Hot dry rock geothermal energy makes use of hot fluids found naturally in basement rock; but such HWR conditions are rare. By far the bulk of the world’s geothermal resource base (over 98%) is in the form of basement rock that is hot but dry—with no naturally available water. This means that HDR technology is applicable almost everywhere on earth (hence the claim that HDR geothermal energy is ubiquitous). Typically, the temperature in those vast regions of the accessible crystalline basement rock increases with depth. This geothermal gradient, which is the principal HDR resource variable, ranges from less than 20 °C/km to over 60 °C/km,
Hot dry rock geothermal energy hydrothermal energy production can only exploit hot fluids already in place in the earth’s crust, an HDR system (consisting of the pressurized HDR reservoir, the boreholes drilled from the surface, and the surface injection pumps and associated plumbing) recovers the earth’s heat from hot but dry regions via the closed-loop circulation of pressurized fluid. This fluid, injected from the surface under high pressure, opens pre-existing joints in the basement rock, creating a man-made reservoir which can be as much as a cubic kilometer in size. The fluid injected into the reservoir absorbs thermal energy from the high-temperature rock surfaces and