Dynamic Visualization and Quantification of Intragranular Microscopic Mechanical Characteristics of Heterogeneous Rocks via In-Situ Nanoindentation

Summary We establish the first use of nanoindentation inside a scanning electron microscope (SEM; in-situ nanoindentation) to dynamically visualize and quantify intragranular microscopic mechanical characteristics in heterogeneous rocks. Microscopic mechanical properties determine macroscopic performances in various subsurface applications, including drilling, hydraulic fracturing, energy storage, and resource recovery. Conventional experimental methods provide statistical averages of mechanical properties [e.g., reduced modulus (Er) and hardness (H)] but are unable to capture relationships between local heterogeneities and their dynamic behavior under stress. Our approach enables collocated measurements which reveal significant mechanical heterogeneities across grain phases of quartz, pyrite, Na-feldspar, dolomite, and carbon within selected shale and coal samples. We directly visualize crack evolution and grain damage dynamics and link these events to abnormal variations in Er and H. This work sets a foundation for a dynamic methodology for ultrahigh-resolution investigation of the role of grain texture on mechanical properties and anomalies in complex sedimentary rock fabrics and other heterogeneous materials.