Field Review 3
The Tyra Field
M.C. Doyle and }.M. Conlin
Mrersk Olie og Gas A/S, Copenhagen, Denmark
The Tyra field, located in the Danish sector of the North Sea, contains free wet gas with an initial condensate gas ratio of
50 stb! mmscfand solution gas in an underlying oil rim. The gas accumulation has a maximum column height of225 ft while
the oil rim, although containing a large amount ofSTOIIP, is present in a long transition zone of up to 200 ft thickness. The
Tyra structure is fairly flat with structural dips ranging from 1.5 to 3.5°. The reservoir rocks are Danian and Maastrichtian
chalks with the Danian chalk contaning nearly 60% of the GIIP. Discovered in 1968, a further three appraisal wells were
drilled, the latest in 1976.
Following the signing ofa gas supply contract in 1979, development of the field started in 1980 and first contractual gas
deliveries were made in October 1984. A gas recycling project was initiated in mid 1987 to take advantage of excp.ss gas
availability relative to sales volumes. Current gas development planning and reservoir management is based on a full-field
three-dimensional field simulation study carried out following development drilling (36 wells). Although field performance
to date is broadly in line with predictions, it has reinforced the importance of reservoir characterization in the assessment
of recovery efficiency and chalk compaction. Only limited development of the oil rim has taken place to date, although
dedicated appraisal drilling is planned in the short term to clarify the nature of the oil rim and assess its real potential.
INTRODUCTION
The Tyra gas field is located in the Danish sector of the
North Sea, 230 km west of the port of Esbjerg (Fig. I).
Companies participating in the Tyra field are A.P. Meller,
Shell and Texaco, forming the Dansk Undergrunds Consortium (DUC). Chevron was a member of the consortium
until 1986. A.P. Meller is the sole concession holder and
the A.P. Meller company, Mrersk Olie og Gas A/S is
operator for the group.
In March, 1979 contracts were signed between the DUC
partners and the Danish state energy company Dansk Olie
og Naturgas A/S (DONG) for the sale and purchase of
2 Tcf (55 bern) of natural gas over a -contract period of
25 years. This agreement, which triggered the Tyra field
development, covers gas reserves from four dedicated
fields; Tyra, Roar, Dan and Gorm. Tyra and Roar are
gas/condensate fields, while Dan and Gorm are oil fields,
whose associated gas production is also a dedicated reserve
under the Gas Sales Agreement. The Tyra field came on
stream in 1984 while the Roar field is as yet undeveloped.
This chapter is based upon data gathered and experience
gained during the development of Tyra and four contract
years of prod uction (1984--1987).
RESERVOIR GEOLOGY
Regional setting
FIELD mSTORY
The first exploration well in the Tyra field, E-I X, was
drilled in 1968 with a primary objective in Tertiary/Upper
Cretaceous chalks. The basis for the test was a mappable,
.'vorth Sea Oil and Gas Reservoirs-II
© The Norwegian Institute of Technology (Graham
broad, low-relief structure at base Tertiary level. Secondary objectives were Lower Cretaceous sandstones and
limestones and Jurassic sandstones. The well was drilled on
the eastern flank of the structure (Fig. 2) and encountered
only traces of hydrocarbons in the chalk sequence. In the
Lower Cretaceous and Jurassic intervals, no significant
reservoir rock was present.
A second well, E-2X, located to the west of the crestal
area, was drilled in the same year and encountered 161 ft of
wet gas-bearing chalk. The two subsequent appraisal
wells, E-3X and E-4X, drilled in 1974 and 1976 respectively, confirmed the presence of a wet gas accumulation
overlying a thin movable black oil rim with a long
(150-200 ft) oil/water transition zone.
Development drilling began in 1982 and was completed
in 1984. The Tyra field was put on stream in October 1984
and dry gas recycling was initiated in June 1987. The
production and evacuation facilities at Tyra and their
position relative to other DUC installations in the Danish
sector of the- North Sea are diagramatically illustrated in
Fig. 3.
& Trotman, 1990).
Denmark's oil and gas fields are located within the North
Sea Central Graben. Rifting of the Graben was initiated in
the Permian or Triassic and was followed by a long and
complex tectonic history. The major elements of the
Danish Central Graben are illustrated in Fig. 4.
The Tyra field is located at the southern end of the Tail
End Graben, where it merges with the Southern Salt
Dome Province. The extensional tectonic regime asso-
48
North Sea Oil and Gas Reservoirs - II
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Field location map.
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