Summary
This paper provides insights on the design of downhole gas separators based
on the laboratory study of various separator designs. A downhole gas separator,
also known as a gas anchor, may be installed below the pump to separate free
gas from the produced liquid. The free gas-produced downhole is usually
separated through the casing-tubing annulus (the casing-tubing annulus acts as
a natural gravity separator) while the liquid is produced through the tubing.
However, inefficient gas anchor designs are widespread and an acceptable guide
for their optimum design does not currently exist.
Laboratory testing of downhole gas separators has been ongoing since January
2005 at The University of Texas at Austin Petroleum Production Engineering
Facility (UTAPPEF) using an instrumented full-scale model of a wellbore and
separator constructed with clear acrylic pipe to visualize the fluid mechanics
of the separation process. An air and water mixture is injected through the
well’s perforations. The air and water flow rate measurements are used to
measure and define a performance plot of each separator design. The separator
designs tested differed in entry-port configuration, size of dip tube, and the
relative position of the separator-fluid entry ports with respect to the well’s
perforations. Based on the results of the tests, a new separator design that
includes the effect of centrifugal forces to separate the gas and liquid phases
was developed.
The results show that, for the conditions in the laboratory, 100% separation
was achieved whenever the entry ports were located 1- to 2-feet (ft) below the
bottom-most casing perforation thereby dispelling the predominant industry-held
opinion that more distance is required between the separator-fluid entry ports
and the bottom-most casing perforation. Similarly, laboratory results equally
show that an optimum dip tube length of 5.5 ft is sufficient for the optimal
separation of free gas from the produced liquid by the separator. This clearly
runs contrary to the accepted industry practice, as many industry models
employed for the estimation of dip-tube length were found to over-estimate the
dip-tube length, and this subsequently results in the undesirable increased
pressure drop across the separator. Lastly, the entry port geometry does not
appear to have a significant impact on the separator performance as long as
sufficient flow area is present. The efficiency of all gravity-driven
separators was limited by the liquid velocity inside the separator annulus.
When the liquid velocity inside the separator averaged approximately 6 in. per
second or less, an almost-to-complete gas separation was achieved. On the other
hand, the centrifugal separator had a liquid capacity 70% greater than any of
the gravity-driven, static-downhole gas separators.
© 2009. Society of Petroleum Engineers
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History
- Original manuscript received:
24 July 2007
- Meeting paper published:
11 November 2007
- Revised manuscript received:
28 January 2009
- Manuscript approved:
29 January 2009
- Published online:
25 November 2009
- Version of record:
25 November 2009
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