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

Reza Khoramian, Ibraheem Salaudeen, Peyman Pourafshary, Masoud Riazi, Riyaz Kharrat

2025

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DOI 10.2118/230344-ms
Data confidence 70 medium
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Abstract

Abstract This study quantifies the impacts of wettability on CO2 trapping mechanisms in deep saline aquifers by comparing water-wet and weakly water-wet conditions. Wettability alteration, often triggered by prolonged CO2 exposure and organic acid generation, is widely observed in storage sites and can shift rock surfaces from strongly water-wet to intermediate states. Understanding how such transitions affect fluid behavior and geochemical trapping is essential for long-term storage security. A fine-scale heterogeneous model was developed using a Gaussian distribution and simulated over 200 years, including 5 years of CO2 injection followed by 195 years of post-injection monitoring. Grid sensitivity analysis was conducted to minimize numerical dispersion. Two wettability states were defined using distinct relative permeability and capillary pressure curves. Capillary effects were examined by calculating critical capillary numbers (NCap) that govern the shift between capillary and viscous regimes. Reactive transport modeling 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 capillary pressure 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 and promoted lateral spreading, especially under water-wet conditions. In weakly water-wet systems, enhanced CO2 mobility facilitated greater dissolution and mineralization by increasing brine contact and lowering pH, thereby mobilizing ions such as Ca2+. Optimal trapping occurred below critical NCap thresholds (~6.31×10-7 for water-wet; ~3.05×10-6 for weakly water-wet), highlighting the importance of rate control. This study demonstrates how wettability shifts influence the balance among trapping mechanisms over time. Recognizing this shift in dominant trapping mechanisms enables better-informed decisions for reservoir management and long-term carbon storage planning. Future research could explore applying these findings to various reservoir types, broadening their impact.