Link: http://cleantechnica.com/2009/11/16/using-co2-to-extract-geothermal-energy/

As part of developing new energy resources that don’t emit carbon dioxide, the DOE is funding 9 trials that use supercritical CO2 to extract more geothermal energy. The idea started in 2000 at Los Alamos National Laboratory; when physicist Donald Brown thought of pumping geothermal fluid using supercritical CO2 - a pressurized form that is part gas, part liquid; instead of water. Theoretically this should flow more freely through rock than water, because it is less viscous than water. Then, six years later; in modeling the technology Lawrence Berkeley hydro-geologist Karsten Pruess projected that not only should it perform as expected but that it would also yield a 50% hotter geothermal resource.