Assessment of calcite scaling potential in the geothermal wells of the NW-Sabalan geothermal prospects, NW-Iran

Abstract

Three deep exploration wells have been drilled in the northwest Sabalan geothermal field in Iran. Two of them (NWS-1 and NWS-4) are productive. The aquifer temperature for both wells is between 220 and 230°C. During discharge tests of these wells, a decline in fluid discharge calcium concentration was observed that was thought to be due to calcite scaling. Also, traces of calcite scale were observed in the wells’ installed surface test equipment. Assessment of the calcite scaling potential in wells NWS-1 and NWS-4 is the object of this study. The large variation in the reported pH, especially in samples from well NWS-1, was considered to be a consequence of inadequate sampling and an analytical error which, in turn, led to large variations in the calculated calcite saturation state of the aquifer waters. The speciation program WATCH was used to calculate aquifer water composition for wells NWS-1 and NWS-4 as well as changes in calcite saturation during adiabatic boiling. At the aquifer temperature, one would expect the un-boiled aquifer water to be close to saturation with respect to calcite. For initially calcite-saturated aquifer water, adiabatic boiling leads to calcite over-saturation and therefore calcite precipitation. Maximum super-saturation is attained at about 180°C in both wells. For equilibrium, (maximum) degassing over-saturation by a factor of about 10 is produced at 180°C. In contrast, 10% of maximum degassing does not produce significant over-saturation. Degassing by boiling, causes the pH and carbonate ion concentrations to increase sharply in the boiled water with a slight increase in free calcium ion concentrations, while CO2 concentrations decrease. The sharp increase in the carbonate ion concentrations is largely responsible for the calcite over-saturation produced by boiling. When the water has been largely degassed (at about 180°C for maximum degassing) ..........