Geochemical studies of geothermal fluid and evaluation of well test results from wells SM-01, SM-02 and SM-03, Sol de Mañana field, geothermal project, Laguna Colorada, Bolivia

Abstract

ENDE (National Electricity Company of Bolivia) carried out well testing of wells SM-01, SM-02 and SM-03 in the geothermal field Sol de Mañana in 2013, twenty six years after they were drilled. Temperature, pressure and spinner logging were carried out under static (non-flow) and discharging conditions. The flow rates and enthalpies were measured using the Russell James and TFT methods and the results obtained showed a good correspondence between the two methods. According to the steam flow obtained by TFT, wells SM-01, SM-02 and SM-03 may generate 7, 7.8 and 8.2 MWe, respectively. Samples of brine and steam were collected during discharge through each of the four orifice plates used during the tests. Samples for silica analysis were not properly preserved. Calculations based on silica concentrations, such as reservoir concentrations, and temperatures, based on geothermometers, were not reliable due to silica polymerization. A Na/K geothermometer was in agreement with temperatures obtained from logs. Ternary diagrams of cations and anions showed a reservoir temperature of 280°C and classifies the fluid as mature water. Amorphous silica is expected to form at 150°C and 4.8 bar-a. The calcite solubility curve showed an uncommon behaviour, perhaps due to unreliable pH values. A mineral called Teschemacherite (NH4HCO3) was observed in well SM-02. Teschemacherite is a possible cause of the clogging of well SM-05. A special approach was performed in order to determine the scaling potential of Teschemacherite but, unfortunately, the necessary thermodynamic information was not available.