**2. EGS technology**

Flow rates on the order of 50 L.s-1 and temperatures of 150° C to 200° C are required to allow an economical generation of electrical energy from geothermal resources (Clauser, 2006). Heat source risk can be quantified via a detailed assessment of surface heat flow together with measurements of temperature in boreholes (for example, for the Desert Peak Geothermal area, at 0.9 to 1.1 km depth, the ambient temperatures is of ~180 to 195° C in rhyolite tuff and argillite; Hickman and Davatzes, 2010). Thermal insulation (measured on core or cuttings samples) together with precision surface heat flow measurements allows the prediction of temperature distribution at depth in one, two or three dimensions (Beardsmore and Cooper, 2009). For regions outside natural steam systems and high surface heat flow (for example Iceland, Indonesia, Turkey, etc.) conditions necessary for electricity production are met at depths below 3 km provided the underground rock heat exchanger is engineered in order to increase the paths available for the fluid flow. Such systems are called Engineered (or Enhanced) Geothermal Systems (EGS) or Hot Dry Rock (HDR). In these techniques, the host rock is submitted to stimulations (Economides and Nolte, 2000) in order to increase the heat exchange surface between the rock and the injected fluid. Stimulations are derived from petroleum technology where they have been used for decades. The first method consists in the injection of water under high pressure to create irreversible shearing and opening of fractures and is called hydraulic stimulation (Portier et al., 2009). The second method, called chemical stimulation (e.g. Nami et al., 2008; Portier et al., 2009) uses various kinds of chemical reactants to dissolve minerals and to increase permeability. Both methods have proven successful for enhancing permeability at depth but it is still a challenge to plan and control the stimulation process. Details about stimulations can be found in the abundant litterature (e.g. Kosack et al., 2011; Nami et al., 2008; Portier et al., 2009 and references therein). During or after stimulations, tracers are used to assess the connectivity between the wells and the speed of fluid transfer. Many examples of use of tracers are found in the literature (e.g. Radilla et al., 2010; Redden et al., 2010; Sanjuan, 2006). In addition, prior to any stimulation or circulation test between the wells, in-situ stress and fracture characterization have to be considered with great attention in order to better constrain the geometry and relative permeability of natural or artificially created fractures (e.g. Hickman and Davatzes, 2010 for the Desert Peak geothermal field). A subsequent modelling of the 3D fracture network (Genter et al., 2009; Sausse et al., 2010, Dezayes et al., 2011) and of flow and transport along the fractures can be profitably performed (e.g. Karvounis and Jenny, 2011) to predict the behaviour of the thermal exchanger and ensure its financial viability. Such modelling is based on the accurate knowledge of the fracture network obtained through seismic records performed during stimulation or production tests (Concha et al., 2010) and

environmental impact when the production of electricity is considered compared to fossil or nuclear generation (MIT, 2006). As concerns the demand, supply and economic point of

Geothermal energy and EGS in particular is studied world-wide and the annual Workshop on Geothermal Reservoir Engineering (Stanford, California, USA) allows scientists and industrials to compare data and improve renewable energy production. Proceedings of the workshop can be downloaded very easily and for free on the Internet. As a consequence, the

Flow rates on the order of 50 L.s-1 and temperatures of 150° C to 200° C are required to allow an economical generation of electrical energy from geothermal resources (Clauser, 2006). Heat source risk can be quantified via a detailed assessment of surface heat flow together with measurements of temperature in boreholes (for example, for the Desert Peak Geothermal area, at 0.9 to 1.1 km depth, the ambient temperatures is of ~180 to 195° C in rhyolite tuff and argillite; Hickman and Davatzes, 2010). Thermal insulation (measured on core or cuttings samples) together with precision surface heat flow measurements allows the prediction of temperature distribution at depth in one, two or three dimensions (Beardsmore and Cooper, 2009). For regions outside natural steam systems and high surface heat flow (for example Iceland, Indonesia, Turkey, etc.) conditions necessary for electricity production are met at depths below 3 km provided the underground rock heat exchanger is engineered in order to increase the paths available for the fluid flow. Such systems are called Engineered (or Enhanced) Geothermal Systems (EGS) or Hot Dry Rock (HDR). In these techniques, the host rock is submitted to stimulations (Economides and Nolte, 2000) in order to increase the heat exchange surface between the rock and the injected fluid. Stimulations are derived from petroleum technology where they have been used for decades. The first method consists in the injection of water under high pressure to create irreversible shearing and opening of fractures and is called hydraulic stimulation (Portier et al., 2009). The second method, called chemical stimulation (e.g. Nami et al., 2008; Portier et al., 2009) uses various kinds of chemical reactants to dissolve minerals and to increase permeability. Both methods have proven successful for enhancing permeability at depth but it is still a challenge to plan and control the stimulation process. Details about stimulations can be found in the abundant litterature (e.g. Kosack et al., 2011; Nami et al., 2008; Portier et al., 2009 and references therein). During or after stimulations, tracers are used to assess the connectivity between the wells and the speed of fluid transfer. Many examples of use of tracers are found in the literature (e.g. Radilla et al., 2010; Redden et al., 2010; Sanjuan, 2006). In addition, prior to any stimulation or circulation test between the wells, in-situ stress and fracture characterization have to be considered with great attention in order to better constrain the geometry and relative permeability of natural or artificially created fractures (e.g. Hickman and Davatzes, 2010 for the Desert Peak geothermal field). A subsequent modelling of the 3D fracture network (Genter et al., 2009; Sausse et al., 2010, Dezayes et al., 2011) and of flow and transport along the fractures can be profitably performed (e.g. Karvounis and Jenny, 2011) to predict the behaviour of the thermal exchanger and ensure its financial viability. Such modelling is based on the accurate knowledge of the fracture network obtained through seismic records performed during stimulation or production tests (Concha et al., 2010) and

latest advances in geothermal technology are available to the scientific community.

view, MIT (2006) provides a rather detailed analysis.

**2. EGS technology** 

thanks to thorough characterization of the fracture network (Hébert et al., 2010, 2011; Ledésert et al., 2009). When the rock heat exchanger is finally operated, careful reservoir engineering and monitoring has to be performed to ensure the viability of the EGS (Satman, 2011). The produced hot fluid is continuously replaced by cooled injected water. After the thermal breakthrough time the temperature of the produced fluid decreases. However if after a time the field is shut-in the natural energy flow will slowly replenish the geothermal system and it will again be available for production. Therefore when operated on a periodic basis, with production followed by recovery, doublets are renewable and sustainable (Satman, 2011). Triplets (one injection and two production boreholes) are now considered as being the best configuration (MIT, 2006; Genter et al., 2009). However, it must be taken into consideration that when an EGS reservoir is developed through hydraulic fracturing, the size of the reservoir might extend too much and attendant high water losses might occur compromising the sustainability of the project as at Fenton Hill (MIT, 2006; Brown, 2009). When shearing occurs through reopening of pre-existing sealed fractures during hydraulic stimulation (e.g. at Fenton Hill; Brown, 2009) or when the mineral deposits are dissolved through chemical stimulation (e.g. at Soultz; Nami et al., 2008; Portier et al., 2009; Genter et al., 2009) the size of the exchanger is better constrained and fluid losses are limited.

Other risks such as technology (reliable supply of produced geofluids with adequate flow rates and heat content), finances (cost of construction, drilling, delays), scheduling, politics, etc…have to be estimated in order to make an EGS project viable. They are presented in MIT (2006) for the different stages of a project. In addition, seismic risk has to be fully taken into account where EGS programs are to be developed in urban areas in order to produce both electricity and central heating (Giardini, 2009): the Basel (Switzerland) experience (see section 4.4) had to be stopped because of a 3.4 magnitude earthquake generated by stimulation in a naturally seismic area. As a consequence to these numerous constraints, no financially viable EGS program is operating at present but the production of electricity that began at Soultz-sous-Forêts (France) in June 2008 is highly promising.


As a conclusion, Table 2 shows some forces and difficulties of EGS programs inferred from the literature.

Table 2. Forces and difficulties of EGS programs inferred form the abundant literature on the subject (see reference list). GHG: greenhouse gases.

The Soultz-sous-Forêts' Enhanced Geothermal System:

A Granitic Basement Used as a Heat Exchanger to Produce Electricity 483

Fig. 2. Location map of the upper Rhine graben (URG) in eastern France and of the Soultzsous-Forêts' site, 50 km North of Strasbourg, in a zone of high thermal anomaly (grey on the map). Six boreholes are present : 4550 (previous oil well), GPK1 (first HDR borehole), EPS1 (entirely cored scientific HDR borehole), GP2-GPK3-GPK4 (5 000m deep boreholes forming the triplet of the EGS). Their horizontal trajectories are shown on the main map. The E-W geological cross section shows the geometry of the upper Rhine graben and of the Soultz horst. Dots represent the granite while inclined grey and white layers correspond to the

sedimentary cover. After Ledésert et al. (2010).
