Summary
Valhall is a field development located in the southernmost part of the
Norwegian Continental Shelf. The chalk reservoir is weak and started to compact
when oil production was initiated in 1982. The reservoir compaction was soon
after observed at the seafloor in form of seafloor subsidence. Drilling was
initially not a major challenge at Valhall, but as the reservoir depletion,
compaction, and subsidence continued through the 90s, one had to develop
drilling strategies to handle the associated reduction in frac gradient in the
reservoir as well as the increasing problems experienced in high-angle ERD
wells in the overburden. The problems in the overburden were associated with
the compaction and subsidence and were most problematic in high-angle wells. An
important factor in supporting the drilling teams in their decision-making
process has been the development of wellbore stability models and the
integration of drilling engineering and rock mechanics with subsurface
technology. This paper briefly reviews the drilling history at Valhall prior to
the ERD program initiated in the mid-90s, and more in detail the increasing
problems in the late 90s in the ERD wells, where several very expensive wells
did not make it to the reservoir target to produce oil. The paper reviews the
work that supported the abandonment of the ERD program in favor of unmanned
platforms in the North and South flank of the field, which resulted in the
current drilling performance of the flank reservoir targets today being
achieved with best-in-class drilling performance.
Introduction
The Valhall field is an overpressured, undersaturated Upper Cretaceous chalk
reservoir located in the North Sea approximately 290 km offshore southern
Norway in 69 m of water. The field is located in the southwestern most corner
of the Norwegian continental shelf (see Fig. 1). Valhall was discovered in 1975
and a three-platform complex with 24 slots was installed in 1981. The 24 slots
have been extended to 30 on the original platform around 1990. In 1996 a new
19-slot wellhead platform was installed next to the existing central complex
for infill and ERD drilling. Because of severe drilling problems during the ERD
drilling a third unmanned wellhead platform was installed in the South flank in
2002 in order to drain this area of the field more cost effectively. A similar
wellhead platform was installed in the North flank in 2003. A new platform will
also be installed in 2003 to waterflood the crestal part of the field. The
field was originally developed to recover 250 MM bbl, but has currently
produced around twice of this, and work is ongoing to recover over 1 billion
bbl from the Valhall structure.
The reservoir is at a depth of approximately 2400 m subsea and consists of
two oil-bearing chalk formations, Tor and Hod. Approximately 2/3 of the oil and
the majority of the productivity are located in the Tor. For more details
around reservoir management at Valhall, see the 10- and 20-year summary papers
by Ali and Alcock (1994) and Barkved et al. (2003).
The effective overburden stress in the crest of the field was around 500 to
1,000 psi at discovery because of significant overpressure (0.82 psi/ft). The
chalk is relatively weak (UCS between 450 and 2,000 psi), and chalk production
has resulted in lost oil production and casing deformations. As reservoir
pressure has been reduced during primary depletion from around 6,500 to 2,500
psi in the crest, the chalk matrix has compacted; a partial transfer of this
compaction through the overburden results in seafloor subsidence. The current
seafloor subsidence is approaching 6 meters and continuing at a rate of less
than 20 cm/year. The compaction and subsidence have also resulted in casing
deformations, as observed in other compacting and subsiding fields. More
details on the well life and well design issues at Valhall can be found in
papers by Kristiansen et al. (2000) and Pattillo and Kristiansen (2002).
© 2007. Society of Petroleum Engineers
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History
- Original manuscript received:
4 June 2004
- Revised manuscript received:
3 July 2007
- Manuscript approved:
17 August 2007
- Version of record:
20 December 2007
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