Long-term commercial development of geothermal resources for electric power production will depend on significant heat extraction from hydrothermal reservoir rock as well as production of hot water. The study of heat extraction in the Stanford Geothermal Program has concentrated on developing a useful model for estimating the potential for heat extraction from fractured hydrothermal reservoirs. The project encompasses a physical model for data acquisition and mathematical models for interpretation of the results. The physical model consists of a pressure vessel containing a rock matrix simulating a fractured hydrothermal reservoir. Early rock loadings consisted of irregular shape rock fragments of various sizes (Hunsbedt, Kruger, and London 1977, 1978) to develop the production characteristics and a one-dimensional cold-water sweep model. The current rock loading consists of granite rock blocks having regular shape, closer packing of the rocks, and a larger average rock size. Flow conditions in this rock configuration can be varied to obtain a large range of rock-towater temperature differences. These conditions were designed to test the equivalent rock radius approach used in the one-dimensional sweep model. The experimental data are also used to assist in the development of more sophisticated numerical models of thermal production from hydrothermal reservoirs. A major parameter in the onedimensional sweep model is the effective number of heat transfer units, Ntu, which indicates the extent of heat extraction from the reservoir rock. Results of the first experiment with the current rock loading at - 7 were presented by Swenson and Hunkdt (1981). Subsequently, two additional heat extraction experiments were performed. One of these was at a higher flowrate to reduce the N parameter to 2 and the other was at atYower flowrate to yield a Ntu parameter of 15. A fourth experiment was performed to calibrate the heat loss calculation in the mathematical model without fluid injection and production. This paper summarizes the experimental results and the current mathematical modeling efforts using the one-dimensional sweep model and the LBL numerical reservoir simulator.

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