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Abstract

One promising potential site for extracting supercritical geothermal fluids is the Krafla volcano, Iceland, into which a borehole was drilled very close to a shallow rhyolite magma chamber at about 2 km depth. Besides the technical challenge to drill in this setting, the extreme temperature conditions of T > 900°C close to the magma body may significantly change the mechanical rock properties leading to low strength and fast closure of fracs, both of which are unfavorable for successfully operating a supercritical geothermal system (SCGS). Here, we determined experimentally the strength of dry felsite and basalt, lithologies that are important in the context of the Krafla Magma Testbed (KMT) project that aims to drill into or at least close to the magma chamber. The bulk triaxial strength of cylindrical samples and the frictional strength of split-cylinder samples with a preexisting shear plane were measured at about 50 MPa pressure and temperatures up to 1100°C using a Paterson gas-deformation apparatus. Peak compressive strength of the felsite starts to decrease strongly at T ≈ 900°C and of basalt at about 700°C. The friction coefficient of felsite drops down at T ≈ 750°C and of basalt at T ≈ 500°C. Strength decrease at high temperature is associated with partial melting and/or pore collapse. The results indicate that fracture closure-related permeability reduction within the felsite contact aureole at high temperature due to passing the brittle plastic transition is unlikely at fast (laboratory) strain rate, but may occur at lower (in situ) strain rate.