Differences in the plastic and brittle properties of Danian and Maastrichtian reservoir chalk from the North Sea

Wellborecollapse, seafloor subsidence, and seismic events are some of the maintechnical and societal issues related to the deformation of producing chalkreservoirs. The triaxial tests represent valuable input data for geomechanicaland rock physics models that document the properties and mechanical response ofthe rock under in situ conditions. Most of the experimental studies are relatedto chalk from the Upper Maastrichtian Tor Fm. that is the main oil-bearing unitin the North Sea. The overlying Danian Ekofisk Fm. that also contains anonnegligible part of the hydrocarbon reserves has however received littleattention. The aim of this study is to assess the plastic and shear behaviourof oil-saturated, Danian chalk containing different amount of quartz by usingan extensive experimental dataset. The hydrostatic yield stresses and theMohr-Coulomb criteria are quantified across a 30–45% porosity range forspecimens with a low (<3%) and high amount of quartz (>4%), referred to asclean and impure chalk. Porosity-dependent functions are proposed to estimatethe geomechanical properties and the yield surface is reconstructed in a plotof mean-deviatoric stress versus porosity. These results are compared withprevious studies on Maastrichtian chalk. The plastic and shear behaviour ofDanian and Maastrichtian chalk appears similar for porous specimens and differsas porosity declines. The differences are of significance changing thegeomechanical properties by a factor of 20%–60%. The mineralogical and diageneticparameters that can explain these observations are discussed. The outcomes canbe integrated in constitutive models to improve the prediction on thecontribution of the Ekofisk Fm. to the total strain occurring in depletedreservoirs from the North Sea. This study is also of interest for other sectorsof industry such as CO₂ and energy storage in chalk that involve economical andsocietal risks associated with rock deformation.