The Sleipner Øst Ty field is a strong waterdrive gas/condensate field, with
in-place volumes of 59×109Sm3 dry gas and
52×106Sm3 unstabilized condensate. The reservoir consists
of deepwater turbidite sandstones and associated mudstones, which act as
baffles to flow. The reservoir qualities are very good, with high porosity and
with permeability in the range of 100 to 1,000 md.
The first production in the area began at the Sleipner Øst Ty field in 1993.
The initial reservoir pressure of 244 bar (2.44×104 kPa) is only a
few bar above the dewpoint pressure. Massive dry-gas reinjection started in
1996, and the reservoir pressure increased during the next two years, which
caused an increase in the condensate-to-gas ratio. During the injection period,
which lasted until 2005, 29×109Sm3 of dry gas was
injected. The main focus during these years was to obtain good vertical and
areal sweep of the dry gas in order to vaporize the dropped-out condensate.
Chemical gas tracers were injected and analyzed for in the production wells to
monitor the movement of the dry gas through the reservoir. This knowledge was
used to identify unswept areas, and to change the drainage pattern by
conducting well interventions and drilling infill wells.
The injection was stopped primarily because of high probability of trapping
gas. Compositional reservoir simulation showed that from 10 to 20% of the
injected dry-gas volume could be trapped in the northern region, which has no
producers. The risk of not being able to back-produce the injected dry gas was
considered high, since the saddle area separating the producers in the south
from the injectors in the north was invaded by the aquifer.
The gas cycling program has increased the condensate recovery factor
substantially; from the originally planned 50% by pressure depletion, to the
current estimated ultimate recovery of 81%. As of July 2007, the condensate
recovery factor is 76%.
The Sleipner Øst Ty field is located in production license PL046 in the
Sleipner area in the southern part of the Viking Graben, Norwegian North Sea,
Fig. 1. It was discovered in 1981 by exploration Well 15/9-9, and appraised and
developed during the next 12 years. A total of four exploration wells, 13
producers and five injectors have been drilled. On the basis of the well
information, the in-place volumes have been calculated to
59×109Sm3 dry gas and 52×106Sm3
unstabilized condensate. The production started in 1993 from the Sleipner A
platform, a fully integrated gravity base platform, and a connected subsea
template with two producers. Water depth in the area is 82 m. The field is
operated by StatoilHydro ASA with their license partners ExxonMobil Norway AS
and Total Norge AS.
The field was originally planned to be produced by pressure depletion, but a
revised drainage strategy recommended dry-gas reinjection to increase the
condensate recovery. Hence, as soon as plateau production was achieved and gas
sales commitments were fulfilled, a recycling program of the surplus dry gas
was started. However, for the first two years, the injected volumes were small.
It was not until the large neighboring gas/condensate field Sleipner Vest came
on stream in 1996 that massive gas reinjection was possible. Since the initial
reservoir pressure was only a few bar above the dewpoint pressure, condensate
started to drop out in the reservoir immediately after production commenced
1993. The dry-gas reinjection has been important both to revaporize this
dropped-out condensate and to keep the pressure high in order to prevent
further condensate drop out in the reservoir.
This paper will present how the reinjection of dry gas into the Ty reservoir
has increased the condensate recovery substantially compared to a pure
depletion drainage strategy. The strong aquifer has given important pressure
support during the production history. When the injection period ended,
blowdown of the field was accelerated to maximize the recovery. The time-line
diagram in Fig. 2 illustrates the major events in the field’s lifetime.
© 2009. Society of Petroleum Engineers
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- Original manuscript received:
1 August 2007
- Meeting paper published:
11 November 2007
- Revised manuscript received:
11 November 2008
- Manuscript approved:
13 November 2008
- Published online:
15 April 2009
- Version of record:
15 April 2009