**6. Conclusion**

498 Heat Exchangers – Basics Design Applications

(Wyborn, 2011). The concept of a 25MWe commercial plant is now designed with 3 injection wells and 6 production wells. The ultimate potential is to supply up to 6500 MWe of long-term base-loadpower, equivalent to electrical supply from ~750 MT thermal

Many research teams are currently working on improvement of existing techniques of innovation developments to ensure better production rates and minimized constraints. Among these innovations, the following are particularly promising but of course non

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

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

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

**5. Forthcoming developments and challenges of EGS projects** 

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

was studied by Rose et al. (2010) for use as a diversion agent in EGS.

coproduced from oil and gas reservoirs because of the high temperature.

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

coal (Wyborn, 2011).

exhaustive.

**5.1 CO2 EGS** 

convection.

Enhanced Geothermal Systems experiences at Fenton Hill (USA), Rosemanowes (UK), Hijori (Japan) and Basel (Switzerland) allowed scientists to develop a European thermal pilot-plant producing electricity in Soultz-sous-Forêts (France) since June 2008. This project is the result of 20 years of active research based on geology (petrography, mineralogy, fracture analysis), geochemistry, geophysics (seismic monitoring, well-logging), hydraulics and modelling. Technical improvements were also necessary to allow deep drilling (down to 5000 m) in a hard (granite), highly fractured rock and circulation of water at great depth (between 4500 and 5000 m). The rock behaves as a heat exchanger in which cold water is injected. The water circulates in the re-activated fracture planes where it warms up. It is pumped to the surface and activates a 1.5 MWe geothermal Organic Rankine Cycle power plant that converts the thermal energy into electricity. In such a project challenges are numerous and difficult since the injected water must circulate at great depths between the 3 wells of the triplet with no or little loss and the flow rate and fluid temperature must be and remain high enough to allow production of electricity. Provided careful monitoring of the reservoir during operation, EGS are a sustainable, renewable and clean way to produce electricity. It has been proven that environmental impacts of EGS are lower than those of nuclear or fossil fuel power plants dedicated to the production of electricity. The Soultz EGS pilot plant is the first one in the world to produce electricity and it should be followed in the forthcoming years by industrial units that will produce electricity at a commercial scale. Many other EGS projects have begun all around the world and a lot of scientific and technical targets are in development to improve the production of energy (electricity and central heating through district networks).
