Why must geothermal reservoirs have permeable rock or faults?

Geothermal reservoirs require adequate permeability or faults to function as effective energy sources because these features allow heated fluids—such as water and steam—to circulate and rise from deep underground toward the surface.1

Permeability, which is the capacity of a porous material to transmit fluids, is essential for the movement of these heated fluids through the rock. Faults and permeable rock structures provide the necessary pathways for groundwater to circulate by convection, where it absorbs heat from deep-seated sources like magma and forms large, accessible reservoirs of hot water or steam. Without these pathways, the heat remains trapped in the rock, making it difficult to extract for practical use.213

While natural reservoirs rely on these existing geological features, developing technologies like enhanced geothermal systems (EGS) are designed to artificially create fracture systems in low-permeability rock. By increasing the rock's permeability, EGS allows for the injection and circulation of fluids to harvest heat from hot dry rock resources that would otherwise be inaccessible.4