Journal of Petroleum Science and Engineering 81 (2012) 122–128
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Journal of Petroleum Science and Engineering
journal homepage: www.elsevier.com/locate/petrol
Rheological properties of heavy & light crude oil mixtures for improving flowability
Mamdouh T. Ghannam a,⁎, Shadi W. Hasan b, Basim Abu-Jdayil a, Nabil Esmail b
a
b
Department of Chemical and Petroleum Engineering, College of Engineering, United Arab Emirates University, Al-Ain, United Arab Emirates
Department of Mechanical and Industrial Engineering, Concordia University, Montreal, Quebec, Canada
a r t i c l e
i n f o
Article history:
Received 29 September 2010
Accepted 24 December 2011
Available online 4 January 2012
Keywords:
heavy crude oil
viscosity
yield stress
thixotropy
loss modulus
storage modulus
a b s t r a c t
The rheological properties of heavy crude oil and its mixture with light crude oil were investigated
experimentally. These rheological properties include steady flow behavior, yield stress, transient flow
behavior, thixotropy behavior, and viscoelastic behavior. A RheoStress RS100 rheometer was employed in
all of the rheological examination tests. The heavy crude oil exhibits a non-Newtonian shear thinning
behavior over the examined shear rate range of 0.1–750 s− 1. The viscosity of the heavy crude oil decreases
considerably with temperature over the range of 25–65 °C. The addition of 10% light crude oil to the heavy
crude oil, to form 10% HLCO mixture, causes a strong reduction in the heavy crude oil viscosity from
10 Pa.s to 1.2 Pa.s at 25 °C. An even higher viscosity reduction from 10 Pa.s to 0.375 Pa.s at 25 °C can be
achieved in the presence of 20% light crude oil. The yield stress of the heavy crude oil reaches 0.7 Pa at a
room temperature of 25 °C and it decreases to 0.4 Pa at 65 °C. The tested samples of heavy crude oil, 10%
HLCO mixture, and 20% HLCO mixture exhibit time independent flow behavior. The heavy crude oil shows
a thixotropic area of 321.65 kPa/s at 25 °C and decreases to 118.62 kPa/s at 65 °C. The presence of the light
crude oil eliminates the thixotropic behavior of the heavy crude oil. The complex modulus of the heavy
crude oil decreases significantly with temperature. The addition of 10% of the light crude oil strongly reduces
the values of the complex modulus of the heavy crude oil. From the comparison of the storage and loss
moduli values for the heavy crude oil, it is found that the heavy crude and its blends flow in a viscous liquid
behavior. The 10% and 20% of the HLCO mixtures display viscous behavior as well. The storage and loss
moduli of the heavy crude oil decrease significantly by the addition of 10% light crude oil.
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
The world crude oil demand grew on an average of 1.76% per year
over the period of 1994–2006, with a high of 3.4% during 2003–2004.
Demand growth is the highest in the developing countries like China
and India whose dynamic economies resulted in a remarkable 1.8%
global growth in demand for crude oil in 2009 (IEA, 2010). Numerous
international reports predicted that in the next two decades, at least
80% of the world's energy demands will rely on the petroleum
products like oil and natural gas (IEA, 2010). According to some IEA
(International Energy Agency) statistics, heavy crude oil represents
almost 50% of the world's recoverable oil resources. During the 20th
century, the demand for heavy and extra-heavy crude oil has been
minimal because of their extra-high viscosity and complexity that
make these oils difficult and expensive to produce, transport and
refine. The involvement of heavy crude oil in the international energy
markets faces serious difficulties that need certain technological
developments in the production, refining and transportation.
⁎ Corresponding author. Tel.: + 97137133635; fax: + 97137624262.
E-mail address: mamdouh.ghannam@uaeu.ac.ae (M.T. Ghannam).
0920-4105/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.petrol.2011.12.024
Heavy crude oil has a vital role of the world's recoverable oil reserves. The viscosities of the heavy crude oils at 25 °C might reach
more than 10 5 mPa.s, however, the desired pipeline viscosity of
crude oils might not exceed 400 mPa.s (Fruman and Briant, 1983;
Nunez et al., 1996; Schumacher, 1980). Crude oil transportation
through a pipeline is the most convenient and economical way for
transportation of crude oil and its products. However, the transportation of heavy crude oil is quite difficult because of the high viscosity,
i.e. low mobility and low flowability. Crude oil is a mixture of a large
amount of hydrocarbons, varying amount of waxes and low content
of asphaltenes (Johnsen and Ronningsen, 2003). The carbon content
normally is in the range of 83–87%, and the hydrogen content ranges
from 10 to 14%. In addition, small amounts of nitrogen, sulfur, oxygen,
nickel and vanadium may be found in the crude oils (Sjoblom et al.,
2002). Crude oils from different regions have different properties
(Nunez et al., 1996; Yaghi and al-Bemani, 2002). There numerous
numbers of chemical compounds present in crude oil, therefore it is
difficult to achieve a complete structure definition and composition
of each individual molecule. Heavy crude oil (i.e. with viscosity ranging from few thousands to millions of mPa.s) and low API gravity of
less than 20 (Centeno et al., 2011) contains high presence of asphaltenes and low proportion of low molecular weight of hydrocarbons
(Castro and Vazquez, 2009; Murillo-Hernandez et al., 2009).
M.T. Ghannam et al. / Journal of Petroleum Science and Engineering 81 (2012) 122–128
123
Rheological properties for petroleum oils are very useful for all
processes in which fluids are transferred from one location to
another. For example, the crude oil migration within the oil reservoir
requires the fluid viscosity of the migrating oils. Other examples for
the demand of the crude oil viscosity are in the crude oil production
from oil wells and the petroleum refining units (Werner et al.,
1996). Limited numbers of crude oil rheological properties are
available especially for very heavy crude oil in which resins and
asphaltenes are present (Evdokimov et al., 2001).
Different studies were made in order to come up with useful techniques to reduce the viscosity of the heavy crude oil for the pipeline
transportation. These techniques include the formation of oil-inwater emulsions (Pal and Rhodes, 1989). Others include preheating
the crude oil or the addition of light petroleum fraction (Ghannam
and Esmail, 2006). Heating technique can be utilized to overcome
the difficulties related to the transportation of the heavy crude oil.
The basis of this method lies in the fact that as the heavy crude oil
is heated, the viscosity of the heavy crude oil is reduced significantly
and it will be easier to pump. However, the capital and operating
costs will be significantly high especially over long distances of pipelines. Another technique which can be employed for the transportation of heavy oil is the formation of the emulsions. In such a method
the heavy crude oil is suspended as micro-spheres of oil stabilized
in a water continuous phase by the use of surfactants and detergents
forming an O/W emulsion, and consequently achieving a reduction in
the apparent viscosity. The main difficulty with using this method is
the selection and cost of the surfactant materials to form a stable
oil-in-water emulsion. Not only the surfactant must be capable of stabilizing the emulsion during transportation of heavy crude oil, but it
also must be capable of separating the crude oil once the destination
point is reached. Blending of heavy crude oils with lighter oils, hydrocarbon gases or alcohols are other potential alternatives to reduce the
viscosity of the heavy oils. The main objective of this study is to investigate the rheological properties of the heavy crude oil as well as the
mixture of heavy and light crude oils (HLCO).
HLCO was stable for at least 6 h, the prepared samples of HLCO
were monitored through graduated glass cone-container.
The rheological measurements were carried out using RheoStress
RS100 from Haake. All measurements were carried out at a room
temperature of 25 °C unless it is mentioned otherwise. A water bath
was connected to the RS100 to control the applied temperature in
the rheometer system. This rheometer has several operating test
modes. It has a universal controlled rate (CR) mode, a controlled
stress (CS) mode, and an oscillation (OSC) test mode. The drive
shaft of the RS100 is centered by an air bearing to ensure an almost
frictionless transmission of the applied stress to the test fluid. The
resulting deformation of the sample is detected with a digital encoder
that processes 10 6 impulses per revolution. This resolution makes it
possible to measure small yield stress values, strains, or shear rates.
The computer controlled rheometer can be easily switched between
both CS and CR modes, and it can provide oscillating stress inputs. A
controlled variable lift speed is used to position the cone on the
plate. A thermal gap size is controlled to compensate for any of the
sensor generated heat. The software package of Haake-Windows
controls both test routines and data evaluation. The rheometer is
equipped with a cone and plate sensor. This cone-plate sensor was
used with a cone angle of 4°, cone diameter of 35 mm, and
0.137 mm gap at the cone tip.
All tested samples were equilibrated at the desired measuring
temperature for 5 min before the test started. A thin layer of light
silicon oil fluid was placed around the plate rim to avoid evaporation
during the experimental test. A fluid of known viscosity (Brookfield,
USA: 55 mPa.s at 25 °C) was used to check the rheometer RS100
accuracy. The measured viscosity differed 3.6% from the viscosity of
the reference fluid. At the beginning of each test, several measurements were taken to check the reproducibility of the Haake RS100
rheometer system. Reproducibility of the experimental measurements was satisfactory with an estimated error of about 3–5%.
2. Experimental work
3.1. Steady flow behavior
The original heavy crude oil was collected from Husky EnergyLloydminster, Canada with density 995 kg/m 3 at 25 °C. The light
crude oil from Shell Canada-Montreal with density 887 kg/m 3 at
25 °C was used as an alternative blending material.Table 1 shows
the API and SARA analysis of the heavy and light crude oils. The
American Petroleum Institute (API) density was obtained by the ISO
12185 method. A SARA analysis was performed in a thin-layer
chromatography-flame ionization detection system as reported by
Jiang et al. 2008.
Heavy crude oil was blended with the light crude oil in different
volumetric proportions to form a sample of 250 cm 3 with different
concentrations of the heavy-light crude oil mixtures. A measured
volume of the heavy crude oil was placed into a mixing container
using a laboratory high speed mixer. HLCO mixtures were prepared
by a gradual addition of almost 3–5 cm 3/min of the light crude oil
into the heavy crude oil while mixing with a speed of 360 rpm for
25 min to form stable and homogeneous mixture. To make sure the
The flow behavior of the heavy crude oil is investigated in terms of
viscosity-shear rate using the CR mode of the RS100. Fig. 1 shows a
typical behavior for the measured results at different temperatures.
It is observed from Fig. 1 that the crude oil exhibits non-Newtonian
shear thinning behavior over the examined range of shear rates in
which the apparent viscosity decreases considerably with temperature when it is heated from 25 to 65 °C. It is also demonstrated that
the viscosity differences are larger at low shear rates than at high
shear rates. As the temperature increases, the high molecular weight
components of the heavy crude oil, such as asphaltenes, resins, waxes
3. Results and discussion
Table 1
Properties and composition of the heavy and light crude oils.
Parameter
Heavy crude oil
Light crude oil
API gravity
SARA analysis (wt.%)
Saturates
Aromatics
Resins
Asphaltenes
10.71
26.6
23.3
41.9
19.6
15.2
51.2
24.2
23.1
1.5
Fig. 1. Viscosity profile of heavy crude oil at different temperature.
124
M.T. Ghannam et al. / Journal of Petroleum Science and Engineering 81 (2012) 122–128
etc., do not have the chance to agglomerate and form aggregates, and
consequently affecting the bonds between the solid particles and
eventually reducing the oil viscosity. From the viscosity investigation
versus different temperatures, it is observed that there is a significant
viscosity reduction over the tested temperatures. This can be attributed to the strong effect of temperature on the viscosity and chemical
structure of heavy components of the crude oil such as wax and
asphaltene (Khan, 1996).
The addition of the light crude oil, with viscosity around 300 mPa.s
at 25 °C, into the heavy crude oil to form HLCO mixture is a potential
alternative for viscosity reduction, which should be investigated
extensively. The idea behind having a HLCO mixture is to form a
less viscous and less dense crude oil that will be more desirable and
more suitable for the pipeline transportation. Figs. 2 and 3 show the
viscosity flow behavior of the heavy crude oil when it is mixed with
10% and 20% by volume of the light crude oil, respectively. The
viscosity flow behavior of the HLCO mixture reports much less
viscosity than that observed for the heavy crude oil in Fig. 1. The
presence of 10% light crude oil causes a strong reduction in the
heavy crude oil viscosity from 10.0 Pa.s to 1.2 Pa.s at 25 °C, which is
12% of the original crude oil viscosity. Fig. 2 shows almost Newtonian
flow behavior for the HLCO mixture when the temperature exceeds
45 °C. However, the addition of 20% light crude oil leads to a
Newtonian flow behavior for a temperature higher than 25 °C with
even higher viscosity reduction from 10.0 Pa.s to 0.375 Pa.s at 25 °C,
which is almost 4% of the heavy crude oil alone.
The heavy crude oils with an API gravity of less than 20 are due to
the high presence of asphaltenes and low content of low molecular
weight compounds, i.e., lack of light hydrocarbons. Asphaltenes play
an important role in controlling the viscous behavior of crude oil.
This has been extensively reported in the literature (Mullins and
Sheu, 1998; Wong and Yen, 2000; Yen and Chilingarian, 1994). Pipeline transportation of these heavy crude oils leads to several challenges such as multiphase flow, clogging of pipelines, high pressure
drops, and production stops. The addition of lighter crude oil, which
is more paraffinic in nature (Nelson, 1987), is an effective option to
reduce the viscosity of the heavy crude oil and facilitate its flow
mobility through the pipeline. The effect of the light crude oil can
be attributed to its high API gravity and its efficiency in the dilution
of the heavy crude oil, which may alter the asphaltene stability
leading to asphaltene flocculation and precipitation. Argillier et al.
(2006) suggested that the breaking of the colloid system of the
heavy crude oil through asphaltene precipitation will have a definitive effect in the crude oil viscosity reduction. Yaghi and Al-Bemani
(2002) found that a mixture of heavy and light crude oils with mixing
ratio of 7:3 produces a viscosity of around 1000 mPa.s (represents 7%
of the original viscosity) and 300 mPa.s (represents 2% of the original
viscosity) at 30 and 50 °C, respectively when compared to the original
heavy crude oil of 15,000 mPa.s at 20 °C.
Fig. 2. Viscosity flow behavior of 10% light–90% heavy mixture.
Fig. 3. Viscosity flow behavior of 20% light–80% heavy mixture.
3.2. Yield stress
The mechanical characteristics of structured liquids modify
significantly over a narrow low range of stress level. Below this stress
level, the material deforms elastically with finite rigidity. Above this
stress level, the applied stress leads to continuous deformation
which causes the sample to start flow, i.e., viscous liquid-like
behavior. The stress level at this transition stage is known as apparent
yield stress τo. The utilization of controlled stress rheometer
(CS-mode) has made the measurement of yield stress more accurate
in comparison with controlled rate technique (Schramm, 1994).
The yield stress measurements of the heavy crude oil were carried
out at different temperatures employing the RheoStress RS100 under
CS-mode. Shear stress in the sample is progressively increased with
no shear flow reported till the yield stress is reached. The heavy
crude oil sample is subjected to stress ramp from 0.09 to 4.0 Pa for
the period of 150 s to establish the rheogram behavior of shear
stress–shear rate curve. Fig. 4-a displays the assigned shear stress
and the response shear rate relationship for different temperatures.
It can be observed that the yield point which is required to start the
flow decreases with temperature. Modeling analysis of Bingham
model was used to determine the yield stress for each sample. The
yield point of the heavy crude oil reaches a value of 0.7 Pa at room
temperature of 25 °C and it decreases to 0.4 Pa when the temperature
reaches 65 °C. When 10% and 20% by volume of light crude oil are
added into the heavy crude oil, it is observed that the apparent
yield stress diminishes completely as can be concluded from Fig. 4-b
and c. This means that the presence of the light crude oil within the
mixture of HLCO will eliminate the need for any further stresses to
initiate the steady flow.
3.3. Transient flow behavior
The characterization of time-dependent non-Newtonian fluid flow
properties is important in establishing fluid flow and piping systems,
quality control, and storage. It is known in the literatures that the
colloidal suspensions show transient flow behavior, i.e., continuous
reduction in viscosity with time under applied constant shear
(Barnes, 1979; Barnes et al., 1989; Gupta, 2000; Macosko, 1994). It
was proposed that the change in viscosity results from the formation
of structure within the colloidal suspension under shear. A transient
test was conducted to study the transient flow behavior of the
heavy crude oil and HLCO mixtures under CR-mode of RS100. The
tested sample was subjected to a constant value of shear rate
between the range of 0.6–740 s − 1 for 300 s. A temperature range of
25–65 °C was covered. Fig. 5 shows a typical example of the transient
flow behavior for the heavy crude oil, 10% HLCO mixture, and 20%
HLCO mixture at 25 °C. Fig. 5 shows that the tested samples exhibit
time independent flow behavior.
M.T. Ghannam et al. / Journal of Petroleum Science and Engineering 81 (2012) 122–128
125
(a)
Fig. 6. Thixotropic behavior of heavy crude oil at different temperatures.
(b)
3.4. Thixotropy behavior
(c)
Fig. 4. Rheogram behaviors for heavy crude oil, 10% HLCO, and 20% HLCO.
Solutions with microstructure usually display thixotropy
phenomenon which determines the magnitude of the changes
occurring from one state of microstructure to another and back
again. The microstructure changes occur by the balance between
the breakdown due to flow stress, the buildup due to non flow
collisions and the Brownian motion. All of these activities dictate
the level of solution viscosity, and all of these activities require time
to change from one state to another under the shear applied. The
hysteresis loop of the area enclosed between the up curve and the
down curve rheograms is one of the recommended methods to
measure thixotropy which is called the thixotropic area (Barnes,
1979; Mewis, 1979; Weltmann, 1960).
To completely break down the thixotropic structure of the crude
oil sample, a number of tests of up and down cycles may be required
to determine the required duration for each up and down curve. Tests
were carried out for the heavy crude oil samples using cycles for 100,
200, and 300 s for each of the up and down curves. The shear rate was
ramped up from 0.6 to 740 s − 1 for the specified period and then
immediately ramped down from 740 to 0.6 s − 1 in equal time. The
hysteresis area is calculated for each test. The test according to up
curve from 0.4 to 740 s − 1 and down from 740 to 0.4 s − 1 for 300 s
each was enough to make sure that the crude oil structure has been
completely broken down. To check the reliability of the measurement
data, three tests of the recommended scheme were repeated for the
heavy crude oil samples. The resulted rheograms coincided on top
of each other within an acceptance tolerance of about ±2%.
The purpose of this investigation is to study the thixotropy
behavior of the heavy crude oil and HLCO mixtures. Measurements
of thixotropy were carried out under the CR-mode. The apparatus
was programmed to gradually increase the assigned shear rate from
0.6 to 740 s − 1. The stresses and the shear rates of the flow curve
were recorded, and the results formed the up-curve. To establish
the down-curve cycle, a gradual decrease of the shear rate was
assigned from 740 to 0.6 s − 1. The duration of each cycle was 300 s.
For non-thixotropic materials, the down-curve rheogram is identical
to the up-curve rheogram; whereas the down-curve rheogram of
thixotropic material is different from the up-curve rheogram. For a
Table 2
Hysteresis area at different temperatures.
Fig. 5. Transient flow behavior for HLCO mixtures.
Sample
Hysteresis area, kPa.s− 1
Heavy crude oil @ 25 °C
Heavy crude oil @ 45 °C
Heavy crude oil @ 65 °C
10% HLCO mixture @ 25 °C
20% HLCO mixture @ 25 °C
321.65
207.83
118.62
3.78
1.07
126
M.T. Ghannam et al. / Journal of Petroleum Science and Engineering 81 (2012) 122–128
Fig. 7. Thixotropic behavior for HLCO at 25 °C.
Fig. 9. Complex modulus for HLCO at 25 °C.
thixotropic material, the up- and down-curves form a hysteresis loop
which provides an area A. The hysteresis area A, Pa/s, of a material is
taken to be a measure of the degree of thixotropy with dimension of
energy per unit volume, which reflects the amount of energy per
volume required to break down the thixotropic structure.
Fig. 6 shows the thixotropic behavior for the heavy crude oil at
different temperatures through which the hysteresis phenomenon is
clear. The area between the up and down curves is the measure of
thixotropy phenomenon. This area is measured for the heavy crude
oil at each examined temperature. Table 2 reports the hysteresis
area for each examined temperature. The measured data in Fig. 6
and the calculated area reported in Table 2 show that the thixotropy
phenomenon decreases significantly from 321.65 kPa/s to
118.62 kPa/s over the temperature range of 25–65 °C. The effect of
the presence of light crude oil within the heavy crude is displayed
in Fig. 7 for different concentrations. The presence of light crude oil
eliminates the thixotropic behavior and leads to reversible processes
through which the up curve almost coincides on top of the down
curve. The results show an approximate identical up and down
curve which results in an unnoticeable thixotropic behavior. Table 2
shows the hysteresis area for the presence of the light crude oil
within the heavy crude at 25 °C. If a thixotropic structural material
is placed in a viscometer, at rest, the microstructure is a combination
of large flocs. When the shear rate increases progressively and
enough time is allowed, the floc size decreases continuously. At
high shear rate, the floc will be disintegrated into its individual
constituents. This process of breakdown happens over some time. If
the opposite process takes place, i.e. the shear rate is instantaneously
decreased, the individual constituents start to colloid and flocculate
with each other till the size of the floc corresponds to the new status.
This process of buildup also happens over time but with a different
rate than the breakdown process (Barnes, 1979).
The dynamic test is an important tool of the rheological
investigation to study the viscoelastic behavior of the heavy crude
oil. This test investigates the effect of oscillating stresses or strains
on a crude oil sample. The complex modulus G*, in Pa, represents
the substance total resistance against the applied strain. The term
storage or elastic modulus, G′, indicates the contribution of the stress
energy that temporarily stored during the test and can be recovered.
Whereas, the term viscous or loss modulus, G″, refers to the energy
which has been used to initiate the flow which is irreversibly
dissipated into shear heat.
Dynamic tests start with a stress sweep to determine the linear
viscoelastic range, then further tests can be utilized to determine
extra features of the viscoelastic behavior. The linear viscoelastic
range is the range where G* is constant with the stress sweep,
indicating that the internal bonds of the sample structure are still
intact. Otherwise, at higher stress range, shear thinning will take
place and a major part of the introduced energy will be irreversibly
lost as heat. Many tests of stress sweep at 1 Hz were carried out for
the crude oil samples over a stress range of 0.1–10 Pa. The linear
viscoelastic range was found to be within the range of 0.01to
0.75 Pa over which the complex modulus shows a constant response.
All the subsequent tests were carried out at a stress value of 0.5 Pa to
avoid the non-linear viscoelastic range conditions.
The complex modulus versus frequency for the heavy crude oil at
different temperatures is plotted in Fig. 8. The measured G* increases
gradually with frequency displaying a linear relationship between G*
and w for all different temperatures. Fig. 8 shows that the
temperature effect has a strong influence on G*. The complex
modulus decreases significantly with temperature, which means
Fig. 8. Complex modulus of heavy crude oil at different temperatures.
Fig. 10. Storage and loss moduli for heavy crude oil at 25 °C.
3.5. Viscoelastic behavior
M.T. Ghannam et al. / Journal of Petroleum Science and Engineering 81 (2012) 122–128
127
Fig. 11. Storage and loss moduli for 10% HLCO mixture at 25 °C.
Fig. 13. Loss modulus for HLCO at 25 °C.
that the substance total resistance against the applied strain
decreases significantly with temperature. Fig. 9 shows the effect of
the light crude oil addition on the profile of the measured complex
modulus. The addition of 10% of the light crude oil causes a tremendous drop in the values of the complex modulus of the heavy crude
oil. G* decreases very slightly for any further addition of the light
crude oil beyond the 10%. Mullins and Sheu, 1998; Wong and Yen,
2000; and Yen and Chilingarian, 1994 have reported that the heavy
crude oils contain suspension of asphaltene colloids stabilized by
resins. The smallest colloid particles diameter is around 2–4 nm and
may form clusters with dimensions of 10–30 nm. Further asphaltene
aggregation may lead to formation of flocs and macrostructures. The
presence of the viscoelastic behavior for the heavy crude oil indicates
that interacting flocs and macrostructures form a certain network
that will be affected by temperature and the addition of light crude
oil as in the following figures.
Fig. 10 shows the frequency dependence profiles of G′ & G″ moduli
for the heavy crude oil. The heavy crude oil shows linear relationship
response over the examined range of frequency. The measurement
results show that the loss modulus, G″, reaches values that are higher
than the values of the storage modulus, G′. This means that the energy stored in the heavy crude oil is less than the energy dissipated as
heat and thus lost per cycle. Therefore, the heavy crude oil tends to
behave in a viscous liquid behavior more than it behaves as a solidlike material. Fig. 11 shows the effect of the light crude oil addition
into the heavy crude oil. The 10% and 20% HLCO mixtures display
viscous behavior since G″ profile is positioned well above the G′
profile as can be seen in Fig. 11. The storage and loss moduli of the
heavy crude oil decrease significantly by the addition of 10% light
crude oil as can be seen in the Figs. 12 and 13, respectively. However,
for the addition of 20% light crude oil, experimental results display a
slight change for both of G′ and G″.
Fig. 12. Storage modulus for HLCO at 25 °C.
4. Conclusions
The current study is carried out to investigate the rheological
properties of the heavy crude oil and its mixture with light crude
oil. The following conclusions can be made:
(1) The heavy crude oil exhibits a non-Newtonian shear thinning
behavior over the examined range of shear rates. The viscosity
of the heavy crude oil decreases significantly with temperature over the range of 25–65 °C.
(2) The addition of 10% light crude oil to the heavy crude oil
causes a strong reduction in the viscosity of the heavy crude
oil from 10 Pa.s to 1.2 Pa.s at 25 °C. The presence of 20% light
crude oil leads to a higher viscosity reduction from 10 Pa.s
to 0.375 Pa.s at 25 °C.
(3) The 10% HLCO mixture shows a Newtonian flow behavior
when the temperature exceeds 45 °C. However, the addition
of 20% light crude oil leads to a Newtonian flow behavior for
temperature higher than 25 °C.
(4) The yield stress of the heavy crude oil decreases with temperature. It reaches to 0.7 Pa at 25 °C and it decreases to 0.4 Pa at
65 °C.
(5) When 10% and 20% by volume of light crude oil are added into
the heavy crude oil, the apparent yield stress diminishes
completely.
(6) The tested samples of heavy crude oil, 10% HLCO mixture, and
20% HLCO mixture exhibit time independent flow behavior.
(7) The heavy crude oil shows a thixotropic area of 321.65 kPa/s
at 25 °C and it decreases to 118.62 kPa/s at 65 °C.
(8) The presence of the light crude oil eliminates the thixotropic
behavior of the heavy crude oil.
(9) The complex modulus of the heavy crude oil decreases significantly with temperature. The addition of 10% of the light
crude oil leads to a strong reduction in the values of the complex modulus of the heavy crude oil.
(10) The heavy crude oil behaves in a viscous liquid behavior more
than as a solid-like material. The 10% and 20% HLCO mixtures
display viscous behavior as well. The storage and loss moduli
of the heavy crude oil decrease significantly in the presence of
10% light crude oil. However, the addition of 20% light crude
oil will cause a slight changes for both of G′ and G″.
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