Drilling Wellbore Stability in the Compacting and Subsiding Valhall Field
Authors: Kristiansen Tron Golder 
Published: December 2007

Abstract
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 90'ies, 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-90'ies, and more in detail the increasing problems in the late 90'ties 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 Figure 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. Due to 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 BO, but has currently produced around twice of this, and work is ongoing to recover over 1 Billion BO 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. About 2/3 of the oil and the majority of the productivity is 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 500–1000 psi at discovery due to significant overpressure (0.82 psi/ft). The chalk is relatively weak (UCS between 450 psi to 2000 psi), and chalk production has resulted in lost oil production and casing deformations. As reservoir pressure has been reduced during primary depletion from around 6500 to 2500 psi in the crest, the chalk matrix has compacted, partial transfer of this compaction through the overburden resulting in sea-floor subsidence. The current seafloor subsidence is around 5 meters and continuing at a relative constant rate around 25 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).

Field Geology
The lithological column presented in Figure 2 illustrates the Valhall field geology. The lithologies above the Cretaceous chalk reservoir are predominantly claystones and mudstones of Tertiary age. The sediments are over pressured as can be seen by the pore pressure profile in Figure 3. For more details see for example Kristiansen, (1998).