a
conclusion recently supported by other authors (Sirota, 2004).
Other common concepts are that at concentrations, typical for
natural petroleum systems: 1) the structure of asphaltene disper-
sion is fairly insensitive to small variations of ambient conditions,
and 2) the state of asphaltenes is thermodynamically controlled.
Hence, usually it is implicitly assumed that far from obvious criti-
cal/transition points the properties of asphaltene colloids are
slowly varying smooth functions of external conditions, e.g. of tem-
perature. Consequently, in a lot of conventional experiments tem-
perature dependencies are investigated with fairly large intervals
between consecutive data points, e.g. 15–40°C (Wong and Yen,
2000); 10–20°C (Priyanto et al., 2001) etc. Over the past several
years we have conducted a series of rheological experiments with
“unconventionally” small temperature increments of 1–2°C
(Evdokimov et al, 2001;2003d;2004). Unexpectedly, we have ob-
served reproducible non-monotonous temperature dependencies of
some rheological parameters at temperatures of 10–50°C. These
results were interpreted as an evidence of noticeable structural
transformations in asphaltene colloids, induced by comparatively
small temperature variations. These new states of asphaltenes
may become long-lived metastable ones at lower temperatures (or
long-lived transient ones due to a strong kinetic control of ther-
mally-induced transformations of asphaltenes). Whatever the ex-
planation, these experiments have shown that the current proper-
ties of an asphaltene-containing fluid may strongly depend on its
thermal history. E.g., Fig. 2 shows that a low-temperature associa-
tion energy of asphaltene particles in a model crude remains at a
level of 8-11 kcal/mol (presumably characteristic of an “equilib-
rium” state of asphaltene colloids), in case the fluid previously has
never been pre-heated at temperatures above 27–28°C. Thermal
pre-treatment at temperatures 28–45°C evidently strongly trans-
8
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