<p>Acid fracturing is a widely used technique to enhance oil recovery in carbonate reservoirs, but its effectiveness depends on several factors, including the presence of iron oxides. Magnetite, commonly found in sedimentary rocks – including carbonates that comprise over 50% of global petroleum systems – can significantly influence catalytic activity, mineral leaching, and dissolution during acid–rock interactions. This study investigates the surface properties and catalytic behavior of magnetite particles ranging from nano - to micrometric sizes under acid injection conditions. Two catalysts were evaluated: (i) homogeneous FeSO₄ and (ii) heterogeneous Fe₃O₄ with variable grain sizes, both characterized through a suite of physicochemical analyses (Scanning and Transmission Electron Microscopes, X-Ray Diffraction, Rietveld refinement and N₂ adsorption-desorption isotherms, for surface area and porous nature). Leaching experiments were performed using synthetic seawater at pH 2.36 and 8.40, focusing on the degradation of naphthalene – a representative hydrocarbon in oil reservoirs – and of H₂O₂. Results demonstrate that while the homogeneous catalyst achieved the highest efficiency in naphthalene degradation, heterogeneous magnetite also catalyzed significant substrate degradation – particularly the nanometric Magtt S, which showed enhanced activity due to greater surface area and iron leaching. Magnetite clustering links specific surface area to cationic site availability, both crucial for catalytic performance. The acid-induced release of Fe²⁺ further enhanced the generation of hydroxyl radicals, promoting more efficient hydrocarbon degradation. These findings highlight that the presence of magnetite in carbonate reservoirs undergoing acid fracturing can promote unintended hydrocarbon decomposition, alter reservoir geochemistry, and compromise mechanical stability through oxide dissolution and porosity changes.</p>