Chemical and isotopic study to define the origin of acidity in the Los Humeros geothermal reservoir

Chemical and isotopic analyses of geothermal fluids were carried out at the Los Humeros, Mexico, geothermal field in order to establish the water-rock equilibrium state and to determine the origin of the acidity in the reservoir. From the perspective of water-rock equilibrium, the Los Humeros geothermal reservoir is out of equilibrium. This is probably the case since it is associated with recent volcanism, with magmatic components in the geothermal fluids that are not neutralized by reaction with feldspar and mica. Superheated magmatic steam contains HCl gas, which condenses or mixes with liquid at moderate temperature (< 374 deg. C) to form acid waters. This appears to have occurred at Los Humeros because high concentrations of HCl were detected in the steam phase. In addition, high concentrations of boron and arsenic also favor the definition of the Los Humeros as a recent volcanic system. Most wells of this field have high fluid enthalpy and almost all produce high steam fractions at the wellhead. Chemical characteristics of the fluids show that the produced brine is low-salinity water, whose geochemical character varies according to both well type and production. Shallower wells produce sodium bicarbonate (H-1) type water, whereas deeper wells produce sodium chloride (H-6) type. It was also found, according to enthalpy data, that well H-1 is located in a zone where the liquid phase is dominant, while the remaining wells are producing in a two-phase zone. As a result, the chemical composition of the total discharge from well H-1 is the same as that of the reservoir fluid. Measured temperatures at bottom hole vary between 260 and 340 deg. C and are in agreement with geothermometer calculations from Na/K of 295±10 deg. C and from H2/Ar of 270 to 340 deg. C. The isotopic composition of the Los Humeros wells shows anδ18O shift which is characteristic of fluids of geothermal origin that have reached equilibrium with the rock at high temperatures. The isotopic compositions of cold spring waters in the area are mostly located on the meteoric line. The sample from Alchichica lagoon has an isotopic composition typical of evaporated water. (author)