 One 1 km³ of 200°C hot granite cooled by 20°C... ...delivers about 10 MW of electric power... ...for a period of 20 years.

The estimated EGS potential is huge: According to a study presented by the German Parliament the total technical potential for electricity production form EGS sources amounts to about 1’200 EJ (300’000 TWh), which corresponds to 600times the annual consumption in Germany.

Source: AXPO Holding, Switzerland

aus GASVERBUND MITTELLAND AG A Swiss vision MWe

There are widely accepted operational numbers, which are necessary for a technically feasible and economically viable EGS system (Garnish 2002): heat exchange surfaces > m 2 in a volume > m 3 production flow-rates of l/s at temperatures °C flow impedance <0.1 MPa/l/s water losses <10%. So far, such numbers have not yet been demonstrated; presently there is no power generation from EGS systems.

Table 1: Goals and achievements in EGS projects world-wide

So there is still quite a bit ahead… Numerous problems must be solved to reach the numerical goals and many unknowns need to be clarified:  irregularities of the temperature field at depth  favourable stress field conditions  long-term effects, rock-water interaction  possible short-circuiting  environmental impacts like man-made seismicity to name only a few.

T(z) Basel ? ?

T(z) T(z) : Static temperature logs Well DP 23-1 Desert Peak/NV, USA - 1 km 230°C

Brown et al. (1999) Yield(t) and recovery factor depend on fracture network 25 MWt Long-term production

(Sanyal & Butler 2005) 500 l/s 245 MW e yr Production stop 20 yrs Long-term effects

(Sanyal & Butler 2005) 125 l/s 250 MW e yr

Induced seismicity Reinjection is increasingly applied at numerous geothermal production areas. This changes the pore pressure conditions and herewith the local stress field. At The Geysers field/California,USA a large-scale reinjection of fluids (piped to the field over long distances from a sewage plant) is underway since a few years. This creates frequent, perceptible tremors. Induced seismicity is especially relevant for the EGS technology.  Monitoring of local seismicity by a suitable seismometer array (starting well before reinjection/fracturing) is indispensable.

The key component: an extended, sufficiently permeable fracture network at several km depth, with suitable heat exchange surfaces.

Key issue is the creation, characterization and management of an extended, sufficiently permeable fracture network at several km depth, with suitable heat exchange surfaces. No direct observation/ manipulation is possible to achieve this; it must be accomplished by a kind of remote- sensing and –control; promising developments to provide the tools needed here are underway (e.g. the HEX-B and HEX-S software of GEOWATT).

Remote Sensing and Control in Reservoir Engineering Fracture network Data range distribution (spacing, aperture, length) Hydraulic boundary conditions Worst case scenarios Most probable scnarios Reservoir domain:Wellhead domain: Hydraulic tests Pressure recalculation wellhead to open hole domain (density changes!) Flow/pressure development at reservoir depth PTQ(t), Chem. ? Production temperatures Cooling between open hole.and wellhead Thermal processes 3D-conductive/advective High flow-rates pT-Borehole Simulator HEX-B FE/FD Applications for coupled hydraulic- thermal processes 3D-Code Cluster

Reservoir engineering tool (1): pT- simulator HEX-B Reservoir properties from wellhead data GPK2/GPK3 wellheads Temperature/ pressure profile, calculated withHEX-B Example: European. EGS Project Soultz- sous-Forêts, France Stimulation GPK3, 2003 ca. 10 Tage p(z,t) tmp(z,t) Flow Exit/Entry points Q [l/s] HEX-B

Example: EGS Project Coso, USA Deterministische Strukturen (UBI) Stochastische Strukturen (  UBI) HEX-S Pressure distribution in the reservoir after 24 hours reinjection with l/s Wellhead pressure Reservoir engineering tool (2): Stimulation Code HEX-S Coupled hydro-rock mechanical code deterministic stochastic structures

ECONOMICS  Various economic models (for example the one at developed by the IEA Geothermal Implementing Agreement) come up with favourable electricity production prices.  Such models are all based on numerous assumptions, which have not yet been substantiated.  So far there is no practical experience with real costs.  In any case, substantial front-up investment is needed since EGS technical feasibility at a given site can be demonstrated by deep drilling and circulation only.  Co-generation (and selling the heat) could secure a better price than electricity generation alone.

There are great challenges but still numerous problems ahead.  The real challenge is to work for problem solutions, through a wide spectrum of disciplines: earth sciences, physics, chemistry, engineering, economics….  What will really be needed is the planning and establishment of successful EGS systems in several, contrasting geological settings;  Key issue will be remote sensing and –control in creating, characterizing and operating the fracture system at depth;  Joining forces by a broad, internationally based interdisciplinary effort like ENGINE is an important step towards the ambitious goals;  The EGS adventure resembles an Alpine tour: the difficulties and struggles underway are numerous and major, the prospect however (“the view from the top”) is rewarding.

Prof. Dr. L. Rybach GEOWATT AG Zurich Dohlenweg 28 CH-8093 Zurich, Switzerland