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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).

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.

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

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.