Abstract
Flooded underground mines provide an innovative opportunity to extract low-grade geothermal energy. To evaluate these geothermal systems, in situ rock sampling and the investigation of thermal (heat conduction) and hydraulic rock properties (thermal convection) is essential. This paper discusses how these geothermal parameters were determined using Freiberg grey gneiss rock samples from the Alter Tiefer Furstenstolln gallery, where a geothermal mine water project has been successfully carried out in Spring, 2009. Unsteady autoclaving with the unsteady-state two-box method allowed us to measure the permeability of the nearly impermeable gneiss. As expected, samples drilled perpendicular to the rock foliation plane had lower permeabilities than those drilled parallel. Furthermore, it was demonstrated that the permeability varied due to the existence of fissures and the amount of the mantle pressure. The thermal conductivity was also anisotropic, and was higher in samples drilled parallel to the foliation than in those drilled perpendicular to it. Since gneiss is nearly nonporous, the effect of waterlogged pores on the thermal conductivity is minimal. The radiogenic heat production of the sampled Freiberg grey gneiss was found to lie in the lower half of the range of upper crustal metamorphic rocks.
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