Cross-scale mechanical softening of Marcellus shale induced by CO₂-water–rock interactions using nanoindentation and accurate grain-based modeling

Mechanical softening behaviors of shale in CO₂-water–rock interaction are critical for shale gas exploitation and CO₂ sequestration. This work investigated the cross-scale mechanical softening of shale triggered by CO₂-water–rock interaction. Initially, the mechanical softening of shale following 30 d of exposure to CO₂ and water was assessed at the rock-forming mineral scale using nanoindentation. The mechanical alterations of rock-forming minerals, including quartz, muscovite, chlorite, and kaolinite, were analyzed and compared. Subsequently, an accurate grain-based modeling (AGBM) was proposed to upscale the nanoindentation results. Numerical models were generated based on the real microstructure of shale derived from TESCAN integrated minerals analyzer (TIMA) digital images. Mechanical parameters of shale minerals determined by nanoindentation served as input material properties for AGBMs. Finally, numerical simulations of uniaxial compression tests were conducted to investigate the impact of mineral softening on the macroscopic Young's modulus and uniaxial compressive strength (UCS) of shale. The results present direct evidence of shale mineral softening during CO₂-water–rock interaction and explore its influence on the upscale mechanical properties of shale. This paper offers a microscopic perspective for comprehending CO₂-water-shale interactions and contributes to the development of a cross-scale mechanical model for shale.