Geochemical Tools as a Contribution to Improve Geothermal Potential on the Azores Archipelago

Abstract

Three high enthalpy geothermal power plants are currently managed by EDA Renováveis S.A. in the Azores archipelago (Portugal), two of them located at São Miguel Island and one at Terceira Island. The volcanic nature of the Azores islands, linked with a complex geodynamic context, converts the archipelago into a natural candidate not only for geothermal exploitation but also to develop research that can then be applied to other areas of the world. Besides the application of traditional geophysical tools, geochemical data can also provide relevant information at different phases, from the exploration to the monitoring and reservoir management. One of the main objectives of the European project HEATSTORE (Geothermica Era-net) aims at constraining advanced numerical models that allow simulating the geothermal system under Fogo Volcano (São Miguel Island) through the use of geochemical data. To this end, chemical compositions of hydrothermal fumaroles, thermal and cold CO2-rich springs have been used as geothermometers for the geothermal reservoir. In addition, thermal energy released in a target area and permeability zones (diffuse degassing structures) were identified by mapping diffuse degassing areas. For the specific case of Ribeira Grande area (north flank of Fogo Volcano, São Miguel Island), equilibrium temperatures ranging between 231 and 258ºC were inferred for the reservoir feeding the fumaroles. Chemical composition of the springs highlighted disequilibrium conditions and oversaturation regarding silica solid phases making it challenging for the application of geothermometers. However, values in the range of 180 to 230ºC have been estimated. Despite some differences, these temperatures are quite similar to those measured in the geothermal wells, where maximum temperatures of 245ºC were detected. Radon (222Rn) and carbon dioxide (CO2) degassing maps carried out at Caldeiras da Ribeira Grande study site showed hidden diffuse degassing structures with general NW-SE direction, which are in agreement with the direction of the main tectonic structures identified in the area. A deep-derived CO2 emission of approximately 63 t d-1 was estimated for an area with 0.218 km2. Integration of this data with chemical composition of the fumaroles has allowed estimating a thermal energy of 7.7 MW for the study site. Alteration minerals present in rock samples from a geothermal well also contributed to define temperatures at depth as well as permeable layers. Integration of all these geochemical parameters will feed and constrain a holistic geothermal reservoir model for the area and shows the potential of these relatively “low cost” tools to any geothermal exploitation area.