EGS — Goodbye or back to the future

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 addition 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 exploitation concept being applied in all major projects since the early 1980´th. Until than the basement had been regarded as a competent rock mass and the leading exploitation scheme was to connect two inclined boreholes by a number of parallel fractures created by hydraulic fracturing in short insulated borehole sections. Realizing that the basement contains open natural 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 major EGS-projects however shows that this is not happening but that generally one large wing-crack 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 various 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 wing-cracks. 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 demanding in various aspects it seems almost certain that geothermal multi-fracture-systems of this kind can be realized in the near future.