The purpose of this study was to evaluate the risk of CO2 leakage from a storage site by 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 phases of CO2 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 1,000 years and much higher than safety index of 0.45 for the up-dip well. More mobile supercritical CO2 had to be sealed by a caprock in the case of up-dip well injection. Injecting CO2 by a down-dip well is a better engineering strategy because the safety index is higher.