I am not a reservoir engineer, I just drill the holes, so don't quote me too closely.
1/For gas fields, there is no EOR, Just depressurize the formation and let the gas come out. Therefore no CO2 need to be pumped into a gas field. 2/Water wet formations, then water is normally considered the preferred pressure maintenance fluid of choice, where as oil wet formations natural gas (hydrocarbon) is the preferred. These are what is called secondary methods of production. 3/CO2 is considered a tertiary method. CO2 under pressure will be a liquid and has the ability to penetrate smaller pore spaces than water and displace greater quantities of oil than water, it also lowers the oil viscosity to allow it to flow more freely but unfortunately lowers the pH, which exposes the tubing to corrosion. CO2, is not widely used world wide due to mainly lack of availability. Linc with UCG has the potential to change this situation on a fairly wide scale.
I may not have answered your question directly, as I said I am not expert in this field, but I hope it has helped some how. Here is a little reading for you. Good Luck
