Multiscale X-ray tomography for shale microstructure and mechanical behaviour

Abstract

Studying shales and other sedimentary rock systems is important to provide a fundamental understanding of their microstructural properties, including the mechanical response and the permeability of liquids and gases. Understanding the pore scale physical behaviour of geological shale formations is crucial for CO2-sequestration, environmental remediation of polluted aquifers, and preservation of cultural heritage, to mention a few. These topics are all highly relevant to the Norwegian society. Shales are inherently difficult to measure and model, as there are hierarchical structural levels, many minerals are involved, the porosity is low, and the structure is inherently anisotropic with sedimentary layers.

This thesis focuses on enhancing the understanding of shales by using a range of X-ray computed tomography (CT)-based techniques. CT is a uniquely suited non-destructive tool, able to reconstruct three-dimensional information. Importantly, CT can be performed also in bulky sample environments mimicking realistic environments with for example high pressure and temperature. Different contrast mechanisms, such as absorption, phase, scattering, and diffraction, can be used to explore different properties of shales.

Understanding the mechanical properties of shales is important for subsurface CO2 storage and for predicting the long-term stability of the reservoir. Conventional triaxial test experiments do not provide microscale insights into the rock behaviour during mechanical testing. We demonstrate that strain localization in shales can be observed during testing using CT, with new findings relating to the elastic response regime of small deformations. With similar methods, we also study shale fragments embedded in cement. Shale waste from oil and gas extraction requires proper management. Investigating the long-term stability of shale waste mixed with cement, as used for example for soil stabilization, is an important and original contribution.

The four papers in the thesis explore the mechanical properties of Draupne shale, fatigue evolution in cement-shale composites, nanoscale morphology and mineral distribution of Pierre shale I, and the 3D orientation of clay minerals in Pierre shale I. Triaxial testing combined with CT digital volume correlation is the main experimental technique. Other recently developed non-destructive X-ray microscopy methods like diffraction contrast X-ray CT, tensorial tomography retrieving sub-resolution orientation information, and coherent X-ray diffraction imaging are with this thesis introduced as tools for geoscience and shown to provide unique insights into mineral orientation and the internal structures of shale. All the papers rely on synchrotron radiation and are based on experiments done at the European Synchrotron Radiation Facility (ESRF) in Grenoble.

In summary, the thesis proposes the application of advanced X-ray CT techniques to mitigate risks in complex subsurface engineering operations by studying the complex and hierarchical nature of minerals and microstructures in shales.