Abstract. Attributes of several thousand fractures were collected in three boreholes of 2.2, 3.6, and 3.8 km depth, penetrating the Soultz Hot Dry Rock reservoir (France). The fractures were sampled from cores and from several high-resolution imaging techniques such as borehole televiewer (BHTV), ultrasonic borehole imager (UBI), formation microscanner (FMS), formation microimager (FMI), and azimuthal resistivity imaging (ARI). A comparison was made between the data collected on cores and those provided by different imaging techniques. The comparison clearly establishes that the different wall-images are not as exhaustive as the core data and cannot provide a complete characterization of the fracture network. Discrete fractures thinner than 1 mm are not properly detected. This is also the case for discrete fractures closer than 5 mm, which appear only as single traces. The imaging techniques are, nevertheless, very powerful for characterizing altered fracture clusters. Whatever the technique used, the fracture strikes were correctly sampled with the different systems. This comparison allowed us to calibrate the fracture population data obtained from the imaging system in order to correct for the filtering effect introduced by the technique itself and by the alteration of the rock mass.
The German Continental Deep Drilling Program comprising a pilot borehole down to 4000 m and a main borehole down to 9101 m in southeast Germany (KTB) is continuing to provide a unique opportunity for the identification of important factors and processes in deep-seated fluid and energy transfer. In situ stress conditions significantly impact flow, transport and exchange characteristics of fracture networks that dominate the permeability of crystalline reservoir rocks. In this paper, several scales of information are combined to present a fully three-dimensional hydraulic finite element model of the principal KTB fault zones, and linked to a geomechanical model describing the alteration of the hydraulic parameters with stress changes caused by fluid extraction. The concept of geomechanical facies is introduced to define and characterize architectural elements in the subsurface system. Evaluation of a long-term pump test in the KTB pilot hole, June 2002-July 2003, coupled with a geomechanical model gives an insight into some of the elastic and nonelastic processes controlling hydraulic transport in the basement rocks. Trends in the decline of the permeability and the degree of storage in the system could only partially be explained by elastic processes, clearly indicating the importance of nonelastic processes. A number of inelastic processes are suggested as areas for further research.
Earth-reservoirs are increasingly exploited today with the extraction of resources, such as heat and hydrocarbons, and the large-scale emplacement of waste, such as CO 2 sequestration. The characterization, site investigation, predictive modeling and long-term monitoring are dependent on the processes being investigated and modeled. In most cases complex coupled processes have to be addressed in a geologically complex rock mass system. In this paper we present a conceptual holistic framework known as geomechanical facies linking all the scales of investigation, characterization and reservoir development methods. We demonstrate this concept on the work undertaken during the design and development of the enhanced geothermal systems (EGS) systems at the forefront of European HotDry-Rock (HDR) technology, Soultz-sous-Forêts (France) and Spa Urach (Germany). Soultz-sous-Forêts is situated within granitic rocks and an active tectonic graben system in the central part of the Rhine Graben. It presents conditions of lithology, temperature, stress, hydraulics and geochemistry that are very different from those at Spa Urach, located in a very dense gneiss formation in the South German crystalline complex. Spa Urach exhibits more elastic behavior and is set tectonically within an almost inactive strike-slip stress field described in more detail in Sects. ''Drill core analysis'' and ''Hydraulic stimulation at Spa Urach''. This paper compares the exploration and field development methods used at these two sites against the back drop of the geomechanical facies concept. Issues addressed include the key parameters for flow and heat transport properties, coupled hydro-mechanical process identification, the success of the HDR reservoir as a heat exchanger and exploration techniques applicable to the different facies. Identification of the key geomechanical facies gives an indication as to which technologies will prove more efficient in the application of HDR technology. The results of this study will hopefully help in developing heat recovery schemes for the long-term economical operation of future HDR plants and EGS as well as assist in the understanding of engineered geosystems.
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