Available online at www.sciencedirect.com ScienceDirect Procedia Earth and Planetary Science 17 (2017) 193 – 196 15th Water-Rock Interaction International Symposium, WRI-15 Geological and geophysical perspective of supercritical geothermal energy in subduction zone, Northeast Japan Noriyoshi Tsuchiyaa,1, Ryoichi Yamadab a Graduate School of Environmental Studies, Tohoku University Aramaki-aza-Aoba 6-6-20, Aoba-ku, Sendai, 982-8579, Japan b Abstract Geological and geophysical characteristics of supercritical geothermal reservoirs in the Tohoku District, NE Japan were investigated in order to evaluate potential of future geothermal energy Geological information in the Tohoku District (caldera, hot spring hydrothermal alteration, mine, and granitoid) and geophysical data (gravity, MT and seismicity) were accumulated into GIS database We can evaluate present volcanism and related events, geothermal structure, depth of heat source, fracture distribution, and existence of fluid by using the GIS database We identify the following geological features in terms of geothermal resources: “Deep fluid input”, ”Magma input”, “Geothermal gradient”, and “High temperature hot spring”, and we can classify the geothermal energy and potentials as follows: “Promising”, “Probable”, ”Possible”, and “Potential” © 2017 2017Published The Authors Published by Elsevier B.V by Elsevier B.V This is an open access article under the CC BY-NC-ND license Peer-review under responsibility of the organizing committee of WRI-15 (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the organizing committee of WRI-15 Keywords: Geothermal Reservoir, Supercritical condition, Geofluid, Subduction zone, EGS, Supercritical geothermal energy Introduction After the Great East Japan Earthquake and the accident at the Fukushima Daiichi Nuclear reactor on 3.11 (11th March) 2011, geothermal energy came to be considered one of the most promising sources of renewable energy for the future in Japan However, there are several geological and geophysical issues First is the National Park Issue (~80% of the potential geothermal energy in Japan lies inside National Parks), second is Onsen (hot springs) problem, and another is induced seismicity related to the development of geothermal energy The temperatures of geothermal fields operating in Japan range from 200 to 300 °C (average ~250 °C), and the depths range from 1000 to 2000 m (average ~1500 m) In conventional geothermal reservoirs, the mechanical behavior of the rocks is presumed * Corresponding author Tel.: +81-22-795-6335 E-mail address: tsuchiya@mail.kankyo.tohoku.ac.jp 1878-5220 © 2017 Published by Elsevier B.V This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the organizing committee of WRI-15 doi:10.1016/j.proeps.2016.12.066 Noriyoshi Tsuchiya and Ryoichi Yamada / Procedia Earth and Planetary Science 17 (2017) 193 – 196 to be brittle In order to minimize induced seismicity, a rock mass that is “beyond brittle” is one possible candidate, because the rock mechanics of “beyond brittle” material is one of plastic deformation rather than brittle failure 1, Geothermal fluid condition under “beyond brittle” is considered to be supercritical state, and one of future targets of geothermal development in Japan is supercritical geothermal reservoirs At Kakkonda in NE Japan, the exploration well WD-1a encountered the partly solidified Kakkonda Granite and inferred reservoir temperatures in excess of 500°C We already studied an exposed Quaternary granitoid (the Takidani Granodiorite), since it is analogous to a deep-seated geothermal reservoir which might be in supercritical geofluid condition4,5 The Takidani Granodiorite is located at the boundary of the Eurasian and North American Plates In contrast, we have investigated hydrothermal activity in order to understand the evolution of supercritical geothermal fluids in certain geological settings Temperatures are over 350°C in the “beyond brittle” condition (a temperature of ~350 °C coincides with the brittle–ductile transition)6-9 Tohoku District, located in northeast part of Japan, is one of representative subduction zone in the world10, 11, where the Pacific Plate is subducting to the Eurasian Plate, and we can recognize obvious volcanic front at back-born mountain range and so-called hot fingers in Tohoku District12 Based on geological and geophysical survey, we could classify characteristics geothermal resources in terms of geothermal reservoir Here, we describe some geological perspectives and feasibilities to evaluate geothermal energy under supercritical conditions Supercritical geothermal reservoir 400͠ 500͠ How about WRI? FRONTIER F RONTIER 㻮㼑㼥㼛㼚㼐㻌㻮㼞㼕㼠㼠㼘㼑 㻮 㼑 㼥㼛 㼚 㼐 㻮 㼞 㼕 㼠 㼠 㼘 㼑 subcritical Geofluid 䡚350͠ supercritical Can we drill? Brittle Can we estimate and monitor fluid flow? Beyond Brittle Geothermal Frontier Mechanics It is possible to extract high enthalpy energy from supercritical geothermal reservoirs, and weak water-rock interactions with respect to silica solubility are estimated in high temperature conditions However, drilling projects to reach the supercritical geofluids were limited, and our knowledge of supercritical geothermal reservoirs has been limited Fig shows a conceptual model of the supercritical geothermal reservoir As mentioned above, conventional geothermal reservoirs in Japan are below ~250°C and ~1500 m in depth, where is under brittle condition in terms of mechanical behaviors of rocks and subcritical geofluid condition Temperature profiles of the conventional geothermal reservoirs indicate hydrothermal convection is dominant mechanisms of heat and fluid transfer The geothermal frontier lies below conventional geothermal reservoirs, where mechanical behaviors of reservoir rocks are “beyond brittle” and physicochemical state of the geofluid is considered to be under supercritical condition Temperatures of the geothermal frontier may be higher than 350°C and heat conduction is considered to be main mechanisms of heat and fluid transport Four big research targets which are evaluation of fluid flow, water-rock interaction (WRI)13, creation of rock fractures and drilling technology for extremely high temperature conditions are suggested Heat Hydrothermal conductive convection 194 How create fractures? Fig Conceptual model of supercritical geothermal reservoir “Geothermal Frontier” Noriyoshi Tsuchiya and Ryoichi Yamada / Procedia Earth and Planetary Science 17 (2017) 193 – 196 Geological data Geophysical Approach Geological Approach We have already published the geosphere informatic universal system (“GENIUS”: DVD on Arc GIS®) for evaluation of the geosphere environment Using “GENIUS”, we complied the distribution and dimension of caldera, geochemical characteristics of granitoids in Neogene and Paleogene, hot springs, abandoned mine, distribution and alteration types of hydrothermal alteration in Tohoku District Fig shows outline of data base (DB) for evaluating geothermal resources Geological data from caldera and hot spring indicate surface manifestation of geothermal activities Hydrothermal alteration and mining data and some of hot spring could suggest geothermal activities in the shallow part Granitoids suggests geothermal and magmatic activity at relatively great depths qz vein Caldera DB Hot spring volcano caldera mineral vein ⾲ᒙ␗ᖖ Surface Hot Spring DB Hydrothermal Alteration Silica self-sealing Eruption, Degassing Fracture cloud High Shallow Geotherm Mine DB Granite DB Volcano, Hot spring Magma ascent Magma chamber Geothermal Gradient Deep Melt upwelling Epicenter DB Seismic DB Gravity DB Subducted slab Aseismic Deep magma input Tectonic Anomaly Low Vp Deep fluid input Fig Geological and geophysical database (DB) for supercritical geothermal reservoirs Geophysical data We compiled geothermal gradient and gravity published by AIST (Advanced Institute of Science and Technology: DVD2), seismic and epicenter data base (http://www.jma.go.jp/en/quake/) on “GENIUS” Additionally, we compiled several results for MT (Megnetotelluric survey) in specific areas Geophysical survey data indicates deep and tectonic anomalies in magma and geothermal activities Geothermal potential Fig shows a classification of geothermal resources in terms of feasibility for development of geothermal reservoirs “Promising” geothermal resources show the highest potential “Probable” is related to high temperature hot spring, but it seems to be small scale, and ”Possible” indicates very high possibilities of high temperatures, but it is not clear evidence for existence for geofluid Finally, “Potential” is a realistic possible area for future development in terms of feasibility and energy potential The classification was based on some considerations of heat (essential geothermal energy source), geofluid (heat carrier) and fractures (fluid transport) https://www.gsj.jp/Map/JP/docs/DGM-P02_dvd_doc.pdf 195 196 Noriyoshi Tsuchiya and Ryoichi Yamada / Procedia Earth and Planetary Science 17 (2017) 193 – 196 High temperature Hot Spring Promisibility Volcano Caldera Alteration Geothermal Gradient Magma Input Mantle Fluid Fig Classification of supercritical geothermal reservoir Conclusions Geothermal energy is one of the promising renewable energy sources in Japan, and high enthalpy energy can be extracted from supercritical geothermal reservoirs, however, no practical supercritical geothermal reservoirs have been developed at this time For future geothermal energy utilization, we propose a conceptual classification of supercritical geothermal reservoirs based on geological and geophysical data We can clarify the geothermal energy and potential as follows: “Promising”, “Probable”, ”Possible”, and “Potential” , and we can evaluate “Deep fluid input”, ”Magma input”, “Geothermal gradient”, and “High temperature hot spring” As technological issues, we have to develop novel drilling technology to reach and mine supercritical geothermal energy Acknowledgements This work was supported by a Grant-in-Aid for Specially Promoted Research, Grant Number 25000009 We would like to say thanks to research members of JBBP (Japan Beyond Brittle Project) References Asanuma, H., Muraoka, H., Tsuchiya, N., Ito, H., 2012 The Concept of the Japan Beyond-Brittle Project (JBBP) to Develop EGS Reservoirs in Ductile Zones, GRC Transactions 36, 359-364 Muraoka, H Asanuma, H., Tsuchiya, N., Ito, T., Mogi, T Ito, H., the participants of the ICDP/JBBP Workshop, 2014 The Japan BeyondBrittle Project, Sci Dril 17, 51-59 Doi, N., Kato O., Ikeuchi, K., Komatsu, R., Miyazaki, S., Akaku, K., Uchida, T 1998 Genesis of the plutonic-hydrothermal system around Quaternary granite in the Kakkonda geothermal system, Japan Geothermics 27, 663-690 Bando, M., Bignall, G., Sekine, K., Tsuchiya, N., 2003 Petrography and uplift history of the Quaternary Takidani Granodiorite: Could it have hosted a supercritical (HDR) geothermal reservoir? 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Classification of supercritical geothermal reservoir Conclusions Geothermal energy is one of the promising renewable energy sources in Japan, and high enthalpy energy can be extracted from supercritical geothermal. .. back-born mountain range and so-called hot fingers in Tohoku District12 Based on geological and geophysical survey, we could classify characteristics geothermal resources in terms of geothermal reservoir