Casing deformation resulting from reservoir compaction has occurred in the Ekofisk field operated by Phillips Petroleum Company, Norway. Since 1981, casing deformation has been recognized as a serious problem in three of the fields at Ekofisk. To date, casing failures have been confirmed in about two-thirds of Ekofisk wells.
Casing deformation and field data were scrutinized and analyzed for the purpose of establishing a relationship between reservoir compaction and casing failure and obtaining useful correlations to estimate future probabilities of casing deformation as a function of reservoir variables and well parameters. It has been determined by statistical analysis, finite element modeling, and the analysis of deformed casing and logs run through some of the collapsed casings, that casing deformation is related primarily to the near-well incremental strain, well inclination, and casing diameter.
A correlation was developed to predict the probability of casing deformation in any given well. The required data are the near-well pressure as a function of time obtained from the reservoir model, and the well angle of inclination. Failure prediction capability is a useful tool for reservoir management and risk analysis in workovers.
The conclusions are that casing failure can be minimized through a pressure maintenance program to reduce strain, by drilling with the highest practical angle and by using the largest possible casing in the well. This work has provided considerable insight into the casing deformation problem at Greater Ekofisk. Casing strain gauges will be installed in several of Ekofisk wells for on-site monitoring, to refine the predictive correlation and the computer modeling. The results should be helpful to drilling and completion engineers and in reservoir management.
Casing failure at Ekofisk was first discovered in 1978. Subsidence and reservoir compaction as the result of pressure depletion was detected in late 1984. Subsidence and reservoir compaction have resulted in two challenging problems, notably, the jack-up project carried out during the summer of 1987, and casing deformation. Casing deformation at Ekofisk is discussed in a recent paper by Anvik and Gibson (1). Casing deformation was first noticed while performing routine workover and wireline operations. Currently, about two-thirds of Ekofisk well casings are reported to have failed.
Casing failures at Ekofisk have been discovered in the producing formation and in the overburden. The overburden deformations have been located in the lower 1500 feet of the overburden, with the highest concentration between 400 and 900 feet above the reservoir. A casing failure in the formation has no noticeable effect on the well' s productivity. It can go undetected until a wire1ine or workover operation is performed. The consequences of a casing deformation in the overburden, on the other hand, are more severe. An overburden deformation typically results in a casing leak, a tubing leak, or both. Either case would cause an unacceptable annulus pressure problem resulting in the well being shut in and worked over.
