Isotopic (δ18O, δD) patterns in Los Azufres (Mexico) geothermal fluids related to reservoir exploitation

Isotopic patterns for the year 2000 in the Los Azufres geothermal reservoir were related to injection of a condensed steam-water-air mixture as well as to the occurrence of reservoir physical processes resulting from exploitation. Reservoir boiling and mixing of reservoir fluids with cooler fluids were the most important processes identified. Boiling takes place in two zones of the field. In the north, the boiling area includes wells AZ-13, AZ-28, AZ-48, AZ-43 and AZ-32, while, in the south, boiling affects wells AZ-16AD, AZ-22, AZ-18, AZ-26 and AZ-36. Mixing of reservoir fluids with cooler waters was identified in wells AZ-2, AZ-33, AZ-16 and AZ-46 located in the southern zone and in well AZ-4 in the north. The isotopic (oxygen-18 and deuterium) patterns of fluids collected in September 2000 show that the original convective process found in the unperturbed reservoir is still taking place, although mixing of reservoir and reinjected fluids is also indicated. According to N2 data, the effects of reinjection on the physical and chemical characteristics of the reservoir fluids can be observed in the northern part of the field. Until now, however, only a steam phase, resulting from boiling and steam separation of the re-injected mixture, is evident in the fluids discharged by the northern wells.