AAPG European Region Annual Conference Paris-Malmaison, France 23-24 November 2009 From HOT DRY ROCKS to ENHANCED G OETHERMAL SYSTEMS: The SOULTZ- sous-FORETS PROJECT

In a traditional industrial geothermal plant, the objective is to explo it the heat stored in the ground through the production of existing geothermal fluids. Like all energy production systems, geothermal plants are characterized by their production power, which is simply the product of the produced flow rate, the heat capacity of the geothermal fluid and the difference between the temperature of the fluid at the production wells and that at the re-injection wells. Traditionally, two different types of geotherm al energy systems are considered: High enthalpy systems that produce electricity and low enthalpy systems that produce heat for direct use, like space heating. But heat cannot be transported over long distances (maximum of 15 to 20 km, in the best of cases), and exploitation of low enthalpy geothermal energy (temperature of geothermal fluid below 100°C) has been restricted to domains where high porosity and permeability (located either in zones of volcanic activity or in proper sedimentary layers) is also a site where heat is needed locally commercially. Further, because of the poor efficiency of the heat to electricity conversion for tem peratures lower than 250°C, high enthalpy systems have been restricted to very specific site conditions, mostly encountered in volcanic regions. Its present potential remains fairly limited. This prompted, in the mid seventies of the last century, some research activity in the so called domain of Hot Dry Rocks (HDR), in which the objective was to develop artificial geothermal systems in rock masses with temperature high enough (larger than 300°C) for electricity production but with no pre-existing geothermal fluids in commercially significant quantity. Unfortunately, the experiment undertaken near Los Alamos, at Fenton Hill (New Mexico) failed to reach most of the anticipated figures for an economical plant and the project was stopped in the mid eighties. But the recent development of binary cycles for electricity production out of geothermal fluids has rendered commercially attractive tem peratures in the 150-200°C range, especially when combined with direct use of heat. This range in temperatures is encountered in vast domains of the earth, around depths of 5 km, but in most places there is too little quantity of geothermal fluid to warrant the development of an economical geothermal plant. Hence the original HDR concept has evolved toward that of Enhanced Geothermal System (EGS) in which the objective is to stimulate the local permeability of a deep rock mass, whether it involves or not pre-exiting geothermal fluids, and whether it concerns the production of electricity, the production of heat for direct use, or a combination of both applications. In this paper, first the m ain features to be considered for the design of an economical EGS system are presented. Then some results obtained at the existing experimental site of Soultzsous-Forets in north eastern France are discussed. Like most EGS systems developed or planned in north-western Europe, the experimental Soultz program aims at producing electricity out of rocks at about 180-200°C encountered at depths between 4 and 5 km. Reservoir Engineering elements that control the design of an economical EGS sys tem