I'm looking for comments on the various level measurement technologies
that are currently being used to find the interface level of an
emulsion. Specifically, the oil/water emulsion in the desalters of a
crude unit. The two technologies that seem to lead the pack are
capacitance and microwave absorption. Does anyone have experience
with either of these technologies in this, or a similar, application?
A couple of decades ago, I wrote a paper about water in crude oil
measurements. A device was tested that was referred to as a 'ration
tube' which used the specific gravity measurements of the crude oil
and water from a particular well. When a give well was diverted to
the test separator, we used very specific values for the crude and the
water from that particular well. It measure the difference in weight
between 2 taps in a vertical section of pipe. The device was
manufactured by ITT Barton (don't know if they still exist). Bottom
line, it worked well in a test separator at prudhoe bay, as long as
the pressures were low (less than 200 psi. Above 200 psi, the water
and oil formed a 'tight emulsion' and the accuracy of the measurements
was much less reliable. This would not be a problem for a reservior
deep into secondary or tertiary recovery, unless the pressures were
The typical pressures of the field separators was over 700psi (that
was when prudhoe was making close to 2M barrels of oil a day!). At
that time capacitance probes were know not to be accurate above a few
percent. This 'specific gravity' ratio tube measurement system did
require that we measure the specific gravity of the crude, on a
frequent basis to ensure it had not changed. Various wells produce oil
at different S.G. within larger oil fields. S.G. also varies quite a
lot over the life of a given well, depending on how the reservior was
managed. In those days, early 1980s, the gas was reinjected and not
many of the 'light-ends' were removed, so the S.G. variations were
At the time I presented the paper to the SPE, in 1981, as a student
paper contest entrant. During the conference, I met a fellow ARCO
engineer that had been studying the same issue for years, and had
tested all sorts of devices. I not certain where ARCO's R&D results
are published. Maybe the SPE has some articles, as it is a fundamental
problem for oilfield operations.....
Surely the equipment has changed significantly.
One thing I learned was to 'verify' the measurements against
traditional analysis of samples.
It depends on the SG of the oil. If it is low enough (the API high enough)
density based devices will work. The kind of crude oil I work with has an
SG very close to 1., sometimes even more. In those cases capacitance or u
wave absorption are the only way to go. Drexelbrooke and Agar are the two
commonly used brands.
Now here is a weird trick for detecting the rag layer (emulsion) which gets
stuck between the oil overflow and the water underflow: These is a grid of
1/2 dozen thermometers spanning the interface. There will be an abrupt
temperature change when there is a well defined interface. The gradient is
blurry when there is an accumulation or emulsion.
Oh, you might ask, how does one use a separator when the API < 10 (SG > 1)?
Everything is upside down. Weird but it works.
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