That's really not the case. The injected CO2 is under such pressure that it stays liquid. We don't worry about massive oil seepage from kilometers underground, and studies have shown we don't have to worry about CO2 either: https://hub.globalccsinstitute.com/sites/default/files/publi...
E.g.: " The CO2 being injected should be in a “dense phase,” which means that it is put under pressure (compressed) until it turns from a gas into a kind of liquid. Because liquids take up much less space than gases, when large amounts of CO2 are injected, less space will be needed underground if the CO2 is in liquid form. It will mix more easily with water and other compounds, helping to secure the CO2 underground. The storage location must be deep enough in the earth that the pressures of the multiple layers of rock above the storage site keep the CO2 in this dense phase, to help it stay in the secure storage location, and not rise up like a gas. It is possible for CO2 to be injected as a gas, but the storage location would have to have a higher storage capacity since a gas takes up more space than a liquid. • Containment: A storage site must also have a layer of dense, impenetrable rock above it (called caprock) to stop any upward movement of CO2. There is often more than one such layer above the target storage formation that offers protection to keep the CO2 in the storage site. "
And: " The oil reservoirs studied in the WMP are about 1500 meters deep, which is deep enough so that the CO2 remains like a liquid. The reservoirs are made of limestone and a similar rock-type called dolostone. Both rocks contain lots of pore space where the oil has been trapped for millions of years. "