ABSTRACT
Reservoir management of the Ekofisk field requires accurate predictions of reservoir compaction that will occur during the production lifetime of the field. Laboratory measurements must provide realistic values of strain as a function of effective stress or predictive models will not give reliable estimates of compaction and surface subsidence.
The compaction behavior of Ekofisk chalk as a function of effective applied stress has been studied in the laboratory. The objective of this study was to obtain data which can be used to predict reservoir resulting from increased stress on the chalk due to reduction in reservoir pressure with production of the field. Compaction data for uniaxial strain conditions and for ultimate strain at a given applied stress are required as input for the predictive model. It was found that results obtained under hydrostatic test conditions are very similar to those obtained in uniaxial tests when the hydrostatic test results are converted by a standard formula to uniaxial conditions in the elastic region of the compaction curve and applied directly in the plastic (pore collapse) region. This observation for this particular material is valuable in providing a means of utilizing more easily obtainable hydrostatic data for many studies, and also is interesting with regard to understanding what is happening to the chalk in the pore collapse region.
Laboratory studies clearly show that strain in chalk is time-dependent. Ideally, compressibility tests should be of sufficient duration to observe all potential strain that would occur during the life of the field. This is clearly not feasible, however, and the analyst must obtain data from tests conducted on a time frame of days or weeks. To deal with this dilemma, a numerical technique was developed that predicts the ultimate compaction chalk will undergo when subjected to a given stress for infinite time. The technique has been applied to a large number of chalk compressibility tests. Correlation coefficients indicate the numerical technique does very well predicting chalk compaction in extended laboratory tests. The numerical technique allows real-time evaluation of strain equilibration during compressibility tests, it allows shorter tests, and it provides predictions of ultimate compaction of the reservoir.
