The role of formation water evaporation in fluid flow processes within porous media for CO2 saline aquifer storage

Formation water evaporation has received limited attention in the context of CO 2 saline aquifer storage. To address this gap, a CO 2 –H 2 O phase equilibrium model was developed and validated to characterize subsurface water evaporation behavior. The results reveal that the presence of gaseous or supercritical CO 2 is a prerequisite for effective water evaporation, which is primarily governed by thermodynamic conditions and phase transitions. While rising temperature consistently promotes evaporation, the influence of pressure exhibits a turning trend–initially suppressing, then enhancing it. Core–scale simulations reveal that formation water migration is governed by two coupled mechanisms: displacement and evaporation. Formation water evaporation eliminates the constraint of irreducible water saturation, thereby expanding the effective pore space for fluid flow. To overcome the limitations of conventional relative permeability curves that ignore evaporation effects, a correction method is proposed to improve the accuracy of gas–liquid flow simulation. Field–scale simulation results demonstrate that evaporation is most pronounced in near–wellbore region, extending from several to dozens of meters due to cumulative effects. This localized phenomenon is governed by high pore–volume displacement and low water vapor saturation. Although salt precipitation induced by evaporation can reduce permeability, this adverse effect is generally outweighed by the enhancement resulting from the reduction in irreducible water saturation. Overall, evaporation influences fluid flow primarily through four mechanisms: reducing irreducible water mole fraction, enhancing gas–phase flow capacity, lowering CO 2 injection pressure, and improving effective pore–throat radius for fluid flow. Collectively, formation water evaporation exerts a predominantly positive effect on CO 2 saline aquifer storage. • The presence of gaseous or supercritical CO 2 is a prerequisite for H 2 O evaporation. • Water evaporation is governed by thermal conditions and H 2 O/CO 2 phase transitions. • Water migration is driven by two coupled mechanisms: displacement and evaporation. • Evaporation is a localized phenomenon, most pronounced in the near wellbore region. • Decline in irreducible water can outweigh the adverse effect of salt precipitation.