| Abstract |
The Los Azufres (Mexico) geothermal field (188 MW installed power capacity), consists of two well-defined areas of production, Maritaro in the north and Tejamaniles in the south. The Comisión Federal de Electricidad (CFE) started field development in 1980 while commercial exploitation began in 1987. Injection started in 1983 and has been beneficial to the reservoir. All reinjection wells are located to the west of the field, at present time, there are two injection wells operating in the south zone and four in the north zone. Reinjection fluids are a mixture of separated water and condensed steam that is highly evaporated at ambient conditions before injection. In order to investigate the main processes occurring in the reservoir due to exploitation/reinjection processes, monitoring of chemical and isotopic compositions of fluids are routinely performed. In this work the distributions of geochemical indicators obtained in 2010 from chemical and isotopic data of fluids were related to reservoir exploitation and reinjection processes. Data for 50 wells provided by Comisión Federal de Electricidad were included. Geochemical indicators studied were: total discharge isotopes (?18O, ?D); Na/K temperatures for two-phase wells and gas FT-HSH2 temperatures of dry steam wells; CO2; chlorides and N2 concentrations, reservoir excess steam and reservoir liquid saturation. The results indicate that chloride concentrations decrease but reservoir temperatures increase toward the northeast of the field where no injection takes place. Injection influence was noticed through minimum CO2 concentrations in fluids. CO2 concentrations increase from the west toward the east, with maximum CO2 concentrations in the southeast but also in the northeast, where important natural upflow seems to occur. Because injection takes place by gravity at atmospheric conditions, air is unintentionally injected to the reservoir, together with separated water and condensed steam. Thus N2 distribution provides actual trajectories of volatile species. Maximum N2 concentrations are seen in the southwest of the field (where injection wells are located) with a decreasing trend toward the northeast. Isotopic results for 2010 showed enrichment at the west of the field, where injection wells are located. The 2010 dD vs ?18O relationship showed that some wells produce different proportions of injection returns and that injection effects are more important in the south zone. Most of the production of well AZ-2A (south zone) seems to consist of injection returns while well AZ-16 produced approximately 50% from injection. In contrast, in the north zone the wells are little affected by injection with the exception of well AZ-42 that shows a relatively enriched isotopic composition. |