CO₂ geological storage in subsurface aquifers as a function of brine salinity: A field-scale numerical investigation

Subsurface aquifers demonstrate a broad range of salinities and salt compositions, affecting the physicochemical characteristics of the CO₂/brine/rock systems, which in turn, influence the CO₂ trapping of the aquifer formation. Available simulation studies generally focus on single NaCl systems and do not adequately account for the variations in salt types. In this study, the impact of salinity and salt type on several key parameters, including wettability, interfacial tension, brine properties, diffusion, and capillary pressure, were considered within the context of underground CO₂ storage. We conducted pore network modeling to assess the impact of salinity (i.e., pure water, 1 M (molality), 3 M, and 5 M) and salt type (i.e., NaCl, CaCl₂, and MgCl₂) on the residual trapping behaviors. Subsequently, these findings were utilized in field-scale simulations to assess the influence of various salinities and salt types on the CO₂ trapping capacity in a single salt brine system. The pore network modeling results showed that residual CO₂ saturation decreases in higher salinity conditions, with the lowest value in MgCl₂ brine system. In field-scale simulations incorporating residual trapping alone, the residual trapping capacity decreases in higher salinity NaCl brine systems. However, in high salinity MgCl₂ brine, increased viscosity and density lead to a widespread CO₂ plume, leading to an increased residual trapping capacity. This plume spread difference also influences the amount of dissolved CO₂ in scenarios considering dissolution trapping alone. When considering both trapping mechanisms, our observations indicate that a decrease in dissolution trapping under high salinity and divalent cations conditions leads to enhanced residual trapping (e.g., ∼51.56% for 5 M MgCl₂) - suggesting an interplay or codependency between these two mechanisms. The influences of diffusion and capillary pressure on the CO₂ geo-storage trapping capacity are also investigated. Overall, an aquifer containing lower salinity brine composed of monovalent ions exhibits lower residual trapping, greater dissolution trapping, and lower mobile CO₂. Especially, the pure water system exhibits the lowest percentage of mobile CO₂ (∼13.72%). We also highlight that this impact is not governed by the corresponding wettability shift alone; rather, the physical properties of native brine (i.e., viscosity and density) play a part too. The findings help evaluate the CO₂ storage potential of aquifers and thus assist in de-risking large-scale storage projects.