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ABSTRACT:This study investigates the distinction between unreacted shale samples and those exposed to CO2-rich brine under elevated temperature (100°C) and pressure (1800 psi) conditions over 28 days. Samples underwent scratch testing under constant loading to ensure independent penetration depth, circumventing variability associated with load-dependent outcomes prevalent in progressive loading methodologies. Vertical hardness profiles revealed significant variations between reacted and unreacted regions, influenced by differential dissolution and precipitation characteristics, while horizontal hardness provided limited insights, particularly in the reacted region where higher tangential forces and deeper scratches indicated greater material compressibility. Distinct scratch path variations were observed, with fractures absent in the ductile reacted region at lower testing forces. The shale samples were sourced from the Eagle Ford Formation, providing insights into the mechanical response of carbonate-rich shale rocks in extreme environments. This research enhances understanding of shale's mechanical properties and material responses under diverse operational conditions, elucidating interactions with influential environmental factors, particularly in CO2-exposed scenarios. Conducted at a microscale level, this study offers detailed insights into material behavior, crucial for predicting long-term stability of geostructures exposed to reactive brine and potential CO2 leakage in subsurface reservoirs. 1. INTRODUCTIONThe investigation into chemical interactions between carbonate rocks and acidic brine is cruical for understanding complex mechanical and microstructural transformations essential for applications like geostructure stability, CO2 storage, and energy exploitation. Under elevated pressure and temperature conditions, the equilibrium between injected fluids and rocks undergoes alterations, leading to geochemical responses, especially with the presence of CO2 as a supercritical phase or in aqueous form (Prakash et al. 2023a; Prakash et al. 2022). In this context, investigating fracture properties becomes essential, aiming to comprehend the development and propagation of fractures within reacted formations to evaluate structural integrity and potential pathways for fluid migration.Prior geochemical investigations have explored the localized repercussions of CO2 attacks on rock permeability, shedding light on alterations attributed to carbonate precipitation sealing fractures and pores or the dissolution of diverse minerals (Burnside et al. 2013; Minardi et al. 2021). Shale rocks exposed to acidic brine predominantly undergo carbonate reactions, particularly carbonates dissolution and precipitation (Prakash et al., 2022; Prakash et al. 2023b). Experimental studies on fracture mechanics and mechanical properties have utilized conventional methods such as single edge notched bend, chevron notched beam, three-point bending, and semi-circular bending tests, acknowledging their inherent limitations (Smith & Chowdary, 1975; Bazant and Kazemi, 1990; Helmer et al. 2014; Dubey et al., 2020).