Beyond wettability: Flow regime and geological heterogeneity controls on CO2 trapping efficiency and dissolution behavior in saline aquifers

CO2 trapping in saline aquifers is governed by the link of capillary, viscous, and gravitational forces, each modulated by wettability, flow rate, and geological structure. This study employs field-scale simulations to quantify how these parameters jointly control trapping efficiency, pressure evolution, and long-term storage security across a range of dimensionless gravity numbers (NGrav). At low NGrav, dissolution trapping is effective in both wettability states, whereas at high NGrav, water-wet systems surpass weakly water-wet ones due to stronger capillary retention that prolongs CO2–brine contact. Over time, a portion of the capillary-trapped CO2 gradually dissolves, reducing the trapped fraction by 3.2 % (Rt = 0.036 % yr−1) in water-wet and 0.6 % (Rt = 0.007 % yr−1) in weakly water-wet systems, confirming a slow capillarity-driven transformation. Further Simulations incorporating lithological heterogeneity and fault structures reveal that geological compartmentalization changes flow regimes and trapping behavior. Moderate heterogeneity enhances both residual and dissolution trapping, while a tight, low-conductivity (1 %) fault reverses conventional wettability behavior: the water-wet case exhibits higher dissolution at both early and late times. This arises because the fault suppresses cross-fault flux and lowers gas velocity, converting a viscous-dominated regime into a capillary-controlled one. Consequently, the effective critical NGrav for crossover behavior shifts to values exceeding ∼500, showing that structural compartmentalization can override expected wettability trends. Pressure analysis confirmed safe operation below the mechanical limit (0.9 × Pfracture = 6500 psi) with ≥50 % safety headroom at moderate rates. These results establish a mechanistic framework for safe, energy-efficient CO2 storage, directly supporting cleaner production objectives.