Statistical nano-chemo-mechanical assessment of shale by wave dispersive spectroscopy and nanoindentation

Abstract

Shale is a common type of sedimentary rock formed by clay particles and silt inclusions, and, in some cases, organic matter. Typically, shale formations serve as geological caps for hydrocarbon reservoirs. More recently, various shale formations have been identified as prolific sources of oil and natural gas and as host lithologies for the disposal of CO2 and nuclear waste. Despite its abundance, the characterization of shale rocks remains a challenging task due to their complex chemistry, heterogeneous microstructure, and multiscale mechanical behaviors. This thesis aims at establishing the link between the composition and mechanics of shale materials at grain scales. A comprehensive experimental program forms the basis for the characterization of the chemical composition and mechanical properties of shale at micrometer and sub-micrometer length scales. The chemical assessment was conducted through a novel experimental design involving grids of wave dispersive spectroscopy (WDS) spot analyses and statistical clustering of the chemical data generated by the experiments. This so-called statistical grid WDS technique was coupled with grid nanoindentation experiments as a means to assess the nanochemomechanics of shale rocks. The similar microvolumes probed by both methods ensure a direct relation between the local chemistry and mechanics response of shale materials. The results of this investigation showed that the grid WDS technique provides quantitative means to determine the chemistries of silt-size inclusions (mainly quartz and feldspars) and the clay matrix. The mineralogy assessments obtained by grid WDS analysis were validated through comparisons with results from X-ray image analysis and X-ray diffraction (XRD) experiments. The direct coupling of the grid WDS and indentation techniques revealed that the porous clay phase, previously inferred from the mechanistic interpretation of indentation experiments, corresponds to the response of clay minerals. The coupling technique also showed that clay minerals located nearby silt inclusions exhibit enhanced mechanical properties due to a composite action sensed by nanoindentation. The new understanding developed in this thesis provides valuable insight into the chemomechanics of shale at nano and microscales. This coupled assessment represents valuable information for the development of predictive models for shale materials which consider the intricate links of composition, microstructure, and mechanical performance.