Effect of formation brine on interfacial interaction: Implications for CO2 storage

Carbon dioxide (CO2) geo-storage in deep saline aquifers is a promising technique to mitigate anthropogenic emissions and achieve net-zero targets. The storage capacity and containment security of CO2 in subsurface systems are influenced by a range of processes, including geochemical and interfacial interactions between rock/CO2/brine and CO2/brine systems. Specifically, CO2/brine interfacial tension (IFT) directly affects capillary sealing efficiency and CO2 storage volume. Accurate characterization of IFT under realistic subsurface conditions, i.e., pressure, temperature, salinity, and formation brine composition, is thus vital to predict storage capacity and ensure containment security. Here we report new data sets of CO2/brine IFT obtained by the pendant drop method under a pressure range of 0-20 MPa at three different temperatures 298 K, 323 K and 343 K, and for three different solutions: fresh water, 21.4 wt% NaCl brine and 21.4 wt% formation brine comprising of mixed salt from a real field in the Middle East. Moreover, a detailed analysis is performed for the factors influencing IFT. Importantly, we also provide insights into the key experimental artifacts pertinent to the IFT phenomena e.g., the impact of CO2/water equilibrium on IFT. For all systems, IFT exhibits a monotonically decreasing trend with pressure until it reaches a plateau at similar to 12 MPa, after which it stabilizes upon a critical pressure called angular point. The system IFT vs. temperature trend exhibits a monotonic increase throughout the studied experimental matrix. The lowest recorded IFT is 22.8 mN/m at 20 MPa and 298 K while the highest recorded IFT is 78.92 mN/m at 0.1 MPa and 298 K. Similarly, IFT also increases steadily when formation brine contains monovalent and divalent ions, while bivalent cations have a greater influence on IFT compared to monovalent cations. Notably, at the same salinity, temperature and pressure conditions, the CO2/brine systems IFT, on average, is greater for formation brine than synthetic NaCl brine. Importantly, we also present the effects of mixed salts on CO2 saturation and the associated storage capacity using core flooding experiments. It was found that CO2 saturation records 38 % in the NaCl saturated core while it reaches 22 % in the formation brine saturated core. The results of this study thus provide new datasets and insights into the interfacial phenomena relevant to subsurface CO2 storage applications.