Chemo-Mechanical Degradation Mechanisms of Shale Minerals Induced by CO2–Water–Rock Interactions: A Targeted Nanoindentation Study under Different CO2 Partial Pressures

Abstract To investigate the chemo-mechanically coupled degradation mechanisms of individual mineral phases during CO2–water–shale interactions, shale samples from the deep-marine Wufeng–Longmaxi Formation in the Sichuan Basin were selected. A series of multiscale characterization techniques, including X-ray diffraction (XRD), inductively coupled plasma optical emission spectrometry (ICP-OES), scanning electron microscopy coupled with energy-dispersive spectroscopy (SEM-EDS), and targeted nanoindentation tests, were conducted on shale samples subjected to high-temperature and high-pressure static immersion under different CO2 partial pressures. The relative mineral content variations, microstructural morphology evolution, and degradation behaviors of the micromechanical properties of different minerals were comprehensively analyzed. The results show that the acidic environment in the CO2–water system significantly promoted the dissolution of calcite, dolomite, and certain feldspar minerals, leading to pronounced increases in the concentrations of Ca2+, Mg2+, Na+, and K+ ions, whereas quartz exhibited relatively high chemical stability. SEM observations revealed the development of evident dissolution pores and honeycomb-like structures on the surfaces of carbonate minerals, while the interlayer contacts of clay minerals were progressively damaged, resulting in an overall loosening of the shale matrix. Nanoindentation results demonstrated that increasing CO2 partial pressure progressively reduced the Young’s modulus and hardness of all mineral phases, with carbonate and clay minerals undergoing the most pronounced softening. The H–E evolutionary trajectories further indicated a gradual transition of the shale framework from brittle to more ductile deformation characteristics. Moreover, mineral-specific empirical exponential functions were established to describe the nonlinear pressure-dependent changes in Young’s modulus and hardness within the investigated CO2 partial-pressure range. The fitted degradation amplitude and characteristic pressure scale provide a new comparative framework for quantifying differences in pressure sensitivity among individual shale minerals. The results establish a mineral-specific empirical exponential framework for nonlinear micromechanical responses under a CO2 partial-pressure gradient, with implications for shale gas recovery and CO2 storage.