This article was downloaded by: ["Queen's University Libraries, Kingston"] On: 09 September 2013, At: 09:54 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK Journal of Dispersion Science and Technology Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/ldis20 STABILITY OF WATER-IN-CRUDE OIL EMULSIONS: ROLE PLAYED BY THE STATE OF SOLVATION OF ASPHALTENES AND BY WAXES O. Mouraille a , T. Skodvin a , J. Sjöblom a & J.-L Peytavy b a Department of Chemistry, University of Bergen, Allégt 41, Bergen, N-5007, Norway b Elf Exploration Production, Lacq, F-64000, FRANCE Published online: 06 Apr 2007. To cite this article: O. Mouraille , T. Skodvin , J. Sjöblom & J.-L Peytavy (1998) STABILITY OF WATER-IN-CRUDE OIL EMULSIONS: ROLE PLAYED BY THE STATE OF SOLVATION OF ASPHALTENES AND BY WAXES, Journal of Dispersion Science and Technology, 19:2-3, 339-367, DOI: 10.1080/01932699808913179 To link to this article: http://dx.doi.org/10.1080/01932699808913179 PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http:// www.tandfonline.com/page/terms-and-conditions
Transcript
This article was downloaded by: ["Queen's University Libraries, Kingston"]On: 09 September 2013, At: 09:54Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House,37-41 Mortimer Street, London W1T 3JH, UK
Journal of Dispersion Science and TechnologyPublication details, including instructions for authors and subscription information:http://www.tandfonline.com/loi/ldis20
STABILITY OF WATER-IN-CRUDE OIL EMULSIONS: ROLEPLAYED BY THE STATE OF SOLVATION OF ASPHALTENESAND BY WAXESO. Mouraille a , T. Skodvin a , J. Sjöblom a & J.-L Peytavy ba Department of Chemistry, University of Bergen, Allégt 41, Bergen, N-5007, Norwayb Elf Exploration Production, Lacq, F-64000, FRANCEPublished online: 06 Apr 2007.
To cite this article: O. Mouraille , T. Skodvin , J. Sjöblom & J.-L Peytavy (1998) STABILITY OF WATER-IN-CRUDE OILEMULSIONS: ROLE PLAYED BY THE STATE OF SOLVATION OF ASPHALTENES AND BY WAXES, Journal of Dispersion Science andTechnology, 19:2-3, 339-367, DOI: 10.1080/01932699808913179
To link to this article: http://dx.doi.org/10.1080/01932699808913179
PLEASE SCROLL DOWN FOR ARTICLE
Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) containedin the publications on our platform. However, Taylor & Francis, our agents, and our licensors make norepresentations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of theContent. Any opinions and views expressed in this publication are the opinions and views of the authors, andare not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon andshould be independently verified with primary sources of information. Taylor and Francis shall not be liable forany losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoeveror howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use ofthe Content.
This article may be used for research, teaching, and private study purposes. Any substantial or systematicreproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in anyform to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http://www.tandfonline.com/page/terms-and-conditions
I. DISPERSION SCIENCE AND TECHNOLOGY, 19(2&3), 339-367 (1998)
STABILITY OF WATER-IN-CRUDE OIL EMULSIONS: ROLE
PLAYED BY THE STATE OF SOLVATION OF ASPHALTENES
AND BY WAXES
0. Mouraille (')."), T. Skodvin (I), J. SjBblom "' and J.-L Peytavy "' (I) Department of Chemistry. University of Bergen, AllCgt 41. N-5007 Bergen.
Noway
(2) Elf Exploration Production. F-64000 Lacq, FRANCE
(9) Present affiliation: Elf Exploration Production, F-64000 Lacq, FRANCE
ABSTRACT
The stability of water-in-crude oil (or model crude oil) emulsions was determined by means of separationtsedimentation tests and high voltage destabilization tests. First the impact of the state of solvation of asphaltenes on their ability to stabilize emulsions were studied. Secondly, we analyzed the role of naturally occurring waxes in the stabilization of emulsions. Finally, the emulsion stability when both asphaltenes and waxes are involved was investigated.
339
Capyrighl O 1998 by Marcel Dekkcr. Inc.
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340 MOURAILLE ET AL.
INTRODUCTION
During the exploitation of crude oil fields, formation or injection water is
extracted together with the crude and emulsions may be formed. Some of the
problems to be encountered in the production process depend on the stability of
these emulsions.
It is of industrial interest to dehydrate (i.e., remove the water droplets from)
the crude oil for several reasons. Due to increased viscosity and volume, water-in-
oil emulsions increase the pumping cost during transport. Gas hydrate formation.
corrosion and scaling are problems that are closely related to the water
contamination, moreover, water is undesirable in any refinery process.
Naturally occurring surfactants in crude oils (mainly asphaltenes and resins)
are important for the stabilization of water-in-crude oil emulsions. Fprdedal et
al. ( I ) have shown that at room temperature the emulsion stability was mainly due
to those surface active fractions (using a crude oil corresponding to the oil named
crude oil BI in this work). It is essential to gain a better understanding of the
mechanisms behind the stabilization processes of water-in-crude oil emulsions in
order to solve the emulsion problem more efficiently.
Frequently in the crude oil industry an electric field is applied in order to
induce coalescence in water-in-oil emulsions, in this way accelerating the separa-
tion process. Several techniques can be employed experimentally to follow this
electrically induced coalescence of emulsions. Among them one can mention light
scattering(2). dielectrophoretic(3) and conductivity measurements(4). Coulter
counter(5), rapid freezing microscopy(6) or dielectric spectroscopy(7).
In this study a dielectric spectroscopic technique has been used in order to
determine emulsion stability. A high electric field induces coalescence which is
monitored by following the change in the dielectric properties of the emulsions, as
described in detail in ref. (7).
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STABILITY OF WATER-IN-CRUDE OIL 34 1
We repon on the stability of two different water-in-crude-oil emulsion
systems. By changing the solvation state of asphaltenes we studied the influence
of asphaltenes on the emulsion stability. We show that if the emulsion stability is
mainly due to the natural surfactants for asphaltenic crudes, waxes and the
interaction between waxes and natural surfactants seem to be important in
emulsion stabilization for paraffinic crudes at low temperatures.
METHODS
Chemicals
Elf Aquitaine provided two crude oils ( Bl and B2) and a crude oil
condensate (F). Crude oil B1 is from a field in south of France, crude oil B2 and
the crude used to produce the condensate are from the North Sea. Crude B I has a
low paraftin (or wax) content (= 4.75%) and high asphaltene (~7%) and resin
(= 25%) content. Crude oil B2 on the other hand is rich in p d ~ n e s (= 12%). but
poor in asphaltenes (4.5%) and resins (~15%). Condensate F is mainly composed
of different alkanes and aromatic molecules. A commercial pour point depressor
supposed to modify the crystallization of waxes, was used as supplied. N-pentane
Cop.) were used in the extraction of the adsorbed fraction (or resins) from the
crudes.
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342
Exwriments
MOURAlLLE ET AL.
Extraction of Surface Active Fractions from Crude Oils
The techniques used to extract the active fractions of the crudes are de-
scribed in more detail in refs. (8.9).
Precipitation of Asphaltenes
The crude oils were diluted in n-pentane (volumetric ratio 1 5 ) and then
centrifuged for 10 minutes at 2000 rpm. The fraction that precipitates under these
conditions will be referred to as asphaltenes. This fraction is somewhat
different(l0) from the asphaltene fraction as defined in paper (1 1) where the part
of the crude oil that precipitates in heptane is named asphaltenes. The supernatant
is kept for the next steps of extraction.
Crude Oil Without Active Fractions
Silica particles were added to the supernatant (i.e. dilute crude oil without
asphaltenes) until the liquid became transparent. The silica particles were
subsequently separated from the liquid by filtration. Pentane was evaporated under
low pressure. The remaining liquid now contains the main components of the
crude oil, excluding the asphaltenes and resins.
Desorbtion of Resins
A mixture of dichloromethane and methanol (in volumetric proportions
93:7) was used in order to desorb the adsorbed fraction from the silica particles.
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STABILITY OF WATER-IN-CRUDE OIL 343
After removal of the silica particles by filtration, the solvent was evaporated under
low pressure. The fraction recovered in this way will be referred to as resins.
Emulsion Pre~aration
A Silverson Laboratory Mixer Emulsifier Model STD I with an emulsor
screen head, running at 1000 rpm, was used for preparation of the emulsion
samples. The emulsification time was from 1.5 to 3 minutes, depending on the
system. For all samples within a given experiment the duration of emulsification
was the same.
Measurements
Interfacial tension measurements were carried out with a KSV Sigma 70
Tensiometer meter (KSV Chemicals, Finland) using the ring method.
Densities were measured by the use of an Eichfahiger Messbereich density
measuring cell.
SedimentationlSe~aration Tests
Immediately after emulsi ification the samples were trans iferred to graded
cylinders, where the sedimentation or separation of the phases were studied under
normal gravity conditions.
Emulsion Stabilitv as Measured bv use of a Time Domain S~ec t ro sco~v Method
The experimental setup for this method is schematically depicted in figure I.
A pulse generator feeds a fast rising electromagnetic pulse through a coaxial line
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MOURAILLE ET AL.
I Im spra
Tam
FIG. 1: The experimental set-up.
to the sample cell. Here. the pulse is reflected and travels back through the line. A
digitizing oscilloscope (HP 54120a) records both the original pulse, v(r), and the
reflected pulse, ifr). For a non-conducting sample lim(v(r) - r(f)) = 0 while for a I+-
conducting sample lim(v(f)- r(f))>0(12). In other words, if the final levels of ,-+- the reflected and incoming pulses are equal, the sample in the cell is not
conductive. If, on the other hand the level of iff) does not reach the level of the
incoming pulse, the sample is a conductor of electric charges.
A DC-voltage supply (Metrix AX 322 power supply) is connected to the
coaxial line via a bias-tee. In this way a potential difference (in the range from 0 V
to 60 V) can be applied between the cell electrodes. Due to the short distance
between the electrodes (controlled by the spacer) strong electric fields can be
applied even at moderate voltages. In the currently used set-up a potential
difference of 60 V leads to an electric field strength of approximately 5 kV/cm.
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STABILITY OF WATER-IN-CRUDE OIL 345
When a water-in-oil emulsion is placed in the sample cell the water droplets
become polarized by the applied field(13-15). This polarization may lead to a
reorganization of the droplets (especially when the aqueous phase contains
electrolytes), forming chain-like structures. The chains are aligned parallel to the
electric field lines. If the applied field is strong enough the droplets coalesce. A
conducting path through the sample, from one electrode to the other, is created
when a sufficiently high number of droplets have coalesced. This will again lead
to a lower final level of the reflected pulse signal as recorded on the
oscilloscope(l6,17). We define the critical electric field (E,) as the minimum DC
electric field that has to be applied over an emulsion in order to observe a
macroscopic conductivity within 2 minutes after application. Thus E,, may be
used as a measure on the wlo emulsion stability (i.e., stability versus coalescence).
RESULTS
In the first series of experiments the oil phase was either pure crude oil B l
or mixtures of 9 1 and an organic solvent. The solvent amounted to up to 50% (by
volume) of the mixture. As solvents were used toluene, heptane, a mixture of
tolueneheptane (I : I ) and condensate F. Samples containing 80% (by volume) oil
phase and 20% aqueous phase were emulsified. The samples were split in two,
one part was used for sedimentation tests while the second part was used for the
determination of the critical electric field. In all the tests we could observe water
droplets forming a sedimentation layer at the bottom of the graduated cylinder.
But even after two weeks there was not enough coalescence of the droplets to lead
to a separation of the aqueous phase. Moreover, the low amount of water in the
emulsion phase made any visual observation of an interface between the emulsion
phase and the oil phase impossible. Hence, as a measure on the emulsion stability
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346 MOURAILLE ET AL.
the level of the sedimented water droplet layer was used. The results from the
sedimentation tests are reported in figures 2 to 5.
In figure 6 the critical electric field for emulsions containing the different
solvents are plotted versus the amount of solvent in the oil phase. The
measurements were carried out immediately after preparation of the samples. It is
seen that when toluene or a tolueneheptane ( 1 : I ) mixture is used to dilute the
crude, the emulsion stability is lower than when either heptane or condensate F is
used. From the figure we also note that the emulsion stability decreases with an
increasing proportion of solvent in the oil phase. The critical electric field
measurements were repeated one week after emulsification, and as seen from
figure 7, one week of aging seemed to have little influence on the emulsion
stability.
In the second series of experiments the oil phase was either condensate F,
heptane, toluene or the I: 1 mixture of toluene and heptane. Asphaltenes and resins
extracted from crude oil B1 were added to this oil phase. The amount of extracted
fractions used ranged from 0.9% (weight% of the oil phase) asphaltenes and
I% resins to 5% asphaltenes and 10% resins. The emulsions were made from
equal volumes of oil and aqueous phases. Sedimentation tests on emulsions with
lower water concentrations (20 % by volume) were also carried out.
In contrast to the sedimentation tests in the first series of experiments, we
could not distinguish between an emulsion phase and sedimented water droplets in
these systems. However, the interface between the emulsion phase and the oil
phase could be observed. Thus in figures 8-10 we report on the amount of oil
phase separated as a function of time. Figure 8 (heptane) and figure9
(condensate F) show that the general trend is a decrease of the separation speed
with increasing amount of extracted fractions added to the oil phase. The
comparison between figures8 and 9 shows that when the oil phase is
condensate F, the emulsions are more stable than when the oil phase is heptane.
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STABILITY OF WATER-IN-CRUDE OIL 347
0 5 10 15 20 25
Time [h]
FIG. 2: The build up of a layer of sedimented water droplets in a wlo emulsion where the oil phase is crude oil B1 or mixtures of crude oil B I and heptane. The water content is 20 % by volume. Symbols: The heptane amount in BI is 0 % a), I0 % (M), 20 % (A), 30 % (A), 40 % (0) or 50 % (e). (All percentages are volume %.)
When the oil phase is based on toluene or the tolueneheplane mixture the
emulsions are not stable at all and separate immediately. These same solvents also
lead to the less stable emulsions when used to dilute crude oil B I (see figure 6 ) . Figure 10 shows how the oil phase separates from the emulsion phase as a
function of time when the volume of water is 20% of the total volume. The oil
phase is based on condensate F. It is noteworthy that the highest amounts of
additives (i.e., 5% asphaltenes and 10% resins) lead to the highest separation
speed. It seems that in this case, a high concentration of resins destabilizes the
emulsions.
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MOURAlLLE ET AL.
I
FIG. 3: The build up of a layer of sedimented water droplets in a wlo emulsion where the oil phase is crude oil BI or mixtures of crude oil BI and a tolueneheplane mixture ( I : I by volume). The water content is 20 % by volume. Symbols: The tolueneheptane mixture amount in BI is 0 % 0, 10 % m, 20 % (A), 30 % (A), 40 % (0) or 50 % (e). (All percentages are volume %.)
Critical electric field tests were carried out immediately after emulsification.
The tests were also repeated several times during one week after the sample
preparation. The aged emulsions were slighily shaken by hand, prior to the
measurements. The results from these tests are reported in Table I and can be
summarized as follows:
After one day the emulsions are more stable than they were just after
making. This increased stability remains relatively constant during the following
days. Similar rime dependent effects were not observed when the oil phase was
composed of crude oil diluted by different solvents.
FIG. 4: The build up of a layer of sedimented water droplets in a wlo emulsion where the oil phase is crude oil B1 or mixtures of crude oil BI and toluene. The water content is 20 % by volume. Symbols: The toluene amount in B1 is 0 % a), 10 %a). 20 %(A), 30 %(A), 40 % (0) or 50 %(a). (All percentages are volume %.)
Crude oil B2 was the continuous phase of the emulsions in the fourth row of
experiments. This crude contains large quantities of paraffines or waxes, and h e
effects of temperature and a commercial pour point depressor on the emulsion
stability were investigated.
Emulsions with a water content of 20%, 30% and 40%, respectively, were
prepared as earlier described. The oil phase was either the pure crude B2, or crude
B2 with the pour point depressor added at a concentration of 200ppm.
Sedimentation tests on these systems were performed at 4°C. room temperature
(23°C) and at 50°C.
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350 MOURAILLE ET AL.
0 10 20 30 40 50 60 70
Time [hl
FIG. 5: The build up of a layer of sedimented water droplets in a w/o emulsion where the oil phase is crude oil B1 or mixtures of c ~ d e . 0 i l BI and condensate F. The water content is 20 % by volume. Symbols: The condensate amount in B1 is 0 % 0, lo % 4). 20 % (4, 30 5% (A), 40 % (0) or 50 % (0). (All percentages are volume %.)
At the lowest temperatures (4'C and 2 3 T ) and without the pour point
depressor, no separation of the oil phase nor the water phase could be observed
within 12 hours. No sedimentation of water droplets was observed either during
this period.
On the other hand, at 50°C, the water phase separated quickly when the
water content was above 30 % (figure I I). For the lowest volume fractions of
water no phase separation was observed but large water droplets did appeared near
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STABILITY OF WATER-IN-CRUDE OIL
0 4 t
0 5 10 15 20 25 30 35 40 45 50
Solvent concenlralion [val. % a1 the allphase]
FIG. 6: E,, for wlo emulsions where the oil phase is a mixture of crude oil B I and a solvent. The water content is 20 % by volume. The solvents are condensate F % ( @ ) , heptanem), a toluenelheptane 1:l mixture(*) or toluene(A), respectively.
the bottom of the graded cylinder. It shou'ld be mentioned that 50°C is well above
the WAT (Wax Appearance Temperature) at 34,5"C (18).
When the pour point depressor is used the water phase separates relatively
fast, even at room temperature (figure 12).
In order to investigate the temperature effect on the emulsion stability more
thoroughly, E,, were measured in the temperature range from 3°C to 2 3 T . It
turned out that the emulsions based on pure crude B2, with 20% or 30% water and
at temperatures below 17 "C or 12 "C, respectively, could not be broken at all.
even with the strongest electrical field applicable (5 kV1cm). For the emulsions
having E,, less than 5 kV1cm the results are summarized in figure 13, where the
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MOURAILLE ET AL.
0 I
0 5 10 15 20 25 30 35 40 45 50
Amount ot solvent in 011 phase (vol. %)
FIG. 7: The effect of aging on E,, for wlo emulsions where the oil phase is a mixture of crude oil B1 and a solvent. The water content is 20 4 by volume. Closed symbols: measurements made immediately after emulsification. Open symbols: Measurements made I week after emulsification. The solvents are condensate F % (@, O), heptane an), a tolueneheptane 1 :I mixture(*. 0) or toluene (A. A), respectively.
critical electric field for the various emulsions are plotted versus temperature. It is
seen that without the pour point depressor the emulsions are very stable at low
temperatures, the stability decreases as the temperature is raised. The slope of the
stability versus temperature curves are steepest for the lowest water contents, and
gradually flattens out as the amount of water increases. The system seems to
display a critical temperature, above which the emulsions are unstable towards an
electric field. This critical temperature is inversely proportional to the water
content of the emulsions. When the pour point depressor is added the stability
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STABILITY OF WATER-IN-CRUDE OIL
I
0 0.5 1 1.5 2 2.5 3 3.5
Time [h]
FIG. 8: Separation of the oil phase from wlo emulsions where the oil phase is heptane. The water content is 50% (by volume). The emulsions are stabilized by asphaltenes and resins. The concentrations of the added compounds are 0.9 % asphaltenes and 1.0 % resins a), 2.0 % asphaltenes and 2.0 % resins a). 5.0 % asphaltenes and 5.0 % resins (A) or 5.0 % asphaltenes and 10.0 % resins (A). respectively.
decreases at all temperatures, and the critical temperature is shifted towards lower
values.
Interfacial tension measurements were carried out in order to check whether
the pour point depressor has surface active properties of its own that would lead to
a competition with the natural surfactants in the crude. The interfacial tension
between the crude oil B2 and distilled water was found to be 25,5mNlm.
Approximately the same value (25,2 dim) was measured when 200 ppm of the
pour point depressor was added to the crude.
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MOURAILLE ET AL.
0 I 0 5 10 15 20 25 30 35
Time [h]
RG. 9: Separation of the oil phase from wlo emulsions where the oil phase is condensate F. The water content is 50% (by volume). The emulsions are stabilized by asphaltepes and resins. Same symbols as in figure 8.
The last experiments were carried put to investigate how the mixing of an
asphaltene-rich and a paraffin-rich crude oil would influence the water-in-oil
emulsion stability. In figure 14 the emulsion stability at room temperature as
expressed by the critical electric field is plotted against the fraction of crude oil B I
in crude oil B2. It is obvious from figure 14 that the emulsion stability increases as
the imount of BI increases and the water content decreases. It is striking that only
a small amount of crude BI is necessary in order to significantly increase the
stability of the emulsions.
In figure 15 the emulsion stability when the oil phase is mixtures of crude
B I and 8 2 is compared at two temperatures, 10°C and 23°C. The water content is
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STABILITY OF WATER-IN-CRUDE OIL
FIG. 10: Separation of the oil phase from wlo emulsions where the oil phase is condensate F. The water content is 20 % (by volume). The emulsions are stabilized by asphaltenes and resins. Same symbols as in figure 8.
Table I. The Effect of Aging on E,, of WIO Emulsions Containing Equal Volumes of the Oil and Aqueous Phases. To the Oil Phases (either Condensate F or Heptane) Different Amounts of Asphaltenes and Resins from Crude Oil BI are Added.
FIG. I I: The amount of water separated from water-incrude oil emulsions as a function of time. The oil is crude oil 82. The temperaNre is 50°C. The water content is 30 % a) or 40 % (n), respectively.
50%. While at 23°C there is an almost linear relationship between E,, and the
amount of B I in the oil phase, a completely different picture emerges at 10 "C. At
this temperature a clear maximum critical electric field is observed when crude B1
constitute 60 % of the oil phase. Further, as long as crude B2 is replaced by more
than approximately 40% 51, the emulsions are more stable than what we find for
the pure crude B 1.
Finally, asphaltenes extracted from crude oil 81 were added to crude B2 and
condensate F. We found that at room temperature less asphaltenes were needed in
order to stabilize the emulsions based on condensate F than those based on crude
8 2 (figure 16). According to the measurements of critical electric fields the
emulsions based on the condensate were more stable at all the asphaltene contents
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STABILITY OF WATER-IN-CRUDE OIL
so T
FIG. 12: The amount of water separated from water-insrude oil emulsions at mom temperature. The oil is crude oil B2 with 200 ppm Swim 5X. The water content is 20 % (0). 30 % a) or 40 % a), respectively.
investigated. When the temperature was reduced to 10 "C the situation was
reversed. The crude 82 based emulsions were more stable than those based on
condensate, and when the asphaltene content exceeded 2% (by weight), very
stable emulsions were formed.
DISCUSSION
Water-in-Oil Emulsions Stabilized bv Asuhaltenes and Resins from Crude Oil B1
The fact that water-in-crude oil emulsions may be stabilized by surface
active components naturally occuning in the crudes is already well documented.
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MOURAILLE ET AL.
Temperature rC1
FIG. 13: E,, versus temperature for wlo emulsions based on crude oil B2. The water content is 20% (6. O), 30% (A, A) or 40% (0,O). Filled symbols: The continuous phase is pure crude B2. Open symbols: The continuous phase is crude 8 2 + 200 ppm pour point depressor.
In particular, Fordedal et al.(l) have shown that at room temperature the stability
of emulsions based on crude oil BI was mainly due to the surface active fractions
(i.e., asphaltenes and resins). It is also relatively well established that the
asphaltenes occur in different states depending on the chemical
surroundings(l9.20).
In order to gain more insight in the mechanisms behind the emulsion
stabilization due to asphaltenes we have modified the asphaltenes solvation state
using two different approaches. In the first approach, where we diluted the crude
with different organic solvents, we found that the emulsion stability decrease with
an increasing proportion of solvent in the oil phase (figures 2 - 6). This trend may
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STABILITY OF WATER-IN-CRUDE OIL 359
FIG. 14: E, of w/o emulsions versus the relative amounl of crude oil BI in the continuous phase (a mixture of crude BI and crude B2). The water content is 20 % (O), 30 % a). 40 % (A) or 50 % (A), respectively.
1.8
- 1.6 E $ 1,4.- I E 1.2.- D .- - U 1 - U (Y g 0.6 - 04 .U 0.6 - .- ' 0.4
0.2
0
be attributed to a dilution of the surface active fractions. It would be difficult to
compare the effect of the different solvents (heptane, heptandtoluene or toluene)
on the emulsion stability only from the sedimentation tests in as much as the
sedimentation speed depends on the oil phase density (density of heptane = 0.69;
density of condensate F = 0.83 and density of toluene = 0.87) and viscosity. For
instance, when heptane or toluene is used as solvents, the observed difference in
sedimentation speed could be explained by solvent densities. The measurements
of E,, do not in the same way depend on the densities and viscosities of the
continuous phase. The results indicate that emulsions made using condensate F o r
heptane as solvents are more stable than those made using toluene or the
I:1 mixture of toluene and heptane. This funher indicates that the solvation state
0 10 20 30 40 M 60 70 80 80 1W Amount of Crude E l in the oil phase pol. %)
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360 MOURAILLE ET AL.
Amount of crude B1 In the oll phase (vol. %)
FIG. 15: &, of w/o emulsions versus relative amount of crude oil B1 in the continuous phase (a mixture of crude BI and crude B2). The water content is 50 %. The temperature is 23 "C (.) or 10 "C (A).
of the asphaltenes is an important factor for the emulsion stability. One can figure
that when the crude oil asphaltenes are well solved in the oil phase (like for
instance in toluene), they do not seek the watertoil interface. On the other hand,
when the asphaltenes are not dissolved at all by the solvent, they become too
agglomerated to efficiently cover the watertoil interface.
The solvent that reduced the stability of the crude oil emulsion the least was
condensate F. Since this condensate is mainly composed of alkanes and aromatic
molecules we can expect that an optimized mixture of heptane and toluene (i.e.,
different from ]:I) could lead to more stable emulsions than toluene or heptane
alone or the tolueneheptane I: 1 mixture. In ref.(21) water in oil emulsions having
an oil phase made of 3%( weight of the oil phase) asphaltene solved in mixtures of
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STABILITY OF WATER-IN-CRUDE OIL 361
0 0.5 1 1.5 2 2.5 3
Asphellene content [w.%]
FIG. 16: E,, of wlo emulsions versus the amount of asphaltenes (extracted from cmde BI) in the continuous phase. The water content is 50 % . The continuous phase is either cmde oil B2 an) or condensate F (A, A). Open symbols: The temperature is 23 "C. Filled symbols: The temperature is 10 "C.
decane and toluene in different proportions were studied. The authors found a
maximum emulsion stability using approximately 15 % toluene and 85 % decane.
In the second approach, where the extracted surface active fractions from the
crude were dissolved in the organic solvents, we again found that condensate F
leads to the more stable emulsions, whereas when the oil phase was toluene or the
tolueneheptane (1:l) mixture no stable emulsions could be formed. The aging
effect observed in Table 1 (an effect not observed in the case of the diluted cmde
oils) may be due to the extraction process of the asphaltenes. The precipitation of
asphaltenes is not fully reversible(22,23), and when the asphaltenes are to be
dissolved again they may need some time to dissociate(24).
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362 MOURAILLE ET AL.
When the oil phase is condensate F, increasing the amount of the extracted
fractions in general leads to an increased emulsion stability. However, too much
resins seems to destabilize the emulsions: Emulsions are less stable when 5%
asphaltenes and 10% resins are used than when 5% asphaltenes and 5% resins are
used.
On the other hand, when the solvent is heptane, 5% asphaltenes and 5%
resins do not stabilize the emulsion while stable emulsions are formed with 5%
asphaltenes and 10% resins.
This may he explained by an optimization of the solvation state of the
asphaltenes. Indeed resins are known not to stabilize emulsion themselves(l).but
rather to solve asphaltenes(25). Pure heptane is a poor solvent for asphaltenes,
possibly the resins disperse the asphaltenes making them more efficient as
emulsions stabilizers.
When the oil phase is condensate F the asphaltenes may already be close to
the state where they the most contribute to the stabilization of the emulsions, as
seen from figure 5. An excess of resins may transfer more asphaltenes from the
waterloil interface to the oil phase, thus reducing the emulsion stability.
Water-in-Crude Oil Emulsions Stabilized by Waxes from Crude Oil B2
Crude oil B2 is characterized by a high wax content (1 12 %) and only small
amounts of asphaltenic compounds. The resin fraction is also relatively small
(15 %) compared to crude B1 where the asphaltenes and resins constitute more
than 3 0 % of the crude.
The stability of the emulsions based on crude 8 2 show a high degree of
temperature dependence. At temperatures well above the WAT the oil and
aqueous phases separated immediately, while for temperatures below the
crystallization temperature very stable emulsions were formed (figure 13).
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STABILITY OF WATER-IN-CRUDE OIL 363
When the pour point depressor was added to crude B2 a significant
reduction in the emulsion stability was observed. Based on surface tension
measurements we assume that this effect is not due to a preference of the additive
on behalf of the asphaltenes at the water-oil interface. The pour point depressor is
according to the specifications supposed to modify the crystallization of waxes.
The results clearly suggest that in crude oils with high wax contents the formation
of wax crystals or particles plays an important role when it comes to stabilization
of water in crude oil emulsions. It is well known that solid panicles are able to
stabilize emulsion by hindering the coalescence(26). and wax crystals are shown
to act in the same way(27). When the wax crystallization is hindered or changed,
the stabilization due to wax panicles becomes less important. It should be
mentioned that the presence of wax panicles is not sufficient to stabilize
emulsions based on crude oil B2 with the asphaltenes and resins removed. Thus
even if the low temperature stability of 8 2 emulsions may mainly be attributed to
wax panicles, the surface active fractions in the crude also play a role.
Water-in-Oil Emulsion Stability when the Crude Oils BI and 8 2 are Mixed
It is reasonable to assume that the mechanisms behind the stability of the
water-in-oil emulsions from the two crudes B1 and B2 are different. Since the
mixing of crude oils from different sources is quite common in the oil industry,
we were interested in investigating what consequences such a mixing might have
on the emulsion stability. At room temperature we found that the emulsion
stability increases when the watercut is decreasing and when the proportion of BI
in the oil phase is increasing (figure 14). This is the same behavior as observed
when crude oil B I was diluted with a solvent. The increase in emulsion stability at
room temperature when the crude 8 2 is diluted with the crude BI may be
altributed both to an increase in the asphaltene concentration and to a modification
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of the surrounding solvent that may affect the asphaltenes ability to stabilize
emulsions. At I0 'C (figure 15), only a small proportion of B1 (20%) in the
mixture is necessary in order to considerably increase the emulsion stability. This
increase is much larger than what would be expected from the addition of
asphaltenes (and resins) alone, and the experimental results suggest that when B1
is added to crude oil 82, there is a considerable interaction between the waxes and
the asphalteneslresins that contributes to a high stability of emulsions. The
conclusion that interactions between the waxes in crude 8 2 and the asphaltenes
from B1 really is responsible for an increased emulsion stability at low
temperatures can be drawn from figure 16. The addition of asphaltenes to
condensate F (which does not contain any wax) leads to more stable emulsions at
23 "C than when the asphaltenes are added to crude B2. In both cases there is an
increase of stability more or less proportional to the amount of asphaltenes added,
but crude oil 8 2 to a smaller degree than condensate F allows the asphaltenes
from crude B1 to play their stabilizing role. At 10°C this picture is changed.
While the stability of the emulsions based on the condensate shows the same trend
as for the higher temperature (the increased stability when going to the lower
temperature may possibly be explained by an increased film viscosity), the
stability of the B2-based emulsions does no longer show a linear dependency on
the asphaltene content. A drastic enhancement of the stability is obtained already
when 2 % asphaltenes has been added. Since the curve for condensate F is
representative of the effect on emulsion stability from adding asphaltenes to an oil
phase, and the stability of the 82 based emulsions when no asphaltenes are added
represents the stability attributed to the waxes, it is reasonable to conclude that
interactions between asphaltenes from B1 and waxes from B2 to a very high
degree influence on the stability.
This explains the shape of the curve in figure 15. As crude oil BI is added to
the oil phase an enhanced stabilization takes place, due to the interaction between
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STABILITY OF WATER-IN-CRUDE OIL 365
the asphaltenes from B1 and the wax crystals from B2. Only small amounts of
asphaltenes from BI are needed before this stabilization appears. Initially, when
the fraction of BI in the oil phase increases, two effects take place simultaneously:
An increased stabilization due to the increased amount of asphaltenes and an
additional stabilization due to the asphaltenelwax interactions. As the addition of
B1 continues, the waxes become more and more dilute, thus the asphaltendwax
interactions become less important and the stability of the emulsions is reduced.
SUMMARY
The state of solvation of asphaltenes in the oil phase plays an important role
in their ability to stabilize emulsions, which may explain the indirect action of
resins on emulsions stability. The temperature is also an important factor when it
comes to the stability of crude oil emulsions, especially true when the wax content
is relatively high. The most important factor influencing the water-in-crude-oil
emulsion stability at low temperature is the interaction at the water-oil interface
between wax crystals and the heavy fractions of the crude.
ACKNOWLEDGEMENTS
Professor Stig E. Friberg is thanked for valuable comments on a first draft.
Elf Exploration Production is thanked for supplying the crudes and for financial
support to Olivier Mouraille.
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