Geomechanical Facies Concept in Exploration Techniques of EGS Sites, Soultz-sous-Forets and Spa Urach

Using the holisitic geomechanical facies concept as a framework, this paper compares techniques for major exploration, evaluation and field development at two European geothermal projects: Soultz-sous-Forets, in the central part of the Upper Rhine Graben; and Urach Spa, in the South German crystalline complex. Since 1971 the Hot Dry Rock/Hot Wet Rock (HDR/HWR) concept has been developed from the conceptual stage to the point where the feasibility of the technology has been demonstrated. The HDR concept has been adapted to fit the natural in-situ conditions of the rock and exemplifies one type of Enhanced Geothermal Systems (EGS) development. Significant developments have been made in drilling, borehole measurements and understanding the processes involved in the creation and operation of stimulated reservoirs, but little is yet known about how to influence joint opening so as to maximize productivity. Soultz, situated in granite rock and an active tectonic graben system, shows different conditions of lithology, temperature field, stress field, hydraulics and geochemistry than the Urach site. In contrast to Soultz, the Urach site is located in a very dense gneiss formation which shows comparatively more elastic behaviour and lies in a tectonically almost inactive strike slip stress field. The design and creation of HDR/HWR enhanced geothermal systems requires the development of simulation models predicting the extension behaviour of hydraulically induced fractures. This paper investigates key parameters for heat and fluid transport properties, coupled hydro-mechancial processes, the success of HDR heat exchanger development, and exploration techniques applicable to the different geomechanical facies, and suggests which technologies may be more or less successful for the development and use of the HDR technology.