Normal casing design in the oil and gas industry is based on uniform pressure load cases, where the load cases are given by the pressure of various fluids and gases that can be encountered during various scenarios during the well life. In some cases, the exploitation for oil and gas can result in substantial changes in in-situ rock stress and resulting deformations in the subsurface that will result in other types of loads on the well structure. An overview of various well structure/tubular deformation modes experienced in the oil and gas industry is given by Dusseault, et al.,2001. There do also exist a substantial literature of tubular deformations in wells going through salt. A summary of some of these cases is given by Willson et. al, 2003.

The Valhall case, from the Norwegian Continental Shelf (NCS) discussed here in this paper, is one type of reservoirs where casing and liners have been deformed. The Valhall type of reservoirs are the ones with large amounts of reservoir compaction that result in large enough deformation in the overlaying overburden to deform casing and liners. Typical reservoirs experiencing this are chalk, diatomite and HPHT reservoirs, but also others with large depletion, weak reservoir rocks, very thick reservoirs or a combination of these attributes. The documentation of the Valhall case is therefore applicable to many reservoirs in the industry.

This paper documents the Valhall case from the first observations of deformations in the overburden, including cases with production of overburden shale and clay through damaged tubulars. We also present the most important parts of the work that was performed to understand the mechanisms causing the deformations. Then we present the well design changes that one selected to implement, like steel grades, thickness/diameter (t/D)-ratio and concentric cemented liner laps. Some of this was covered in Kristiansen et al., 2000. We also present a prioritized list of design options in terms of their impact on well life. We also look at how the results of these design changes is estimated to have increased the average well life at Valhall from 7 to more than 24 years around 20 years after the changes to the well design had been implemented.

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