Progress in energy extraction modeling of hydrothermal reservoirs has been achieved in two major directions. One was the development of an analytic model of heat transfer in a fractured geothermal system. The other was an experimental study of the effects of thermal stressing on rock strength and porosity.

The analytic model of linear sweep heat transfer was developed by Iregui et a1 (1978) and described by Hunsbedt et a1 (1979). This onedimensional model calculates the water temperature as a function of time and position in the idealized geothermal system depicted in Fig. 1. Cold water enters the reservoir through a series of injection wells at one end and flows horizontally to a series of production wells at the other end. The injection and production flow rates are steady and the permeability of the formation is such that the flow is considered uniform. Reservoir pressure prevents boiling at any point in the formation. Results from the analytic model are compared with experimental data obtained from the Stanford Geothermal Program (SGP) large reservoir model, shown schematically in Fig. 2.

In fractured hydrothermal reservoirs, circulation of colder water induces tensile thermal stress at and below the rock surface. Murphy (1978) has shown analytically that such stresses have the potential to create self-driven cracks of sufficient depth and aperture to enhance energy extraction and prolong useful production life. of producing self-driven cracks, thermal stressing may also influence both the mechanical and thermal properties of the rock. Changes in these properties may alter ;he heat transfer characteristics of the rock over the production period of a reservoir. To gain some insight into the potential significance of the effects of thermal stressing on heat transfer characteristics, an exploratory study was conducted to observe the effects of thermal stressing on rock strength and porosity.

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