Changes in Gas Composition of the Los Azufres (Mexico) Geothermal Fluids Related to Exploitation

Abstract

Gas data from four representative wells of Los Azufres geothermal field were interpreted in order to study the effects of exploitation on the gas phase discharged. Equilibrium of the gas phase with mineral buffers controlling H2S concentration in fluids was assumed to occur according to the Fischer-Tropsch (FT) and the combined pyrite-hematite-magnetite (HSH2) reactions. For steam wells the FT-HSH2 results showed a trend of decrease in reservoir temperature and increase in reservoir excess steam, suggesting steam phase returns from the reinjection fluid in different proportions. In contrast, for two-phase wells the FT-HSH2 results showed a trend of decrease in both, temperature and excess steam, indicating the recovery of gas depleted liquid phase. Original reservoir fluids and reinjection returns can be distinguished because of their different H2S/ H2O ratios, which are higher in the reservoir fluids compared to those in the returns. By this approach the FT-HSH2 equilibria can be used to identify the presence of vapor phase reinjection returns in steam wells discharges.