**5.1 CO2 EGS**

The use of supercritical CO2 as a heat transfer fluid has been first proposed as an alternative to water for both reservoir creation and heat extraction in EGS (Brown, 2000). Numerical simulations have shown that under expected EGS operating conditions, CO2 could achieve more efficient heat extraction performance than water (Magliocco et al., 2011). CO2 has numerous advantages for EGS: greater power output, minimized parasitic losses from pumping and cooling, carbon sequestration and minimized water use. Magliocco et al. (2011) have performed laboratory tests of CO2 injection while Plaskina et al. (2011) made a numerical simulation study of effects of CO2 injection to provide a new method to improve heat recovery from the geopressured aquifers by combining the effects of natural and forced convection.

## **5.2** *In situ* **formation of calcium carbonate as a diversion agent**

During stimulation of EGS wells, water is injected in order to open sealed fractures through shear failure. When the fractures are open, the stimulation fluid flows into them and becomes unavailable for stimulation elsewhere. Fluid diversion agents can serve to temporarily plug newly stimulated fractures in order to make the injected water available to stimulate new fractures (e.g. Petty et al., 2011). The diversion agent is subsequently removed to allow flow from those previously sealed fractures. As demonstrated by Ledésert et al. (2009) and Hébert et al., (2010), calcite is found naturally in fractures of EGS reservoirs and prevents the fluid from flowing into fractures. The in situ precipitation of calcium carbonate was studied by Rose et al. (2010) for use as a diversion agent in EGS.

#### **5.3 Use of oil and gas reservoirs for EGS purposes**

A lot of oil and gas reservoirs have been or will be abandoned in petroleum industry. According to Li and Zhang (2008) these oil and gas reservoirs might be transferred into exceptional enhanced geothermal reservoirs with very high temperatures. Air may be injected in these abandoned hydrocarbon reservoirs and in-situ combustion will occur through oxidization. The efficiency of power generation using the fluids from in-situ combustion reservoirs might be much higher than that obtained by using hot fluids coproduced from oil and gas reservoirs because of the high temperature.
