Impact of Wettability on CO2 Plume Migration and Trapping: Sand-Tank Experimentation and Modeling

Abstract The wettability of rocks at geologic CO2 storage sites is often assumed to be strongly water-wet; however, it remains one of the uncertain factors affecting CO2 plume migration. Rock compositions can vary widely, leading to different wettability states within a petrophysically and geologically heterogeneous aquifer. This study provides a comprehensive analysis of how wettability states influence CO2 plume migration and trapping through meter-scale sand-tank experiments. The sand-tank experiments utilized two types of synthetic sands—fine and coarse—to represent the caprock and storage media, respectively. Soltrol 220 and a glycerol-brine mixture served as analog fluids that mimic the density, viscosity ratios, and interfacial tension of the in-situ CO2-brine system. The entire sand tank was coated with stearic acid to create a less water-wet condition (Experiment 1) and with silica nanoparticles to produce a more water-wet condition (Experiment 2). The relative permeability and capillary pressure curves of the fine and coarse sands under these two wetting conditions were determined through numerical history matching of the experimental data. The two sand-tank experiments revealed notable differences in structural trapping, hysteresis trapping, and the phase distributions of the resident glycerol/brine mixture and the injected Soltrol 220. For instance, the structural trapping of Soltrol 220 increased by 12% due to the higher capillary entry pressure, PCe, for the caprock in Experiment 2. Additionally, Experiment 2 showed a greater degree of hysteresis in the relative permeability, resulting in 48% more non-wetting phase entrapment. This enhanced hysteresis immobilized the non-wetting phase, limiting the column height below the caprock and preventing capillary sealing failure. Phase-saturation distributions in the sand-tank experiments highlighted the potential impact of wettability on both structural and hysteresis trapping, confirmed through history-matched simulations. However, the relative importance of these two trapping mechanisms under varying wetting conditions still needs to be evaluated for more realistic petrophysical and geological scenarios.