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

Clay-rich caprocks play a critical role in sealing subsurface energy storage formations, yet the mineral-scale mechanical controls on their sealing performance remain insufficiently quantified. This study addresses this gap by systematically investigating the micromechanical response of ripidolite and illite using nanoindentation under varying peak loads, holding times, and loading rates. Load-displacement curves reveal distinct deformation behaviors: ripidolite frequently displays pop-in events associated with localized plasticity, whereas illite deforms more smoothly. Under all testing conditions, illite shows shallower indentation depths, higher hardness, and higher elastic modulus, reflecting its stiffer microstructure and the presence of hard mineral inclusions. Ripidolite demonstrates 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 confirms contrasting deformation modes, with ripidolite characterized by pronounced pile-up without visible cracking, while illite shows limited pile-up but radial and lateral cracks at higher loads. Creep tests reveal a two-stage time-dependent response for both minerals; however, illite displays higher creep ratios despite its greater strength, implying enhanced relative viscoplastic deformation. These results indicate a fundamental trade-off between short-term resistance and long-term stability: illite offers greater resistance to immediate penetration but is more vulnerable to brittle fracture and time-dependent strain, whereas ripidolite better resists crack growth but undergoes more immediate plastic deformation. This parameter-dependent mineral behavior provides critical insights for evaluating the sealing integrity of clay-rich caprocks under variable subsurface stress histories. • Nanoindentation reveals distinct micromechanical behavior of ripidolite and illite. • Illite is harder and stiffer, while ripidolite shows higher fracture toughness. • Ripidolite deforms by pile-up, whereas illite forms cracks under higher loads. • Illite shows higher creep ratios, indicating greater long-term deformation. • Mineral-scale behavior affects caprock sealing in underground energy storage.