Energy extraction from fractured geothermal reservoirs in low-permeability crystalline rock

Two hot dry rock geothermal energy reservoirs were created by hydraulic fracturing of Precambrian granitic rock on the west flank of the Valles Caldera, a dormant volcanic complex, in the Jemez Mountains of northern New Mexico. Heat was extracted in a closed-loop mode of operation, injecting water into one well and extracting the heated water from a separate production well. The first reservoir was produced by fracturing the injection well at a depth of 2.75 km, where the indigenous rock temperature was 185°C. The relatively rapid decline in temperature of the water produced from the first reservoir, 100°C in 74 days, indicated an effective fracture radius of about 60 m with an average thermal power extracted of 4 MW. A second, larger reservoir was created by refracturing the injection well 180 m deeper. Downhole measurements of water temperature at the reservoir outlet as well as temperatures inferred from chemical geothermometry showed that the thermal decline of this reservoir was negligible; the effective heat transfer area of the new reservoir must be at least 45,000 m2, nearly 6 times larger than the first reservoir. In addition, reservoir residence time studies employing visible dye tracers indicated that the mean volume of the second reservoir is 9 times larger. Other measurements showed that flow impedances were low and that downhole water losses from these reservoirs should be manageable. The geochemistry of the produced water was essentially benign, with no scaling problems apparent. Moreover, the level of induced seismic activity was insignificantly small.