Advanced modeling of enhanced CO₂ dissolution trapping in saline aquifers

Abstract

A consistent thermodynamic model based on a combination of the Peng-Robinson equation of state for gas components with an activity model for the aqueous phase is utilized in this study to accurately evaluate the performance of geologic CO₂ storage in saline aquifers. To account for thermal effects, the phase enthalpy and conductivity are derived with consideration of CO₂ dissolution in brines. Several conceptual numerical setups with variations in parameters and physics are constructed to investigate the enhanced CO₂ dissolution in saline aquifers. In addition, a realistic geological structure is considered to evaluate the potential for storing CO₂ at the field scale. Our simulation results show that the presence of a capillary transition zone (CTZ) can significantly reduce the onset time and enhance the dissolution rate. The temperature has a crucial effect on the dissolution trapping compared to isothermal assumptions. Thermal conduction and convection suspend the onset of mass convection, while the final mass flux is augmented. Heat conduction smears out the temperature difference between rocks and fluids, thus suppressing thermal fingering. We demonstrate that the dissolution trapping in realistic 3D aquifers can be significantly different from their 2D analogues. The field-scale simulation results show that the dissolution rate increases sharply once the convection dominates the CO₂ migration which corresponds to a reduction of residual trapping. Besides, a high-resolution 3D model is required for accurately resolving the enhanced CO₂ dissolution in realistic heterogeneous reservoirs. An efficient multi-scale framework for an accurate evaluation of CO₂ trapping is proposed for this purpose.