CESS Seminar: STIMTEC – stimulation experiments and their validation in the Reiche Zeche underground laboratory, Freiberg, Germany

The presentation will cover the ongoing research of the STIMTEC underground experiment designed to investigate the permeability enhancement associated with hydrofracturing and activation of shearing as for example targeted in deep geothermal projects. We combine periodic pumping tests, high resolution seismic monitoring, structural analysis and drilling into stimulated volumes in an effort to improve near-real-time monitoring, phenomenological models of stimulation processes, and prognosis strategies. The ongoing experiment is located at the Reiche Zeche underground laboratory below Freiberg in Saxony/Germany at a depth of about 130 m below surface in metamorphic gneisses. A structural analysis of the test volume enclosed by tunnels preceded the experimental program. The installed seismic network consists of 12 broad-band acoustic emission sensors (sensitivity 1-100 kHz) and three 1-component Wilcoxon accelerometers (sensitivity 50 Hz-25 kHz). The stimulation borehole with 63 m length was drilled with 15° northward inclination. Previous investigations suggest a strike-slip regime with the maximum horizontal stress striking N350. Field testing was accompanied by a series of laboratory tests on core samples to estimate fracture toughness and elastic properties of the gneiss. After an extended campaign characterizing the hydraulics of the test volume, ten stimulation tests were performed in the injection borehole during which a total of > 10000 high frequency events were recorded. Each stimulation stage consisted of a frac stage, several refracs, and a subsequent hydraulic testing period. On the basis of the ongoing analyses of test records we are currently deciding on the location of three validation boreholes to be drilled in spring 2019.

Bio
After his dissertation at the Ruhr-Universität Bochum (RUB), Jörg Renner held postdoctoral fellowships and appointments at Massachusetts Institute of Technology with intermittent stints at GFZ Potsdam as guest scientist and at a borehole-service company as senior engineer before he became Professor for Experimental Geophysics at Ruhr-Universität Bochum end of 2001. His major research interests are in two strongly linked topics, subsurface fluid transport and rheology of rocks. He addresses problems from groundwater flow near the surface to resource recovery from the upper crust to melt transport in the Earth's mantle by performing and analyzing field and laboratory experiments.