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Potentially exploitable supercritical geothermal resources in the ductile crust

January 23, 2017

The hypothesis that the brittle–ductile transition (BDT) drastically reduces permeability implies that potentially exploitable geothermal resources (permeability >10−16 m2) consisting of supercritical water could occur only in rocks with unusually high transition temperatures such as basalt. However, tensile fracturing is possible even in ductile rocks, and some permeability–depth relations proposed for the continental crust show no drastic permeability reduction at the BDT. Here we present experimental results suggesting that the BDT is not the first-order control on rock permeability, and that potentially exploitable resources may occur in rocks with much lower BDT temperatures, such as the granitic rocks that comprise the bulk of the continental crust. We find that permeability behaviour for fractured granite samples at 350–500 °C under effective confining stress is characterized by a transition from a weakly stress-dependent and reversible behaviour to a strongly stress-dependent and irreversible behaviour at a specific, temperature-dependent effective confining stress level. This transition is induced by onset of plastic normal deformation of the fracture surface (elastic–plastic transition) and, importantly, causes no ‘jump’ in the permeability. Empirical equations for this permeability behaviour suggest that potentially exploitable resources exceeding 450 °C may form at depths of 2–6 km even in the nominally ductile crust.

Citation Information

Publication Year 2017
Title Potentially exploitable supercritical geothermal resources in the ductile crust
DOI 10.1038/NGEO2879
Authors Noriaki Watanabe, Tatsuya Numakura, Kiyotoshi Sakaguchi, Hanae Saishu, Atsushi Okamoto, Steven E. Ingebritsen, Noriyoshi Tsuchiya
Publication Type Article
Publication Subtype Journal Article
Series Title Nature Geoscience
Index ID 70181006
Record Source USGS Publications Warehouse
USGS Organization National Research Program - Western Branch