Supercritical geothermal systems are very high temperature geothermal systems that are located at depths near or below the brittle-ductile transition zone in the crust where the reservoir fluid is assumed to be in the supercritical state, e.g., for pure water temperature and pressure are respectively in excess of 374°C and 221 bar. These systems have garnered attention in recent years as a possible type of unconventional geothermal resource that could yield much higher well productivities due to their very high enthalpy fluids. Supercritical conditions are often found at the roots of volcanic-hosted hydrothermal systems. Deep wells drilled in geothermal fields such as The Geysers and Salton Sea, USA, Kakkonda, Japan, Larderello, Italy, Krafla, Iceland and Los Humeros, Mexico, have encountered temperatures in excess of 374°C, and in some cases have encountered fluid entries. The IDDP-1 well at Krafla encountered magma, and ended up producing very high enthalpy fluids; however these fluids were very corrosive and abrasive. Innovative drilling and well completion techniques may be needed to deal with the extreme temperatures and aggressive fluid chemistry compositions of these systems. New efforts are underway in Japan (northern Honshu), Italy (Larderello), Iceland (Reykjanes peninsula), Mexico (Los Humeros), and New Zealand (Taupo Volcanic Zone) to investigate supercritical systems. The Japan Beyond Brittle Project (JBBP), the New Zealand Hotter and Deeper Project (HADES), the FP7 project IMAGE as well as the Horizon 2020 projects DEEPEGS, DESCRAMBLE, GeoWell and GEMex, funded by the European Commission, provide an unprecedented opportunity for international collaboration to help solve the technical challenges associated with characterizing, drilling, and developing these high temperature systems. To facilitate interaction, METI Japan organized a meeting with invited representatives from selected G7 countries to discuss international collaboration for supercritical geothermal development as a part of the Innovation for Cool Earth Forum (ICEF) 2016. This paper, which grew out of the ICEF workshop, reviews past studies, describes current research efforts, and outlines the challenges and potential opportunities that these systems provide as geothermal resources.

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