Numerical simulation of CO 2 leakage in a shallow subsurface layer from a CO 2 geological storage site

In an effort to detect and quantify potential CO 2 leakage from geologic storage sites, this work investigates the migration process of CO 2 leaked in a shallow subsurface associated with the CO 2 sequestration project at Shaanxi, China. A shallow subsurface model with aquifer and soil layers was established, and the sensitivities of leakage rate and geological conditions were analysed. The results show that the distribution morphology of the CO 2 plume is generally mushroom-shaped when the leaked CO 2 enters the formation as a point source. Maximum leakage occurs at the soil surface due to the comprehensive effects of pressure, concentration, and density differences between CO 2 and soil gas. The mass fraction of CO 2 in gas is approximately 0.38–0.48 near the surface leak point, and leaked CO 2 moves laterally along the bottom of the soil vadose zone. The leakage rate of CO 2 significantly affects the pressure build-up as observed for an increase in the CO 2 leakage rate from 0.0002 to 0.02 kg/s. In addition to the loess beam area, the migration process of CO 2 leakage in the valley terrace area was also studied. The results show that the CO 2 concentration in the leakage centre area of the valley terrace is not much different from that in the loess beam. Moreover, the surface leakage range in the valley terrace is much smaller than that in the loess beam, which is because of the differences in the formation composition and thickness. Heterogeneity has limited effect on leakage.