Rock Mechanics Of The Ekofisk Reservoir In The Evaluation Of Subsidence

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 behaviour 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 behaviour. 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.