Determination of Mechanical Property Evolutions of Shales by Nanoindentation and High-Pressure CO₂ and Water Treatments: A Nano-to-Micron Scale Experimental Study

Fluid–shale interaction mechanisms and their implications on shale mechanical property evolutions are essential for both shale gas exploitation and CO₂ sequestration. To understand the potential influence of water and CO₂ exposure on shale, three shale samples from different formations were treated with dry supercritical CO₂, supercritical CO₂–water, and pure water for up to 30 days. Nanoindentation was used to measure the localized elastic modulus and hardness before and after fluid treatments. Combined nanoindentation with XRD–SEM–EDS analysis, we obtained the probing mechanical properties of three mineral phases in shales: quartz, organic matter, and mixed composites. The mechanical properties and creep behaviors of the mineral phases before and after fluid treatment were analyzed. XRD analysis and nanoindentation results reveal that the three shales are distinct in mineralogy and mechanical properties. The mechanical heterogeneity is related to the mineral constituents, which can be observed in the deconvolution results. The fluid treatment results indicate that dry supercritical CO₂ does not impair the nanomechanical properties of shale, whereas water-contained fluid shows a significant weakening effect. The mechanical evolution of shale is both time-dependent and mineralogy-related. After supercritical CO₂ and water treatment, the maximum deformations of three shales demonstrate an increasing trend, indicating a deterioration of the resistance to load. The elastic moduli and hardness of quartz, organic matter, and mixed composites exhibit apparent decreases after fluid treatment. The time-dependent mechanical behaviors of mineral phases were analyzed. CO₂ and water play critical roles in the reactions with shale and the involved mineral reactions are discussed. The experimental findings provide insight into the nanomechanical property evolutions of shale in field operations involved in CO₂ and water, such as fracturing and CO₂ sequestration.