On the eve of celebrating the 30th anniversary of starting electrical production from the Cerro Prieto Geothermal field a new stage of development is taking place by adding 100 MW to its 620 MW installed capacity. Although this field has been producing for almost 30 years, reservoir conditions such as pressure and temperature remain as high as 200 bar and 300?C in a large part of the reservoir. Cerro Prieto can thus be considered not only as the largest Mexican geothermal fields but as one of the oldest and most reliable geothermal systems in the world. Even with its large capacity several changes are taking place in this liquid-dominated reservoir, mostly in the west part of the field where production started. Chemical and isotopic changes as well as pressure decline have been observed as a result of the 12 000 t/h of brine withdrawal from the reservoir. Temperature logs and geochemistry monitoring have identified a intensive inflow of groundwater as well as reinjected fluids below the shallow Alpha reservoir, showing a temperature inversion in some deep wells. In this paper, a practical approach is presented to management of a liquid-dominated reservoir, where critical decisions have to be taken on reinjection and make up wells locations, along with long term reservoir monitoring programs and work over policy.

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