Chemical and isotopic studies in the Miravalles geothermal field, Costa Rica

Abstract

Commercial generation in the Miravalles geothermal field began in 1994 with the installation of 55 MW condensing power plant and at present the total generation is about 70 MW. The reservoir fluids have a sodium-chloride composition with a TDS of 5500-6000 ppm, a neutral to slightly alkaline pH, and temperatures around 230-240 °C. According to isotopic measurements the recharge zone is located in the northern part of the Miravalles area. After four years of exploitation the bicarbonate, sulphate and chloride spring waters (i.e. San Bernardo, Las Hornillas and Salitral Bagaces) present a constant isotopic composition during that time, while the majority of the well waters shows significant increase in heavy isotopes when comparing the pre-1997 to the 1997 data whereas very little change is observed between the 1997 and 1998 data. In most of the wells minor chemical changes can be detected in the production fluids with time, mainly in the chloride content. Contour maps show two trends of mixing of deeper fluid with the reinjection water. These trends will become larger when the second unit starts to operate in September 1998. Evaluation of the saturation index with respect to chrysolite, talc and calcite suggests that the deep water is slightly under-saturated with respect to chrysolite and talc and slightly over-saturated with respect to calcite. After adiabatic boiling these minerals become over-saturated giving higher potential for scaling. Calcite over-saturation has decreased with time.