Permeability Evolution and Heat Extraction in Superhot and Supercritical Geothermal Systems: From Brittle–Ductile Mechanics to Long-Term Reservoir Performance

Superhot and supercritical geothermal resources provide higher-enthalpy fluids than conventional systems. Their feasibility, however, depends on a connected flow system that survives extreme temperature, pressure, stress, and fluid chemistry. This critical narrative review evaluates field, laboratory, and numerical evidence for the creation, evolution, and management of permeability. It distinguishes superhot rock from supercritical fluid. The latter requires temperature, pressure, and composition to be evaluated together. Kakkonda and the Iceland Deep Drilling Project (IDDP) demonstrate access to very high temperatures. The IDDP-2 reported downhole temperature and pressure above commonly cited seawater critical values. Newberry demonstrates propped interwell circulation at 331 °C and provides a field milestone toward enhanced geothermal systems (EGS) at higher temperatures. Laboratory studies show that permeability can persist or be created near the brittle–ductile transition through hydraulic loading, distributed deformation, microfracturing, and cooling. Recent experiments also show that lithology, water phase, creep, silica precipitation, and salt scaling can alter fracture permeability or fault stability. Numerical studies identify reservoir pressure, transmissivity, fracture architecture, injection strategy, and thermal sweep as first-order controls on heat extraction. The evidence is integrated into a permeability lifecycle framework. It links initial state, reservoir creation, coupled thermal–hydraulic–mechanical–chemical (THMC) evolution, flow redistribution, heat extraction, degradation or stabilization, and adaptive management. The main uncertainty is scale. Several mechanisms are laboratory-demonstrated or numerically predicted, whereas durable reservoir-scale circulation at or above 375–400 °C remains an active area of field validation. Priorities are long-duration field tests, dynamic permeability models, reactive transport, quantitative monitoring, and shared benchmark datasets.