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2.2. Apparatus and experimental procedure
The dynamic viscosity was measured at 8–32°C, for shear
rates up to 150 s
-1
, in a Brookfield Programmable DV-II+ Viscome-
ter. In each experiment a fresh crude oil sample was introduced
into the viscometer’s cell at room temperature (19–20°C) and was
given an individual thermal pre-treatment. The sample precondi-
tioning was conducted at rest for 1 hour at a constant pre-
treatment temperature T
pt
(15°C to 65°C), followed by cooling to
the lowest measurement (flow) temperature (8°C) at the rate of
1–1,5°C per minute. At each measurement temperature T
m
, the
system was initially held at rest for 20 min. The tests began at the
lowest shear rate and continued in an ascending-rate order up to
the maximum torque limit. At each shear rate the torque and
shear stress data were collected for 90–120 s and averaged. This
procedure was repeated at new values of T
m
, increased by 2°C in-
crements.
For the optical studies we used oil solutions in toluene with a
standard concentration (C≈0.1 vol.% of oil). The optical absorption
spectra have been measured within 10–15 minutes after prepara-
tion of each solution, using a KFK–2 photocolorimeter equipped
with a set of narrowband light filters (cf. [15]). The absorptivities ε
were calculated by using the Beer–Lambert’s law: log (I
0
/I) = εbC ,
where I
0
is the intensity of the light emerging from a reference
(toluene) cuvete, I is the intensity of light emerging from a crude
oil solution, b is the optical path length of cuvets.
2.3. Determination of the activation energies
of a viscous flow and of the pour points
In all oil samples a measurable “shear thinning” was regis-
tered, characteristic for heavy crudes [6]. The observed dependen-
cies of the shear stress σ on the shear rate
we approximated by
the two-parameter Bingham’s model [11]: σ = σ
γ
&
e
+ η
p
γ
&
, where σ
e
is
the apparent yield stress and the constant η
p
usually is referred to
as the plastic viscosity. In all studied samples, for shear rates ex-
ceeding 30–50 s
-1
, η
p
practically coincided with the Newtonian dy-
41
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