Impact of Wettability on CO2 Mineral Trapping in Carbonate Saline Aquifers: A Reactive Transport Simulation Study

Long-term containment of CO2 in geological formations depends on both physical and chemical trapping mechanisms. Although capillary and solubility trapping have been widely studied, the role of reservoir wettability in governing geochemical interactions remains poorly understood, particularly in reactive carbonate systems. This study investigates how contrasting wetting states influence multiphase flow and mineralization in carbonate saline aquifers. Reactive transport simulations were conducted using a compositional simulator under water- and CO2-wet conditions over a 60-year period. The model incorporates hysteresis in relative permeability, capillary pressure variation, and calcite reaction kinetics to evaluate the evolution of capillary, solubility, and mineral trapping mechanisms. Results show that wettability strongly affects both phase distribution and geochemical reactivity. Under CO2-wet conditions, mineral trapping more than doubled compared to the water-wet case (6.5% vs. 2.8%) due to enhanced gas–rock contact and sustained local acidification. Solubility trapping also increased markedly (38.3% vs. 20.9%), facilitated by continuous CO2 pathways that improved convective mass transfer. However, capillary trapping was significantly lower, resulting in reduced total retention (63.5%) compared to the water-wet scenario (84.9%), where capillary forces immobilized more CO2 but restricted mineralization. This analysis demonstrates that wettability directly influences geochemical reactions by controlling CO2 access to mineral surfaces and shaping local pH conditions. The findings suggest that selectively adjusting wettability, depending on reservoir lithology and storage goals, may enhance long-term CO2 immobilization through mineral trapping without compromising containment.