Nano-scale mechanical properties of constituent minerals in shales investigated by combined nanoindentation statistical analyses and SEM-EDS-XRD techniques

The mechanical properties of the constituent minerals in shale rock are fundamental to a better knowledge of multi-scale shale behaviors. It benefits the engineering applications and predictive physics modeling of the shale formation. In this work, nanoindentation testing combined with scanning electron microscopy - energy dispersive spectroscopy (SEM-EDS) were used to obtain the in situ mechanical properties of individual mineral phases in four shale samples. Engraved cross-marks were employed to locate the indented areas on a micron scale for subsequent SEM-EDS measurements and analysis. The elastic moduli and hardness of the quartz, iron-type minerals, muscovite, clinochlore, organic matter, and the mineral assortments in the matrix were analyzed and relevant deformative behaviors were compared. Results show that the identified mineral groups exhibit a diverse set of mechanical properties at the nanoscale. The iron-type minerals have the highest elastic modulus (104.7 GPa), then followed by quartz, muscovite, clinochlore, and organic matter. Quartz shows the highest hardness (10.2 GPa). The identified minerals demonstrated different elasto-plastic characteristics to the indentation loads. Quartz exhibits the lowest plastic behavior, while phyllosilicates exhibited large plastic behavior owing to the layered structure. Organic matter showed both elastic-dominant and plastic-dominant behaviors, which may be related to the chemical compositional differences and various thermal maturity of kerogen in the shale samples. The mechanical properties of three mineral assortments in shale matrices vary significantly due to the wide variety of the mineral compositions. Radial cracks were observed on the boundary of brittle minerals and relatively weak minerals, while shear cracks were observed on strength-weak minerals such as layered silicates and organic matters. The mechanism of the induced cracks was discussed. This study obtains a basic understanding of shale behavior on nano-scale and provides reliable basic data for multi-scale modeling of shale reservoirs without the need for large-scale mechanical tests.