Differences in the plastic and brittle properties of Danian and Maastrichtian reservoir chalk from the 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. The triaxial tests represent valuable input data for geomechanical and rock physics models that document the properties and mechanical response of the rock under in situ conditions. Most of the experimental studies are related to chalk from the Upper Maastrichtian Tor Fm. that is the main oil-bearing unit in the North Sea. The overlying Danian Ekofisk Fm. that also contains a non-negligible part of the hydrocarbon reserves has however received little attention. The aim of this study is to assess the plastic and shear behaviour of oil-saturated, Danian chalk containing different amount of quartz by using an extensive experimental dataset. The hydrostatic yield stresses and the Mohr-Coulomb criteria are quantified across a 30-45% porosity range for specimens with a low (< 3%) and high amount of quartz (> 4%), referred to as clean and impure chalk. Porosity-dependent functions are proposed to estimate the geomechanical properties and the yield surface is reconstructed in a plot of mean-deviatoric stress versus porosity. These results are compared with previous studies on Maastrichtian chalk. The plastic and shear behaviour of Danian and Maastrichtian chalk appears similar for porous specimens and differs as porosity declines. The differences are of significance changing the geomechanical properties by a factor of 20%-60%. The mineralogical and diagenetic parameters that can explain these observations are discussed. The outcomes can be integrated in constitutive models to improve the prediction on the contribution of the Ekofisk Fm. to the total strain occurring in depleted reservoirs from the North Sea. This study is also of interest for other sectors of industry such as CO2 and energy storage in chalk that involve economical and societal risks associated with rock deformation.

Automated summary

The plastic and shear behavior of oil-saturated Danian chalk with different amounts of quartz was assessed. Porosity-dependent functions were proposed to estimate geomechanical properties, and differences in behavior between Danian and Maastrichtian chalk were identified.