Assessment of local phase to mechanical response link: Application to the chemo-mechanical identification of rock phases subjected to reactive environments

Abstract

An original approach to the problem of chemo-mechanical characterization of shale rocks is presented, which consists of combining grid nanoindentation technique, scanning electron microscopy-energy dispersive X-ray spectrometry (SEM-EDS), and multispectral image analysis. X-ray microanalysis of the major elements is performed over the indented region, resulting in high-resolution images of the spatial distribution of different elements over the indentation grid. The individual elemental maps of the image fields are mathematically merged to create a multispectral image proper for segmentation into distinct clustered phases and mapped on the indentation points. Unsupervised clustering analysis is performed on the multispectral image to determine the number of statistically definable mutually exclusive material phases.The method is illustrated through application to two types of shale rocks, demonstrating spatially varying microstructures as a result of exposure to reactive environments. Mancos shale subjected to water at ambient pressure and temperature, and Eagle Ford shale exposed to CO2-rich brine at high pressure and high temperature conditions. Clay-, carbonate-, and feldspar-rich phases in the Mancos shale show sensitivity to water exposure, demonstrating alterations in mechanical or compositional properties compared to similar phases in unreacted samples. Carbonate-rich phases in the Eagle Ford shale show three different regions of mechanical properties: dissolution region, followed by precipitation and no reaction regions. A discussion of chemical reactions responsible for mechanical variations in different material phases is included. The results show that the method is capable of successfully isolating different material phases at the indentation scale; thus, providing an efficient means to link chemical and microstructural properties to the mechanical response.