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.