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.
