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Shale unconventional reservoirs are currently and expected to remain substantial fossil fuel resources in the future. As CO₂ is being considered to enhance oil recovery and for storage purposes in unconventional reservoirs, it is unclear how the shale matrix and fractures will react with CO₂ and water during these efforts. Here, we examined the Utica Shale and its reactivity with CO₂ and water using scanning electron microscopy, N₂ and CO₂ sorption isotherms, mercury intrusion porosimetry, and X-ray scattering methods. During CO₂ exposure, the presence of water can inhibit CO₂ migration into the shale matrix, promote carbonate dissolution, and dramatically change the pore scale variability by opening and closing pore networks over the macro- to nano-scale range. These alterations in the shale matrix could impact flow pathways and ultimately, oil recovery factors and carbon storage potential.