VARIABILITY IN THE GAS-PHASE COMPOSITION OF FLUIDS DISCHARGED FROM LOS AZUFRES GEOTHERMAL-FIELD, MEXICO

Studies related to hydrological structure of the Los Azufres geothermal field and its effects on the exploitation of the field for generation of electrical energy have included a program of sampling and chemical analysis of fluids discharged by a number of deep wells in continuous production. Chemical analysis of the gaseous phase includes monitoring of CO2, H2S, NH3, H-2, He, N2, CH4 and Rn-222. Five wells in Los Azufres field were periodically sampled during 1983-1988. The monitoring program has shown considerable variability in the gas concentrations of fluids. Before mid-1985, the 'baseline' concentrations of the gases showed standard deviations between 8 and 28%. During the later period, the average concentrations of different gases ranged from 37% to much higher values above the 'base-line'. The largest variations are observed in He, CO2 and Rn-222. This variability is interpreted in terms of (1) addition of 'excess steam', (2) a possible relationship with earthquake events that occurred in Mexico during September 19-21, 1985, and early May to early June of 1987 and, (3) more recent variations (after mid-1987), increased exploitation of the field.