Probing Nanomechanical Properties of a Shale with Nanoindentation: Heterogeneity and the Effect of Water-Shale Interactions

The mechanical properties of shale are critical for the extraction of gas and oil from nanoporous shale. In this study, nanoindentation tests with the accelerated property mapping mode were used to obtain the localized quantitative mechanical properties (the elastic modulus and hardness) of the tested shale sample. X-ray diffraction was used to analyze the bulk mineral composition. The Gaussian mixture model and k-means algorithm were used to fit the data and identify distinct mineral phases based on load-displacement curves. Three clusters of different mineral phases were recognized through nanoindentation curves. Experimental results and analyses show that the nanomechanics of shale are conditioned by the mineral composition and its spatial distribution. The spatial distribution of the elastic modulus and hardness showed the high heterogeneity of shale at the nano-to-micron scale, closely related to its localized mineralogical distribution and microstructures. It was found that the plastic work induced in the nanoindentation process was different between the three mineral clusters with a plastic work ratio of 0.06-0.18, 0.32-0.38, and 0.46-0.53 in high-, medium- and low-hardness clusters, respectively. A two-term exponential relationship was observed between hardness and plastic work. For the tested Marcellus shale, obvious mechanical weakening was observed after 20 days of water vapor treatment. The elastic modulus of shale exhibited a reduction of 0.6-15%, and the hardness decreased by 8.6-17.8%. Further investigation is recommended to quantify the variations in its indent-specified mechanical properties and its direct relationship with localized mineralogy and microstructure.