The European Union's Soultz-sous-Fôrets project in Alsace, France, the most advanced such project worldwide, has taken 20 years to reach just 1.5 megawatts of power generation (enough to supply roughly 1,500 homes). And the process has antagonized nearby communities because of the small earthquakes sparked by the aggressive fracturing required.
In 2000, Los Alamos National Laboratory physicist Donald Brown proposed replacing water with supercritical carbon dioxide, a pressurized form that is part gas, part liquid. Supercritical CO2 is less viscous than water and thus should flow more freely through rock. Brown noted that a siphoning effect should help cycle the carbon dioxide, thanks to the density difference between the supercritical CO2 pumped down and the hotter gas coming up, slashing power losses from pumping fluid. Plus, Brown argued, instead of using precious fresh water resources, a carbon dioxide-based project could sequester the equivalent of 70 years worth of CO2 emissions from a 500 megawatt coal power plant.
It does sound like it could be better than injecting water, but it still needs to be proven in the field, and sadly, geothermal doesn't move very fast. I used to think that it would scale up rapidly, but the more I learn about it, the more complicated things seem.
There's also always the danger of having people think "Oh, we'll be able to sequester the CO2 at some point, so we can keep burning coal in the meantime." This is the wrong way to think about it because current coal plants cannot capture carbon (or at least, not easily) and they might not be anywhere near the proper types of underground rock formations that would be needed to sequester the CO2 (and pipelines are expensive). Our best bet is still to phase out coal as fast as possible.
