The concept of Hot Dry Rock (HDR) geothermal energy originated at Los Alamos National Laboratory in the early 1970s, to exploit the heat contained in those vast regions of the earth's crust that contain no fluids in place—by far more widespread than natural hydrothermal resources.

Two separate HDR reservoirs were created in deep, hot crystalline rock, at the Fenton Hill HDR Test site about 40 miles west of Los Alamos. These reservoirs, at depths of 2800 m and 3500 m and temperatures of 195°C and 235°C respectively, were created with technology that was rapidly evolving at the time. They were flow-tested for a period of almost a year each. Thermal power production ranged from 4 MW for extended routine production intervals to as high as 10 MW for a 30-day period. The testing proved beyond a doubt that it is technically feasible to recover useful amounts of thermal energy from HDR.

From tracer testing of the deeper reservoir, it was found that the flow patterns became more diffuse with time, suggesting that more of the reservoir was being accessed as flow continued—with flow definitely not tending toward short-circuiting, which had been a worry.

The major finding of the work at Fenton Hill is that an HDR reservoir should first be created from the initial borehole, and then accessed by two production boreholes. It is almost impossible to create an effective system by drilling the boreholes first and then trying to connect them by hydraulic pressurization.

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