David V. Duchane

Scientific progress on the Fenton Hill HDR project since 1983

The modern HDR concept originated at the Los Alamos National Laboratory and was first demonstrated at Fenton Hill, NM. Experience gained during the development of the deeper HDR reservoir at Fenton Hill clearly showed that HDR reservoirs are formed by opening pre-existing, but sealed, multiply connected joint sets. Subsequent flow testing indicated that sustained operation of HDR systems under steady state conditions is feasible. The most significant remaining HDR issues are related to economics and locational flexibility. Additional field test sites are needed to advance the understanding of HDR technology so that the vast potential of this resource can be economically realized around the world.

Unbacked cylindrical metal foils of submicron thickness

Abstract Implosion experiments often utilize cylindrical thin metal foils. Previously these foils have been made either from flat sheets, with a seam where the edges joined, or in the form of a composite polymer/metal laminate, in which the plastic film acts as a supporting substrate. A method has been developed to produce unbacked cylindrical metal foils of submicron thicknesses. This process utilizes a temporary substrate consisting of a water-soluble polymer film as a base for the electron beam deposition of the metal layer. After formation of the metal foil, the polymer is removed by immersion of the assembly in water. Unbacked metal foil cylinders as thin as 0.17 μm with extremely smooth wrinkle-free surfaces have been produced by this technique.

Vacuum deposition of high quality metal films on porous substrates

A composite mandrel has been developed consisting of a core of low density polymethylpentene foam overcoated with a thin layer of film‐forming polymer. The surface tension and viscosity of the coating solution are important parameters in obtaining a polymer film which forms a continuous, smooth skin over the core without penetrating into the foam matrix. Water soluble film formers with surface tensions in the range of 45 dyn/cm and minimum viscosities of a few hundred centipoises have been found most satisfactory for coating polymethylpentene foam. By means of this technique, continuous polymer fims with thicknesses of 10–20 μm have been formed on the surface of machined polymethylpentene foam blanks. Aluminum has been vacuum deposited onto these composite mandrels to produce metal films which appear smooth and generally defect free even at 10 000 times magnification.

The Future of Hot Dry Rock Geothermal Energy

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.

Attempts to Create a Deeper Reservoir, Redrilling of EE-3, and Completion of the Phase II Reservoir

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.”

Phase I Drilling and Initial Attempts to Establish Hydraulic Communication

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 Enormous Potential for Hot Dry Rock Geothermal Energy

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.

Planning and Drilling of the Phase II Boreholes

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).

Long-Term Flow Testing of the Phase II Reservoir

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.

Phase I Reservoir Development—Redrilling and Flow Testing

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).