Identification of Microstructural Properties of Shale by Combined Use of X-Ray Micro-CT and Nanoindentation Tests

Abstract

Shale, a fine-grained sedimentary rock, is composed of highly compacted clay particles, nanometric porosity and silt-sized particles of other minerals. In this paper, a combined use of X-Ray micro-CT and nanoindentation tests is proposed for the qualitative and quantitative characterization of this material. The individual methods are utilized at different scales of observation: nanoindentation at nano and micro level while the X-ray microtomography at the meso scale. For the purpose of mechanical morphology evaluation, the grid nanoindentation measurements are carried out resulting in the overall histograms of indentation properties, i.e. indentation modulus and hardness. Tests are performed at different values of applied load and in two directions: parallel and normal to the plane of isotropy, x1 and x3, respectively. The deconvolution technique is used for the segmentation of the material constituents with respect to mechanical properties. Based on the results obtained from nanoindentation testing at nano scale, the shale matrix composition is identified, namely: the number of constituents, their volume fractions as well as the mechanical properties. In addition, at micro scale, the homogenized parameters of the intact shale material are determined. On the other hand, based on the X-ray microtomography imaging results, the geometry of the shale structure at meso scale is investigated. The studies focus on observation of the evolution of 3D crack network in the samples of shale under uniaxial compression at different levels of load. The research includes three series of scans for eight samples, previously subjected to loading. Based on the acquired images, some statistical morphological measures are calculated, i.e. volume fraction of cracks, tortuosity and crack aperture spatial distribution. In addition, the effective parameters of anisotropic permeability and their changes induced by different levels of loading are estimated using Kozeny-Carman formula.