On the Importance of Capillary-Pressure Gradient on Water Backflow during Subsurface CO₂ Storage

Salt precipitation near the injection well is a major industrial challenge during CO₂ injection into saline aquifers, as it can severely reduce injectivity. This phenomenon has been attributed to strong capillary-driven backflow of brine toward the wellbore during the dry-out process. However, a widespread misconception holds that the absolute magnitude of capillary pressure controls the intensity of this backflow and the resulting salt accumulation. Using radial two-phase flow simulations with six bounded capillary-pressure–saturation functions, we demonstrate that it is the capillary-pressure gradient (∂Pc/∂x = ∂Pc/∂Sw · ∂Sw/∂x), not its absolute value, that governs water redistribution and backflow strength. Steeper gradients generate significantly stronger capillary forces, reverse brine flow near the dry-out front, enlarge the unsaturated zone, and slow front propagation. Critically, the most pronounced backflow and greatest potential for salt precipitation occur in systems with the largest |∂Pc/∂Sw|, even when capillary-pressure magnitudes differ substantially. These findings reveal that sharp contrasts in capillary-pressure gradients, caused by lithofacies transitions, layering, or material interfaces, can drive intense capillary fluxes across boundaries. Accurate characterization of gradient-dependent capillary behavior is therefore essential for reliably predicting injectivity impairment and salt-precipitation risks in geological CO₂storage operations.