Exploration for Enhanced Geothermal Systems (EGS) requires a combination of imaging methods to characterize the subsurface and to develop a geoscientifically integrated understanding of the reservoir. With an integrated geophysical-geological-geomechanical understanding at a variety of scales it is possible (I) to quantify and characterize the reservoir, (II) to define the location of a drill site, and (III) to support the drilling process in terms of geological risk mitigation. Here, we describe an exploration case study from the geothermal test site Groß Schönebeck, approximately 40 km North of Berlin/Germany, located in the Northeast German Basin. The target for exploration is a hydrothermal system in 4,200 m deep sandstones and volcanic rocks undergoing development for an EGS. The initial investigation of the subsurface was based on pre-existing 2D seismic and well data from former gas exploration campaigns. From the existing data we developed a 3D geological model to describe the general structure and main fault systems to a depth of 5,000 m. New geophysical field experiments, using innovative magnetotelluric and wide angle seismic techniques, provided new data and deeper insights into the characteristics of the site.

In 2006 a new geothermal production well GtGrSk4/05 was drilled as part of a well doublet at the Groß Schönebeck site. For the drilling operation it was necessary to develop methods to avoid formation damage and subsequent permeability impairment by mud solid infiltration and borehole breakouts. Previously, experiments on drill cores were designed to simulate the mechanical behavior of some geological formations and to test for the development of fractures and borehole breakouts under varying in situ conditions. With these data we could define specific mud density windows ensuring a safe drilling process. Hydraulic stimulation of a well is commonly used to increase the productivity of a reservoir, i.e. enhancing a geothermal system. A successful application of EGS technologies requires detailed knowledge of the stress field and reactivation potential of existing faults in the reservoir. We therefore applied the so called slip tendency method to estimate the likelihood of fault reactivation in both sandstone and volcanic rock successions which suggested an orientation of NNE-SSW faults with high slip-tendency. These results were confirmed later by microseismicity records during the hydraulic stimulation. This integrated geothermal exploration strategy covers all aspects from geosystem analysis, reservoir characterization, and reservoir geomechanics. Such an integrated approach might be essential for an economic and sustainable exploitation not only of EGS but of all geothermal systems.

Press the Back button in your browser, or search again.

Copyright 2010, International Geothermal Association: Readers who download papers from this site should honor the copyright of the original authors and may not copy or distribute the work further without the permission of the original publisher.