Nanoindentation study of ripidolite and illite: Micromechanical controls on the sealing capacity of clay-rich caprocks

Clay-rich caprocks are essential for sealing underground energy storage formations, yet their micromechanical behavior under varying stress conditions is not well constrained. In particular, the influence of loading parameters on hardness, elastic modulus, fracture toughness, and time-dependent deformation of individual clay minerals remains insufficiently explored. This study addresses this gap through systematic nanoindentation experiments on ripidolite and illite, evaluating the effects of peak load, holding time, and loading rate on their mechanical response. Load-displacement curves revealed distinct behaviors: ripidolite frequently displayed pop-in events associated with localized plasticity, whereas illite deformed more smoothly. Across all conditions, illite showed shallower indentation depths, higher hardness, and higher elastic modulus, consistent with its stiffer microstructure and harder mineral inclusions. Ripidolite demonstrated a stronger indentation size effect in hardness and generally higher fracture toughness, indicating a greater ability to dissipate energy before crack propagation. Post-indentation imaging confirmed contrasting deformation modes, with ripidolite producing pronounced pile-up without visible cracking and illite showing limited pile-up but radial and lateral cracks. Creep tests revealed a two-stage response in both minerals; however, illite displayed higher creep ratios despite its greater strength, suggesting enhanced time-dependent deformation. Overall, these results highlight a fundamental trade-off: illite offers superior short-term resistance but is more vulnerable to brittle fracture and viscous strain, while ripidolite better resists crack growth but is more prone to immediate plastic deformation. Understanding such parameter-dependent behaviors improves predictions of caprock sealing performance under variable stress histories, informing risk assessments for subsurface energy storage operations.