Numerical simulation of CO2 leakage in a shallow subsurface layer from a CO2 geological storage site

Abstract

In an effort to detect and quantify potential CO2 leakage from geologic storage sites, this work investigates the migration process of CO2 leaked in a shallow subsurface associated with the CO2 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 CO2 plume is generally mushroom-shaped when the leaked CO2 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 CO2 and soil gas. The mass fraction of CO2 in gas is approximately 0.38–0.48 near the surface leak point, and leaked CO2 moves laterally along the bottom of the soil vadose zone. The leakage rate of CO2 significantly affects the pressure build-up as observed for an increase in the CO2 leakage rate from 0.0002 to 0.02 kg/s. In addition to the loess beam area, the migration process of CO2 leakage in the valley terrace area was also studied. The results show that the CO2 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.