This study evaluates the risk for CO₂ leakage from a storage site using a risk assessment criterion, the safety index, which considers the contributions of residual gas, solubility, ionic, and mineral trapping mechanisms. We present a case of CO₂ storage in a deep saline aquifer in Yutengping (YTP) sandstone, Tiehchanshan (TCS) field, Taiwan. The numerical method was used to estimate the amount of different CO₂ phases sequestered by the various trapping mechanisms. The CO₂ injection rate was 1 million tons per year for 20 years. The total simulation time was 1000 years. In the case of down-dip well injection, the safety index was 0.77 at the storage time of 1000 years and much higher than the safety index of 0.45 for the up-dip well. More mobile supercritical CO₂ had to be sealed using a caprock in the up-dip well injection case. Injecting CO₂ using a down-dip well is a better engineering strategy because the safety index is higher.