Multiscale Geomechanical Characterization of Continental Shales Using a Combination of High- and Low-Load Nanoindentation

Continental shales are known for their intricate depositional nature and significant vertical and lateral sedimentary variations. Characterizing their mechanical properties requires robust, efficient, and cost-effective methods. Nanoindentation has emerged as an excellent choice, delivering rapid and dependable results. A dual-pronged approach, integrating both high- and low-load nanoindentation, was employed herein to establish a structured framework for statistically assessing the multi-scale mechanical properties of compositionally diverse continental shale samples. High-load nanoindentation experiments on outcrop Kazakhstani shale samples, representative of world class source rocks in the subsurface, revealed the progressive merging and stabilization of initially discrete mechanical attributes within shale constituents (i.e., clay, organic matter, and quartz-feldspar) with increasing indentation depth, suggesting the presence of a minimum probing depth for investigating the bulk mechanical behavior of heterogeneous shale rocks. The acquired bulk mechanical parameters exhibited a robust correlation with the mineralogy of the samples. Additionally, the low-load nanoindentation data were statistically deconvoluted to extract phase-specific mechanical properties of the samples, which were subsequently extrapolated to the macroscopic level using the well-established Mori-Tanaka homogenization technique. The homogenized elastic modulus closely aligned with the empirical data obtained from high-load nanoindentation, further emphasizing the potential of the latter not only for quantifying nano- and micro-scale but also the macro-scale mechanical characteristics of composite materials. Analogies of Kazakhstani shale with Cuyana Basin shale samples, sharing similar characteristics but higher thermal maturity, make these data particularly relevant to local unconventional development plans. Furthermore, the findings offer promise for advancing geomechanical investigations of heterogeneous and anisotropic rocks in the petroleum industry, particularly when core plugs are unavailable. The expected implications of this study are poised to offer valuable insights for determining optimal indentation depths in the empirical assessment of multi-scale mechanical properties in highly heterogeneous shale and other related rocks.