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Potentially exploitable supercritical geothermal resources in the ductile crust

Nature Geoscience volume 10, pages 140–144 (2017)Cite this article

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Abstract

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.

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Figure 1: Relations between permeability and effective confining stress at 350 °C for granite containing multiple fractures and basalt containing a single fracture.
Figure 2: Log–log diagrams of the relation between permeability and effective confining stress for granite samples containing either multiple fractures or a single fracture.
Figure 3: Comparison between the experimental results summarized in equations (1)–(3) and a permeability–depth relation from the literature10.
Figure 4: Temperature and effective confining stress (depth) conditions for the base of high-temperature geothermal reservoirs, and potentially exploitable supercritical geothermal resources.

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Acknowledgements

The present study was supported in part by the Japan Society for the Promotion of Science (JSPS) through a grant-in-aid for Specially Promoted Research (no. 25000009). The authors would like to thank Toei Scientific Industrial Co., Ltd. for manufacturing the experimental system.

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Authors and Affiliations

  1. Department of Environmental Studies for Advanced Society, Graduate School of Environmental Studies, Tohoku University, Sendai, Miyagi 9800845, Japan

    Noriaki Watanabe, Tatsuya Numakura, Kiyotoshi Sakaguchi, Atsushi Okamoto & Noriyoshi Tsuchiya

  2. Renewable Energy Research Center, National Institute of Advanced Industrial Science and Technology (AIST), Koriyama, Fukushima 9630298, Japan

    Hanae Saishu

  3. US Geological Survey, Menlo Park, California 94025, USA

    Steven E. Ingebritsen

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  1. Noriaki Watanabe
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Contributions

N.W. conducted overall study design and N.T. supervised the study. N.W., T.N. and K.S. conducted the experiments, analysed the results, and estimated the permeability–depth relation. H.S., A.O. and N.T. summarized temperature and depth conditions at the base of high-temperature geothermal reservoirs. N.W., H.S. and S.E.I. wrote the manuscript, and all authors commented on the manuscript.

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Correspondence to Noriaki Watanabe.

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Watanabe, N., Numakura, T., Sakaguchi, K. et al. Potentially exploitable supercritical geothermal resources in the ductile crust. Nature Geosci 10, 140–144 (2017). https://doi.org/10.1038/ngeo2879

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