DGG/EAGE Workshop - Geophysics for Deep Thermal Energy

Geophysical exploration for geothermal resources is often challenging because information on the parameters of interest such as porosity, permeability, fluid content, etc., cannot be observed directly. Conventional (seismic) structure images are usually not sufficient to locate potential geothermal targets. Magnetotelluric (MT) and seismic methods provide information about the resistivity and velocity distributions of the subsurface in similar scales and resolution. The lack of a fundamental law linking the two parameters, however, limits a joint interpretation to a more qualitative analysis. Using a statistical approach in which resisitivity and velocity models are investigated in the joint parameter space, we can identify regions of high correlation between the two model parameters. Back-mapping of these regions onto the spatial domain allows us to identify common classes which can then be compared with lithological information. Application of this technique to a seismic - MT profile in the area of the Groß Schönebeck geothermal site, allows us to identify a number of classes in accordance with local geology. In particular, a high velocity - low resistivity class is interpreted as related to salt lows, where highly fractured anhydrite might produce enhanced permeability.

Aside from volcanic regions, most places in the world do not exhibit natural geological structures where temperature and rock-permeability meet the conditions required for a geothermal power production. The EGS (Enhanced Geothermal Systems) concept implies the creation of an artificial subsurface heat exchanger by enhancing the rock permeability in a spatially confined region. Such reservoir stimulation is achieved by injecting large volumes of fluid into the host rock under high pressures. Stimulation activities are frequently accompanied by induced seismicity and there exist a direct relationship between induced seismicity and hydraulic conductivity enhancement. A major challenge in EGS technology is an optimum positioning of the injection and production wells with respect to the stimulated zones. This is critically dependent on the spatial image of in situ hydraulic conductivity. In the current paper we present a new technology for determining 3-D images of enhanced reservoir conductivity based on the observed shear-displacement of the induced seismicity.

The development of geothermal energy utilization for electricity production has suffered strong drawbacks with the recent events of induced seismicity occurred at the site of Basel (Switzerland) and Landau (Germany). Typically, the population only got aware of the local geothermal activities when being affected by vibrations and noise originating from the shallow underground. Subsequently, the mass media, authorities expressed their discomfort with the technology applied. In Baden Württemberg, having one of the most important geothermal resources in Germany, the geothermal development has nearly stopped today. Public campaigns, citizens' initiatives, and the lack of knowledge within the administrative body lead to missing acceptance. These social conditions lead to a further weakening of a geothermal technology that already had to digest some failures in the recent years.

Geothermal energy is a clean, globally available and nearly inexhaustible energy resource. However, the necessary deep drill holes demand financial investments on the order of a few tens of millions of Euros, without guarantee that the required flow rates and temperature of the produced fluids allow an economical use. This exploration risk often turns into a major show-stopper for a commercial use of geothermal energy. Improved methods for estimating the distribution of subsurface hydraulic and thermal properties will result in a reduced exploration risk. Therefore, reliable estimates of these properties are mandatory for the economic and technical planning and operation of a geothermal installation. Not in the least, this comprises quantifying the uncertainty of these estimates at an early stage of reservoir exploration and continuous updates and modifications as reservoir development progresses.

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