Microscopic investigation of chemical and physical alteration behaviors in sandstone minerals induced by CO-HO-rock interactions during CO saline aquifer storage.

CO saline aquifer storage represents a promising strategy for mitigating the environmental impact of greenhouse gas emissions. However, the long-term effects of CO dissolved in formation water on rock minerals remain insufficiently understood. This study utilizes cast thin section analysis, scanning electron microscopy, and energy dispersive spectrometry techniques to perform a comprehensive microscopic investigation on this issue. Experimental results from sandstone core samples drilled from the Ordos pilot field reveal that feldspar minerals predominantly undergo geochemical dissolution, while quartz and clay minerals primarily exhibit physical alterations. Feldspar minerals, including albite, potassium feldspar, and anorthite, exhibit significant geochemical dissolution, characterized by cleavage plane dissolution, swelling, selective dissolution, and in-situ accumulation of dissolution products. This process leads to the formation of secondary minerals such as quartz, kaolinite, and illite, along with various microscopic structures like vugs, pits, and filamentous remnants. Alterations in quartz include the formation of stress-induced microfractures, the attachment of mineral clasts, the precipitation of geochemical reaction products, and pore blockage. In clay minerals, the formation, closure, interconnection, and reconfiguration of microfractures are evident characteristics, particularly at the nanoscale. The products of CO-HO-rock interactions typically comprise a complex mixture of physical and chemical products, marked by intricate elemental compositions and diverse structural forms, including blocky, granular, powdery, filamentous, and floc-like structures. This work reveals the distinctive chemical dissolution mechanisms of sandstone with complex mineral compositions, clarifies the CO-induced physical alteration behavior of multiple minerals and identifies the multi-scale migration mechanisms of products generated by CO-HO-rock interactions. Moreover, these interactions have a dual impact: they enhance the porosity and permeability, while also potentially compromising the structural integrity of the rock and formation. This research provides a foundation for assessing the impact of CO storage on fluid flow and evaluating environmental safety concerns, such as CO leakage and geological subsidence.