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In early 1977, the condition of the two deep boreholes at Fenton Hill was as follows: The GT-2 borehole had been drilled to a depth of 9,619 ft (2,832 m) and a 7 5/8-in. scab liner had been placed and cemented from 8,973 ft to 9,581 ft, leaving a 38-ft “rat hole” below the liner (see Fig. 3.12). The EE-1 borehole had been drilled to a depth of 10,053 ft (3,064 m) and cased from 9,599 ft to the surface with a composite casing string (999 ft of 7 5/8-in. casing from 9,599 to 8,600 ft, and 8 5/8-in. casing above 8,600 ft—see Fig. 3.26).

The world’s first demonstration of the hot dry rock (HDR) geothermal energy concept took place at Fenton Hill, New Mexico, in the mid to late 1970s. The objective was to create a large, man-made HDR reservoir in rock at an appropriate temperature (~200°C) and accessed by two deep boreholes, completing the pressurized earth circulation loop.

The concept of hot dry rock (HDR) geothermal energy originated at the Los Alamos National Laboratory. In 1970 it was proposed as a method for exploiting the heat contained in those vast regions of the earth’s crust that contain no fluids in place—by far more widespread than natural geothermal resources (HDR represents over 99% of the total U. S. geothermal resource). Although often confused with the small, already mostly commercialized hydrothermal resource, HDR geothermal energy is completely different from hydrothermal energy.

The Phase I reservoir at Fenton Hill, which extended over the approximate depth interval 8,000–10,000 ft, had indeed demonstrated the technical feasibility of the HDR concept, but at a temperature (157°C) and thermal power (3 MW) lower than desirable for commercial power production. The Phase II reservoir was planned for development at a depth of 12,000–14,000 ft, to test the HDR concept at a temperature and rate of geoheat production more appropriate for a commercial power plant, and with a reservoir large enough to sustain a high level of thermal power for an extended period (at least 10 years).

With the very difficult task of directionally drilling the lower portions of the EE-2 and EE-3 boreholes complete, in early 1982 the critical next step in the development of the Phase II reservoir began. Recall that at this time, the theory that a penny-shaped, vertical hydraulic fracture could be created in jointed basement rock still held sway with the HDR Project management (although many of the Project staff had by then abandoned it). This, of course, was why the lower portions of EE-2 and EE-3 had been inclined 35° from the vertical by costly and time-consuming directional drilling, with EE-3 terminating about 1,200 ft vertically above EE-2. Mort Smith, in his Abstract for the 1982 Annual Report (HDR 1983b), stated that “During Fiscal 1982, emphasis in the Hot Dry Rock Program was on development of methods to produce the hydraulic fractures required to connect the deep, inclined wells of the Phase II system at Fenton Hill.”

In the wake of initial Phase II reservoir testing and the ICFT, fundamental questions about the commercial potential of HDR technology remained to be answered. How reliable is power production from an HDR reservoir? What might be the longevity of such a reservoir? To answer these questions, and to demonstrate that geothermal energy could be extracted on a sustained basis, more extensive testing would be required. The Long-Term Flow Test (LTFT) for the Fenton Hill Phase II reservoir was designed to simulate as closely as possible the conditions under which a commercial HDR power plant might operate.

As detailed in preceding chapters, the technical feasibility of HDR geothermal energy was clearly demonstrated at Fenton Hill, with the testing of two separate confined reservoirs. The major task now in view is moving this revolutionary new technology to its appropriate place in the world’s energy supply mix. Obviously, technical feasibility is not enough: HDR must also be capable of supplying useful amounts of energy economically. For that requirement to be met, several issues—which have yet to be adequately addressed—will need to be resolved.

Following the ICFT, work began to complete the design and installation of a permanent surface plant for the Long-Term Flow Test (LTFT), which was planned as a realistic demonstration of the viability of HDR technology under conditions closely approximating those of a commercial HDR power facility. The major difference was that no power would be produced (the thermal energy brought to the surface during circulation would simply be wasted to the atmosphere). The decision not to produce power was based on the following considerations

How far back in our past did humans begin to use hot water and steam coming from vents in the earth’s surface to improve their lives? Did they make stops at such sites while moving from place to place, bathing in the warm pools or using the waters for cooking, and eventually construct villages near hot springs? We can only imagine in what ways man first availed himself of the earth’s heat; but we can assume that human populations in various areas sooner or later encountered hot waters that were bubbling up to the surface after having been raised to high temperatures by circulation through deep, hot rock—and that they made use of the heated water.

The demonstration of flow connectivity between the redrilled EE-3A wellbore and the EE-2 reservoir zone (Expts. 2059 and 2062) made it clear that a viable HDR system had finally been established, although that stage of the Fenton Hill HDR Project took about 5 years (1980–1985)—much longer than originally anticipated. Throughout that period, Germany and Japan contributed both funds and manpower to the effort. The Germans withdrew from the Project at the end of 1985, but the Japanese maintained a presence through the following year. In early 1986, they began pressing strongly for the thermal and flow characteristics of the HDR system to be evaluated.