Geochemical Evolution of the Los Azufres, Mexico, Geothermal Reservoir. Part I: Water and Salts

Abstract

The Los Azufres, Mexico volcanic reservoir is characterized by fluids of neutral sodium chloride type at depth. High temperatures, and different concentrations of soluble components in the rocks affect the amount of chemical elements such as Cl, B, Br, Li, Cs and As observed in its water. There are in the liquid other less soluble components such as SiO2, Ca, Mg, Rb, K, Na, SO4, NH4 and HCO3. This geothermal field has been under continuous exploitation since 1982; reinjection of waste liquid and air is carried out parallel to exploitation. These actions contribute to modify the reservoirís geochemistry. Chemical composition of fluid feeding the wells is not the same everywhere, indicating that there are several production sections from different formations. Measured enthalpies show that boiling occurs within the reservoir. Initial chemical data showed that the concentration of volatile components in the fluid such as CO2 and deuterium, decreased with depth, while concentration of non-volatile components such as oxygen-18 and chloride, increased. In a few lateral portions of the field there is isotopic evidence of mixture of meteoric water and shallow groundwater with geothermal fluids. An efficient caprock exists only in the southern sector where the main recharge must be lateral and deep. Data from many two-phase production wells, show that concentrations of chloride, boron, sodium, calcium, lithium and cesium, dissolved in the liquid, have been increasing since 1984, while rubidium and arsenic decreased in the same wells, during the same period. This effect is closely connected to the constant injection of cold liquid into the reservoir. Reinjection produces successive concentration of those salts in the water within the production zones. Wells not affected by injection are increasing their steam quality and show the opposite behavior. Potassium, sulfates and silica are constant or vary slightly in all the wells, independently of whether or not they are affected by reinjection. Some wells do not show any significant trend in their overall chemical behavior, suggesting the existence of large, well-connected high porosity zones around such wells.