CASING DEFORMATION IN EKOFISK

Casing deformation resulting from reservoir compaction has occurred in the Ekofisk field operated by Phillips Petroleum Co, Norway. Since 1981, casing deformation has been recognised 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 scrutinised and analysed 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 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 minimised 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 modelling. The results should be helpful to drilling and completion engineers and in reservoir management.