Influence of residual oil saturation on wormhole propagation during carbonated seawater injection under supercritical CO₂ conditions

Carbonated water injection (CWI) under supercritical CO₂ (ScCO₂) conditions has gained attention as a viable method for improving CO₂ sequestration efficiency. In particular, deep saline aquifers and depleted hydrocarbon reservoirs are considered key candidates for long-term CO₂ storage due to their availability and proven containment capacity. While previous studies have established wormhole formation during carbonated brine injection into carbonate rocks, the reactive behavior of carbonated seawater in depleted reservoir conditions remained unexplored. To address this research gap, four Indiana limestone core samples were used: two representing deep saline aquifer and the other two conditioned with residual oil saturation (ROS: 7.3 % and 10.2 %) to mimic depleted reservoir conditions. Core flooding experiments were conducted at a flow rate 1 mL/min under ScCO₂ conditions (60 °C and 2000 psi) using carbonated Arabian Gulf seawater for all saturated core samples. The results were evaluated using pressure monitoring, porosity measurements, micro-CT imaging, and mechanical testing (Young's modulus). Wormhole structures were observed in all samples. However, ROS cores exhibited delayed and localized wormhole development with slightly higher pore volume to breakthrough (PVBT) of 36.9 and 33.1 PV, compared to 27.4 and 30.1 PV in non-ROS cores. Wormhole volume fractions derived from micro-CT imaging closely matched porosity changes in non-ROS (0.85 % and 0.62 %) cores but diverged in ROS cores (0.60 % and 0.71 %), highlighting the limitations of bulk porosity in capturing localized dissolution. Post-flooding Young's modulus values decreased in all samples with ROS cores showing greater stiffness loss (27.7 % and 28.6 %) than non-ROS cores (20.7 % and 18.5 %), indicating mechanical weakening of the carbonate matrix. Overall, the results confirm that carbonated seawater under ScCO₂ conditions can induce reactive dissolution and structural degradation in both fresh and depleted carbonate systems, with implications for CO₂ injectivity, storage efficiency, and reservoir stability.