Fluid residence times and fluid-rock contact surface areas are important parameters in geothermal reservoir design and prediction. To determine them, tracer tests are the method of choice. In contrast with geophysical and seismic methods, the use of tracer tests for characterizing candidate geothermal formations at several km depth is relatively new in Germany. A more systematic campaign of fluid spiking applications, originally emerging from theoretical research interests but finally tailored to the specific needs of German pilot geothermal projects was made possible, since 2003, by a basic research project funded by the German Research Foundation (DFG) within its Priority Program engagement to the ICDP (International Continental Scientific Drilling Program). So far, the tracer testing campaign comprised single-well push-pull tracings, as well as a single-well and a inter-well flow-path tracing, in crystalline (KTB, Urach) and sedimentary (Horstberg) formations in ~4km depth; these tests' main endeavor was to help understanding processes associated with fluid transport in the deep crust, and subsidiarily also to assist in evaluating the effect of hydraulic stimulation measures, which were either short-term, high-rate (Urach, Horstberg) or long-term, moderate-rate (KTB). For the Gro?Schˆnebeck site in the Northern-German sedimentary basin, a sequence of short-, mid- and long-term hydraulic experiments is planned, with single-well fluid spiking at four different stages and one inter-well flow-path tracing.

Good knowledge of the tracers' physicochemical behaviour under the given reservoir conditions, and reliable tracer analytics is a prerequisite for the correct interpretation of test results. Clearly, more research will be needed in this area.

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