The subsidence occurring in the Ekofisk field originates from the compaction of the reservoir rock due to the increasing stress placed upon it as reservoir pressure is reduced with production from the field. The mechanical properties of the reservoir rock determine how much compaction will take place for given conditions in the field and are therefore a key factor in determining the degree to which subsidence will occur. These mechanical properties can be combined with other reservoir information (pressure, overburden load, structure, etc.) in simulators to predict the amount of compaction and surface subsidence that will occur in the life of the field. For this to be done accurately, there must be sufficient information to describe the compaction behavior of all the rock within the reservoir for all conditions encountered.
The Ekofisk reservoir consists largely of chalk, a very fine-grained, high porosity, mechanically weak rock. A large number of laboratory tests have been run under simulated reservoir conditions to provide a description of the mechanical properties of this chalk. Stress states were applied to reservoir chalk samples to duplicate those encountered in the field, and the resulting compaction was measured. At low stress the chalk compacts elastically with moderate compressibility, but at the higher stress levels encountered in the field during production large amounts of plastic deformation occur. Basic mechanisms of chalk compaction were examined to provide better understanding of the chalk behavior. The chalk properties that primarily influence compaction were identified as porosity and quartz content; the dependence of compaction on these was determined to provide a description of all the chalk within the reservoir. Time dependence of chalk compaction was studied so that laboratory results could be properly applied to the production life of the field. The influence of the waterflooding of the Ekofisk reservoir on the strength of the chalk was studied and found to have no effect.
The origin of the subsidence in the Ekofisk field is the compaction of the reservoir rock due to production. The effective stress on the rock (the difference between the overburden load on the rock and the pore pressure within the rock) increases as hydrocarbons are withdrawn and reservoir pressure declines. Reservoir rock will compact under these circumstances by an amount determined by its mechanical properties. This compaction can lead to an amount of surface subsidence which is determined by the degree of compaction and the structure and properties of the reservoir and overlying layers.
Accurate prediction of subsidence requires that the mechanical properties of the reservoir rock be determined. This can then be combined in simulators with such information as reservoir pressure, overburden properties, and field structure to predict the amount of compaction and resulting surface subsidence which will occur. The objective of the study described here was to determine the mechanical properties of the Ekofisk reservoir rock for use in such simulators. This requires determination of the mechanical properties of all types of rock in the reservoir for all conditions that would be encountered during its production life.
