Fossil fuels account for more than 90% of the world total energy consumption. The emission of CO2 to the atmosphere can be reduced by the development and implementation of carbon capture and storage technologies. The geological formations considered for CO2 storage are saline aquifers, depleted and semidepleted hydrocarbon reservoossil fuels account for more than 90% of the world total energy consumption. The emission of CO2 to the atmosphere can be reduced by the development and implementation of carbon capture and storage technologies. The geological formations considered for CO2 storage are saline aquifers, depleted and semidepleted hydrocarbon reservoirs, and unminable coal seams. The efficient short-term injection and the stable long-term geological storage of carbon dioxide are affected by complex hydro-chemo-mechanical interactions that take place in the formation, including water acidification, mineral dissolution, and stress and volume changes. Positive feedback mechanisms may lead to runaway effects. These hydro-chemo-mechanical coupled processes and emergent phenomena may hinder the storativity of injected carbon dioxide. Technological developments such as adequate geophysical tools for injection and reservoir monitoring, are needed for the safe geo-storage of CO2.