Summary
The terms auto, natural, and in-situ gas lift all refer to artificial lift
systems that use gas from a gas-bearing formation to gas lift a well. The gas
lift gas is produced downhole and bled into the production tubing via an auto
gas lift valve designed for gas operations.
The value of auto gas lift is probably easier to demonstrate than for other
types of intelligent well because it provides a direct replacement for
conventional gas lift equipment, compressors, and pipelines, and the ancillary
equipment they require.
An estimated 60 auto gas lift systems have been installed at the time of
writing of this paper, most of them in the Scandinavian sector of the North
Sea. Several papers have discussed this technology, but so far none has
presented a rigorous analysis or solution of the wells’ production from a gas
lift perspective.
This paper presents the basic theory behind auto gas lift and how to apply
it. The components of the theory are well known and commonly used in nodal
analysis and conventional gas lift design. Properly combining these components
enables an auto gas lifted well’s performance to be calculated and downhole
equipment to be correctly sized and located.
Introduction
Auto, natural, and in-situ gas lift systems use gas from a gas-bearing
formation or gas cap to lift an oil-producing zone artificially, as shown in
Figs. 1 and 2. Unlike conventional gas lift in which gas is pumped down the
annulus from surface, an auto gas lift well has a downhole gas zone completion
from which gas is bled into the tubing at a controlled rate. The flow of gas
into the production tubing is controlled by a downhole flow control valve with
a capability to adjust the flow area from surface by hydraulic or electric
means. The use of downhole flow control valves means that auto gas lift belongs
to the category of intelligent or smart wells. Auto gas lift systems can
generate significant value by:
1. Increasing oil production rates through the use of a cost-effective
artificial lift system.
2. Mitigating the effects of high water-cut in both well production and
start-up.
3. Maintaining tubing-head pressure in subsea wells.
4. Eliminating the capital cost of gas-compression facilities or gas-transport
pipelines.
5. Reducing platform load requirements caused by gas lift compression.
6. Eliminating the need for annular safety valves in places where they are
required in conventional gas lift environments.
7. Allowing nonassociated gas to be produced without recompleting the
well.
8. Eliminating interventions for resizing or replacing conventional gas lift
equipment.
9. Providing the ability to control gas and water coning (Betancourt et al.
2002).
An estimated 60 auto gas lift systems have been installed at the time of
writing of this paper, the majority of them in the Scandinavian sector of the
North Sea. Various papers have discussed applications of this technology
(Betancourt et al. 2002; Al Kasim et al. 2002; Clarke et al. 2006), but so far
none has presented a rigorous solution for the performance of such wells. This
situation is reflected in the software domain (or perhaps reflects it), where
most commercially available nodal analysis packages cannot easily model auto
gas lift wells.
Interestingly, the flow control valve technology developed for auto gas lift
has found applications in subsea and deepwater wells using conventional gas
lift. The reasons for using these variable valves are usually their higher
pressure ratings, their ability to deliver a wider range of gas lift rates as
well conditions change, the elimination of stability concerns resulting from
oversized orifices, and faster annulus unloading during well commissioning.
© 2008. Society of Petroleum Engineers
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History
- Original manuscript received:
30 August 2006
- Meeting paper published:
5 December 2006
- Manuscript approved:
16 May 2007
- Version of record:
20 February 2008
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