Isotopic evolution of wells in the geothermal field of Los Azufres, Michoacan, Mexico

Isotopic (delta(18)O and deltaD) data from production and reinjection well fluids from the Los Azufres geothermal field were interpreted in order to define reservoir evolution and the occurrence of physical processes as a result of exploitation. The study included data of 30 wells, which were sampled in September, 2000. General results indicate that different phenomena seem,to occur in both zones of the field. In the southern zone there are two different trends of behavior: a mixture of fluids evidenced by the deltaD versus delta(18)O trend with a positive slope, where the isotopically enriched-fluid is constituted by the reinjection fluid. The other trend, with a negative slope, was interpreted as the result of reservoir vapor separation at a temperature above 220 degreesC, since for temperatures above 220 degreesC deuterium behaves as a volatile component No well-defined deltaD vs delta(18)O trend was found for the northern zone, but some points seem to fit the same negative slope trend found in the southern zone. The study of reservoir temperatures estimated by different approaches, for particular wells through time, as well as temperature results obtained with a heat and flow well simulator suggest that reservoir boiling occurs in localized areas in both zones of the field. This process is probably due to exploitation.