Regular chemical sampling and analysis of fluids produced from the hot-water geothermal system of Cerro Prieto, Mexico has provided early warning of reservoir processes (Manon et a l.,1977). The changes in chloride concentration, sodium to potassium ratio and measured fluid enthalpy are shown in the figures for wells M- 5, M-26, M-21A, and M-11 of the Cerro Prieto field. The concentration of chloride, a conservative" constituent, is characteristic of different water masses and is affected by a change of water source, by mixing of waters and by boiling and steam loss but not by reaction with rock minerals. geothermal index resulting from rock-water reaction and is not affected by boiling and steam loss or by mixing of water masses provided these processes occur at constant temperature. The enthalpy is related to the fluid temperature and to boiling in the aquifer with "excess" steam entering the well. These indices provide a reasonably complete picture of major reservoir processes occurring in hot water systems. should be used in addition to Na/K as a temperature indicator. Analysis of fluids from a producing geothermal field must of course include other constituents for study of environmental effects, scaling, corrosion, etc.

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