Summary
Oil exploitation is always accompanied by the nondesired formation of
emulsions caused by the presence of naturally occurring surface-active
molecules such as asphaltenes and resins. Because their presence stabilizes the
oil/water interface, it is necessary to break the emulsions by adding other
surface-active molecules. Formulations based on polysiloxane molecules were
selected as effective nontoxic products to enhance the oil/water phase
separation. Establishing the relation between the efficiency of formulations
and the interfacial properties of silicone molecules is the objective of this
study. Formulation selection is based on bottle tests using turbidimetric
measurements and on dynamic tests using a “dispersion rig” setup that allows
the formation of emulsions under pressure and temperature conditions and
injection of additives into the formed emulsion online. Rapid kinetics of
separation and high levels of separated water were observed. Dynamic
interfacial measurements were performed using complementary techniques. The
drop-volume technique allows the measurement of the evolution of
crude-oil/water interfacial tension with time. The Langmuir trough technique is
used to obtain 2D compression isotherms and to deduce the elastic properties of
interfaces. The coalescence of water droplets leading to the destabilization of
emulsions and consequently to the oil/water separation efficiency can be
related to the rheological properties of water/crude-oil interface. Therefore,
such comprehensive study based on a specific methodology can lead to a strict
and effective selection of emulsion breaker additives in relation with the oil
composition.
Introduction
As crude oil is always produced with water, many problems occur during oil
production because of the formation of emulsions.1 Most common emulsions in the
oil field are water-in-crude-oil emulsions. Their formation is mainly caused by
high shear rates and zones of turbulence encountered at different points of
production facilities, especially at the wellhead in the choke valve.2 These
emulsions can be very stable due to the presence of polar compounds such as
aphaltenes and resins that play the role of “natural emulsifiers” and also
because of the occurrence of many types of fine solids (e.g., crystallized
waxes, clays, and scales)3–6 that strongly participate in the formation of
resistant films at the crude-oil/water interface.7,8
Effective separation of crude oil and water is an essential operation in
order to ensure not only the quality of crude oil but also the quality of the
separated water phase at the lowest cost. Crude-oil dehydration is generally
performed in separators by classical physical treatments such as heating or
electrocoalescence.1 Unfortunately, these physical means are generally not
sufficient to respect required times of residence, especially in offshore
production, and chemical additives have to be added in order to disrupt the
interfacial film and enhance and speed up emulsion breaking. Chemical
demulsification appears, therefore, to be an essential step in crude-oil
dehydration.9
Demulsifiers are generally polymeric surfactants (e.g., copolymers ethylene
oxide, propylene oxide, polymeric chain of EO/PO of alcohols, ethoxylated
phenols, nonylphenols, alcohols, amines, resins, and sulphonic acid salts).
Commercial demulsifiers are formulated in solvents such as short-chain
alcohols, aromatics, or heavy aromatic naphtha and can contain a mixture of
several active matters.10,11
It is believed that most of these products are not safe from an
environmental point of view, even if their toxicity or mutagenic effects have
not been clearly demonstrated from a scientific point of view. The increase of
environmental constraints makes necessary, therefore, the development of safer
formulations in order to replace toxic chemicals such as aromatics or
nonylphenols.
In a previous study,12 a large screening of commercial demulsifiers was
performed by classical bottle tests in the laboratory. Then, nontoxic
polysiloxane surfactants were selected. These molecules were tested here on two
types of crude oil in order to characterize their efficiency and to select
high-performance blends. Best products were also tested in a dynamic dispersion
rig that allows reconstituting crude-oil emulsions in more realistic conditions
under temperature and pressure. Finally, dynamic interfacial measurements were
performed with the Langmuir trough and the drop-volume techniques in order to
determine the dynamic and viscoelastic properties of the crude-oil/water
interface in the presence of these types of demulsifiers.
The objective of this work was to define a rigorous methodology in order to
develop efficient and safe formulations under static and dynamic conditions,
but also to understand the mode of action of polysiloxane in emulsion breaking
in order to correlate the efficiency of coalescence to the interfacial film
properties.
© 2005. Society of Petroleum Engineers
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History
- Original manuscript received:
29 July 2003
- Revised manuscript received:
2 December 2004
- Manuscript approved:
14 December 2004
- Version of record:
15 March 2005
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