**Abstract**

The heat content of the crystalline basement is by far the biggest energy resource of the earth crust. First attempts to access this resource date back to the early 1970´th and more than a dozen research and industrial projects have been performed since than in various countries. But still the technique, known as HDR (Hot-Dry-Rock) or EGS (Enhanced-Geothermal-Systems) is not mature and the thermal power achieved so far does not meet economical standards. In addi‐ tion further development is now hindered by the risk of induced earthquakes.

A critical review of results and observations shows that the main reason for the poor progress is the exploitationconceptbeing appliedinallmajorprojects since the early 1980´th.Untilthanthe basementhadbeenregardedas a competentrockmass andthe leadingexploitationschemewas to connect two inclined boreholes by a number of parallel fractures created by hydraulic fracturinginshortinsulatedboreholesections.Realizingthatthebasementcontainsopennatural fractures even at great depth this multi-fracture-concept was abandoned and replaced by the EGS-concept. The intent of this concept was to enhance the permeability of the natural joint network by massive water injection in very long uncased borehole sections. The results of all majorEGS-projectshowever shows thatthis isnothappening butthat generally one largewingcrack is created by the stimulation process regardless of the length of the test-interval. These wing-cracks require significantly bigger fluid volumes for the envisaged fracture-area, have a highly heterogeneous and anisotropic transmissibility and are a plausible explanation for the intense and strong induced seismicity as well as for the strong after-shocks observed at vari‐ ous EGS-locations. These findings suggest a return to the original multi-fracture concept with the only difference that the tensile fractures are to be replaced by the same number of wingcracks. Directional drilling and packer technology improved significantly during the last three decades and multi-fracture concepts are applied with great success in unconventional gas reservoirs. Though the conditions and requirements in geothermal applications are more

© 2013 Jung; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

demandinginvarious aspects it seems almost certainthatgeothermalmulti-fracture-systemsof this kind can be realized in the near future.

stimulation. The second borehole is then directionally drilled into the region of enhanced permeability. Since the stimulated region is elongated in the direction of the maximum horizontal stress, the boreholes are aligned in this direction which is 90 ° off the direction of

EGS — Goodbye or Back to the Future http://dx.doi.org/10.5772/56458 97

HDR-Concept EGS-Concept

Due to the enormous size of the created or enhanced fracture systems mainly water or brine without proppants were considered as frac-fluids since it seemed too costly or technically impossible to place proppant material over such large areas. All tests in the crystalline basement were accompanied by intense induced seismicity. Localizing and mapping the sources of induced seismicity thus became the most important tool for investigating the evolution of the fracture systems during water injection and most of the projects used this method to define the target for the second or third well. On the other hand induced seismicity has become a major obstacle for further development of the HDR- or EGS-technology since on some locations the population was shocked by events with magnitudes bigger than 3 [7].

The HDR-concept was followed only during the first years of development. Warned by the inability to create vertical fractures in the pioneering Los Alamos project and convinced by the arguments of the EGS-proponents that shearing of natural fractures is the predominant failure mechanism this concept was abandoned [8] and all projects after the 1980th followed the new EGS-concept. The rapid adoption of this concept was to a big part due to its technical simplicity. In particular, it required no high-temperature open hole packers, which created enormous technical problems in the Los-Alamos-Project. The change in the leading concept had severe

the HDR-concept.

**Figure 1.** Basic concepts

consequences:
