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Significance Statement
Hydrogeothermal usage for sustainable energy supply (heat, electricity) from low-enthalpy resources (temperature gradient of about 30 °K/km) at 3-5 km depth is common practice in hot limestone aquifers in the Bavarian Molasse north of the Alps. However, hardrock (petrothermal) reservoirs hold a much higher geothermal potential in Germany than these conventional hydrogeothermal reservoirs. The latter contain in-situ hot water, while petrothermal reservoirs are characterized by dry heat at depths (‘hot dry rock’) where cold water needs to be injected first.
The aim of the 3D seismic survey was to image and characterize steeply dipping fault and fracture zones in granites and the outcropping crystalline rocks of the western Erzgebirge (Ore Mountains) in Saxony where cold water could be injected into a naturally fractured fault zone. The target depth is 5-6 km where temperatures between 150 °C and 180 °C can be expected.
Application and experience regarding 3D seismic measurements in the crystalline environment are different compared to the sedimentary environment. There is normally no stratification as in sedimentary basins, where seismic exploration for oil and gas reservoirs is best practice since a long time. Unconventional und novel procedures are required in data acquisition and data processing. Therefore, the second aim was to test the 3D seismic technique under these challenging conditions. A feasibility study suggested the area Aue-Bad Schlema-Schneeberg as a deep petrothermal project, mainly based upon favorable temperature conditions and the huge amount of knowledge available from mining activities.
The 3D seismic measurements were performed using the Vibroseis technique and consisted of source and receiver lines spaced nominally at 400 m with 30 m station spacing within a 10 km x 12 km area. Three simultaneously acting 27 t vibrators were used as a seismic source with a 12-96 Hz sweep signal of 10 s length, with 8 sweeps per source point. A patch of up to 6000 recording channels was moved in role-along mode over the area for a total of 8146 recording stations. There were 5348 vibrator points, giving a nominal common-midpoint (CMP) coverage up to 250 in the centre of the area for a bin size of 15 m x 15 m.
First conventional common midpoint (CMP)-processing revealed encouraging results already. Important steps of the pre-processing were noise-editing, static corrections and surface-consistent amplitude scaling and deconvolution (high cultural noise). Common-reflection-surface (CRS) processing and pre-stack time migrations (PSTM) were finally applied which provided images of high and unexpected quality.
Figure Legend
Vertical sections and depth slice through the data volume after CRS stacking and finite difference (FD) migration. The “Roter Kamm” normal fault is imaged directly and indirectly by the offset of its conjugate faults (sealed faults containing ores). The properties of the “Roter Kamm” fault exhibit a fractured zone which could provide the necessary permeability for geothermal usage.
Journal Reference
Ewald Lüschen1,*, Sascha Görne2,Hartwig von Hartmann1, Rüdiger Thomas1, Rüdiger Schulz1.
[expand title=”Show Affiliations”]- Leibniz Institute for Applied Geophysics (LIAG), Hannover
- Saxon State Office for Environment, Agriculture and Geology, Dresden
Abstract
A 3D seismic survey was recorded in 2012 to explore a petrothermal reservoir in a late-Variscan granitic pluton within the Erzgebirge (Ore Mountains) in Saxony, Germany. The main objective was to test this area in the context of the Enhanced Geothermal System concept and to test the 3D seismic technique as an exploration tool. The intention and challenge are to image and characterize potentially permeable fracture zones at target depths of 5–6 km, with temperatures above 150 °C. Unconventional methods were applied for field acquisition and data processing. The vibrose is technique was used in the core experiment, accompanied by a special explosive seismic experiment. Field acquisition was characterized by severe noise conditions and a highly irregular layout. These conditions required extensive preprocessing and data conditioning. The imaging started with conventional Common Midpoint processing for quality control and for a first reference. Better images were obtained by Common Reflection Surface processing with subsequent post-stack time migration. Prestack time migration was also used for comparison. Outstanding results were obtained by the ‘operator-oriented’ version of the Common Reflection Surface technique. A rich repertoire of structures within the granite pluton was imaged, including steeply dipping fault zones and conjugate faults. Images and indications of fracture and crack porosity of a prominent fault zone provide the background to define an optimum drill path. This is considered as the next stage for a possible geothermal plant, if a decision is taken to drill a research well in the future. The 3D seismic reflection technique was shown to be an indispensable tool for geothermal exploration, even in crystalline basement rocks.
© 2015 European Association of Geoscientists & Engineers
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