We can hope CO2 will be used as fracking fluid in enhanced geothermal systems. Such enhanced geothermal system would need to have DRY hot rock reservoirs to use CO2 as fracking fluid. If there is already salt water in hot rock reservoirs of proposed geothermal system, seawater is the most suitable fracking fluid (I'm also assuming we'll use stainless steel pipes. And of course leave fresh water for farmers for irrigation! CO2 is more likely to remain underground if it is doing something useful for the firm that put it there.
Fracking/hydraulic/heat-transfer fluid in DRY hot rock reservoir would be doing something useful: it will bring geothermal heat to aboveground turbines of an enhanced geothermal system. It takes two closed loops: one, a lower loop that fracking fluid brings heat up from, to an heat exchanger. That heat exchanger transfers heat to a second, closed loop that's filled with a hydrocarbon. The hydrocarbon is similar to the hydrocarbon used in refrigerators and air-conditioners. That energy release would spin a turbine to generate electricity, which in its own turn will then transfer otherwise wasted heat to district heating (in winter) or the compressor of an air-conditioner in summer; or even to store waste heat underground in summer that can be retrieved in winter.
Geothermal systems need to be re-drilled every 6 years and topside generating equipment has an expected service life of 30 years. Therefore, a stack of 5 hot rock reservoirs should last as long as topside equipment. Assume it is reasonable to move a few kilometers along transmission lines to put another field of geothermal hot rock reservoirs next to an old field; then, put a new set of topside equipment above center of a new field. After the 2nd thirty (30) years, would it be more cost-effective to re-drill another stack of 5 hot rock reservoirs under each one already established in the first field, or move a few more kilometers, along a transmission line, to start over?
With expandable well liners, there no longer is a hard limit of 10 kilometers depth. It is supposed to take anywhere from 50 to 300 years for a fallow hot rock reservoir to reheat enough for reuse. Starting, let's say, after 60 years, we get to check the first set of hot rock reservoirs every 30 years to see if it is ready to reuse. Plans have to start somewhere. Why not simply start where we are, do what we can?ï»¿