There's a lot of literature on this; Wikipedia has a decent starting point: https://en.wikipedia.org/wiki/Carbon_sequestration#Geologica...

TL;DR: if you compress CO2 to above 73 bar, it becomes supercritical fluid, which has a density almost like water and which does not mix with water. If we inject this fluid into underground saline aquifers, we can trap it indefinitely. Basically exactly the reverse of producing oil and gas. In this case you're transporting and injecting pure CO2, not water with 0.1% - 1% CO2.

It's been done at large scale since the mid-1990s, so we know it's viable. Lots of research and development on details/optimization still to be done, but we know it will work and we know we can scale it up.