Are we sure about that ? I must admit I'm not a geologist, but CO2 has a different chemistry as methane, it becomes acid when dissolved in water, how would rocks behave when exposed to acid for thousands of years ? Are the rocks still gas-tight when they have been emptied, which causes movement and potential fractures, or when fracking was used for extraction ? How does it work to pump back CO2 in the same field you extract methane from ? Do we even have enough room to store the massive volumes of CO2 were generating ? An accidental release of such a store of CO2 would be a disaster of enormous proportions.