Summary
Aibel (former ABB Offshore Systems) has for a long while carried out
fundamental research on compact separation. A significant part of the
development has been to incorporate robust electrostatic coalescers into three
phase separators. The current technology enables single-stage separation and
accelerates separation of oil and water. The most recent product developed to
enable single-stage separation is the Low Water Content Coalescer (LOWACC). The
LOWACC is an electrostatic coalescer element, enhancing and accelerating the
separation in an oil/water/gas separator. It is designed to operate as a
polishing element for removal of the remaining droplets of water and is
designed to be located downstream the Vessel Internal Electrostatic Coalescer
(VIEC), which is a bulk separation device. A prototype polishing element,
together with the bulk separation device, has been tested on several different
real crude oils ranging from API 17 to API 29. For the API 29 crude, the water
in oil (WiO) content was successfully reduced below 0.5% without the use of
chemicals. For the challenging API 17 oil, the oil in water (OiW) was reduced
to below 2–5%. All the test results verified good separation even with tough
separation conditions that created stable emulsions and small droplets. The
produced water leaving the separator contained 2 to 200 ppm oil in water.
Introduction
Separation of water from crude oil is of great importance for the petroleum
industry. The traditional oil, water, and gas separation train consists of a
first-stage gravity separator, a second-stage gravity separator, and an
electrostatic coalescer unit with bare electrodes. The free gas and most of the
water is removed in first and second stages so that a low-water-content oil
phase is passed on to the coalescer. A careful tuning of the water content,
retention time, and temperature, together with de-emulsifying chemicals, is
necessary to obtain a satisfying separation process. For heavy-oil emulsions,
the process becomes very difficult and often unstable.
Electrocoalescence is a mechanism for enhancing oil/water separation that
has been known since the beginning of of the 20th century, when it was patented
by Cottrell (Eow 2001). Many of the commercially available electrostatic
coalescers are made with bare metal electrodes. When the WiO level is too high,
it becomes difficult to establish the electrical field in the fluid because of
short-circuiting or electrical arcing. This is why the traditional
electrostatic coalescer is located after the first- and second-stage gravity
separator. A coalescer with insulated electrodes will tolerate higher water
contents and can therefore be placed earlier in the process. A more thorough
discussion of the different configurations has been presented by Noïk et al.
(2006).
By introducing an electrostatic coalescer with insulated electrodes already
in the first-stage separator, a much more efficient separation process can be
achieved (Wolff and Knutsen 2004; Kvilesjø et al. 2005; Mosland et al. 2005).
This is further demonstrated through the combined use of the VIEC and LOWACC.
The LOWACC is an electrostatic coalescer element, enhancing and accelerating
the separation in an oil/water/gas separator. It is designed to operate as a
polishing element for removal of the remaining droplets of water and is located
downstream the VIEC, which is a bulk separation device. VIEC and LOWACC both
have insulated electrodes that allow the coalescers to operate in all kinds of
fluids without short-circuiting. One additional feature with the coalescer
elements is that they have built-in transformers that will produce a
high-voltage electric field across the electrodes, but only require low-voltage
penetrators on the power supply side.
A single-stage separator is a separator that separates the water from the
crude oil so efficiently that there is litte need for a separate electrostatic
coalescer, or even a second-stage separator. This paper describes how insulated
electrode coalescers can enable single-stage separation and related
experimental results. The experimental data demonstrates that this technology
greatly enhances oil/water separation and improves the produced water
quality.
Possible applications of this technology are for oil producers on fields
struggling with separation problems, and for solving problems related to
hydrate formation in subsea separation processes. By implementing the
single-stage separation, the footprint and weight of separation equipment
topside will be significantly reduced. In addition, the operational expences
could be lowered as the requirement for emulsion breaker chemical is also
significantly reduced. The technology can be very useful for subsea
applications as it enables long tiebacks because of reduced water content in
oil.
© 2008. Society of Petroleum Engineers
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History
- Original manuscript received:
6 February 2006
- Meeting paper published:
1 May 2006
- Revised manuscript received:
3 March 2008
- Manuscript approved:
11 March 2008
- Version of record:
15 June 2008
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