What stops the CO₂ from coming back to the surface once injected?

The geology is selected to ensure there is a very low permeability (solid) caprock above the injection zone where the CO₂ is stored. CO₂ is unable to pass through the caprock. Above the caprock are additional layers of rock with low permeability, keeping the CO₂ trapped underground. The well itself is constructed with a cement casing around the steel pipe injection line that runs the entire length of the well and is impenetrable by CO₂.