This paper presents the numerical modeling study to investigate the leakage possibility of CO 2 in a deep saline aquifer where caprock is discontinuous or does not exist. As a result of simulations for 30 years of injection and 5000 years of monitoring, injected CO 2 rises by buoyancy. However, it does not reach the surface, and the height of CO 2 moving upward is not high from the effect of residual and solubility trap mechanisms. Meanwhile, when the vertical permeability is higher than the actual, CO 2 moves highly upward and the phase of CO 2 changes into a gas phase. However, it is shown that CO 2 does not affect the surface during the monitoring period. When CO 2 is injected into a deeper aquifer, it is more effective and stable for CO 2 sequestration. Finally, CO 2 may leak faster when there are flow networks composed of faults and porous mediums. For this reason, we should pay attention to the formation lies on the target aquifer to prevent the possible leakage of injected CO 2 .