Pressure decline associated with production induces compaction of high Porosity reservoirs and overburden subsidence in fields of the North Sea chalk Basin. More than ninety casing failures have been observed in this area which are related to the increase of axial and radial loads on the wellbore. Anumerical technique was developed to describe the mechanism of casing failurein compacted reservoirs as well as in associated overburdens. A 3D finite element modelling procedure was set up to simulate and predict the onset of casing failures. Important characteristics of this numerical model are the introduction of the concept of sliding interfaces and the transfer of deformation parameters from a large scale displacement (field)to a more refined scale analysis (casing). This 3D finite element simulation identified the failure modes and allowed an appropriate revision of casing design throughout the field area Furthermore, it has been demonstrated that the revised design solutions extend the casing lives for a large number of years. Introduction In the fields of the North Sea Chalk Basin more than 90 casing failures have been observed. They are caused by the compaction of the high porosity chalk during reservoir depletion and the associated overburden subsidence. In the Ekofisk field, up to 58 casing failures have been reported. At 38 wells, the failure is located in the reservoir, while for 20 wells the failure occurred in the overburden. Fig. 1 shows the location of wells for which casing failures were observed up to the end of 1988. The tremendous impact of these failures on the productivity of the field prompted the development of the numerical model which is presented in this paper. This model may serve: - to understand the phenomena of casing failure to evaluate casing design improvements - to predict future performance. The uncertainty of the stress conditions causing the casing failures and the necessarily limited knowledge of the large scale stress strain behaviour of the associated reservoir and overburden implies several assumptions and idealizations for such a model. Only by close cooperation of the involved engineers and geologists can a realistic model be defines in addition, numerical sensitivity studies have proven to be very useful to identify the effect of the individual assumptions. Nevertheless, it proved essential to qualify and calibrate the model. For the analysis of casing failures in the reservoir this was achieved by the back analysis of four observed casing failures in the Danian reservoir of the Ekofisk field- One of these calculations will be described in more detail in the following section- Only itis alter the location and the date of these failures were successfully simulated with the numerical model that pammetric studies were performed to evaluate the influence of casing weight, steel quality, wellbore inclination, etc. on the casing stability. Finally, the results of all calculations performed were combined in order to derive global recommendations for the field. A similar concept of qualification and calibration was used for the analysis of casing failures in the overburdened.. Horizontal movements in the overburden rock have been identified to cause most of the failures. In order to evaluate the large scale displacement field, a numerical model was developed which serves for the calculation of compaction and associated overburden subsidence in the Ekofisk field. This finite element model, which is closely coupled with a reservoir model, was calibrated by the back analysis of the observed compaction and subsidence measurements. Afterwards, in a very original manner, the results of the large scale displacement field analysis were transferred to a numerical model which served for the detailed analysis of casing stability of individual wells.