Temperature logs, which have been run routinely in The Geysers geothermal wells, have been used to indicate the depth corresponding to the top of the steam zone ( 1 ). This steam chest is marked by temperatures which exceed 400?F and by a sharp change in temperature gradient. Above the steam chest heat transfer is largely by conduction, so that the gradient depends on heat flux and thermal conductivity. Within the steam chest, which is highly fractured, heat is transferred via the vertical fractures by convective reflux as well. This being a much more effective mechanism, the temperatures are more nearly isothermal ( 2 ). The existence of this abrupt gradient change has been confirmed directly in U.S. Geothermal C - 4 and C-5, where the temperature was logged from the surface into the upper unproductive portion of the steam chest.

This report describes a model of the heat transfer within the steam chest. By comparing the model with temperature gradient data from a well, one can estimate the average vertical permeability with in the reflux system.

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