`Inintially fracking was done with high explosives, with the discovery of vast reserves under inhabited areas, a different method had to be found for these low producing rich fields. Enter Haliburton and their magic elixer . This was a slurry of mostly waste saltwater and sand, but those two would only release about 10-20 % of the trapped product. The secret ingredients have been leaked to be deisel, benzene, other aromatic petrochemicals, industrial waste both hazardous and non hazardous. Thae added punch allows an additional 20-40% recovery. Stean injection has also been used, but it is very costly and is about as effective as the elixer recovery method. CO2 is a very cold liquid, and the oil doesn't flow well when it is cold even under extreme pressure, so I wouldn't see it as efficent as just water and sand, maybe less. Then there is the problem ofsequestering the CO2, by definition you have holes drilled into the field that povide an easy egress. I have lived near storage wells that were over 10,000 feet deep and product was constantly escaping so my vote is find a better way. We already know that the chemicals leak back into the groundwater in a short period of time through cracks and fissures, so a liquid under high pressure seems like trying to fill a bucket with a sieve.