| Abstract |
Northwest Sabalan geothermal field is one of the four main high temperature fields in Iran that has practically been exploring and developing in the recent years. In the second stage of development and reservoir assessment, the discharged fluids from four wells, including NWS-6D, NWS-7D, NWS-9D and NWS-10D were analyzed. The water samples are classified as high chloride, neutral-pH and mature liquids that partially equilibrated with host rock. Chemistry of the waters also indicates that they are originated from an andesitic old hydrothermal system. The chemistry of deep aquifers charging wells is slightly different that can be due to presence of more than one aquifer with relatively different composition. Quartz geothermometers suggest the average temperature of 246°C, while cation geothermometers propose higher average temperatures; 257°C (K-Mg), 269 °C (Na-K), 279°C (Na-K-Ca), implying higher temperatures of aquifers at depth rather than adjacent area of boreholes. Gas (CO2 and H2S) and isotope (18O) geothermometers represent much higher temperature ranges; 280°C-301°C and 299 °C, respectively. The δD and δ18O isotope concentrations in the discharged samples of well NWS-6D (average of -69.8‰ and -7.9‰, respectively) indicate that charging water of geothermal system originate from higher elevations with lower humidity. Mineral saturation states of reservoir liquids display that amorphous silica is anticipated to be deposited in temperatures lower than 90°C, whereas super-saturation state of aquifers with regard to quartz will occur in temperatures lower than 240°C and will not cause serious problems during production. The deep aquifer liquid, however, is super-saturated with calcite in temperatures lower than 231°C and it is expected to give rise to scaling problems in the wells, even if the reservoir liquid is not super-saturated with regard to anhydrite and fluorite. Variation of saturation state trend of deep aquifers, charging each well along adiabatic boiling, can be considered as another sign of different chemical composition of deep multi-aquifer reservoir. |