Tracer-Assisted Evaluation of Hydraulic Stimulation Experiments for Geothermal Reservoir Candidates in Deep Crystalline and Sedimentary Formations

Abstract

Tracer tests provide information on transport properties essential for heat exchange in geothermal reservoirs, namely: fluid residence times, fluid-rock contact surface areas ? which are not properly captured by hydraulic or geophysical methods. Mostly, tracer tests can be conducted in parallel with hydraulic or hydro-mechanical experiments, without major additional effort. Here, the use of push-pull and flow-path tracing tests to evaluate the effect of hydraulic stimulation measures is illustrated with different field settings, for short- and long-term experiments in deep crystalline and sedimentary reservoirs in Germany. From the tracer breakthrough curves in the push-pull tests (at the KTB site), the specific (i.e., per-volume) area of the fluid-rock contact surface could be estimated, and its change was used to appreciate the effects of massive stimulation. From tracer BTCs in the hydrofrac spiking test (at the Horstberg site), a fluid residence time distribution was obtained, from which the reservoir?s flow?storage characteristics could be derived, and extrapolated tracer recoveries provided an estimate of the flow capture angle to the target horizon.