Mechanical Properties of Sandstones Damaged by CO ₂ Reactions and Flow Characteristics under Complex Mixed-Wettability

The acid production reaction between CO2 and water in rocks changes the pore structure, thereby altering the rock mechanical properties. To ensure the safety of CO2 storage, it is critical to identify the rock damage mechanisms, clarify the variation patterns of mechanical properties under damage conditions, and investigate the flow characteristics of two-phase CO2-water systems under complex mixed-wettability. This study established a constant-temperature and -pressure multiphase CO2-rock immersion experimental system. Additionally, a comprehensive experimental procedure integrating CO2 immersion, CT imaging, and uniaxial compression was designed. A comparative study revealed that reactions involving gaseous CO2 and ScCO2 with water increased the core porosity by 1.2% and 2.9%, respectively. During the immersion period, gaseous CO2 accumulated on the upper section of the rock, causing a sharp increase in the surface porosity of the upper section (Δϕ = 0.9%). By contrast, ScCO2 accumulated on the middle section, causing a substantial increase in the surface porosity of the middle part (Δϕ = 2.8%). After the CO2 saturation in different phase states, the pore connectivity of the core was enhanced. When exposed to gaseous CO2, some isolated micropores became interconnected, thereby increasing connected porosity from 13.0 to 13.9%. When exposed to ScCO2, the connected porosity increased from 11.3 to 11.7%. Under damage conditions, the primary factors contributing to the deterioration of mechanical properties were the expansion of the pore volume and the increase in the number of connected pores. Compared to gaseous CO2, ScCO2 generated stronger carbonic acid upon reaction with water. After the acidification reaction, the number of both interconnected and isolated pores within the core increased, leading to considerable changes in the mechanical properties. Specifically, the compressive strength and elastic modulus decreased by 19.08 and 16.2%, respectively, and the Poisson’s ratio increased by 26.9%. Under the single-phase wettability, the displacement efficiencies of gaseous CO2 and ScCO2 were enhanced under weak and strong wettability conditions, respectively. The displacement efficiencies of gaseous CO2 and ScCO2 under complex mixed-wettability decreased by 5.7 and 15.3%, respectively, compared with those observed under single-phase wettability. Therefore, neglecting the complex mixed-wettability of pore wall surface often leads to the overestimation of displacement efficiency.