Cyclic Nanoindentation of Clay Minerals: Micromechanical Insights Into Caprock Integrity for Subsurface Energy Storage

Abstract Clay minerals are key constituents of subsurface caprocks and play a critical role in the long-term containment performance of underground energy storage systems. However, their cyclic mechanical response remains insufficiently understood, despite repeated pressure fluctuations being inherent to subsurface energy operations. This study investigates the cyclic nanoindentation response of four representative clay minerals, including ripidolite, illite, illite–smectite, and beidellite, using constant, progressive, and degressive loading protocols. Load–displacement behavior, indentation depth evolution, apparent mechanical properties derived from the Oliver-Pharr method, and post-indentation microstructure were analyzed to evaluate the influence of mineral structure and loading history. Illite exhibits the smallest indentation depths and the highest hardness and elastic modulus under single-cycle loading, consistent with its compact non-expandable structure. Ripidolite and illite–smectite show intermediate mechanical response with pile-up and moderate indentation depths, whereas beidellite displays the deepest penetration and the strongest time-dependent deformation. Under constant cyclic loading, the apparent hardness and elastic modulus remain comparatively stable for all minerals except beidellite, which shows substantial modulus reduction. Progressive loading increases indentation depth and apparent elastic modulus, whereas degressive loading progressively reduces the apparent mechanical response because of residual deformation and evolving contact geometry. Pile-up correction confirms that although contact-area evolution influences the absolute mechanical property values, the overall cyclic trends remain consistent. These findings indicate that the observed cyclic indentation response is governed by the combined influence of mineral structure, loading history, contact-geometry evolution, and time-dependent deformation, providing micromechanical insight into the cyclic response of clay-rich caprocks during subsurface energy storage operations.