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1. Introduction
In petroleum fluids, phase transformations, involving asphaltenes,
may trigger the processes (e.g. flocculation and precipitation) [1–3],
which cause troubles in the petroleum production, transportation
and processing.
Recently [4–6] we have observed thermal anomalies of some
“model oils” which we attributed to a previously unreported struc-
tural phase transition in the asphaltene subsystem. We concluded
that this transition is defined by specific molecular interactions of
asphaltenes, viewed not just as a solubility class, but as true
chemical species [7–9]. Hence, the characteristics of the observed
transition point should be fairly insensitive both to the geographi-
cal/geological origin of asphaltene species and to the nature of the
bulk petroleum fluid.
In the present study, we have measured thermal effects on the
dynamic viscosity of an unprocessed crude oil. The results defi-
nitely prove that in this complex fluid strong structural transfor-
mations occur in the same temperature range as in our “model
oils”. The viscosity results are supported by the measurements of
optical absorptivities, indicating that the structural transforma-
tions may be accompanied by a macroscopic phase separation in a
heated crude oil.
2. Experimental
2.1. Oil samples
The virgin crude was collected directly from a well-head at the
Romashkinskoye reservoir (Tatarstan) and separated from reser-
voir water by mechanical methods. The waterfree crude had a den-
sity of 895 g/l, contained ~3.5 wt. % asphaltenes, ~20 wt. % resins,
~0.3 wt. % waxes. The obtained crude oil was stored at room tem-
perature (19–22°C), in the dark, in air, and no special precautions
were taken to prevent its possible oxidation. Oil solutions for light
scattering studies were prepared with a “chemically pure” grade
toluene.
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