You should know that injecting old oil wells with CO2 helps them produce more oil. The CO2 becomes carbonic acid under pressure, which then dissolves pores in the rock wider, allowing more oil and gas to escape.

Also, it looks pretty doubtful that CO2 will stay in gas wells for a long time. Eventually it'll find some fissure and due to the acidity any crack will be eroded wider till all the CO2 comes out like a fizzy drink.

But don't worry. Oil and gas companies will happily collect government subsidies for pumping CO2 underground to get more oil out to sell, in the knowledge that when the gas escapes in a few decades they can act all surprised...