The Target for Exploration of Deep High-Temperature Geothermal Reservoirs

Abstract

It is well-known that the saturated water for an unit volume has a maximum enthalpy at 330°C. This fact by itself indicates that the highest-grade geothermal reservoir will be of 330°C because the “ore grade” of a reservoir is consideres to be proportional to the heat content stored in definite dimensions. On the other hand, the maximum reservoir temperatures of very active sysems in the world increase with increasing the depth of the reservoir nearly along the boiling curve of water in which the density of high temperature waters is taken into consideration (Hayashi et al., 1981). Consequently, the depth of the highest-grade geothermal reservoir can be predicted before deep drilling as follows: 1750 m for a field with an activity index of 100, 2700 m for one of 90, and below 3500 m for one of 80 . Since the enthalpy of the saturated water for a unit volume varies a little over a temperature range of 300 to 350°C, reservoirs almost equivalent to the highest-grade ones will be expected at depths ranging from 1100 to 2350 m for a field registering 100 in the activity index, below 1600 m for one of 90, and below 2550 m for one of 80.