Chemistry is another issue with the CO2. Borates are the cheapest (and probably the most commonly used) chemical used to create a higher viscosity gel for fracturing. The technical term would be “cross-linking” the molecules making up the gel. In many cases it is vital to be able to crosslink the gel so that proppant can be carried more effectively, but CO2 changes the pH of the fluid and destroys the crosslink. There are other methods of crosslinking gel (titanates, zirconates), but they can get very expensive and have their own disadvantages. So, in very specialized applications, CO2 can be very effective, but as a general rule, cost and chemistry rule it out.