Experimental and Computational Insights into the Nanomechanical Characterization of Ultramafic Rocks for Geologic Hydrogen Production and Storage

Hydrogen is a clean and sustainable energy that holds significant promise in the global transition towards renewable energy sources. An emerging intriguing technology for clean hydrogen generation lies in the utilization of geologic ultramafic rocks through serpentinization reaction. This subsurface process can have a profound impact on the rock mechanical behavior and its structural attributes at the nanoscale which can control rock micro-mechanical behavior, and hence inform the macro-mechanical behavior of ultramafic rock at field scale. However, this knowledge is limited in geologic hydrogen to understand the pore-scale mechanical and microstructural changes. This study presents the first pore-scale thermo–hydro–chemo-mechanical characterization and analysis of ultramafic rocks to understand the nanomechanical and microstructural alterations due to serpentinization and potential geologic hydrogen generation via coupled experimental and numerical methods, and further assess the potential for hydrogen storage in ultramafic rocks. Nanoindenter and scanning electron microscope (SEM) were used to assess the nanoscale Young’s modulus (E), hardness (H), and fracture toughness (K_C) of the rock specimens in addition to microstructural changes. Furthermore, a lab-scale numerical model was developed, and numerical results were validated against the experimental results on the modified nanomechanical properties and microstructure, confirming the proposed approach can be used to predict mechanical response of ultramafic rock at the nanoscale. The results indicate that after the serpentinization reaction, there is significant increase in nano-scale mean hardness (+ 19% to + 32% H), stiffness (+ 31% to + 191% E), and fracture toughness (+ 22% to + 40% K_C) of the ultramafic rock specimens. The numerically homogenized elastic moduli before and after serpentinization are consistent with the experimentally observed nanomechanical alterations in the rock specimens. Further, serpentinization of ultramafic rock can lead to the transformation of olivine and pyroxene minerals into serpentine and magnetite which is indicative of hydrogen liberation. In addition, the serpentinization period can influence the rate of nanomechanical alteration and potential hydrogen generation in ultramafic rocks with comparative changes after 4 h vs. 9 h in stiffness (+ 31% vs. 191%), hardness (+ 19% vs. + 32%), and fracture toughness (+ 22% vs. + 40%). The findings in this work provide new insights that can stimulate future investigations into the mechanical response of serpentinized rocks at multiscale to advance geologic hydrogen production and/or storage in ultramafic rocks.