The critical role of yield stress uncertainties on chalk reservoir compaction

In the context of hydrocarbon production and CO2 and thermal storage in offshore/onshore Denmark, accurate prediction of the compaction behaviour of subsurface chalk is key to mitigate risks so that suitable development plans can be implemented in a safe manner. The experimental data used for model calibration show a non-negligible data scattering, thereby raising questions about the reliability of the predicted compaction. Such uncertainty associated with the laboratory data is not yet addressed in the literature. This study assesses how the uncertainty associated with the determination of one parameter, the hydrostatic yield stress, manifests itself in a compacting field. Two scenarios are carried out per study area (Dan, Halfdan, Gorm, and Kraka fields) by considering in the constitutive equations the initial (conservative approach) and final yield stresses (optimistic approach) that delimitate the elastic and plastic regime. The creep deformation does not contribute to the contrasts in compaction prediction between the conservative and optimistic model. Besides, changing the yield stress shifts the onset of compaction. This shifting modifies by a factor of 26% to 73% the total amount of computed plastic strain. The method that estimates the yield stress of chalk can thus modify crucial decisions taken during a field development.