Tracer Tests for Characterizing Malm Geothermal Reservoirs Within the German BMWi Project TRENDS: A Feasibility Study

The research project TRENDS (“Tracer-assisted evaluation of reservoir behavior under expansive deployment schemes for Malm geothermal resources in the Munich area”) aims at quantifying the hydrogeology of Malm geothermal resources with a focus on fluid transport parameters, and identifying possible restrictions to reservoir exploitation resulting from either hydraulic or geothermal heat supply 'competition' effects between 'adjacent' reservoirs. While it is beyond doubt that these aims cannot be achieved without the use of some fluid-based tracking procedure for quantifying fluid transport (viz.: tracer tests), the use of artificial tracers is challenged by the very large size of Malm geothermal reservoirs, implying very long residence times (RT) of circulating fluids and strong dilution of tracers therein. A series of M. Sc. thesis projects conducted at the University of Göttingen addresses tracer test design, evaluation and interpretation problems specifically associated with the 'very long RT' issue. Preliminary findings from these projects, and some recommendations for future tracer test design are presented.