Creep deformation of Alum shales: insights from micromechanical testing

The Alum Shales are a geological formation exposed over large areas of south-eastern Norway. However, its mechanical and petrophysical behaviour is poorly known. Drilling and production operations usually result in large disturbances of shale formations and are often associated with time‐dependent changes in the structure, mechanical, and transport properties of the rocks. Thus, over time, fracture apertures evolve, subsidence sometimes occurs, transient instabilities may trigger catastrophic failures, and, in some cases, the altered effective stresses may trigger induced seismicity.

Time-dependent mechanical behaviour most likely originates from inelastic creep within shales. Unfortunately, details concerning the physical mechanisms of creep and their controlling parameters are poorly constrained to make predictions of the mechanical behaviour of shales. Shales are usually both heterogeneous and anisotropic. Heterogeneities at the grain scale originate from mineral inclusions, soft organic matter, as well as pores and other defects. Together with the anisotropic fabric of shales, which normally display strong continuous foliations, the grain-scale heterogeneities are likely to affect the mechanical response of shales to stress.

Nanoindentation, coupled with chemical analyses of the indented spots, provides a detailed image of the heterogeneities and of their contribution to the deformation of shales. Nanoindentation is a technique in which a small (< 20 microns) diamond tip of different shapes is loaded against a sample surface, causing plastic deformation of the sample, leaving a residual impression (an indent). The primary data obtained in indentation tests are hardness, elastic modulus, and creep behaviour (stress vs strain curves).

The overarching aim of the project is to determine the role of heterogeneities on the creep strength of the Alum shales. To achieve this, you will apply the following methods to samples of the Alum shales (the relative contribution of each method will be discussed in the course of the project):

  • Micromechanical testing with nanoindentation;
  • Microstructural analysis with polarized light microscopy and scanning electron microscopy (SEM), including energy dispersive spectroscopy (EDS) for microchemical analysis;
  • Three-dimensional microtomography of selected samples to investigate the spatial distribution and volume fraction of mineral grains of specific composition.

You will get training in rock mechanics, advanced material characterization with electron microscopy, and micromechanical testing. You will have the opportunity to use the facilities of the Friction Lab and of the Goldschmidt Lab, a Norwegian national infrastructure for the advanced characterization of solid earth materials run by UiO.

To undertake the project, a good background in geomechanics is recommended.

Published Aug. 24, 2022 8:56 AM - Last modified Aug. 24, 2022 8:56 AM

Scope (credits)