Wettability-Driven Variations in CO2 Trapping: Coupled Impact of CapillaryForces and Reactive Transport in Saline Aquifers

This study quantifies the impacts of wettability on CO2 trapping mechanisms in deep saline aquifers bycomparing water-wet and weakly water-wet conditions. Wettability alteration, often triggered by prolongedCO2 exposure and organic acid generation, is widely observed in storage sites and can shift rock surfacesfrom strongly water-wet to intermediate states. Understanding how such transitions affect fluid behaviorand geochemical trapping is essential for long-term storage security.
A fine-scale heterogeneous model was developed using a Gaussian distribution and simulated over 200years, including 5 years of CO2 injection followed by 195 years of post-injection monitoring. Grid sensitivityanalysis was conducted to minimize numerical dispersion. Two wettability states were defined using distinctrelative permeability and capillary pressure curves. Capillary effects were examined by calculating criticalcapillary numbers (NCap) that govern the shift between capillary and viscous regimes. Reactive transportmodeling in a carbonate reservoir captured the evolution of solubility, residual, and mineral trapping.
Results show that wettability exerts a first-order control on CO2 trapping. Incorporating capillarypressure notably enhanced residual trapping, with water-wet systems achieving up to ~80% immobilization,compared to ~55% in the absence of capillary forces. Capillary pressure restricted vertical migration andpromoted lateral spreading, especially under water-wet conditions. In weakly water-wet systems, enhancedCO2 mobility facilitated greater dissolution and mineralization by increasing brine contact and loweringpH, thereby mobilizing ions such as Ca2+.
Optimal trapping occurred below critical NCap thresholds (~6.31×10-7 for water-wet; ~3.05×10-6 forweakly water-wet), highlighting the importance of rate control. This study demonstrates how wettabilityshifts influence the balance among trapping mechanisms over time. Recognizing this shift in dominanttrapping mechanisms enables better-informed decisions for reservoir management and long-term carbonstorage planning. Future research could explore applying these findings to various reservoir types,broadening their impact.