The process of capturing carbon dioxide (CO2) and injecting it into deep saline aquifers is becoming an important method to reduce future atmospheric emissions of CO2. Key challenges facing carbon capture and storage (CCS) are the storage reservoir's size and safety. The storage size can be addressed by focusing on large saline aquifer reservoirs. The safety concern may be lower if CO2 is injected into depleted hydrocarbon reservoirs because their cap-rock integrity is already proven, but such capping systems are generally potentially compromised by poorly cemented abandoned wells, and compared to saline aquifers, their storage size is small. Therefore, the long-term focus of CCS is on saline aquifers. To reduce the corresponding risks, comprehensive long-term monitoring is inevitable.