Calibration and Applications of a Rate-Dependent Geomechanical Model for Danian and Maastrichtian Reservoir Chalk (Danish North Sea)

Wellbore collapse, seafloor subsidence, and seismic events are some of the main technical and societal issues related to the deformation of producing chalk reservoirs. Despite the overall consistency of chalk in terms of reservoir properties across Danish North Sea fields, most of the constitutive models were successfully applied to one single hydrocarbon field. This study aims at building a comprehensive geomechanical model capable of predicting reservoir deformation under different geological settings and production strategies. To this purpose, a total of 215 mechanical tests archived in the JCR database ( http://jointchalkresearch.org/ ) are analysed and the tensile, shear and plastic properties of both Danian and Maastrichtian pelagic chalks from the North Sea are characterised separately. The yield surfaces of both types of chalk are also reconstructed in plots of mean-deviatoric stress. The studied specimens display a 30% to 45% porosity range and are saturated with oil and water to cover a wide spectrum of reservoir rock types that are likely to deform plastically in a producing field. The contrasts and similarities in the mechanical behaviour of chalk from the Ekofisk and Tor Fms are quantified and the controlling factors are discussed. Besides, the calibrated constitutive model is applied in 1-D to two Danish North Sea fields and the simulation outcomes are quality-checked by seafloor subsidence data. Two runs of simulation are carried out for each study window by considering the initial and final yield stress values in the constitutive equations. Whereas the first simulation run can be considered as a conservative approach, the second one is an optimistic approach that minimises deformation. This work represents the first attempt to quantify the range of possible seafloor subsidence scenarios by integrating in the modelling workflow the uncertainty associated with the determination of the specific yield stresses for subsurface chalk. The capabilities of this simulation workflow can assist engineers during risk assessment associated with the design of offshore infrastructure and during field monitoring surveys.