Flow Test At Fenton Hill HDR Geothermal Area (Dash, Et Al., 1983)

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Exploration Activity: Flow Test At Fenton Hill HDR Geothermal Area (Dash, Et Al., 1983)

Exploration Activity Details
Location Fenton Hill HDR Geothermal Area
Exploration Technique Flow Test
Activity Date 1978 - 1980
Usefulness useful
DOE-funding Unknown

Exploration Basis
Two shallow reservoirs were created during the early stages of the project, and are collectively referred to as the Phase I reservoirs. Flow testing of the Phase I reservoirs was conducted between 1978 and 1980 to evaluate the effectiveness of hydraulic pressure stimulation and test the viability of extracting a thermal resource from hot rocks in an engineered reservoir lacking in-situ geothermal fluids.
A shallow reservoir was originally created in jointed biotite granodiorite at a depth of about 2,500 m with an initial rock temperature of 185°C. Heat-extraction and flow testing experiments were conducted in this reservoir from January 27 to April 13, 1978. A second, larger reservoir was created in March 1979 after recementing the fracture-to-wellbore connections in well EE-1 at 2,930 m depth and refracturing 200 m deeper, in the first of a series of pressure stimulation experiments that would become known as the Massive Hydraulic Fracturing (MHF) Tests (refracturing was accomplished during experiments 203 and 195). Flow and heat-extraction tests were run in the second reservoir from October 23 to November 16, 1979. The Phase I reservoirs grew continuously during Run Segments 2 through 5 (January 1978 to December 1980). Although the reservoir is relatively small in volume, 3-5 MW(t) of heat were produced for more than 9 months with only an 8°C decline in production temperature. The heat-transfer area and fracture volume with the reservoir grew from 8000 to 50,000 m^2 and from 11 to 266 m^3, respectively. Growth of the Phase I reservoirs was caused both by pressurization and hydraulic fracturing, and by the effects of heat-extraction and thermal-contraction. The rate of diffusional water loss to the margins of the Phase I reservoir was 0.4 L/s after 5 months of flow testing. The final volume of the Phase I reservoir was determined to be 10 x 10^6 m^3, defined by an envelope containing the majority of microseismic events measured during flow testing (i.e. the seismic volume). Samples of the geothermal fluids and gases were collected at regular intervals during each of the heat-extraction experiments from the production wellhead, the injection wellhead, and at the make-up pump that supplied water from storage ponds to replace the water lost downhole by permeation into the reservoir walls. Samples were analyzed to identify compositional changes in the produced fluid in order to study the reservoir behavior under normal (recirculating) operating conditions. Concentrations of certain dissolved species appear to be derived from the displacement of an indigenous pore-fluid, while others appear to be derived from dissolution of minerals present in the reservoir rock. Water sampled from the production well is relatively low in total dissolved solids and shows little tendency for corrosion or scaling. The authors attempt to relate the changes in the liquid and gas chemistry of the geothermal fluids to geochemical processes that resulted from the heat-extraction and flow testing experiments. Specifically, the application of the silica and Na-K-Ca geothermometers and the implications of the pore-fluid displacement theory are examined to evaluate the long-term effects of fluid geochemistry on heat extraction from HDR reservoirs. The results of the flow testing and heat-extraction experiments and of the chemical analysis of the geothermal fluids are reported by Dash et al. (1983) and Grigsby et al. (1983), and are summarized in numerous reports by other authors.


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