Variability in the gas phase composition of fluids discharged from Los Azufres geothermal field, Mexico

Abstract

Abstract 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 CO 2 , H 2 S, NH 3 , H 2 , He, N 2 , CH 4 and 222 Rn. 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 ‘base-line’ 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, CO 2 and 222 Rn. 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.