I think as long as they throw a 10 lb bag of sugar down the hole before they start pumping then you don't have to worry about it accidentally becoming a diet Coke.

Geological reservoirs are thousands metter depth and several dozen of km wide. Pressure is a few MPa, and temperature hundreds of °C. Condition are so extrem that filling them with gaz barely change anything. Especially if they were already filled with gaz dozen years ago. Furthemore, they are not big vacum like most people imagine. It's more like giant spongy rock, like sand. It's not a baloon you inflate or deflate.

CCS facilities are not in competition with forest. It's a complementatry solution. If you manage to capture carbon next to poluting factories, you don't spread Co2 on the atmosphere, waiting it to be captured by a forest the other side of the globe. And they can be powered by solar panels.