Field measurements and modeling have shown the potential for using a Hot Dry Rock (HDR) geothermal reservoir for electric load following: either with Power-Peaking from a base-load operating condition, or for Pumped Storage of off-peak electric energy with a very significant thermal augmentation of the stored mechanical energy during periods of power production. For the base-load with power-peaking mode of operation, an HDR reservoir appears capable of producing over twice its nominal power output for short -- 2 to 4 hour -- periods of time. In this mode of operation, the reservoir normally would be produced under a high-backpressure condition with the HDR reservoir region near the production well highly inflated. Upon demand, the production backpressure would be sharply reduced, surging the production flow. Alternatively, for Pumped Storage, the reservoir would be operated in a cyclic mode, with production shut-in during offpeak hours. When the produced thermal energy of such a pumped-storage system is considered, an HDR reservoir would be capable of returning considerably more energy to the surface during the production phase than would have been consumed in inflating the reservoir during the off-peak storage phase. Pumped Storage reservoir operation was actually demonstrated experimentally during a brief series of cyclic reservoir tests at the end of the Long-Term Flow Test (LTFT) of the HDR reservoir at Fenton Hill, NM in May 1993. The analytical tool used in these investigations has been the transient finite element model of the an HDR reservoir called GEOCRACK, which is being developed by Professor Dan Swenson and his students at Kansas State University. This discrete-element representation of a jointed iock mass has recently been validated for transient operations using the set of cyclic reservoir operating data obtained at the end of the LTFIí.

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