| Abstract |
Chemical and isotopic analysis of geothermal fluids were carried out at the Los Humeros, Mexico, geothermal field in order to establish the water-rock equilibrium state and to know the origin of acidity in the reservoir. According to the waterrock equilibrium state, the Los Humeros geothermal reservoir is in an unbalanced state. This is probably due to recent volcanism, which indicates that the magmatic components in the geothermal fluids are not neutralized by reaction with feldspar and mica. The superheated steam contains HCl, which condenses or mixes with liquid at moderate temperature (< 300ÅãC). This is in agreement with the Los Humeros case, because high concentrations of HCl were detected in the steam phase. In addition, high concentrations of boron and arsenic also favor the dfinition of the Los Humeros as a recent volcanic system. Most wells of this field have high fluid enthalpy and almost all produce high quality steam at the wellheads. Chemical characteristics of the fluids show that the produced brine is low-saline water, whose geochemical character varies according to both well type and production. Shallower wells produce sodium bicarbonate (H-1) type water, whereas deeper wells exhibit a sodium chloride geochemical type (H-6). It was also found, according to vapor excess 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 in the reservoir. Measured temperatures at bottomhole vary (between 260 and 340ÅãC) and are in agreement with geothermometer calculations (Na/K of 295Å} 10ÅãC), (H2/Ar of 270 to 340ÅãC). The isotopic composition of the Los Humeros wells shows an ɬ18O shift which is characteristic of fluids of geothermal origin that have reached equilibrium with the rock at high temperatures. The springs in the area are mostly located on the meteoric line. The Alchichica sample has an isotopic composition of evaporated water. |