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 a 55 MW condensing power plant, and at present, the total generation is about 117 MWe. 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. During the five years of exploitation, the bicarbonate, sulphate and chloride spring waters (i.e. San Bernardo, Las Hornillas and Salitral Bagaces) present a constant isotopic composition. The majority of the well waters show significant increases 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.