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Abstract: The onshore Barendrecht CO2 storage project in the densely populated West of the Netherlands is designed to prevent CO2 to migrate out of the deep underground through geological formations and outside of wells. Achieving maximum safe containment is facilitated by selecting depleted gas fields as CO2 storages having stored natural gas over millions of years. This also allows choosing a final storage pressure that is lower than the ambient natural pressure in the surrounding rock formations. The inverse pressure gradient prohibits potential outflow from the storage reservoir. Additional proven safety barriers are the existence of sealing caprocks and large saline aquifers that overlie the storage reservoir. These saline aquifers alone have the capacity to sequester multitudes of the CO2 volumes that are planned to be stored in the depleted gas reservoirs. It is widely accepted that wells hold the main potential of forming a migration path. In the case of Barendrecht this would mainly mean potential migration of reservoir fluids of overlying formations into the CO2 storage reservoir as pressure equalization–returning to a natural equilibrium as existed prior to start of gas production without any overpressures–is not expected to occur before hundreds to thousands of years. Nevertheless assessing the evolution of current and future well integrity with CO2 injection is crucial for ensuring containment of the injected CO2 during injection and in the long term. The Barendrecht fields have respectively two and three drilled and completed wells penetrating the CO2 storage reservoir. This paper describes the most relevant subsurface related highlights from various studies that were carried out in connection with the Barendrecht CO2 storage project. [Copyright &y& Elsevier]