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, that is 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 due to their very high enthalpy fluids. Supercritical conditions are often found at the roots of volcanic-hosted hydrothermal systems. More than 25 deep wells drilled in geothermal fields such as The Geysers, Salton Sea, and on Hawaii (USA), Kakkonda (Japan), Larderello (Italy), Krafla (Iceland), Los Humeros (Mexico), and Menengai (Kenya) have encountered temperatures in excess of 374 °C, and in some cases have encountered magma. Although fluid entries were documented for some of these wells, it remains an open question if permeability can be maintained at high enthalpy conditions. 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 are therefore 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 and Krafla), Mexico (Los Humeros), USA (Newberry), and New Zealand (Taupo Volcanic Zone) to investigate supercritical systems. Here, we review past studies, describe current research efforts, and outline the challenges and potential opportunities that these systems provide for international collaboration to ultimately utilize supercritical geothermal systems as a geothermal energy resource.

Both boreholes of the German Continental Deep Drilling Program encountered a fluid‐conducting crust over a 9101 m vertical profile. Low matrix permeability was observed on the laboratory scale (geometric mean value 7×10⁻²⁰ m² with something more than one decade standard deviation) and in situ higher values of the order of 5×10⁻¹⁸ to 3×10⁻¹⁶ m². With respect to the scatter of measured matrix permeability there was no lithology dependence observed, whereas permeability parallel to the foliation (geometric mean value 3×10⁻¹⁹ m²) is significant higher than those perpendicular to the foliation (2×10⁻²⁰ m²). With increasing depth the in situ permeability varies within a few orders of magnitude, showing, however, no clear depth dependence. The in situ permeability increases over 3 orders of magnitude while the effective pressure decreases by 50 MPa, whereas in contrast, the permeability of laboratory core specimens changes by 1 order of magnitude. The pilot drill hole “Vorboh‐rung” and the main drill hole “Hauptbohrung” communicate through a network of conductive fractures at the bottom hole level of the Vorbohrung as well as between both bottom hole sections. The formation pressure increases with a mean gradient of 11.5 MPa km⁻¹ from surface to 103 ± 3 MPa to 9101 m depth. Overhydrostatic increasing pore pressure and so a strong stress reduction can be excluded as a reason for the low permeability decrease observed at the bottom of the hole. Formation pressure remains hydrostatic with respect to probable increasing salinity.