Enhanced Geothermal Systems inject cold water down an injector well into hydraulically fractured hot dry rock several kilometres deep. The water sweeps through the artificial fracture network, absorbing heat from the rock matrix, and is pumped back up a separate production well as hot water or steam to drive a turbine at the surface.
Q = ṁ · cp · ΔT (extracted thermal power)
T_rock(depth) ≈ T_surface + gradient × depth
reservoir cooling rate ∝ injection_rate / (rock_volume × permeability)
- Well depth — deeper wells reach hotter rock (typical gradient ≈ 25–30 °C/km), raising extracted temperature and power.
- Injection rate — more water flow extracts more total heat per second but cools the reservoir faster.
- Rock permeability — controls how easily water threads through the fracture network; low permeability starves flow and lowers power.
- Fracture network toggle — shows/hides the stimulated fracture planes that make deep hot rock permeable enough to circulate fluid through.
Projects such as Fenton Hill (USA) and Soultz-sous-Forêts (France) pioneered exactly this injector/producer well-pair EGS design to make geothermal power viable outside naturally permeable hydrothermal fields.